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


<morris.slutsky@xxxxxxxxx> wrote in message
news:23bf259d-94c0-4080-b4b2-dc19d5cfd77f@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Now it's gotten to the point to where I'd like the schematic.



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

Please do and be sure to triple check your work so it won't have
any mistakes.

Good luck, I'm sure something can be done to make this amp
do nearly what you would like it to do, if not completely and
without too much alteration. IOW you certainly don't need
to making unneccesary changes.



Thanks for the encouragement. I'm pretty sure this shouldn't be too
hard.

Here is the schematic - a power supply and one of the channels,
they're both the same as far as I can tell:

http://img248.imageshack.us/img248/6381/powersupplyaj6.png

http://img527.imageshack.us/img527/6518/channelty9.png

Thanks for the graphics.

You're right about the "death caps" - eliminate them and install
a 3 prong cord.
I see you have a split load PI for 2 output tubes.
Not so sure it would be advisable to use a single triode PI to
drive all 4 outputs. If you used a long tail PI, you'd be
back to square one.
You certainly do not want to add more preamp tubes unless
you add a filament transformer for their heaters. They are
available and by adding more preamp tubes you could
drive those outputs into saturation easily and sonically
pleasing if designed right.
However, do you have the room for an additional
filament transformer and a couple of preamp tube
sockets?

No comment on the power supply unless I examine it
more closely, but as you know it's a good idea to replace
the chemical filled electrolytic caps which actually
degenerate with non-use. But did you or someone
reform them? However, as long as you aren't gonna gig with
the amp, then I would leave them in there until they
fail and that would result in a loud hum or they'd
start erupting at their portals. If you plan on gigging
with the amp, then for sure, replace the electrolytics
as a preventative. And I'm sure you know to use
the same or close MFD value and you can go up
in voltage ratings if you have the space.

Do not replace the foil caps unless they are defective.
They rarely go bad, but some can start leaking in due time.
I wouldn't replace the ceramic caps either unless you
can find some NOS PFD Mica caps that are NOT silver-dipped.

Definately check all of the resistors and if you rebuild the
power supply and plan to gig, also replace the power supply
resistors and double their watt ratings of their replacements
but keeping the same resistance value. However, if you
try out the amp and it sounds great before fiddling with it
too much and you read a power supply resistor out of tolerance,
replace it with a new one of the same out of tolerance value,
but later try out the correct value with a new one and determine
which way you like the amp to sound, and certainly do not
lose your schematic of the original values.

Good luck again.

Ed


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