Rebuilding the HF100R Seeburg Amplifier (MR A5-L6) - Some Comments

I have been discussing the MR A5-L6 amplifer with some engineers and
they came up with the following recommendations. I wanted to post this
to the group for comments. The direction to the engineers was to come
up with recommendations on which capacitors / resistors / components
were the most critical to replace and/or upgrade to improve the sonic
quality of the amp.

Recommendations:

The tubes should all be replaced with new tubes. The transformers and
inductors should be fine. The resistors have probably changed value
over the years but will be close to the values needed. If you really
want to "rebuild" the amp, you could replace them all with metal film
(power supply) and carbon film or designer resistors. I can send some
URLs for sources of all of these parts. The capacitors, electrolytic
(labeled "Lytic" in the parts list) and paper will have dried out and
you should replace most of them. The most crucial capacitors are the
two groups below:

The coupling capacitors pass the signal from one stage to another.
Using a quality polypropylene film capacitor here will smooth out the
sound. You must the same or higher voltage rating and it is best to
keep the same value as in the original amp. Except: Capacitors C27 &
C29 can be increased to 1.0uF and there should be an increase in the
bass extension. The bass won't be louder, just extend further into
the low bass notes.

They a C6, C11, C15, C16, C27, C29 & C33.

The power supply capacitors are likely old and can be "leaky." It is
always best to replace them. In this case slightly larger or equal
values at the same or higher voltage ratings are the way to go.

They a C1, C2, C3a, b, c, d. The last is four capacitors in a
single "can."

Between the second half of V3 (6SN7GTA) and the first half of V6
(12AX7) is your tone controls. The two tubes "buffer the action of
the tone controls. The contacts in these four switches cause a lot of
noise when switching and can cause the circuit to open and no sound be
produced. If you don't need them you can bypass this part. If you
need the control, maybe you can set it up to have a fixed tone
contour, that is, without the switches.

The last part of the amplifier is the "Phase Splitter" 9second half of
the 12AX7) and the output tubes (6L6s). It is important for this to
be "balanced." The resistors of the phase splitter should be tightly
matched. The values of the resistors should be measured using a
digital meter. Buy 6 or 8 and you can match them to 0.1%. This helps
smooth out the sound. You get the output tubes (last stage) working
perfectly. They are R50 and R52 (0.39M Ohm).

Between this stage and the output tubes are the final "coupling
capacitors that can be increased as I mentioned above.

Finally, the two 6L6 tubes are your output tubes. They should be
matched. Matched pairs can be had from many vendors for under
$30/pair.

There are a number of tone shaping circuits that I have not divined
yet. Some of these can be compensation for the speaker's response
curves.

The "speaker switch" selects different taps from the output
transformer. When you find the one that sounds best, leave it there.
This isn't something that changes. It matches the output of the amp
to the impedance of the speakers. This is a selector switch that
could be bypassed, permanently soldered. But since it is at the
highest voltage (signal) level, it is going to give the least
problems. The lower the signal level the better it is to get rid of
switches. Especially old ones.

URL links of various parts suppliers for people rebuilding these amps:

http://www.tubesandmore.com/
http://www.angela.com/
http://store.yahoo.com/triodeel/capbarbin.html

The Solen/Axon caps are inexpensive Polypropylene caps. Try the
bargain bin at Triodeel. Of course, MCM electronics, Mouser and Newark
are large suppliers who
will sell one and two to home builders.

Hi Fred,

A good deal of what I am reading here seems like vast overkill. I don't think the overall sonics of a juke (speaker system
included) are such that this much attention needs to be afforded to the amp. Of course nothing you stated would result in any
problems, it would just be a great deal of expense and work for little gain IMO). It seems to me that with amount of re-work
they are recommending, you'd be better off to bypass the amp all together and connect the cartridge to a modern solid state or
tube Hi-Fi amp and run this to a external set of modern speakers. I'd bet that the rumble, wow and flutter, response of the
old Pickering cartridge, and mechanism stability will still place a limit on how good the overall system would sound. All this
aside, what kinds of records do you intend to play on the box? Will all this attention result in better sound if you load the
box with older used records from the 50's? If you load the box with new pristine records, how will the tracking force of
almost 6 grams affect the sound over time?

Here are some of my comments.

1. I would replace all electrolytic's and paper caps with new units. The paper caps are almost always leaky and will cause
problems if not addressed. The electrolytic's are usually in the process of dry out and if they fail shorted, could cause
damage to the transformer (not to mention hum). You do not need to use exotic expensive replacements for the paper caps. The
basic polyester cap will work fine.

2. Leave the resistors as is. After re-build fire up the amp and check AC and DC voltages to see if they are in line with the
schematic and troubleshoot from there.

3. You can test the tubes for basic operation on a tester. If they appear ok, leave them in and try running them. I would not
replace them unless they are dead, microphonic, or noisy. Tubes last a long time. You can make a case that the older tubes are
better constructed than the modern imports from China.

4. Clean all the switches with De-Oxit and you should be fine.

5. Re R27 & R29. These are currently .05uf coupling caps. The grid resistors of the 6L6's fed by these caps are 270K. This
results in a low frequency cutoff of about 11hz. I see no need to extend the response lower. I doubt the output transformer
will pass a signal this low and there just is not any useful information on records down there. I didn't check, but I doubt
the earlier stages have low frequency cutoff's down that low either.

Hope this is of some help.

Mark Robinson

Definite overkill - it is a 1954 mono tube amp. FWIW - from my archives here
is what I did on one of mine here.

Seeburg R - I rebuilt the entire unit a year ago - pinbank, mech,
everything, replaced fried select coils, etc. It sounded pretty good, so I
lived with until recently I started getting distortion that was a bear to
get rid of.

