On Sat, 26 Feb 2011 02:25:00 +0000, Arfa Daily wrote:
"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
Looks like a good bit of work in store for you. The hard part is to end
up with tone that doesn't sound like ass.
--
Live Fast, Die Young and Leave a Pretty Corpse