Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tkm on November 03, 2021, 03:37:48 pm
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Hello, this is my first time posting here. What a great resource.
This place reminds me of my time at GroupDIY. And there are even some familar faces. I built an 8 track studio, tons of discrete stuff, compressors etc over there.
But for the last few years I've just been using it all. Anyhoo HOWDY!
OK- here's the situation-
I've built a Hoffman Stout Reverb.
It's a super happy amp, but I'd like to clean it up a little.
I've changed/added the NFB circuit that's on the Hoffman Stout and Stout TMB.
I'm thinking about switching that in and out.
Curious about something.
- The Stout TMB has NFB and the Stout Reverb doesn't, and the adjacent circuits are slightly different.
The role of C6 (sr) /C7 (stmb) confuses me.
why is C6/C7 moved around in the 2 different versions?
As I said, I'm working on a switching solution to turn On/OFF the Negative Feedback but since it's in a Phase Shifting circuit, I'm hesitant to just wing it, since my OScope is in storage.
Thanks!
Sleeper
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- The Stout TMB has NFB and the Stout Reverb doesn't, and the adjacent circuits are slightly different.
The role of C6 (sr) /C7 (stmb) confuses me.
why is C6/C7 moved around in the 2 different versions?
Welcome. :icon_biggrin:
C6/C7 are different because of the -FB loop. C7 is where the -FB is injected into the LTPI's grid of V2, pin 7.
R19 forms a voltage divider with R20 for the amount of -FB.
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Thanks Willabe.
It's that first LTPI, on the hoffman stout reverb,without NFB - that had been confusing me.
Maybe that's the outlier.
In that one the signal goes from pin 2 through a couple of 470k resistors AND the C6 to pin7 with a 47k referencing ground.
On an AC-30 that Cap (C6) parallels the 47K going to ground, more like a bypass cap and the signal reaches Grid pin7 without passing through any caps at all, just resistors.
Point being, with the AC30 configuration of C12 the NFB voltage divider can be switched in and out simply,
With the original Stout Reverb version I think I would need something much more complicated to keep C6 in it's original state.
My parts just arrived, so I'm going to try the AC30 config first, then add the NFB... go from there.