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previous: neddyboy Potentially Stupid Question -- 10/27/2000 7:28 PM View Thread

Long response... sorry

10/28/2000 12:36 AM
SteveFLong 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  
setting.  
 
Or....  
 
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  
point.  
 
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.  
 
Regards,  
Steve F.