AMi "K" amp -- bad output transformer?

Hi, all. Over the weekend I got my K-100 up and pretty much working,
but I suspect I may have a bad output transformer.

This is the mono amp (R-2017/L-2156). I replaced caps, checked
resistors, cleaned controls and sockets, checked/replaced tubes,
etc.... and it comes up and sounds pretty good. Bass is somewhat
disappointing, but highs are really impressive and there is PLENTY of
volume.

Here's the annoying (and scary) problem: When the amp is first powered
up, after about five or ten seconds, I can hear a crackling sound
coming from the output transformer. This crackling sound lasts for
several seconds, and it can be heard even with no speakers hooked up!
With the speakers attached, the crackling sound is amplified, rather
loudly, regardless of the setting of the volume control.

After that the amp works pretty well, as described above. But after
it's been on for an hour or two, some noise can be heard over the
speakers when the amp is quiet, even while sitting idle and muted.
It's nothing as obnoxious as the power-up noise, but it's noticeable.

So is my output transformer bad? I can't imagine what else would cause
this. There were a few resistors that were slightly out of tolerance,
but not by much, and I don't see how that could cause this problem.

(In case it makes any difference: I'm using 6CZ5's in place of the
6973's. I did jumper the tube sockets as needed. When I'm sure the amp
is totally healthy, I'll probably put 6973's in.)

Russel Willoughby wrote:

Hi, all. Over the weekend I got my K-100 up and pretty much working,
but I suspect I may have a bad output transformer.

This is the mono amp (R-2017/L-2156). I replaced caps, checked
resistors, cleaned controls and sockets, checked/replaced tubes,
etc.... and it comes up and sounds pretty good. Bass is somewhat
disappointing, but highs are really impressive and there is PLENTY of
volume.

Here's the annoying (and scary) problem: When the amp is first powered
up, after about five or ten seconds, I can hear a crackling sound
coming from the output transformer. This crackling sound lasts for
several seconds, and it can be heard even with no speakers hooked up!
With the speakers attached, the crackling sound is amplified, rather
loudly, regardless of the setting of the volume control.

After that the amp works pretty well, as described above. But after
it's been on for an hour or two, some noise can be heard over the
speakers when the amp is quiet, even while sitting idle and muted.
It's nothing as obnoxious as the power-up noise, but it's noticeable.

So is my output transformer bad? I can't imagine what else would cause
this. There were a few resistors that were slightly out of tolerance,
but not by much, and I don't see how that could cause this problem.

(In case it makes any difference: I'm using 6CZ5's in place of the
6973's. I did jumper the tube sockets as needed. When I'm sure the amp
is totally healthy, I'll probably put 6973's in.)


Can't say for sure the output transformer is bad without having it here
on the bench, but if it is, one of these days it might short out and
take the power transformer along with it. I wouldn't recommend using it
until the problem is found and repaired. Actually, it could be the
power transformer arcing, or even the filter caps. The voltage at the
filter caps goes up an extra 50 to 100 volts before the tubes all warm
up and put a load on the supply. You don't happen to be using a SS
diode replacement for the 5U4 are you? That will REALLY cause the
voltage to go sky high upon power up!

If the problem is a transformer, I personally forget about finding an
original to replace it with. I've had too many used ones go bad. I use
brand new replacement transformers (and sleep better at night).

--
Tony

On Mon, 18 Aug 2003 20:53:35 GMT, Tony Miklos
wrote:


Can't say for sure the output transformer is bad without having it here
on the bench, but if it is, one of these days it might short out and
take the power transformer along with it. I wouldn't recommend using it
until the problem is found and repaired. Actually, it could be the
power transformer arcing, or even the filter caps. The voltage at the
filter caps goes up an extra 50 to 100 volts before the tubes all warm
up and put a load on the supply. You don't happen to be using a SS
diode replacement for the 5U4 are you? That will REALLY cause the
voltage to go sky high upon power up!

The 5U4 is a real 5U4GB. I have a couple of those (one NOS, one good
used) and the problem remains with either of them.

I have replaced all caps (except the ceramic discs) with new ones. I
even tested them before installing them. We can probably rule out
caps.

I went through and compared DC voltages at all of the tube pins to the
chart on the schematic, and all of them that I could good readings on
were correct. I haven't checked the voltage at the filter caps on
powerup, though.

If the problem is a transformer, I personally forget about finding an
original to replace it with. I've had too many used ones go bad. I use
brand new replacement transformers (and sleep better at night).

I'm hoping the power transformer is okay... that thing screams out
"expensive and hard to find". But the weird crackling noise is
definitely coming out of the output transformer, not the power
transformer.

