Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jim on December 28, 2023, 12:29:59 pm
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I built a stand alone power amp in an old Bogen chassis to use with my DSP pedalboard. It looks like a mess but it's stable and sounds great. I used a DRRI PT and a Hammond 1645 OT--an UL oddball I had sitting around. The controls are fixed/cathode bias switch, Resonance, Presence, and Volume from L to R. The issue is that the Presence pot squeals when I turn it up. The Resonance pot works but amp volume is zero at control minumum--not sure if this is normal. The blue and green wire you see coming to the Resonance pot is the jumpered 4 ohm configuration for the OT. The output Impedence I selected is 8 ohms which also required jumpering OT secondary leads. I used a 10K NFB resistor instead of the 39K in the schematic because I thought I should. Otherwise all matches the schematic except the individual bias pots, fixed/cathode bias switching and the volume control. Can you help me with the squealing presence control? Thanks, Jim
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The Resonance pot works but amp volume is zero at control minumum--not sure if this is normal.
Not normal. If it is built as the schematic shows, the NFB would vary between a moderate amount (eg, based on the 10k NFB resistor) to very little (10k + 1M in circuit).
A full schematic "as built" would be helpful. How did you implement the cathode/fixed bias switch?
Not sure about your squeal, but you are passing your speaker output right across your amp with those jumpered green leads from the 4ohm secondary.
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I hand drew a full schematic which I need to amend and I will post--meanwhile Ill chopstick the NFB loop out of the way and see if that helps. Thanks, Jim
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... ... I used a 10K NFB resistor instead of the 39K in the schematic because I thought I should.... ....
Explain please.
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Whenever I hear the word "squeal" associated with a new build I have to ask, "Did you verify the NFB phase is correct by swapping the OT plate leads?"
Replace that 10K with a 39K just because it's the right thing to do. :icon_biggrin:
The Resonance pot works but amp volume is zero at control minumum--not sure if this is normal.
Should not happen. Most likely it's not wired IAW Rob's schematic.
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OK...chopsticking the 4 ohm NFB leads does nothing. Here is the schematic. I will swap the 10K NFB for 39K as in in Robinette's schematic and if that does not work Ill swap OT secondary leads. I want clean and it simply looked like not enough feedback.....but NFB from the 4 ohm output apparently increases feed back from what I read. Ill try it, thanks, Jim
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but NFB from the 4 ohm output apparently increases feed back from what I read.
The 8Ω tap will produce more NFB signal than the 4Ω tap.
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WOW. The 29kohm change in the feedback loop fixed everything...I got a little volume and presence and resonance works but very subtlely. I don't quite understand it. Compared to the ice pick tone I got from my Twin power amp input ...it is heaven. My noisey strat also seems so much quieter--I used 5.5 K grid stoppers instead of the 1.5 K resistors on the original schematic--do you think it helps with guitar proximity noise? I would like to add a XLR balanced input--could you provide me with a circuit? Many thanks! :worthy1: Jim
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My noisey strat also seems so much quieter--I used 5.5 K grid stoppers instead of the 1.5 K resistors on the original schematic--do you think it helps with guitar proximity noise?
No. The grid stoppers in the power amp are present primarily to reduce the risk of oscillation. They should not affect noise or have anything to do with your guitar picking up RF signals in the wild (that would be one of the tasks of the pre-amp).