Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: waldner on June 09, 2022, 02:16:02 pm
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Wanted to try some NFB on my 18 Watt Stout Reverb. Can anyone point me to where on the circuit to connect wire and resistor- from OT? Also if you have any recommended resistor values that would be great.
Thanks!
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This is how Hoffman did it on the Stout...
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Perfect! Thanks Steve.
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Ah, I thought it would be simple but no dice. I ran from the 16 ohm tap of the OT with a wire to a switch and then an 82K resistor to the junction of the .022 and 470K resistor are on the pic. Note that the layout for my Reverb version is slightly different than the regular Stout where Doug shows this NFB.
I got thumping and no sound when switching the NFB into play. Any ideas on how to make the NFB work for the Reverb version?
I put some notes and arrows on the pic of the internals of my amp. I implemented a master volume that works perfectly a few years back. You can see a clipped resistor that was no longer needed with the MV. I also noticed a resistor that I point out with the yellow arrow that does not appear on the Stout Reverb layout but when I try to remove it I get almost no sound from the amp.
Any help would be greatly appreciated. Thanks!
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Any ideas on how to make the NFB work for the Reverb version?
Yes. Simply wire it EXACTLY like the layout I posted. You ain't done that yet. Notice the extra resistor and notice a board jumper has been removed.
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Ok, so clip the jumper between the .022 and 470K resistor. Then what about the two resistors (47K and 4.7K) that go to the same turret on the Feedback Stout Diagram? Do I add need to add both of those? Actually move one that's already there but in the wrong place and add the other one?
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I think you got it. The 47K and 4.7K connect to the .022cap along with the NFB wire from the 82K.
Be prepared to swap the OT primary plate leads if the amp howls or just sounds crappy.
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Ok, this time I believe I have it wired exactly as shown in the diagram and I'm still getting the same result. Normal sound with the NFB switch off and the thumping and no sound with the NFB switch on.
I clipped the jumper, got both resistors and the NFB connection wired to the .022 cap but still no luck. See the attached pic of latest wiring.
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I'll try swapping the OT primary wires, though what I'm getting is different than any kind of squeal I've experienced before. Let me try and report back....
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Is it working now?
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Ok, I reversed the OT primary wires and now she screams like a banshee when I engage the NFB. So they need to be back to where they were. In the previous non-screaming OT primary position I just had weird thumping and no real signal- maybe a tiny bit coming though. Works perfectly once the NFB is switched off.
Could it be something different about this 'reverb' version of the Stout that is preventing the same NFB implementation from working? The amp is fine with no NFB, but it always sounded a little too ragged / raw, so I thought some feedback might help. It seems like Doug felt the same way and eventually implemented it on his non-reverb Stout. I don't think his originally had NFB?
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Comparing schematics, I don't see any reason why the NFB circuit won't work in your amp. Looks like your feedback resistor is 82Ω rather than 82000Ω. Check that resistor with your meter.
EDIT... added pic
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Holy shit! I think that’s it. Makes me want to kill myself! Also makes sense as such a small value resistor would allow way too much feedback to be injected into the circuit.
I will have to wait until tomorrow when I’m back at my shop to try. Thanks for your never ending patience Steve.
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Yup, that was it. My feedback resistor was off by a factor of 1000! Works perfectly now and sounds great both ways. Raw and more aggressive with no feedback and much more civilized, less loud and less noise when NFB is switched in.
Makes this amp much more versatile at the flip of a switch. Thanks again Steve!
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Some people like to use a SPDT center off switch (3 positions) to switch two different values of NFB resistors.
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Some people like to use a SPDT center off switch (3 positions) to switch two different values of NFB resistors.
Yes, I have done that on a couple of other builds and may do it on this one too. When playing though my 2x12 speaker cab and certain pedals, the difference in loudness between NFB on/off is pretty severe with the 82k value. When playing through my 1x10 cab it's subjectively much less of a difference and just about perfect. I'll try a 3 position switch with another value that's somewhere in between and be optimized for both.
Thanks!