"Phil Allison" wrote in message
...
"Gareth Magennis"
Hi,
what is the purpose of the reverse biased diodes on the OPT/Cathodes as
in this Fender circuit?
** They are not simple diodes, nor are they on the cathodes.
They ARE diode strings wired from each plate to ground.
http://www.schematicheaven.com/fende...er_60_rack.pdf
** The two " R3000 " prevent the plate voltage from swinging negative by
more than 2 volts or so - something all tube amps do when overdriven into
a reactive ( ie speaker) load.
Peak negative plate voltage can exceed 2000 volts and damage the
insulation of the tube, the socket or the OT.
I need to replace a faulty one and am having difficulty sourcing the part
Fender use (R3000). Does this need to be a specific type of diode or can
I make up the 3kV diode required using standard 1Kv rectifier types?
** The later is generally OK.
Use " fast " diodes, if possible
Guitar amp technicians seem to refer to them as "flyback" diodes.
This may or may not be a general term.
Iain
....... Phil