PROSONIC POP

Caused by a few things:
1)  Lousy relays.  I have prototype #2 (Proto #1 went to the factory).  
It only pops if...
2)  Bad preamp tubes.  Sovtek  preamp tubes have a tendency to get 
heater to cathode hum.  Makes for a signal level difference when you 
switch, which can give you the annoying "Pop".  A spiral filament type, 
such as the Electro Harmonix 12AX7 is a  good thing for the first two 
preamp tubes.

MAJOR CAUSE:
What you are getting is induced back EMF from the relays leaking into 
the audio.  R19 (on the Footswitch jack) was designed to  always have 
some signal flowing through the relays.  Problem is, sometimes the value 
is too small and the relays hang, or sometimes it is too big and you get 
pop.  Experiment with this value.  6.8k  was the initial one, later 
changed to 9.1k (so the amps with abberent relays would still pass QC).
[I now use very tiny low current 5V relays from NEC [ED2-5NJ] (mouser sells 'em).]
...

ALSO:

R13  Is 1 Meg.  Reduce this value.  This cap keeps the voltage at the 
input of this tube [V2a] down when the relay is switching, and the circuit is 
otherwise open.  A lower value will reduce the pop.  Gain will be 
reduced slightly as well.


IMPROVEMENTS-  See all those little disc ceramic caps on the tube 
sockets?  cut them out with extreme prejudice.  Not needed, and the 
sustain and tone will improve.  These do some funny business (Massive 
local feedback) that does affect the audible signal.  I don't know quite 
what I was thinking here.

GAIN REDUCTION-
Some people like to remove some or all of the cathode caps in the preamp 
section to reduce the gain.  If you never need the high gain, it's 
something to try.

...

Last Prosonic mod-

I would change the stock transformer to our Zinky unit, model 
ZOT604816.  This transformer will give much better bass response than 
the original, AND the overdrive will not have any of the "Buzzy" or 
"Fizzy" character you may hear with the stock one (Especially at low 
volumes.  Clean tones will be bright and airy, the big changes will be 
on overdriven tones).  Does not fit the standard bolt pattern (Slightly 
larger), so you'll have to drill.  $150 but well worth it.  Has 4/8/16 
Ohm taps.  Will give you the liquid overdrive even at highest gain 
settings, even with high treble settings.  Get dynamics AND tone....

Bruce Zinky