* Plug a humbucker equipped guitar into a '67 Marshall
100 watt with Sylvania 6CA7 tubes.

* Plug the AC power cord from the Marshall into a
Variac set to 90 VAC. Be sure to rebias the amp with
the amp running on 90 VAC so you're not running the
output tubes cold.

* Take the output of the Marshall and run it into a load
box (a THD Hot Plate will do). Run the output of the
load box into a power amp (Ed used H+H amps). Run
the output of the H+H amp to a 4x12 Marshall cab.

* DO NOT RUN THE MARSHALL ON REGULAR
120VAC AFTER HAVING IT BIASED TO RUN ON
90VAC! YOUR TUBES WILL BURN UP IN
MINUTES!

* The downside of running an amp on 90VAC is that
the heaters are getting about 5 volts instead of the
required 6.3 volts. This results in cathode stripping that
will shorten tube life. Some people run the heaters off
of a seperate 6.3 volt transformer running 120 VAC on
its primary.

* Don't forget to practice. 


Why go to the trouble to install an auxiliary heater
tranny to keep the cathodes from stripping.....why not
just install a zener to drop only the plate voltage and
keep the rest of the voltages normal?

FWIW, I have built some amps with this mod included
in the design. I still own one of them. I can switch to
low voltage mode whenver the mood strikes. This amp
also can use tube or SS rectos so the range of plate
voltage I can get is very wide. From about 480volts
down to about 280. It's a pretty useful tool, especially
for recording.

Peter 


220K plate resistors are an extremely BAD idea,
especially in a Marshall type amp. If you want to
increase gain, adjust the cathode resistors, or change
the values in a voltage divider.

Peter