I thought I would post what I did and found so other readers may benefit
from my labors. Many thanks to all of the advice got from quite a number of
people on different aspects. I just did the grunt work - they were the
brains behind this, so I wanted to share the advice I got.
------------------
This week I totally, (& I mean totally, cleaned all connections with
cleaner, pipe cleaners & dremel tool with wire brush), all tube sockets,
plugs contacts, switches, controls. Use a good cleaner like DeOxit.

I cleaned contact block/cam switches with bond paper & control cleaner.
Business cards of good quality work great for this. Don't forget the
contacts on the treble & bass controls. Also, when you can get at it, an ink
eraser works great. Don't use abrasive substances - turns out the plating is
thin & you will compound the felony by using things like sandpaper.

I gave amp a really good once over - verified caps, resistors, voltages. I
found a bad loading connection on the input (47K - which means the cartridge
was intermittently floating). Serial numbers on that amp above 5000 take a
47K loading resistor, not 1M and R8 is 1K not 620 ohms.

I had low cathode voltage on the 6L6's, actually the whole input was about
20 volts low off pin 2 of the rectifier.

I checked the output xfmer as recommended. I got 59 ohms on the first 6L6,
49 on the other; 208 on the first 6L6, 230 ohms on the other. The R33
reads as 270 ohms VS 260 called for. I found out that the windings
resistance on the transformer will NOT be the same. There has to be an
identical number of turns per side, but not the same ohms because one
winding is inside; the other outside; therefore one requires more wire to
make the same number of turns and hence will not have the same resistance.

I replaced the 5U4 with another and the input voltage (pin 2 5U4) is now
at 425 vs. 410 indicated. On both 6L6's - Pin 4 is 404 vs. 395v, and pin 3
is 392 Vs 390v. Pin 8 is at 33 indicated 30V. I would say I am ok on that
section now. I also found out the voltages are running a bit high because
the line voltage into the amp today is higher than when designed. Don't
worry about it as long as it doesn't go above 425 on the main B+. There is
also a small difference since you have two tubes no longer drawing current
from the AVC circuit. Just be sure that the capacitor from pin 8 to ground
is rated at 35 v or 50 v...if it is a 25 volt, upgrade it.
In addition to the AVC tubes I pulled and the AVC control set to max, you'll
need to clip one side of the main AVC coupling capacitor...in that amp, it
goes from pin 4 of the 6SN7 to the first of the two AVC tubes...it was a
..06. Just cut one end and make sure it can't reconnect or short
anything...or even better, just remove the whole capacitor.
I found one side speaker was not in the best of shape and was distorting at
certain frequencies - more or less, depending on record. I replaced them
last night with a couple of 8" speakers I had.

I still have to set up the tone arm for proper balance & tracking, but this
thing sounds awesome now!.

Disconnect that AVC! Clipping out that capacitor, I now have volume in
spades and much, much fuller sound. I also now have my bass back on that
unit!

Hope all this can assist someone else.

Bruce in Canada


"Fred" wrote in message
om...
I have been discussing the MR A5-L6 amplifer with some engineers and
they came up with the following recommendations. I wanted to post this
to the group for comments. The direction to the engineers was to come
up with recommendations on which capacitors / resistors / components
were the most critical to replace and/or upgrade to improve the sonic
quality of the amp.

Recommendations:

The tubes should all be replaced with new tubes. The transformers and
inductors should be fine. The resistors have probably changed value
over the years but will be close to the values needed. If you really
want to "rebuild" the amp, you could replace them all with metal film
(power supply) and carbon film or designer resistors. I can send some
URLs for sources of all of these parts. The capacitors, electrolytic
(labeled "Lytic" in the parts list) and paper will have dried out and
you should replace most of them. The most crucial capacitors are the
two groups below:

The coupling capacitors pass the signal from one stage to another.
Using a quality polypropylene film capacitor here will smooth out the
sound. You must the same or higher voltage rating and it is best to
keep the same value as in the original amp. Except: Capacitors C27 &
C29 can be increased to 1.0uF and there should be an increase in the
bass extension. The bass won't be louder, just extend further into
the low bass notes.

They a C6, C11, C15, C16, C27, C29 & C33.

The power supply capacitors are likely old and can be "leaky." It is
always best to replace them. In this case slightly larger or equal
values at the same or higher voltage ratings are the way to go.

They a C1, C2, C3a, b, c, d. The last is four capacitors in a
single "can."

Between the second half of V3 (6SN7GTA) and the first half of V6
(12AX7) is your tone controls. The two tubes "buffer the action of
the tone controls. The contacts in these four switches cause a lot of
noise when switching and can cause the circuit to open and no sound be
produced. If you don't need them you can bypass this part. If you
need the control, maybe you can set it up to have a fixed tone
contour, that is, without the switches.

The last part of the amplifier is the "Phase Splitter" 9second half of
the 12AX7) and the output tubes (6L6s). It is important for this to
be "balanced." The resistors of the phase splitter should be tightly
matched. The values of the resistors should be measured using a
digital meter. Buy 6 or 8 and you can match them to 0.1%. This helps
smooth out the sound. You get the output tubes (last stage) working
perfectly. They are R50 and R52 (0.39M Ohm).

Between this stage and the output tubes are the final "coupling
capacitors that can be increased as I mentioned above.

Finally, the two 6L6 tubes are your output tubes. They should be
matched. Matched pairs can be had from many vendors for under
$30/pair.

There are a number of tone shaping circuits that I have not divined
yet. Some of these can be compensation for the speaker's response
curves.

The "speaker switch" selects different taps from the output
transformer. When you find the one that sounds best, leave it there.
This isn't something that changes. It matches the output of the amp
to the impedance of the speakers. This is a selector switch that
could be bypassed, permanently soldered. But since it is at the
highest voltage (signal) level, it is going to give the least
problems. The lower the signal level the better it is to get rid of
switches. Especially old ones.

