green vault / bias mod 93 concert/super

[Andy Le Blanc]

Got one of those fenders with the upside down pcbs, I'm looking to mod the
bias with an adjustable without completely disassembling the beast.

its exhibiting cross over distortion symptoms after a tube change

R 103 and R 104 are some what accessible on both sides.

r103 is 15k series and r104 is a 33k to ground, I think I can patch in a trimpot on the back side
of 104 and cut one trace to install a 1 ohm to get a bias.

any better ideas?

[Andy Le Blanc]

on the pcb pin 1 and 8 on the power tube socket are joined to ground thru a trace

looks like the trace can be removed and a jumper can take its place,
easy spot to scab on a 10 or 1 ohm, still looking to locate a pot or trimmer
on the pcb.

[Andy Le Blanc]

This one has been fun, the bias is good, and the real issue turned out to be...
A bad plate resistor on the inverter. The description of symptoms is the same.
Distortion on the clean channel after a tube change.

This amp is tough to service, in order to minimize dis-assembly I had locate
and remove the offending part with forceps and reinstall from the backside.
I t would be easy but the design has critical gain stages in chassis mounted
shields and sockets where the rest of the tubes a pcb sockets.
The mix make service problematic, if you start moving the dress wires around
that go to the chassis mounted sockets they'll start to disintegrate and make
more problems in a very awkward design.

once you get the bias setting resistor to the other side its easy to sub out and
replace with a selected value, I had to install a 1 ohm on the back of a octal
socket and remove a short trace.