Since output transformers seem to be common failure items on these
amps, I'm thinking I should just go ahead and put a new one in. I've
noticed Vern Tisdale sells new ones for $75. Any idea if these are a
good replacement?

Russel Willoughby wrote:

On Mon, 18 Aug 2003 20:53:35 GMT, Tony Miklos
wrote:



Can't say for sure the output transformer is bad without having it here
on the bench, but if it is, one of these days it might short out and
take the power transformer along with it. I wouldn't recommend using it
until the problem is found and repaired. Actually, it could be the
power transformer arcing, or even the filter caps. The voltage at the
filter caps goes up an extra 50 to 100 volts before the tubes all warm
up and put a load on the supply. You don't happen to be using a SS
diode replacement for the 5U4 are you? That will REALLY cause the
voltage to go sky high upon power up!


The 5U4 is a real 5U4GB. I have a couple of those (one NOS, one good
used) and the problem remains with either of them.

I have replaced all caps (except the ceramic discs) with new ones. I
even tested them before installing them. We can probably rule out
caps.


Quite often the ceramic caps will cause problems. Especially since they
often have 400 volts across them. I am assuming you replaced the filter
caps since you said you did all the others.



I went through and compared DC voltages at all of the tube pins to the
chart on the schematic, and all of them that I could good readings on
were correct. I haven't checked the voltage at the filter caps on
powerup, though.


If the problem is a transformer, I personally forget about finding an
original to replace it with. I've had too many used ones go bad. I use
brand new replacement transformers (and sleep better at night).


I'm hoping the power transformer is okay... that thing screams out
"expensive and hard to find".


They cost about the same as an output transformer.

But the weird crackling noise is
definitely coming out of the output transformer, not the power
transformer.

Since output transformers seem to be common failure items on these
amps, I'm thinking I should just go ahead and put a new one in. I've
noticed Vern Tisdale sells new ones for $75. Any idea if these are a
good replacement?


I would guess they are fine, I've never heard of a problem with them.
Probably the same kind I use.

--
Tony

Russel Willoughby wrote:

On 18 Aug 2003 10:57:39 -0700, (JP White)
wrote:



I restored an amp on the earlier H machine and it had a bad output
transformer. The windings were open so I never powered it up with the
bad transformer.
I called around for an original replacement and one vendor said he had
a shelf full of the amps I needed - but ALL had bad transformers. He
said the output transformers were often bad on my model. Not sure if
the same applies to the K.


This one uses 6973 tubes, and it is my understanding that a bad 6973
will fry an output transformer pretty quickly. One of the 6973's was
bad when I got the machine, so this is certainly a possibility.

I checked resistance of the transformer windings before I ever powered
up the amp, and it seemed okay. I didn't have a known good one to
compare it to, but there were no shorts or opens.

What I'm really curious about is what's causing that crackling sound.
Usually transformers fall into one of three categories: A) 100%
working, B) open winding, or C) burned to a crisp. This problem is new
to me.


Transformers like these run at close to 400 volts, (actually 800 volts
since the rated voltage is center tapped) often arc internally. So it's
not always a dead short that you can measure. That arcing could be the
crackling you hear.

--
Tony

On Mon, 18 Aug 2003 23:39:27 GMT, Tony Miklos
wrote:


Transformers like these run at close to 400 volts, (actually 800 volts
since the rated voltage is center tapped) often arc internally. So it's
not always a dead short that you can measure. That arcing could be the
crackling you hear.

Makes sense to me. I'm definitely replacing that transformer before I
run the amp anymore. I will re-check all of the ceramic caps too,
although I checked them all for shorts when I rebuilt the amp.

Thanks again for the advice!

In article ,
Russel Willoughby wrote:

I checked resistance of the transformer windings before I ever powered
up the amp, and it seemed okay. I didn't have a known good one to
compare it to, but there were no shorts or opens.

I use a different method to test output trannies, and it works quite
well if you're careful.

Remove the output tubes.

Then, hook a low voltage AC source - I'm talking like 1 - 10 volts here
to the speaker terminals (no speaker hooked up Measure the voltage
between ground and the plate on each output tube. They should be nearly
the same, and might be quite high (I generally use a 12V tranny on a
variac and bring the voltage up until I see 50V on the plates). If
they're equal, or dead close, the tranny's probbably ok. If there's a
big difference, then you're probbbly dealing with a bad unit.

This works better than resistance measurements.

A real tip off is low volume, distored sound, or one output tube MUCH
hotter than the other. Push-pull setups should work the tubes equally.

I'd look at the B+ as the amp comes up, I bet it spikes quite high
(that's at least somewhat normal though). Might have a flakey power
transformer - they do sometimes arc long before going bad. A weak
tranny and bad electrolytics will do it too.