URL links of various parts suppliers for people rebuilding these amps:

http://www.tubesandmore.com/
http://www.angela.com/
http://store.yahoo.com/triodeel/capbarbin.html

The Solen/Axon caps are inexpensive Polypropylene caps. Try the
bargain bin at Triodeel. Of course, MCM electronics, Mouser and Newark
are large suppliers who
will sell one and two to home builders.

On Fri, 23 Jan 2004 06:39:40 -0800, Fred wrote:

I have been discussing the MR A5-L6 amplifer with some engineers and
they came up with the following recommendations. I wanted to post this
to the group for comments. The direction to the engineers was to come
up with recommendations on which capacitors / resistors / components
were the most critical to replace and/or upgrade to improve the sonic
quality of the amp.

I think the main issues will be safety and reliability, as opposed to
"sonic quality" which tends to border on the absurd, like $300 power
cords and RCA jumper cables. We are dealing with a monaural coin-operated
jukebox that plays 45-RPM records here. Green magic marker for CDs and
the like does not apply.

Recommendations:

The tubes should all be replaced with new tubes. The transformers and
inductors should be fine. The resistors have probably changed value
over the years but will be close to the values needed. If you really
want to "rebuild" the amp, you could replace them all with metal film
(power supply) and carbon film or designer resistors. I can send some
URLs for sources of all of these parts. The capacitors, electrolytic
(labeled "Lytic" in the parts list) and paper will have dried out and
you should replace most of them. The most crucial capacitors are the
two groups below:

The tubes are likely to be just fine, and possibly of higher quality
than new tubes made today. I have a Seeburg R amplifier with the
original Seeburg branded Coke-bottle 6L6s that sounds just great.

On the other hand, the transformers and possibly the tubes may be in
trouble if the amplifier has been run for any length of time with old
leaky coupling capacitors.

I agree with replacing all of the paper capacitors and the smaller
electrolytics. The power supply filters may be OK if re-formed by
bringing the voltage up slowly.

If you are the new owner of a Seeburg R that has been gathering dust
in a storage room for the last 30 years, you're lucky. If someone
"plugged it in and it worked for a while, then started sounding funny
or quit", you are more likely to have some transformer and tube
issues.

As far as the resistors, I'd replace any of the carbon power resistors
(1 watt or higher), and any theat measure out of tolerance. Shotgun
replacement with metal film is way overkill and likely to introduce
problems with mechanical troubles, soldersplash shorts, etc.

The coupling capacitors pass the signal from one stage to another.
Using a quality polypropylene film capacitor here will smooth out the
sound.

Any plastic film capacitor should be fine. Mylar, Orange Drop, etc.

You must the same or higher voltage rating and it is best to
keep the same value as in the original amp. Except: Capacitors C27 &
C29 can be increased to 1.0uF and there should be an increase in the
bass extension. The bass won't be louder, just extend further into
the low bass notes.

Probably not necessary and possibly a bad idea. This amplifier will
be playing 45-RPM records on an insustrial-style turntable, not techno
CDs. The output transformer isn't going to have enough iron to handle the
extended bass, and all you're likely to boost is turntable rumble and
cabinet resonances. The originals have an Xc of about 180K at 20 hertz,
resulting an about a 2dB rolloff with the associated grid resistance of
the output tubes in parallel with the biasing resistors. Figure 1 dB down
at 40 Hz, about the lowest you're going to find on a 45, and below what
the cabinet speakers can handle. Boosting response below 20 Hz is just
asking for trouble in terms of rumble and the like.

They a C6, C11, C15, C16, C27, C29 & C33.

The power supply capacitors are likely old and can be "leaky." It is
always best to replace them. In this case slightly larger or equal
values at the same or higher voltage ratings are the way to go. They
a C1, C2, C3a, b, c, d. The last is four capacitors in a single "can."

Agreed.

Between the second half of V3 (6SN7GTA) and the first half of V6
(12AX7) is your tone controls. The two tubes "buffer the action of
the tone controls. The contacts in these four switches cause a lot of
noise when switching and can cause the circuit to open and no sound be
produced. If you don't need them you can bypass this part. If you
need the control, maybe you can set it up to have a fixed tone
contour, that is, without the switches.

I have restored many Seeburg amplifiers and haven't ever had a problem
with bad tone control switches. A shot of GAIG De-Ox-It is usually
all that's needed, if anything. You want these adjustable for different
room acoustics and record type.

The last part of the amplifier is the "Phase Splitter" 9second half of
the 12AX7) and the output tubes (6L6s). It is important for this to
be "balanced." The resistors of the phase splitter should be tightly
matched. The values of the resistors should be measured using a
digital meter. Buy 6 or 8 and you can match them to 0.1%. This helps
smooth out the sound. You get the output tubes (last stage) working
perfectly. They are R50 and R52 (0.39M Ohm).

Waaaaay overkill. The originals are 5%, if out of tolerance standard
carbon film 5% resistors should be fine.

Between this stage and the output tubes are the final "coupling
capacitors that can be increased as I mentioned above.

Finally, the two 6L6 tubes are your output tubes. They should be
matched. Matched pairs can be had from many vendors for under
$30/pair.

If you need to replace them, this isn't a bad idea, but two random
same-brand same-vintage NOS tubes may last longer and give better
preformance than a matched pair of current production Chinese tubes.

There are a number of tone shaping circuits that I have not divined
yet. Some of these can be compensation for the speaker's response
curves.

The "speaker switch" selects different taps from the output
transformer. When you find the one that sounds best, leave it there.
This isn't something that changes. It matches the output of the amp
to the impedance of the speakers. This is a selector switch that
could be bypassed, permanently soldered. But since it is at the
highest voltage (signal) level, it is going to give the least
problems. The lower the signal level the better it is to get rid of
switches. Especially old ones.

It should be left at the highest (25 watt) position unless you are
adding remote speakers. Otherwise you're running with a higher
speaker impedance than designed. This will result in higher plate
voltage swings on peaks.

