OT (sorry)...6sj7 and octal fatness question.

[csj]

The octal fatness amp...
cool sandwiching of an old 5C1/GA-5 preamp and power amp with a 12ax7 gain stage and a cathode driven Bassman tone stack.

On the 6sj7 amps I've always wondered about that .047 cap from screen to ground. If it's meant to isolate the screen from the power supply in reference to the plate wouldn't you expect it to be across the 2.2 meg resistor and then to ground using an isolation resistor to separate it from the plate supply? This way the screen could draw what it needs from the top of the resistor/cap combination - staying away from coupling with the plate and  impacting the PS? Is the screen to ground connection just a simpler, cheaper way to decouple?

Sorry about asking this question here.
I know there are other places to get answers but I just don't use them.

[Paul Marossy]

I don't know that much about the 6SJ7, but as far as I know, the Octal Fatness that Doug built started out as the preamp section called Octal Fatness that was designed by David Jones. I can't round up a link for that anymore schematic by him, but either he wasn't the world's greatest tube circuit designer or there is a specific reason why he did it that way. Maybe a look at the data sheet for that tube might help to answer your question?

I have looked at David's schematic, and Doug's schematic, and there are differences between the two. I don't know the whole story here, maybe Doug will chime in, I think he's still on vacation, though.

Here's a link to the schematic by David Jones, I just scanned it in: http://www.diyguitarist.com/Schematics/OF-DJ-OriginalSchem.jpg

Hope that helps...

[csj]

was designed by David Jones

The guys at Westinghouse/RCA, were they still alive, might find that interesting  :wink:

I'd never actually taken a look at the Octal Fatness until your last post on it... (yesterday?).

It's cool...
I've used the 6SJ7 as a driver for SE amps before but never as a preamp tube. It's a common driver for alot of old PAs. I've seen an old Gibson Les Paul Jr. amp and I've worked on a someone elses homebuilt  5C1 champ... that's when I started to wonder about that screen cap to ground. The OF front and back halfs are almost identical to the GA5 and the 5C1 except for a few parts swaps. The 12AX7 takes it to a different level.

[Paul Marossy]

Those were used in old PAs, huh? That's interesting. I never heard of that tube until Doug announced his amp was up and running...

[Lonestarjohnny]

Gibson BR4,BR6F,GA20,20RVT,GA25,GA30,GA50,50T,GA75,75W,
these are just a few of the early amps that used the 6SJ7, the list goes on, valco, National, Supro, and many more amp company's used these tube's, 6SC7, 6SL7, 6SN7, 6SQ7, and a bunch of other tube's that anybody under 25 has probably never seen,, some of these old tube's like the 6SJ7 are really very touch sensitive when used in a guitar amp and a lot of Harp people pay high dollar for an old National amp that use these tube's, just watch on ebay sometimes on a old National, Valco, or Supro amp, the bidding can get plum Stupid, but Supply and demand is what turn's the Buck around, wish I had a truck load of them, LOL
JD

[Paul Marossy]

Hmm.... that's interesting. I'm 37, but I didn't know about this tube before. But then again, I was raised in a cave by wolves.  :wink:

[Eric H]

The octal fatness amp...
cool sandwiching of an old 5C1/GA-5 preamp and power amp with a 12ax7 gain stage and a cathode driven Bassman tone stack.

On the 6sj7 amps I've always wondered about that .047 cap from screen to ground. If it's meant to isolate the screen from the power supply in reference to the plate wouldn't you expect it to be across the 2.2 meg resistor and then to ground using an isolation resistor to separate it from the plate supply? This way the screen could draw what it needs from the top of the resistor/cap combination - staying away from coupling with the plate and  impacting the PS? Is the screen to ground connection just a simpler, cheaper way to decouple?

Sorry about asking this question here.
I know there are other places to get answers but I just don't use them.

Good question, and I can't answer it. I can point out that the 5C1, and the early Gibson's are grid-leak biased --note that the cathode is tied directly to ground (and the need for an  input-cap), whereas the OF is cathode-biased in the modern fashion --like the Vox AC10, or early AC15. My understanding is these two methods sound quite a bit different.
http://www.ampwares.com/ffg/schem/champ_5c1_schem.gif
I believe the touch-sensitivity JD refers to, is due to the grid-leak bias, and I've read that those amps react strangely to being driven by stompboxes.



-Eric

[Paul Marossy]

"I believe the touch-sensitivity JD refers to, is due to the grid-leak bias, and I've read that those amps react strangely to being driven by stompboxes."

That's interesting. I was playing my Octal Fatness at church yesterday just to see if it could be loud enough to hang with the band (we play loud, sometimes approaching 100dB), and it did! Not bad for a 5-6 watt amp. Anyhow, I didn't notice any strange reactions being driven by my pedalboard.

There is some differences between the original circuit and Doug's circuit. Maybe Doug's version of the Octal Fatness differs some from David Jone's version to account for the weirdness that can happen in grid leak bias topologies? Doug?! Where are you? Are you back from vacation yet?!  :wink:

[Eric H]

I didn't notice any strange reactions being driven by my pedalboard.

There is some differences between the original circuit and Doug's circuit. Maybe Doug's version of the Octal Fatness differs some from David Jone's version to account for the weirdness that can happen in grid leak bias topologies?

The OF is cathode-biased, Paul. It is  the older amps (early Champ, Gibson GA9, etc.) with grid-leak biasing (which is a --very-- old technology).  Note this is hearsay --I havent played through any of them

-Eric

[Paul Marossy]

Duh! I wasn't quite awake this morning when I wrote that.  :oops:
I'll go hide now...  :wink:

[Lonestarjohnny]

Hey Paul, Don't feel bad, you don't have to be an Old Geezer like me to know about Grid Leak Bais, But it Help's sometimes,  :lol:
if you'll look at the 2 topoligy's you'll see the diff in the vox= cathode Bais and the Gridleak bais, i don't know why the Cap is there either, but when 1 would go bad it was a Dicken's to find until you were more familiar with the circuit.
These amp's don't like high gain distortion pedal's but can sound very good with a mild O/D or boost pedal.
JD

[csj]

Sorry to dredge this thing back up.
I've thought about that little cap for days.
What it does.

If I had a better sense of "matteringness" I probably wouldn't have focused so much on decoupling and seen that it's probably just a simple shunt capacitor. I can't see how it could affect decoupling...at least as I understand decoupling.

Because the screen and the plate sit so closely together the capacitance between them is pretty large. The plate and the screen act like the 2 plates of a capacitor. The idea of "interelectrode capacitance".

The capacitance serves to protect the grid since any unwanted frequencies which have entered the tube would create a voltage/current which would ping pong a small quantity of electrons back and forth, as the signal goes, in and out of the grid circuit. This would be a problem. The screen shields the plate from the grid. It collects these electrons and uses the capacitor's "back and forth" properties to push the unwanted signal voltage to ground. I think it's as simple as that. It's just a shunting cap which helps to bypass unuseable frequencies.

[Paul Marossy]

That sounds like a reasonable explanation to me.