How to increase bass response in Wurlitzer 1100 (Model 506) amplifier?

Because of restrictions imposed by other family members, I am often
forced to listen to my Wurlitzer 1100 at lower volume levels than I
would like. Can anyone suggest alternative values of capacitor(s) or
resistor(s) - together with their identifying numbers on the diagram -
which would give a slight boost to the bass response - rather like the
'loudness' control effect featured on hi-fi amplifiers for low level
listening. Incidentally, I have rebuilt the amp with new caps,
resistors and valves and it sounds great at mid and higher levels.
Many thanks in advance to anyone who may respond.

Paul in the UK.

Because of restrictions imposed by other family members, I am often
forced to listen to my Wurlitzer 1100 at lower volume levels than I
would like. Can anyone suggest alternative values of capacitor(s) or
resistor(s) - together with their identifying numbers on the diagram -
which would give a slight boost to the bass response - rather like the
'loudness' control effect featured on hi-fi amplifiers for low level
listening. Incidentally, I have rebuilt the amp with new caps,
resistors and valves and it sounds great at mid and higher levels.
Many thanks in advance to anyone who may respond.

Paul in the UK.

Paul, have you considered a simple set of pre-amp controls in-line
between the receiver and the amp? The following is a link for a cheap
mono pre-amp with tone controls. That might work, but I'm honestly
not familiar with the impedance I/O values of your particular setup.

http://www.qkits.com/serv/qkits/diy/pages/FK627.asp

Tube-based pre-amps with tone controls are significantly more
expensive, but the UK has a greater following of valve technology than
the US.

You may wish to simulate the amp using a program like PSpice and then
experiment with various changes to see their effect on the output
signal's frequency response.

Perhaps a cheap and easy mod would be to replace the woofer and
tweaters with higher efficiency ones (of course it might result in
your ears melting when the volume increases).

Hi Paul,

I took a quick look at the 506 schematic from Vern Tisdale's site. You could add a simple low freq. shelving boost by adding a
series RC network from the pin 1 grid of the 6SN7 to ground. This is on the right hand schematic side of R27. The scan is
hard to read, but I think it is a 560K resistor. You can control the amount of boost by adjusting the value of the added
resistor (lower values yield greater boosts). The crossover freq is then set by the series cap. Since this is a passive
network, you will lose some gain (amount depending on the value of the series resistor). The final response using such an
network looks like this:


--------------------
-
-
------------------------


The boost (in db) is determined by:

boost = -20 * log(R2/(R1+R2))

and the approximate cap value for a given turnover frequency is found by:

C = 1/(2* PI * (R1+ R2) * Turnover)

where R1 is the 560K in the circuit and R2 is the new resistor added.

So, if you want a 6 db boost below 100hz,

R2 = 560K

-20* log(.5) = 6db

and

C = .015uf

1/(2*3.14159 * 1.12e06 * 100) = 1.42e-9


I don't own this box so I can't try it out, but it is easy to install so it might be worth a try. Hope it helps you out. Let
us know if you try is and it works out for you.

Mark Robinson





"Paul Taylor" wrote in message om...
Because of restrictions imposed by other family members, I am often
forced to listen to my Wurlitzer 1100 at lower volume levels than I
would like. Can anyone suggest alternative values of capacitor(s) or
resistor(s) - together with their identifying numbers on the diagram -
which would give a slight boost to the bass response - rather like the
'loudness' control effect featured on hi-fi amplifiers for low level
listening. Incidentally, I have rebuilt the amp with new caps,
resistors and valves and it sounds great at mid and higher levels.
Many thanks in advance to anyone who may respond.

Paul in the UK.

Hi Again,

Oops, that cap value should be .0015uf. Sorry about that.

Mark