URL links of various parts suppliers for people rebuilding these amps:

http://www.tubesandmore.com/
http://www.angela.com/
http://store.yahoo.com/triodeel/capbarbin.html

The Solen/Axon caps are inexpensive Polypropylene caps. Try the
bargain bin at Triodeel. Of course, MCM electronics, Mouser and Newark
are large suppliers who will sell one and two to home builders.

Mouser is a good choice, but I seriously doubt if the name brand of
the capacitor will be detectable by ear. If you're just doing one
amplifier, there are a number of sources for "kitted" capacitors as well
as tubes, etc. These have good instructions and labeling for those who
don't do a lot of jukebox amplifier rebuilding. One is Vern Tisdale,
http://www.verntisdale.com/ . A bit more money than the individual parts
from Mouser, but a lot easier than ordering dozens of line items.

I'd like to add the recommendation of using a three-wire grounding power
cord on both the amplifier and the box itself, replacing the two-wire
power cord and cabinet-mounted receptacle. The exposed metal parts should
also be grounded.

I replaced the .05 mfd coupling caps with .47mfd, 715P ''Orange drop'' added
more mfd
to the power supply caps. to stop motorboating.... So far I have modded 3
G's an R, 2 C's modded a 201 in a similar fashion ,modded a Q160 with .25mfd
coupling caps and re- configured the 7199 tube with a coupling cap and grid
resistor....necessary to stop motorboating which is feedback through the
power supply to the low level stages.... the G amp has a small hole that you
can mount a switch to disable the AGC ....the 345-03D cartridge helps alot
..........George






"Jay Hennigan" wrote in message
news
On Fri, 23 Jan 2004 06:39:40 -0800, Fred wrote:

I have been discussing the MR A5-L6 amplifer with some engineers and
they came up with the following recommendations. I wanted to post this
to the group for comments. The direction to the engineers was to come
up with recommendations on which capacitors / resistors / components
were the most critical to replace and/or upgrade to improve the sonic
quality of the amp.

I think the main issues will be safety and reliability, as opposed to
"sonic quality" which tends to border on the absurd, like $300 power
cords and RCA jumper cables. We are dealing with a monaural coin-operated
jukebox that plays 45-RPM records here. Green magic marker for CDs and
the like does not apply.

Recommendations:

The tubes should all be replaced with new tubes. The transformers and
inductors should be fine. The resistors have probably changed value
over the years but will be close to the values needed. If you really
want to "rebuild" the amp, you could replace them all with metal film
(power supply) and carbon film or designer resistors. I can send some
URLs for sources of all of these parts. The capacitors, electrolytic
(labeled "Lytic" in the parts list) and paper will have dried out and
you should replace most of them. The most crucial capacitors are the
two groups below:

The tubes are likely to be just fine, and possibly of higher quality
than new tubes made today. I have a Seeburg R amplifier with the
original Seeburg branded Coke-bottle 6L6s that sounds just great.

On the other hand, the transformers and possibly the tubes may be in
trouble if the amplifier has been run for any length of time with old
leaky coupling capacitors.

I agree with replacing all of the paper capacitors and the smaller
electrolytics. The power supply filters may be OK if re-formed by
bringing the voltage up slowly.

If you are the new owner of a Seeburg R that has been gathering dust
in a storage room for the last 30 years, you're lucky. If someone
"plugged it in and it worked for a while, then started sounding funny
or quit", you are more likely to have some transformer and tube
issues.

As far as the resistors, I'd replace any of the carbon power resistors
(1 watt or higher), and any theat measure out of tolerance. Shotgun
replacement with metal film is way overkill and likely to introduce
problems with mechanical troubles, soldersplash shorts, etc.

The coupling capacitors pass the signal from one stage to another.
Using a quality polypropylene film capacitor here will smooth out the
sound.

Any plastic film capacitor should be fine. Mylar, Orange Drop, etc.

You must the same or higher voltage rating and it is best to
keep the same value as in the original amp. Except: Capacitors C27 &
C29 can be increased to 1.0uF and there should be an increase in the
bass extension. The bass won't be louder, just extend further into
the low bass notes.

Probably not necessary and possibly a bad idea. This amplifier will
be playing 45-RPM records on an insustrial-style turntable, not techno
CDs. The output transformer isn't going to have enough iron to handle the
extended bass, and all you're likely to boost is turntable rumble and
cabinet resonances. The originals have an Xc of about 180K at 20 hertz,
resulting an about a 2dB rolloff with the associated grid resistance of
the output tubes in parallel with the biasing resistors. Figure 1 dB down
at 40 Hz, about the lowest you're going to find on a 45, and below what
the cabinet speakers can handle. Boosting response below 20 Hz is just
asking for trouble in terms of rumble and the like.

They a C6, C11, C15, C16, C27, C29 & C33.

The power supply capacitors are likely old and can be "leaky." It is
always best to replace them. In this case slightly larger or equal
values at the same or higher voltage ratings are the way to go. They
a C1, C2, C3a, b, c, d. The last is four capacitors in a single
"can."

Agreed.

Between the second half of V3 (6SN7GTA) and the first half of V6
(12AX7) is your tone controls. The two tubes "buffer the action of
the tone controls. The contacts in these four switches cause a lot of
noise when switching and can cause the circuit to open and no sound be
produced. If you don't need them you can bypass this part. If you
need the control, maybe you can set it up to have a fixed tone
contour, that is, without the switches.

I have restored many Seeburg amplifiers and haven't ever had a problem
with bad tone control switches. A shot of GAIG De-Ox-It is usually
all that's needed, if anything. You want these adjustable for different
room acoustics and record type.

The last part of the amplifier is the "Phase Splitter" 9second half of
the 12AX7) and the output tubes (6L6s). It is important for this to
be "balanced." The resistors of the phase splitter should be tightly
matched. The values of the resistors should be measured using a
digital meter. Buy 6 or 8 and you can match them to 0.1%. This helps
smooth out the sound. You get the output tubes (last stage) working
perfectly. They are R50 and R52 (0.39M Ohm).

