Re: My quad EL84 build, schematics included.


"Gary Gerhart" <Gary@xxxxxxxxxxxxxxx> wrote in message
news:ZJRMj.653$26.439@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
morris.slutsky@xxxxxxxxx wrote:
Hi everyone,

Somehow I hate to interrupt the politics and flaming by talking about
guitar amplifiers, I hope this is an appropriate group. Anyway I
would like to thank everyone who advised me in my earlier post,
"Converting a quad-EL84 power amp into a guitar amplifier". I've got
it pretty much done, although I might tweak it a little more when I
have time, and I thought it would be cool to post schematics and see
what everyone thinks.

I'm pretty sure the bridged-mono output is working okay. At least,
power seems to get to the speakers just fine. Those are some weird
output transformers, but they probably really are designed to run a 3
ohm load. I have no idea why anyone would build an amp that way, but
bridging them sure seems to make them kick. This thing, back when my
friend tried it as a hifi amp, really did sound weak into 8 ohm
speakers.

There are some seemingly useless resistors - a 1M at the first stage
and a 1.5M at the third stage. Those basically terminate cable runs,
I am using shielded cable from the front panel (grounded at the panel,
shield left open on the amp side). I found that it was less buzzy and
hummy if I did that. I know I'm losing gain there, but I think it's
worth it to have a quiet amp. Unless anyone has better ideas? And I
am aware that I am heavily loading the first gain stage, the 100K
resistor there is pretty small, it pulls the voltage gain of that
stage down to a bit over 10 instead of the 40 or so that it would give
otherwise with a more typical 470K resistor there. I did that because
I wanted to cut the RC time constant, to avoid the nasty grid
conduction and bass farting cutoff I was getting when I pushed it, and
I didn't have another high-voltage cap to put in. I know really I
should have reduced the cap and not the resistor.

The master volume - I was originally going to just connect it as a
variable resistor between the two PI outputs, labeled PI1 and PI2 on
the schematic, but it just didn't quite sound right doing it that
way. I mean, yeah, it worked, it just sounded a bit off somehow in a
way I can't really explain.

If the tone stack looks a bit goofy, again, I was using parts that I
had or that I could buy at Radio Shack because I just didn't feel like
waiting for mail order. It's an odd looking thing - I used the Duncan
Labs tonestack calculator, it has the same mid notch frequency of a
Marshall one, higher than that of a Fender stack, but it doesn't push
the highs as much as a Marshall one would. But I guess I didn't want
to waste a tube doing a cathode follower to push a proper Marshall
circuit, and yeah, I wanted to use the parts I had. For example, the
0.020 cap is my two ceramic high voltage "death caps" that I removed
while doing the 3-prong conversion. So overall it's a recycle bin of
a project.

But I like the sound. I'm going to have to play it a lot more before
I really form a complete opinion of it's sound. Definitely though
it's got good articulation and plenty of punch, and through my Carvin
12" (installed in a Peavey Bandit combo), it's way too loud for my
small apartment. Fun though. I've always wanted one of these..

Schematics of the power supply, the preamp, and the power amp:


http://img255.imageshack.us/img255/8799/miniquadbuild1supplyhx9.png
http://img255.imageshack.us/img255/8126/miniquadbuild1preamphj3.png
http://img225.imageshack.us/img225/3903/miniquadbuild1powerampwo7.png



I must opine...

It is very refreshing to see someone thinking outside the
"this is a clone of ..." box. Bridged output stage, non
*standard* component values, a dude thinking for himself.
I, for one, am very pleased. I hope you continue to enjoy the
tone. And tweak it, as required.

Gary Gerhart
Gerhart Amplification



4x EL84 is going to be about 36W give or take. That's going to be loud
enough for a small-medium club and to get over the drummer. No doubt about
it, you've built yourself the monster you're not prepared to use. So what
can you do about it?

I'm not sure. I see discussions over at www.18watt.com and at
www.ampgarage.com about power scaling. As I said, I'm not sure, but it
might be something to look into. As I understand it, pulling two tubes will
lower perceived volume by about 10% and that isn't anywhere near meeting
your needs for an apartment dweller.

As for the re-use of old parts, those who know me here, know that I'm a huge
fan of re-use and adaptive use. I'd like to follow Gary in offering my
congratulations on getting this done.

Now, what we really want are "beaver shots" of the amp. Got a camera?

Regards,
Phil


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