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?
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I checked the input jack grounds and they're good.
With no preamp tubes and a pair of 6L6GC tubes the amp has no noise.
Add a new 12AX7 LTPI V5. No noise.
Add a new 12AX7 tremolo LFO and driver V4. No noise.
Add a new 12AX7 Reverb Recovery and Vibrato Preamp 3 V3. Lots of noise at both 60Hz and 120Hz, with the 120Hz being 6dB louder. I traced all the wiring from the tube to the board and it's wired correctly.
Unplugging the reverb tank had no effect.
Grounding the grid of V3a pin 2 has no effect.
Grounding the grid of V3b pin 7 is a mixed bag. The 60 Hz hum goes away, and the 120Hz hum is 11 dB louder, and I have an additional 180Hz hum that wasn't there before.
The wire lead from V3 pin 7 to the Reverb attenuator grid leak resistors seems to be an antenna, so I'm tracking that down.
Thanks.
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The 120Hz and 180Hz hum is gone.
I changed the heater elevation from 45V to 68V.
I changed the wire from the reverb attenuator to V3 pin 7 to a shielded cable.
I changed the NFB resistor values.
The only issue left is this crazy microphonic on V3, the reverb recovery/stage 3 preamp tube. The microphonic manifests itself especially on the turret where the wire that connects pin 7 to the 470k attenuator resistor. When I put a chopstick on the tube socket shield base and then slide it to the body of the tube, you can hear every movement through the speaker.
Okay, bad tube. Well, I used at least four different tubes - JJ, EHX, NOX GE, etc., and it's still microphonic. Well, what about the socket itself? It's a Beldon Micalex, and I'v cleaned and re-tensioned the pin sockets, and validated every solder joint.
Quite baffling at this point.
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There's a likelihood that the 10pF bypass cap is microphonic. Also that part of the circuit is always pretty noisy in my experience. It's very high impedance. I try to give it every advantage with respect to noise, so the shielded wire is a good move. I also make sure the 3.3M is a good low noise metal film. I also tend to use 3M here, as it's still technically in tolerance for the AB763.
You can also try a 22k gridstopper right at the socket, but I don't see that helping much.
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Latest Update.
First of all, thanks to all for your input! We are complete with a fantastic sounding amp.
Some of the changes:
- Replace C33 & C34 with two 100µF 350V caps in series with parallel 220k resistors.
- Separate the A node grounds (to power amp ground) from the B, C, and D node grounds (to preamp ground).
- 70V heater elevation.
- Add 100k resistor before C22.
- Changed R45 & R51 NFB resistors to more preferable and less noisy values. The stock values of 820Ω and 100Ω attenuates the output signal 89%. My number is closer to 85% for this particular amplifier.
- Changed C28 to .022µF.
- Changed R63 to a lower value than 470k
- Changed C31 to .047µF.
- Tubes: V1=E83CC; V2=12AU7; V3=12AY7 Gold pin; V4=12AX7; V5=12AX7; V6 & V7=6L6GC.
- 5AR4 replaced by diodes.
Various other secret sauce mods . . .
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Replace C33 & C34 with two 100µF 350V caps in series with parallel 220k resistors.
I bet they are not really connected like this... :icon_biggrin:
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Replace C33 & C34 with two 100µF 350V caps in series with parallel 220k resistors.
I bet they are not really connected like this... :icon_biggrin:
Yeah, I know, it's just a drawing error. LOL
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and that's why we troubleshoot with schematics not layouts, schematics get a better quality draftsman :icon_biggrin: