"Phil Allison" wrote in message
...

"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.


Yes Phil. I too wondered whether it was an error. At first glance, I thought
that the two screen grids were commoned, and then taken back to a single
resistor, but when I looked at the print layout diagram a bit more
carefully, I could see that what appeared to be the case was that one screen
went to the 'cold' side of the resistor, and the other went to the 'hot'
side. A squint at the schematic then confirmed this ...



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.


Yes indeed, my thoughts as well. Anode supply is quoted at 330v nominal, so
a switch rated to 250v ac, as most are, should be good for that. At the end
of the day, many tube amps use the same switch for the HT standby switching,
as they do for the mains, and lots of them run with an HT of much more than
330v.



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.


I hadn't considered that there might be any changes to the phase splitter
output as a result of such a change. What leads you to think that this might
be the case ?



Not recommended.

Well, not something that I would normally recommend either, and the owner is
happy to live without the mod. He just thought that he would like to give it
a try if it was practical, whilst I was in there doing the other work.
Apparently, he would like to use it in 'triode' mode when he is at home
practicing. It's not actually a hard mod to carry out, and would be
reasonably easy to implement with the current print layout. I think what I
will probably do is to just 'hang' the mod in temporarily to see how it
performs, before doing any chassis drilling for switch fitting. If it seems
to work ok, then I'll finish it off. If not, I'll just recommend against it.


.... Phil

Thanks for your comments Phil. Useful

Arfa