Tube Amps / Music Electronics
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|previous: neddyboy Potentially Stupid Question -- 10/27/2000 7:28 PM||View Thread|
|10/28/2000 12:36 AM|
|SteveF||Long response... sorry|
Like Dietmar said, it's not a good
idea to simply run the speaker output of
one channel into the input of the
other. Even if you did get away
with it as far as damaging the
equipment, it probably would be
unacceptably noisy because you would have
to run the first amp channel at such a low
level that the signal/noise performance
would be terrible.
Having said that, I have made resistive loads
for small amps that I wanted to use as preamps to
bigger solid-state equipment that worked OK.
What you would do is to provide a 8 ohm (or whatever
the speaker impedance is) power resistor (10 watts
should be big enough) to load the output of the amp, and parallel across that with a voltage divider
that would reduce the voltage swing of the amp
down to an acceptable level. All this requires
are two resistors, with values of a ratio that
represents the amount of signal reduction you
want. For example, 10K and 100K resistors in
series will provide greater than 1:10 reduction in the signal level if you connect across the 10K
resistor. 110K in parallel with an 8 ohm
resistor is not a significant change
to the load seen by the amp, and you would get
very little current flow thru the divider.
This is NOT a good idea on big amps, but seems to be not terribly stressful to small ones, especially if the side
of the amp being used as a preamp is not being
driven very hard. I'm not sure what amount of reduction you would be looking for, so you might have to experiment a little. It really depends on
the sensitivity of the amp's input. Reducing the
signal to one tenth of its original level
might be a good place to start. I'd play with
the value of the smaller resistor until you
get the amount of drive you are looking for.
You could wire a pot in there as a variable
resistor (in place of the smaller R) to tweak it, and when you have got the drive you want, replace
it with a resistor of comparable value to the pot
The above will work, and it's pretty safe
if you have not got much experience working
inside amps, but it wastes half the
power you have available, and is likely to
be a little noisy, especially with respect to hum.
From your earlier post you said you
had two 12AX7's and two EL84's - I'm
assuming it's a stereo amp with one
of each tube per side. If you are willing
to go into the amp, and are able to do so
safely, you COULD rob one
section of one 12AX7 from one side and use
it to preamplify the signal going into the
input of the other side. The other thing you'd
have to do would be to parallel the inputs to
the driver stages of each channel so you would still have both output sections working.
Then you'd have two stages of gain, driving the
paralleled inputs of the driver stages, each of
which drives its own power tube and speaker. There
are some commercial examples of amps that use
two discrete output stages like this, I think Gibson
made some. This might get you the gain you want,
but I'd be surprised if the result sounded much like
a guitar amp. An electric guitar played through an
amp with a flat frequency response sounds, well, flat. But,there are simple things you can do to alter the frequency response, the most simple thing is to change the value of the coupling caps between stages. Look at some guitar amp schems to get an idea of typical values. This may provide a starting
As several other people have pointed out, if you don't feel confident you can do this without grabbing the wrong thing in there, don't do it.
Respect for high voltages is the first prerequisite
to working on tube gear of any size.
Sorry about the windy post. Hope this helps some.