Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Madison on August 20, 2011, 08:38:30 am
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A pseudo "Presence" for an SE amp?
I have always wanted to try an adjustable NFB on an SE so this is an attempt.
I have been messing with with for an hour or so on an amp.
Sounds so so.
The pot doesn't react the way I'd like. Taper is weird.
I have a 25KA or a 22KL that might be better?
Thanks.
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First thing I would try is to remove the ground connection from the pot.
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...or connect it like this:
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I think both those adjustable designs are not good: the pot is in parallel with the cathode resistor, so the pot will alter the bias of the tube. Worse, when the pot is at -0-, there will be -0- cathode resistance = no bias. This should fry the tube.
The global NFB loop is a voltage divider with a series resistor & a shunt resistor. In this circuit the cathode resistor serves double duty as the NFB shunt resistor. To make NFB adjustable, replace the fixed NFB series resistor (all the way on the right in your schematics) with a pot wired as variable resistor -- into lugs 1+2 wired together, out lug 3 no (ground connection). You can bypass the pot with a cap if you want to.
N.B. If you add a bypass cap to the cathode resistor, this cap will bleed NFB voltage to ground. You'll need a split cathode resistance. See the Vibrochamp circuit.
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I think both those adjustable designs are not good: the pot is in parallel with the cathode resistor, so the pot will alter the bias of the tube. Worse, when the pot is at -0-, there will be -0- cathode resistance = no bias. This should fry the tube.
I have to respectfully disagree with you on this one. When the pot is on zero the cathode still has the whole pot parallel with the cathode resistor.
N.B. If you add a bypass cap to the cathode resistor, this cap will bleed NFB voltage to ground. You'll need a split cathode resistance. See the Vibrochamp circuit.
This is more like a presence control where the cap is supposed to bleed the high frequency to ground. In this circuit the cap also boosts the high frequency gain of the tube, so I think you get double effect from the presence control.
Only problem I see is that the tone stack and volume controls are inside the NFB-loop.
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I have to respectfully disagree with you on this one. When the pot is on zero the cathode still has the whole pot parallel with the cathode resistor. You are right -- the NFB cap blocks DC, so for DC bias purposes the cathode resistor is in parallel with the full value of the pot. I guess it still changes the bias of the tube, which now has about a 2K combined cathode resistance.
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It works pretty okay.
The taper on the pot is really strange though.
It's like it doesn't have much effect until just the max end.
I have tried audio and linear pots.
No real difference.
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The way the pot is wired, is more a preamp Variable Cathode Resistor than a V. NFB.
Put the pot in SERIES and upgrade the 22k NFB resistor with a 5k6 or similar...
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It works pretty okay. The taper on the pot is really strange though. It's like it doesn't have much effect until just the max end. I have tried audio and linear pots. No real difference.
The cap is large - maybe passing enough signal to mostly defeat the pot. But don't simply remove the cap - I think that will defeat the tube's cathode resistance, per the above posts. For adjustable NFB use a series pot in place of the fixed series resistor; or as chocopower says.