Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TexasTone on May 13, 2024, 02:29:41 pm
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First of all, the amps sounds great and plays great.
The board is the Hoffman AB763-1 board.
Heaters center tap is elevated 70V via a voltage divider off the C node.
All the input and speaker jacks are isolated from the chassis. Inputs are grounded to the preamp ground buss- board mounting screw closes to left-front of the amp, near where the first input jack would be.
The Power Amp ground includes:
Speaker jacks
B+ Center Tap
Heater CT via 70VDC voltage divider from Doghouse "C" power [C4].
Bias supply circuit - caoc
Power tube cathodes via 1Ω resistors.
NFB circuit: 1kΩ NPB resistor to 100Ω NFB tail; tied to LTPI 27k tail resistor and 0.1µF LTPI AC shunt.
C1/C2 100µF 350V Power Supply reservoir caps, Doghouse "A". (I moved all filter caps to this ground for troubleshooting purposes, and this had no effect.
The issue is the annoying buzz.
- Not affected by any front panel controls
- Not affected by pulling V1-V4
- Pulling V5, the long tail phase inverter, stops the buzz.
Here's what I've done so far:
- Swapped the LTPI tube.
- Reflowed solder on G1 ground bus.
- Grounded 4.7k Mid resistor to ground lug of volume pot.
- Moved LFO ground to preamp buss on turret board.
- Validated heater phase. All preamp pins 9 daisy-chain to both power tube pins 7.
- Added external 22µF-500V electrolytic across C1, C2, C3, C4, C5, one at a time.
- Grounded all filter caps to power amp ground - per Hoffman_AB763_1.PDF page 2 grounding. Previously, B, C, and D nodes were on preamp ground.
Nothing has made any difference.
Asking for help.
Thanks.
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Since pulling your PI eliminates hum, I would think it could be caused by your speaker and nfb being grounded to your reservoir.
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Does the buzz go away if you disconnect the NFB wire from the speaker jack or the board?
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Latest.
I moved some grounds. The issue now is that the entire amp chassis is microphonic.
Pull V1 preamp. No change.
Pull V2 reverb driver. No change.
Pull V3, reverb recovery / PI driver. Totally silent.
I tried three different tubes in V3, a new JJ, and NOS GE, and a NOS RCA. The entire amp is microphonic no matter which one.
I'm going over the V3 circuit with a fine-tooth comb, but so far, nothing.
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There is a component in the signal path with a loose connection.
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Latest.I moved some grounds. The issue now is that the entire amp chassis is microphonic.Pull V1 preamp. No change.Pull V2 reverb driver. No change.Pull V3, reverb recovery / PI driver. Totally silent.I tried three different tubes in V3, a new JJ, and NOS GE, and a NOS RCA. The entire amp is microphonic no matter which one.I'm going over the V3 circuit with a fine-tooth comb, but so far, nothing.
There is a component in the signal path with a loose connection.
Check that all input jack tip-switch contacts are making firm connection with the jack tips when there is no plug inserted. Even a slightly open connection will rattle microphonically. You can verify this by pressing the tip switches against the tips to see if the hum stops. The fix is to re-tension the tip switches slightly to make firm contact.
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Can you isolate the problem? I happens with the reverb level at 0? What happens if you unplug the reverb tank? Ground the grid of V3a pin 2? Ground the grid of V3b pin 7?