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. I've seen one screen resistor
feeding both tubes of a pair, or one resistor per screen, but not just one
tube having a screen feed resistor. Apart from anything else, I wouldn't
normally have considered it very good design practice to have no current
limiting at all in place. Also, it will mean that the screen voltage will be
higher on one tube of the pair, than the other. 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. Anyone got any opinions about just strapping the
screens to the anodes on each pair via a switch, to implement a triode mode
?

Arfa