"Meat Plow" wrote in message
news
On Fri, 25 Feb 2011 17:00:41 +0000, Arfa Daily wrote:
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
Several amp and tone stack mods. Variable cathode bias, OD mod on CH2, OT
replacement, speaker replacement.
blueguitar.org/new/articles/blue_gtr/amps/peavey/c30_origmod.pdf
Hi Meat. Yes, it is Blue Guitar mods that he wants to do. He is not
interested in the changes to the tone stack. He wants the output bias
changing from fixed grid to cathode auto, he wants the C4 cap value change
in the overdrive channel, and he wants the input simplification mod, where
all the C-R garbage between the I/P socket and the first 12AX7 grid is
removed, and a simple series resistor and grid return resistor are put in
their place, a la typical Fender and Marshall I/P schemes.
Arfa