Waaaaay overkill. The originals are 5%, if out of tolerance standard
carbon film 5% resistors should be fine.

Between this stage and the output tubes are the final "coupling
capacitors that can be increased as I mentioned above.

Finally, the two 6L6 tubes are your output tubes. They should be
matched. Matched pairs can be had from many vendors for under
$30/pair.

If you need to replace them, this isn't a bad idea, but two random
same-brand same-vintage NOS tubes may last longer and give better
preformance than a matched pair of current production Chinese tubes.

There are a number of tone shaping circuits that I have not divined
yet. Some of these can be compensation for the speaker's response
curves.

The "speaker switch" selects different taps from the output
transformer. When you find the one that sounds best, leave it there.
This isn't something that changes. It matches the output of the amp
to the impedance of the speakers. This is a selector switch that
could be bypassed, permanently soldered. But since it is at the
highest voltage (signal) level, it is going to give the least
problems. The lower the signal level the better it is to get rid of
switches. Especially old ones.

It should be left at the highest (25 watt) position unless you are
adding remote speakers. Otherwise you're running with a higher
speaker impedance than designed. This will result in higher plate
voltage swings on peaks.

URL links of various parts suppliers for people rebuilding these amps:

http://www.tubesandmore.com/
http://www.angela.com/
http://store.yahoo.com/triodeel/capbarbin.html

The Solen/Axon caps are inexpensive Polypropylene caps. Try the
bargain bin at Triodeel. Of course, MCM electronics, Mouser and Newark
are large suppliers who will sell one and two to home builders.

Mouser is a good choice, but I seriously doubt if the name brand of
the capacitor will be detectable by ear. If you're just doing one
amplifier, there are a number of sources for "kitted" capacitors as well
as tubes, etc. These have good instructions and labeling for those who
don't do a lot of jukebox amplifier rebuilding. One is Vern Tisdale,
http://www.verntisdale.com/ . A bit more money than the individual parts
from Mouser, but a lot easier than ordering dozens of line items.

I'd like to add the recommendation of using a three-wire grounding power
cord on both the amplifier and the box itself, replacing the two-wire
power cord and cabinet-mounted receptacle. The exposed metal parts should
also be grounded.

Forgot the 1000mfd 6L6 cathode bypass caps...George


"george craig" wrote in message
...
I replaced the .05 mfd coupling caps with .47mfd, 715P ''Orange drop''
added
more mfd
to the power supply caps. to stop motorboating.... So far I have modded 3
G's an R, 2 C's modded a 201 in a similar fashion ,modded a Q160 with
..25mfd
coupling caps and re- configured the 7199 tube with a coupling cap and
grid
resistor....necessary to stop motorboating which is feedback through the
power supply to the low level stages.... the G amp has a small hole that
you
can mount a switch to disable the AGC ....the 345-03D cartridge helps alot
.........George






"Jay Hennigan" wrote in message
news
On Fri, 23 Jan 2004 06:39:40 -0800, Fred wrote:

I have been discussing the MR A5-L6 amplifer with some engineers and
they came up with the following recommendations. I wanted to post this
to the group for comments. The direction to the engineers was to come
up with recommendations on which capacitors / resistors / components
were the most critical to replace and/or upgrade to improve the sonic
quality of the amp.

I think the main issues will be safety and reliability, as opposed to
"sonic quality" which tends to border on the absurd, like $300 power
cords and RCA jumper cables. We are dealing with a monaural
coin-operated
jukebox that plays 45-RPM records here. Green magic marker for CDs and
the like does not apply.

Recommendations:

The tubes should all be replaced with new tubes. The transformers and
inductors should be fine. The resistors have probably changed value
over the years but will be close to the values needed. If you really
want to "rebuild" the amp, you could replace them all with metal film
(power supply) and carbon film or designer resistors. I can send some
URLs for sources of all of these parts. The capacitors, electrolytic
(labeled "Lytic" in the parts list) and paper will have dried out and
you should replace most of them. The most crucial capacitors are the
two groups below:

The tubes are likely to be just fine, and possibly of higher quality
than new tubes made today. I have a Seeburg R amplifier with the
original Seeburg branded Coke-bottle 6L6s that sounds just great.

On the other hand, the transformers and possibly the tubes may be in
trouble if the amplifier has been run for any length of time with old
leaky coupling capacitors.

I agree with replacing all of the paper capacitors and the smaller
electrolytics. The power supply filters may be OK if re-formed by
bringing the voltage up slowly.

If you are the new owner of a Seeburg R that has been gathering dust
in a storage room for the last 30 years, you're lucky. If someone
"plugged it in and it worked for a while, then started sounding funny
or quit", you are more likely to have some transformer and tube
issues.

As far as the resistors, I'd replace any of the carbon power resistors
(1 watt or higher), and any theat measure out of tolerance. Shotgun
replacement with metal film is way overkill and likely to introduce
problems with mechanical troubles, soldersplash shorts, etc.

The coupling capacitors pass the signal from one stage to another.
Using a quality polypropylene film capacitor here will smooth out the
sound.

Any plastic film capacitor should be fine. Mylar, Orange Drop, etc.

You must the same or higher voltage rating and it is best to
keep the same value as in the original amp. Except: Capacitors C27 &
C29 can be increased to 1.0uF and there should be an increase in the
bass extension. The bass won't be louder, just extend further into
the low bass notes.

Probably not necessary and possibly a bad idea. This amplifier will
be playing 45-RPM records on an insustrial-style turntable, not techno
CDs. The output transformer isn't going to have enough iron to handle
the
extended bass, and all you're likely to boost is turntable rumble and
cabinet resonances. The originals have an Xc of about 180K at 20 hertz,
resulting an about a 2dB rolloff with the associated grid resistance of
the output tubes in parallel with the biasing resistors. Figure 1 dB
down
at 40 Hz, about the lowest you're going to find on a 45, and below what
the cabinet speakers can handle. Boosting response below 20 Hz is just
asking for trouble in terms of rumble and the like.

They a C6, C11, C15, C16, C27, C29 & C33.

The power supply capacitors are likely old and can be "leaky." It is
always best to replace them. In this case slightly larger or equal
values at the same or higher voltage ratings are the way to go. They
a C1, C2, C3a, b, c, d. The last is four capacitors in a single
"can."

Agreed.

Between the second half of V3 (6SN7GTA) and the first half of V6
(12AX7) is your tone controls. The two tubes "buffer the action of
the tone controls. The contacts in these four switches cause a lot of
noise when switching and can cause the circuit to open and no sound be
produced. If you don't need them you can bypass this part. If you
need the control, maybe you can set it up to have a fixed tone
contour, that is, without the switches.

I have restored many Seeburg amplifiers and haven't ever had a problem
with bad tone control switches. A shot of GAIG De-Ox-It is usually
all that's needed, if anything. You want these adjustable for different
room acoustics and record type.

The last part of the amplifier is the "Phase Splitter" 9second half of
the 12AX7) and the output tubes (6L6s). It is important for this to
be "balanced." The resistors of the phase splitter should be tightly
matched. The values of the resistors should be measured using a
digital meter. Buy 6 or 8 and you can match them to 0.1%. This helps
smooth out the sound. You get the output tubes (last stage) working
perfectly. They are R50 and R52 (0.39M Ohm).

Waaaaay overkill. The originals are 5%, if out of tolerance standard
carbon film 5% resistors should be fine.

Between this stage and the output tubes are the final "coupling
capacitors that can be increased as I mentioned above.

Finally, the two 6L6 tubes are your output tubes. They should be
matched. Matched pairs can be had from many vendors for under
$30/pair.

If you need to replace them, this isn't a bad idea, but two random
same-brand same-vintage NOS tubes may last longer and give better
preformance than a matched pair of current production Chinese tubes.

There are a number of tone shaping circuits that I have not divined
yet. Some of these can be compensation for the speaker's response
curves.

The "speaker switch" selects different taps from the output
transformer. When you find the one that sounds best, leave it there.
This isn't something that changes. It matches the output of the amp
to the impedance of the speakers. This is a selector switch that
could be bypassed, permanently soldered. But since it is at the
highest voltage (signal) level, it is going to give the least
problems. The lower the signal level the better it is to get rid of
switches. Especially old ones.

It should be left at the highest (25 watt) position unless you are
adding remote speakers. Otherwise you're running with a higher
speaker impedance than designed. This will result in higher plate
voltage swings on peaks.

URL links of various parts suppliers for people rebuilding these amps:

http://www.tubesandmore.com/
http://www.angela.com/
http://store.yahoo.com/triodeel/capbarbin.html

The Solen/Axon caps are inexpensive Polypropylene caps. Try the
bargain bin at Triodeel. Of course, MCM electronics, Mouser and Newark
are large suppliers who will sell one and two to home builders.

Mouser is a good choice, but I seriously doubt if the name brand of
the capacitor will be detectable by ear. If you're just doing one
amplifier, there are a number of sources for "kitted" capacitors as well
as tubes, etc. These have good instructions and labeling for those who
don't do a lot of jukebox amplifier rebuilding. One is Vern Tisdale,
http://www.verntisdale.com/ . A bit more money than the individual
parts
from Mouser, but a lot easier than ordering dozens of line items.

I'd like to add the recommendation of using a three-wire grounding power
cord on both the amplifier and the box itself, replacing the two-wire
power cord and cabinet-mounted receptacle. The exposed metal parts
should
also be grounded.

Probably not necessary and possibly a bad idea. This amplifier will
be playing 45-RPM records on an insustrial-style turntable, not techno
CDs. The output transformer isn't going to have enough iron to handle
the
extended bass, and all you're likely to boost is turntable rumble and
cabinet resonances. The originals have an Xc of about 180K at 20 hertz,
resulting an about a 2dB rolloff with the associated grid resistance of
the output tubes in parallel with the biasing resistors. Figure 1 dB
down
at 40 Hz, about the lowest you're going to find on a 45, and below what
the cabinet speakers can handle. Boosting response below 20 Hz is just
asking for trouble in terms of rumble and the like.

I would agree with extending the bass being a bad idea.Probably will cause
rumble or feedback howl.On my home stereo I have a pro dj turntable with an
equalizer.I have the large 15 inch woofer style speakers and I like the bass up
high like my jukeboxes and like to play it loud.Could never get a home
turntable to work without a terrible how at the volumes and bass settings I
like.One speaker is in a corner and farther away from the turntable about 4 ft
in my small living room and one's closer about 3 feet and in order to stop
turntable feedback and howl I have to set the lowest eq contro(30hz)l on the
left one in the corner all the way down(I guess it resonates more at low
frequencies in the corner),the right ones happy at half.At this setting I can
go almost all the way up at 100w per channel and bass on the amp at 8/10 with
loudness on without feedback.Now what's going to happen in a jukebox with much
lower isolation if you try to extend the bass?Probably will get that annoying
howl I used to on my stereo until I got it set just right.

