I have one of these and have made a few changes. I changed
the gain set resistors and caps for the first stage and tone
stack to Marshall values. I wanted the amp to "crunch" a little
more and wanted to focus the tone a little toward the mid
spectrum. I kept the stock "mid boost" which is just a larger
treble cap added. I also changed the bass control to Vox
wiring which controls both bass and mids (turn the bass up,
the mids go down, turn the bass down the mids come up.) 
I cobbled a full size 3 spring reverb tank down to 15" to fit in
the cabinet and replaced the internal springs. After I pulled the
little cardboard tank, I found a spring had come off. The
replacement tank improved things a little, but the speaker
driven design is pretty limited. 

After changing the first gain stage, the distortion channel
sounds pretty good. Very similar in fact to the clean channel
just being cranked. 

I also elevated the heater supply to lower hum. 

I plan on replacing the speaker but haven't decided on the
replacement. I also plan on putting in a proper reverb drive
circuit, although it will be SS. 
Steve Dallman 

If you want to soften the B+ rail a little without a
rectifier tube, you can use a dropping resistor after the
FWB diode block.
Maybe something like a 500ohm 10 watter might sound
good to you.
Bruce

You won't be changing any resistors, you'll be adding
one or a couple 5 watt 150-500 ohm resistors.
I'd try two, one in each lead from the power tranny
high voltage secondary to the rectifiers.
I think this is really a voltage doubler though.
Anyhow, current from the high voltage taps will flow
through the resistors before they hit the recitifiers which
will... (sort of) simulate the drop across a rectifier tube.
I'll email you later if you don't understand this.
Bruce

If you are handy with a soldering iron and want to pick
up the ball, my next move would have been to remove
the reverb *tank* from inside the chassis and connect it
to the PCB with shielded wires . . .
 
I think most of the hum comes from the voltage
doubler B+ supply. I ended up putting in a
choke/capacitor pi filter. That helped a lot. Email me if
you want details of any mods.
 
Dave
DHarris@VGGas.com

I recognise the problem from my own SE EL84 design.
There is a slight hum present in any SE design (at least,
that's what I've been told), but in my case, it turned out
to be mostly from the filament supply. To counter the
problem, I installed a seperate filament trans and
rectified it to supply 6.5v DC. The difference was
huge! I was lucky enough to find the perfect trans in
my parts-bin, but seperate filament transformers are
available at very reasonable prices from Hammond.
Use a bridge rectifier and some filtering and you've got
the most silent filament supply you can imagine - easy
and fool-proof. I guess you could also rectify your
existing filament supply, but the voltage would need to
be brought down with a resistor, and then there might
not be enough current left. It should be worth a try,
anyhow.
 
Henrik

I am
going to remove the artificial center-tap by removing
the two 47 ohm resistors. I'll be getting the voltage from
B+ (496 VDC) via a 220K ohm (1W) resistor, I
calculated, using the resistor divider formula, I figured I
need about 17K ohms (1/2W) to ground to get 35 VDC
(didn't take into account though the load induced be the
heaters). I am going to plug in this 17K ohm resitor in
one of the 47 ohm resistor slots, and then connect the
positive bias supply to the filament side lead of the 17K
ohm resistor. I'll post the results.

You could connect the heater centre tap to the cathode
of the 6L6. It’s at +27V. If you prefer to use the
potential divider from B+ you need to keep the two 47
ohm resistors. Disconnect the junction of the 47 ohms
from ground and connect it to the junction of the 220k
and 17k. Connect a 47u cap across the 17k.
 
The PT runs very hot. For the DC filament supply I’d
use a separate transformer. Rectifying the existing
supply will make the PT run even hotter.