ve been looking through the
schematics, and the standard 1986
preamp is a shared cathode first
stage preamp about identical to the
JTM45 and early 50 watt amps. The
only real difference appears to be the
feedback resistor and OT tap the f.b.
is connected to. And of course EL34
instead of 6L6 when compared to
the JTM45.

to do this, you'd need a new power
transformer to give you a 5v winding for the
GZ34, and a new OT

COpy the 1987 circuit, I'd recommend a
.68uf presence cap vs. a .1 uf, you may
also want to cut the .05uf bright cap on the
bright volume, or go with a smaller value
like 500pf. YOu coudl jumper the briht and
noirmal channel, and play with the settings
on each, to get the right mix of punchy vs.
warm,bluesey. The OT WILL make a
significant difference, since the 70's and
80's ots will fold up and compres rather
quickly, whereas a good plexi OT such as
one from OEI, will give you more punch,
and clarity. 

I would forgo the 5000pf brite cap on the volume (either skip it or
go for 500pF), I personally like the .68 on the 2nd stage, the .02's
after the PI, and 100K bias splitter resistors. Really well controlled
overdrive in this set up at no cost to the clean tones. 

Man do I hate those CTS pots! I think the one thing that bothers
me most is that the turning torque isn't very consistent from pot to
pot. One will be real smooth to the point of feeling almost broken
and another one will feel too tight. I'm actually starting to use the
Magic Parts pots more and more. They're basically a Marshall pot
with solder lugs. Taper is a little more useful too. I'd have to
measure to be sure but the audio tapers are around 15% at half
rotation. 

The clarostat pots are really sweet though! I wonder why Randall
Aiken can't get audio tapers in conductive plastic.

The reason I'm inquiring about the multi-turn pots is that they'd
make for a nice fit on a tag or eyelet board without taking up the
room of a conventional pot. Plus the added benefit of getting a
more precise bias adjustment.

I can get audio tapers in conductive plastic, they are just very long
lead time, so I opted to get a batch of the 1Meg Precision
hot-molded carbon RV4's instead, which are also a very fine pot. I
will be getting some conductive plastic audio tapers in, but it will be
a couple of months.

Randall Aiken 

Chinese KT66 in '67 Plexi.



The Valve Art tubes will have a relatively small (for the size of the
tube) brown base.