If you encounter turntable rumble check the motor coupling , the rubber
turntable gromets,
and a rubber bushing under the motor...they dry out and get hard & brittle,
which introduces rumble ......cabinet resonances and reflections can be
controlled by installing fiber glass batting , carpet padding or Sonex
around the speakers .Installing these materials around the speakers can
SIGNIFICANTLY change the sound of your juke box ....as far as 45 rpm having
a limited response,I must differ, some records are good some are not... 10
in Club 45 rpm sound more musical than CD's... CD's do not have a high
enough sample rate.. Vinyl frequency response can go up to 50khz. The 4
channel system of the 70's known as CD-4 used a a high frequency sub-carrier
for the extra channels ......as far as the mods I posted in this thread,
they effect frequency response, not bass boost which is user selectable,if
crank up the bass and the volume all the way and it clips what do you
expect?? Audio output transformer saturation is amplitude and frequency
dependant, most people adjust the volume so you can hold a conversation in
the same room , and the best sound is with the output tubes running class
''A'' about 8 to 10 watts MAX, which has good dampening factor ,lowest
distortion and nice warm tube sound, not at the 25 watt level......Books
to read ; "Radiotron Designers Handbook''
............George



"Jjmscf" wrote in message
...
Probably not necessary and possibly a bad idea. This amplifier will
be playing 45-RPM records on an insustrial-style turntable, not techno
CDs. The output transformer isn't going to have enough iron to handle
the
extended bass, and all you're likely to boost is turntable rumble and
cabinet resonances. The originals have an Xc of about 180K at 20
hertz,
resulting an about a 2dB rolloff with the associated grid resistance
of
the output tubes in parallel with the biasing resistors. Figure 1 dB
down
at 40 Hz, about the lowest you're going to find on a 45, and below
what
the cabinet speakers can handle. Boosting response below 20 Hz is
just
asking for trouble in terms of rumble and the like.

I would agree with extending the bass being a bad idea.Probably will cause
rumble or feedback howl.On my home stereo I have a pro dj turntable with
an
equalizer.I have the large 15 inch woofer style speakers and I like the
bass up
high like my jukeboxes and like to play it loud.Could never get a home
turntable to work without a terrible how at the volumes and bass settings
I
like.One speaker is in a corner and farther away from the turntable about
4 ft
in my small living room and one's closer about 3 feet and in order to stop
turntable feedback and howl I have to set the lowest eq contro(30hz)l on
the
left one in the corner all the way down(I guess it resonates more at low
frequencies in the corner),the right ones happy at half.At this setting I
can
go almost all the way up at 100w per channel and bass on the amp at 8/10
with
loudness on without feedback.Now what's going to happen in a jukebox with
much
lower isolation if you try to extend the bass?Probably will get that
annoying
howl I used to on my stereo until I got it set just right.

Hi George,

I love my jukes but I don't happen to think that 45's sound more musical than CD's. I also don't believe sample rate has
anything to do with the matter at hand (16 bit 44.1khz is fine). While its true that you can get response out to 50khz with
vinyl (as if you need it), it requires a special playback stylus and great care in the mastering process (I believe CD-4's were
half speed mastered). The subcarrier's were FM/PM modulated on the CD-4 system BTW, so the extra channels were not sensitive
to distortion and amplitude variations so much. Given the stylus and cartridge used on the R, along with the typical records
that you might want to play on the box, I don't think you will find any useful information much above 15Khz. I'm sure you are
right that you can significantly change the sound of your juke by doing some of the things you outlined, but I still have to
ask why.

In your experience have you found that the R mech (when properly restored) has wow and flutter and rumble specs that are on the
same order of a good quality Hi-Fi turntable? Is there any useful information recorded on the typical Pop record of the era
below 40hz? It seems to me, that extending the response down very low just wastes amplifier power on inaudible grunge (not to
mention opening up the motorboating can of worms).

As far as I can see, the wattage selector on the amp does not change the operation class of the output stage in any way. It
just dials in a different impedance output tap on the transformer. You set the tap based on the configuration of external
speakers. As the manual states, you must set the switch to the 25 watt position if you are using only the internal speakers.
Did you re-bias the amp to run class A or perform other mods?

I wanted to correct what I said about the low freq cutoff of the phase splitter. Jay's analysis is much closer to the right
value. I did not take into account the surrounding resistors (only the grid resistance of 270K).

BTW, I agree with you the Radiotron is an excellent resource.

Thanks for the discussion,

Mark

"george craig" wrote in message ...
If you encounter turntable rumble check the motor coupling , the rubber
turntable gromets,
and a rubber bushing under the motor...they dry out and get hard & brittle,
which introduces rumble ......cabinet resonances and reflections can be
controlled by installing fiber glass batting , carpet padding or Sonex
around the speakers .Installing these materials around the speakers can
SIGNIFICANTLY change the sound of your juke box ....as far as 45 rpm having
a limited response,I must differ, some records are good some are not... 10
in Club 45 rpm sound more musical than CD's... CD's do not have a high
enough sample rate.. Vinyl frequency response can go up to 50khz. The 4
channel system of the 70's known as CD-4 used a a high frequency sub-carrier
for the extra channels ......as far as the mods I posted in this thread,
they effect frequency response, not bass boost which is user selectable,if
crank up the bass and the volume all the way and it clips what do you
expect?? Audio output transformer saturation is amplitude and frequency
dependant, most people adjust the volume so you can hold a conversation in
the same room , and the best sound is with the output tubes running class
''A'' about 8 to 10 watts MAX, which has good dampening factor ,lowest
distortion and nice warm tube sound, not at the 25 watt level......Books
to read ; "Radiotron Designers Handbook''
...........George

"Mark Robinson" wrote in message
...
Mark Robinson wrote,

I love my jukes but I don't happen to think that 45's sound more musical
than CD's. I also don't believe sample rate has
anything to do with the matter at hand (16 bit 44.1khz is fine). While
its true that you can get response out to 50khz with
vinyl (as if you need it), it requires a special playback stylus and great
care in the mastering process (I believe CD-4's were
half speed mastered). The subcarrier's were FM/PM modulated on the CD-4
system BTW, so the extra channels were not sensitive
to distortion and amplitude variations so much. Given the stylus and
cartridge used on the R, along with the typical records
that you might want to play on the box, I don't think you will find any
useful information much above 15Khz. I'm sure you are
right that you can significantly change the sound of your juke by doing
some of the things you outlined, but I still have to
ask why.

