"Arfa Daily"

A muso customer has asked me to carry out some tonal mods to his Peavey
Classic 30. All pretty straightforward cap changes and so on. However, he
also asked me to look into fitting a pentode / triode switch whilst I was
at it. At first glance, this didn't look like a problem. The output stage
is a fairly classic AB class using 4 x EL84 tubes as two paralleled pairs.
Anodes of each pair commoned. Cathodes all commoned and decked. Grids of
each pair commoned via individual 47k stoppers. But then, things get a
little odd around the screen grids. One tube of each pair, has its screen
grid fed by a 100 ohm 5 watt resistor off the "screen" supply rail, whilst
the other of each pair, has its screen grid fed *direct* from the "screen"
supply rail.

I don't think I've seen this done before.

** Peavey have been doing that since their first 4 tube ( 6L6GC) models.

See Peavey Deuce from 1975:

http://www.el34world.com/charts/Sche...vy_deuce-b.pdf

Strangely, the 6 tube models have 6 screen resistors.

http://www.el34world.com/charts/Sche...Mace_Deuce.pdf


I doubt that it would have a significant effect on the operation of the
stage, but just interested as to whether anyone else has come across this
configuration, and knows the design reasoning behind it.

** Same question has puzzled me for ages - I suspect it could be an error
that has just got repeated over the decades. No-one but Peavey knows for
sure.

Anyone got any opinions about just strapping the screens to the anodes on
each pair via a switch, to implement a triode mode ?

** It better be a damn good switch, insulation and voltage rating wise -
most makers who have such triode/pentode switching use a very hefty slide
switch.

Also, the power output will of course drop in triode mode and each EL84 will
dissipate more heat under overdrive. Combined with their use of a
"concertina" phase splitter, I suspect that overdrive clipping will not be
symmetrical.

Not recommended.


..... Phil