Liam,
Sounds about right! I always leave the cap off the summing
resistors..... I can't stand the brightness on the bright channel and
what this cap adds in brightness to the bright channels is also what
it shaves off of the already bass heavy muddy channel. If you leave
this cap out there will be better balance between the two channels.
Then I add a 120pf bright cap to the volume control of the bright
channel so darker guitars have a fair chance of sounding half way
decent at very low settings of the volume control. Teles and Strats
get to use the higher gain bottom heavy mud channel. With this
setup, the amp is more usable to my ears. You can also try a 390pf
treble cap with a 47k slope resistor. The amps are so easy to
adjust to suit your needs. Isn't it great!?

Peter S 

Peter is right,some of the reasons of the different channels sound, is
the cathode caps and resistors values, so tweaking here may make
a difference in what you want. Also the plate caps.. use .02.. and
like peter said.. you could use a different value, 250pf cap for the
treble. and bypass the bassier sounding channel with a 120pf
cap..tone slope should be 56k or 47k.. and change the middle
pot..double the value.. it takes trial and error to find the right thing
your looking for. but you'll get it..Thats why tweaking them is so
good. either you have a channel thats too bright or the other is to
bassy.. then they jumper to try and blend them. but if you tweak
them for sound you like you won't need to do this and have two
channels you can use..I like the sound of them modded like the
JTM45/5F6A with a few tweaks..standard. 56K slope.. 250pf
treble,middle pot changed.. a feedback switch .,and .02
caps..plate,, and tweak the cathode caps and resistor values..and
some voltage changes depending on the amp.With EL34 tubesThis
setup gives the more Hendrix tone. chuncky and thick, but still has
plenty of treble.. Thats the sound and tone i always wanted..and
what i try to always get from my amps..{for me} Selecting the right
preamp tubes also makes these amps have there own voice.So
much depends on so many things.. guitar used,pickups and speaker
cab..
Richie{~}==::: 

Richie, do you prefer the shared cathode or the split cathode? Also
what happens by doubling the mid pot?

Thanks 

I like to split them.. the mid pot change gives alot more mid
control.. and a little boost..  

? 0.02 plate caps?
Richie{~}==::: 

Give this a try.

1) On the Normal channel have a 500pf on the Volume pot. This
will give it just enough high-end. The original 330uf cap on the
cathode has drifted, it actualy reads over 450uf on this amp,
something to note.
This is the perfect Strat channel, big bottom yet spanky and fresh
without getting harsh. I like it best with the volume at about "6 1/2"
to "7 1/2".

2) The Bright channel in this circuit has always given me fits. Way
too shrill. Rather than use the .0022 coupling cap off the first plate,
I went with a .01uf, that improved things a lot. The biggest battle
though was the fight with that pesky 500pf at the 470K mixing
resistor, I've always thrown the best silver/mica cap money could
buy for the 500pf and would end up taking it off, but with it off it
wasn't right either. Finaly I thought I'd just this one time see if there
was anything to the ceramic cap mojo. I happened to have a 500pf
ceramic cap I had taken out of an ancient amp and threw it in that
spot. Wow! It's true! I really thought it was going to be bullshit but
it really did make a very noticable improvement. The final thing I do
for this channel is have a 500pf on the Volume pot, for this I
managed to find a 500pf in polypropylene/foil, I think the Parts
Connection had it.
This turns this into the perfect channel to match up with a Les Paul.

3) Be sure there is a .68 on the cathode of the second stage.

4) Because I prefer to use paper-in-oil .12uf caps for the coupling
in the push-pull circuit, I use 33K for the negative feedback
resistor. Originaly I had a 27K here, (1986 schematic), the 1987
schematic says 47K, so I experemented by putting a 50K pot in
series so I could play around with the negative feedback resistance
in the 27K to 77K range. To my surprise even 47K started to
make the amp loose the articulation I like, above 55K it sounded
horrible, the best sound, (to my ears), was around 35K so I chose
the closest value I could to that, 33K, sounds great.

5) I keep the 1987 tone stack, (33K/500pf).

Try the above, let me know if it works for you. This should be a
interesting thread, too bad I'll be out of town for two weeks.

John Kelley Brown 

sounds like good mods from here, thanks for posting!
FWIW I prefer the .02's in the output stage paired with 100K bias
splitters (keep the RC short, grid current down) and this works
well IMHO with a 47K R -fb. 
.12 is Huge! Wow.. "talk about mud flaps.." ;-) 

speed

I agree with speed 100%, in fact, the baddest tones aren't super
high gain. Too much gain just turns your sound into mush. There's a
point of diminishing returns with just about everything in life and
gain is no exception. You can't beat a well constructed Plexi-style
amp using good parts, and a power attentuator to control the
volume. I just fired up a brand new amp. Took a different
approach this time. I copied the 50 watt plexi circuit note for note
with the exception of a 32uf PS cap for the preamp, and instead of
using a 2.7k cathode resistor, I used an 820ohm cathode resistor
on channel I and an 820ohm cathode resistor on channel II, a and
switchable ss or tube rectifier. I built it on a heavy 1/4" thick
aluminum chassis all PTP. This is pretty much a first for me, cuz I've
always used tag and eyelet and steel chassis. I used a 50 watt OEI
OT and a Hammond 272JX 250ma 300-300 PT. With the GZ34,
the amp is running at 410 volts with just under 40ma per EL34 at
idle. I just ballparked it so I could see what I needed to fix or
redress if anything. I haven't tried the ss setting yet cuz I need to
work on the bias circuit so it switches the bias voltage when I select
the ss rectifier. I fired this Gem up expecting to have to tweak it
quite a bit, but I have to say, right out of the starting gate she's
sounding MIGHTY fine! UN-REAL!!!!!!!!!. Thanx Speed! You're
the man! This amp has more than enough gain for any and all
chores and sounds AWESOME cranked wide open. If I want the
tone to dissapear, all I have to do is clean boost the front end. I
think I'm going to be building a bunch of these once people hear
'em. KKKKKKILLER!!!!!

Peter S
Kimerik Amplifiers