snip

To me, CD's sound too processed and manufactured....some of the best
vinyl was the
''Direct to Disc'' pressings of the late 70's mostly classic music....but
as far as pop music,
Pink Floyd's "Dark side of the Moon'' has significant low frequency
energy...I have some
45rpm 80's Euro dance music on the ''Beat Box'' label, mostly synthsizer
music that have significant amounts of powerful clean low frequency bass..
There are some really good records and there is alot of junk ...''Pink
noise''used in audio testing, decreases 3DB per octive and is constant
energy per octive, simulates the spectrum output of an orchestra.....the
most output is at the bottom of the spectrum.....The Pickering 345-03D was
a modern replacement for the ''RedHead'' which left alot to be desired, and
the 345-03D has problems in the ''R'' as it does not weigh as much as the
''Redhead'' and has tracking problems... You could experiment with solder
wrapped around the tone arm ......As far as rumble, I don't think it will
equal a fancy belt drive or direct drive turntable and for some reason
rumble seems to come out on some records more than others....The various
rubber grommets, bushings and motor coupling should be checked and replaced
,and possibly new ones could be made with RTV silicone rubber ,which has
different characteristics that may further reduce rumble .....My friends
Seeburg ''C''
had the rubber bushing missing under the motor and the rumble was beyond
belief!!!...Those clamps that hold the motor in place do not have any rubber
grommets and may be a significant
source of rumble.....this issue of rumble needs more investigation.



In your experience have you found that the R mech (when properly restored)
has wow and flutter and rumble specs that are on the
same order of a good quality Hi-Fi turntable? Is there any useful
information recorded on the typical Pop record of the era
below 40hz? It seems to me, that extending the response down very low
just wastes amplifier power on inaudible grunge (not to
mention opening up the motorboating can of worms).

As far as I can see, the wattage selector on the amp does not change the
operation class of the output stage in any way. It
just dials in a different impedance output tap on the transformer. You
set the tap based on the configuration of external
speakers. As the manual states, you must set the switch to the 25 watt
position if you are using only the internal speakers.
Did you re-bias the amp to run class A or perform other mods?

snip

I normally run my amps in the 25 watt position this provides 25 watts to
the main field coil speaker with no external speakers and with the AVC set
at ''0''. I added a switch between the 6SK7 plate and the .06 mfd cap to
remove the the AVC ....most people yank the 6SK7 tube to de-activate the
AVC....... Seeburg ''G'' and newer amps are configured class AB1 ,
Ultra-linear which is feed back via screen taps on the output transformer it
will run class ''A'' up to 8 to 10 watts... as the level is increased it
goes into class ''B'' at max output, each tube couducts for aprox . 180 deg.
this can cause switching transients that can cause high order harmonic
distortion, but the negitive feedback loop should take care of that...one
possible mod would be to convert the output tubes to triode mode, disconect
the screen tap and tie the screen to plate via 100ohm 1 watt resistors, this
will lower distortion, lower output level about 8 to 10watts
and improve dampening factor...Tighter bass..In general, I don't play my
jukes all that loud and they stay in class ''A'' operation........In alot
of audio circles people do the math or CAD design and after building an amp,
pre-amp , crossover or speaker box found that it didn't sound right and
start tweeking the design...it took me 3 months to mod the ''G'' amp and
it's an on going project.... quite often a manufacturer will select
component values based on price and avliability.... some parts were not
avaliable in the 50's.. ''Mad Man Muntz'' used to go into
the lab with wire cutters and snip out parts on prototype equipment and
would say to the engineers ''Make it work with out the parts'' this analog
audio can be alot of time and on going tweeking and it's not real
structured like digital design or computer programming I worked in a
millimeter wave design lab on 77ghz radar, which is analog taken to an
extreme, despite CAD,
alot of stuff is trial and error .........padding on the inside of the juke
cabinet reduces reflections
in the important 300-3000 hz range... a lack padding causes muddy-ness in
the vocals if your juke is boomy, putting the padding directly over the
speaker basket will dampen the boomy ness....I think every hifi speaker box
ever made has padding, but the juke manufacturers never incorporated
it......as a project put a beach towel underneath the 12 in speakers in your
juke
and a beach towel over the top of them and let me know how it
sounds...Report back to the group.....


I wanted to correct what I said about the low freq cutoff of the phase
splitter. Jay's analysis is much closer to the right
value. I did not take into account the surrounding resistors (only the
grid resistance of 270K).

BTW, I agree with you the Radiotron is an excellent resource.

Thanks for the discussion,

Mark

"george craig" wrote in message
...
If you encounter turntable rumble check the motor coupling , the rubber
turntable gromets,
and a rubber bushing under the motor...they dry out and get hard &
brittle,
which introduces rumble ......cabinet resonances and reflections can be
controlled by installing fiber glass batting , carpet padding or Sonex
around the speakers .Installing these materials around the speakers can
SIGNIFICANTLY change the sound of your juke box ....as far as 45 rpm
having
a limited response,I must differ, some records are good some are not...
10
in Club 45 rpm sound more musical than CD's... CD's do not have a high
enough sample rate.. Vinyl frequency response can go up to 50khz. The 4
channel system of the 70's known as CD-4 used a a high frequency
sub-carrier
for the extra channels ......as far as the mods I posted in this
thread,
they effect frequency response, not bass boost which is user
selectable,if
crank up the bass and the volume all the way and it clips what do you
expect?? Audio output transformer saturation is amplitude and frequency
dependant, most people adjust the volume so you can hold a conversation
in
the same room , and the best sound is with the output tubes running
class
''A'' about 8 to 10 watts MAX, which has good dampening factor ,lowest
distortion and nice warm tube sound, not at the 25 watt
level......Books
to read ; "Radiotron Designers Handbook''
...........George