But don't run it like that - you're hearing something arcing. Arcing ==
bad!
--
To email me, chage 'usermale' to 'usermail'.

Philip Nasadowski wrote:
In article ,
Russel Willoughby wrote:


I checked resistance of the transformer windings before I ever powered
up the amp, and it seemed okay. I didn't have a known good one to
compare it to, but there were no shorts or opens.


I use a different method to test output trannies, and it works quite
well if you're careful.

Remove the output tubes.

Then, hook a low voltage AC source - I'm talking like 1 - 10 volts here
to the speaker terminals (no speaker hooked up Measure the voltage
between ground and the plate on each output tube. They should be nearly
the same, and might be quite high (I generally use a 12V tranny on a
variac and bring the voltage up until I see 50V on the plates). If
they're equal, or dead close, the tranny's probbably ok. If there's a
big difference, then you're probbbly dealing with a bad unit.

This works better than resistance measurements.

I like that test! I do use a Z meter but still will try your method.

--
Tony

Russel Willoughby wrote:

On Mon, 18 Aug 2003 23:39:27 GMT, Tony Miklos
wrote:



Transformers like these run at close to 400 volts, (actually 800 volts
since the rated voltage is center tapped) often arc internally. So it's
not always a dead short that you can measure. That arcing could be the
crackling you hear.


Makes sense to me. I'm definitely replacing that transformer before I
run the amp anymore. I will re-check all of the ceramic caps too,
although I checked them all for shorts when I rebuilt the amp.

Thanks again for the advice!

Did you use one of those hand held capacitor testers? Or a *real* one
that applies the rated voltage to the cap for the leakage test? The
hand held jobs are pretty much useless testing leakage on high voltage
caps. Putting a couple volts across a cap rated for 600 volts just
doesn't cut it. If it's rated for 600 volts, you need to apply an
actual 600+ volts to the cap. (watch your fingers!) For those caps,
you should find NO leakage at all. If you are going to be wanting to
test high voltage caps often in the future, and don't want to spend a
grand or two on new equipment, I suggest finding an old "eye tube" type
capacitor tester, and if working on monitors or other high frequency
circuits, also a ESR meter (John Robertson sells the ESR meter). I'd
recommend simply replacing those caps anyway.

And I made a mistake above. Only the power transformer will have a
voltage potential near the 800 volt range, the output transformer is
around 400 volts. But still they can arc, and so can those ceramic caps.

--
Tony

I had a similar problem earlier this year.
After I had overhauled/rebuilt the power amp in my "K" , I tested it
with a dummy load for output. It could only produce about 1 watt RMS
instead of 16 watts RMS.
Drive signals were OK up to the 6973's and I knew the 6973's were OK
because a friend tested them on his AVO valve tester. I devised a
setup that could deliver 400 volts AC into the primary of the output
transformer and found that increasing the input voltage anymore than
that which would produce 1 watt in the dummy load caused the primary
current to increase alarmingly.
I had proven the output transformer to be bad so I found a local
friendly transformer company to rewind it for me. When I installed the
"new" transformer, the amplifier tested to a full 16 watts RMS output.
I pulled the old transformer windings apart and part way through I
found some minute green spots. These green spots were copper corrosion
where the windings had arced between insulation layers effectively
shorting turns when the applied voltage became great enough to cause
the breakdown.

Mark

Hi, all. Over the weekend I got my K-100 up and pretty much working,
but I suspect I may have a bad output transformer.

This is the mono amp (R-2017/L-2156). I replaced caps, checked
resistors, cleaned controls and sockets, checked/replaced tubes,
etc.... and it comes up and sounds pretty good. Bass is somewhat
disappointing, but highs are really impressive and there is PLENTY of
volume.

Here's the annoying (and scary) problem: When the amp is first powered
up, after about five or ten seconds, I can hear a crackling sound
coming from the output transformer. This crackling sound lasts for
several seconds, and it can be heard even with no speakers hooked up!
With the speakers attached, the crackling sound is amplified, rather
loudly, regardless of the setting of the volume control.

After that the amp works pretty well, as described above. But after
it's been on for an hour or two, some noise can be heard over the
speakers when the amp is quiet, even while sitting idle and muted.
It's nothing as obnoxious as the power-up noise, but it's noticeable.

So is my output transformer bad? I can't imagine what else would cause
this. There were a few resistors that were slightly out of tolerance,
but not by much, and I don't see how that could cause this problem.

(In case it makes any difference: I'm using 6CZ5's in place of the
6973's. I did jumper the tube sockets as needed. When I'm sure the amp
is totally healthy, I'll probably put 6973's in.)