Converting a quad-EL84 power amp into a guitar amplifier. Advice requested.

Dear All,

I was given an EL84 stereo power amp by a friend, and have recently
decided to convert it into a guitar amp. I did trace the circuit and
draw a schematic, but I haven't made a proper drawing yet and don't
think scanning my sloppy drawing is going to entertain anyone - I will
post a schematic soon. Hopefully I can get some advice from experience
people here.

I understand that it would basically be sacrilege to hack up a nice
tube stereo, but this thing isn't exactly a Dynaco or a Marantz. So
I'm giving a short history of this mysterious, unbranded, power amp:

It was a component of an integrated system, that my friend had found
in the garbage, and saved for use in his stereo system. He had his
uncle hack it up so it would work - the power switch and mains fuse
were hooked up through a wiring harness, and there is a tap on the
power tranny that was also sent out through this harness - this is
long gone. So it's just a power amp. Two line-level inputs, and two
speaker outputs. He found it, by audiophile standards, to be far
inferior to the expensive solid-state amp he already had. There was
some serious 120 Hz hum and hash, which we largely solved by replacing
the old yellow hat-shaped solid-state rectifiers with modern fast-
recovery diodes and replacing the main filter caps. Still it didn't
match his expensive MOSFET amp, not even close, so he donated it to me
in the hope that I'd do something useful with it. At first I just
mounted it in an old computer case, with a dummy load and resistor
network on each output, for use as a sound processor to 'warm up'
audio signals or as a dynamic mike preamplifier. It did do this
pretty well, and also lit the room fairly prettily, but I didn't
really use it that much.

Recently I built a little solid state amp, JFET based with a TDA2003
output, and I'm really happy with it, but I want to build a tube amp
so that I can have one of each kind. A bit more volume would be nice
too, 8 watts is great for practice but 20 or 30 would actually be
useful outside my tiny apartment. Meanwhile this thing is just
tempting me to do something useful with it.

I will be able to post a schematic later, but I can give a quick
overview of the construction of the amp now:

The power supply of the amp is a half-wave doubler putting out about
300 Volts B+ supply. There is a 6.3 center-tapped filament winding,
which was originally grounded but currently I have the center-tap
floated up to about +20V with a voltage divider to try to keep hum
down. There is no B+ fuse, but there is a 10 ohm resistor right
between the winding and the rectifiers which I imagine serves as a
fuse itself. There is no heater fuse. This seems sort of cheap, but
hey, it can be improved. There is no bias supply, as the EL84s are
cathode biased.

The line wiring seems to me, from my modern eyes, to be bizarre. This
is a 2-prong device. Each leg of the mains is connected to the
chassis ground through a 0.01 uF ceramic. I could understand putting
MOVs there, but I don't know what these caps are for - is this an
attempt to provide some sort of grounding without creating the safety
hazard that guessing which prong is 'neutral' would? I hope this
isn't a 'death cap' situation waiting to happen, if one gets leaky,
but I'm not sure I should just remove them either. I am currently
using a 3-prong cord and grounding the case that way, doesn't seem to
make any trouble to do that.

The amp is stereo, and is symmetrical as I would expect. Each channel
has 3 stages of B+ decoupling, which power the output transformer
center taps, the EL84 screens, and the preamp stage, respectively.
Both preamp supplies are connected together, 2 caps in parallel, but I
think it would do no real harm to separate them. All the caps seem to
hold charge well.

Each channel has one 12AX7 preamp tube and, as I previously mentioned,
two EL84s. The first 12AX7 stage is a basic voltage amplifier, with
negative feedback returning to the cathode resistor. The plate is
capacitor-coupled to a phase inverter stage, the one-tube kind with
100K cathode and plate resistors - these are connected through caps
and 1K resistors to the EL84 grids. The EL84s of each channel have a
common 130 ohm wirewound cathode resistor. I'm not really looking to
squeeze the extra watts out by changing it to a fixed-bias stage, but
if this is not a good way to run a guitar amp I guess I should know.
Negative feedback is done with a 47K resistor parallel by a 250 pf
capacitor, connected to the 4K cathode resistor of the first 12AX7
stage. I imagine this sets a voltage gain of about 10 with a 30 KHz
upper rolloff, but I'm not definitely sure. If it was an op-amp
that's what I'd think.

So things I'd like to do, and welcome advice about doing:

1) I don't think it takes much to drive an EL84 grid. Is this true?
Could I let one phase inverter drive both channels? This would let me
have 3 gain stages instead of one, and I could actually do a
reasonable preamp circuit with a tone stack and drive/master volume
controls.

2) Can I either parallel the outputs or bridge them? If both power
sections are outputting the same thing anyway, would it do any harm to
have them either paralleled into a 4 ohm load, or alternatively to
switch the phase on one channel and let it run bridged into 16? What
if I just bridged it into the 8 ohm 12" I have handy? Lower impedance
isn't supposed to damage tube outputs is it?

3) How do I manage negative feedback, if I have one input stage
driving two sets of EL84s? If I parallel the outputs, there's only be
one negative feedback path anyway. But if I bridge it, or just give
up and drive two separate speakers, would I see weird problems if only
one of the dual outputs was involved in feedback? Or would it just
happily deal with it? I figure I need some negative feedback for
speaker damping, even if I do reduce it for more overall gain. Am I
right in thinking so?

4) What should I do about the power supply wiring and the mysterious
caps? I am so tempted to just yank them.

Thanks for your time, and if anyone wants, I can have a schematic up
as soon as I make a proper drawing.




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