Hoffman AB763 Single Channel - Everything Works! ...but still have questions

[tdvt]

 
First a big thank you to everyone for all the knowledge shared here. An invaluable resource.


I fired up a new single-channel Hoffman AB763 over the weekend & had the good fortune that everything worked as intended out of the gate.


But there was a low-level mid-frequency hum that would change in loudness with the sweep of the volume control. Slight hum all the way off, dead quiet around 9 o'clock (clock dial), progressively louder until about 3 o'clock then quiet again wide open. Cell phone app had it's frequency around 465hz, awfully close to 480hz (or 60hz X 8)


Changing tubes had no effect, but pulling V3 (recovery/pre-PI gain) would silence it, as did grounding pin 7 of V3. PI to speaker is dead quiet.


After chasing lead dress, adding shielded cables, trying to isolate the reverb section, etc, I noticed that sometimes there was a faint guitar signal with the volume all the way down.
Replacing the brand new volume pot (CTS) seems to have remedied all, or at least most, of that issue.

Now, there is some slight hum introduced as the volume control is turned up (but linear), going away at fully open. But it is so slight it could be attributed to being in an open chassis (no shielding on top) or just tube noise. (waiting for a new tube set)

The part I don't understand is why there was the absolute quiet point at around 9 o'clock with the original pot. Seemed like there was some sort of balance/cancellation happening but that is over my head at this point.





 

[Lectroid]

I recently built this and I have noticed a hum similar to what you describe.  But only once or twice; whatever I have, it's intermittent and rare.  I hope you get an answer.

What value is your volume pot?  Do you have the schematic exactly as you built it?

[tdvt]

I recently built this and I have noticed a hum similar to what you describe.  But only once or twice; whatever I have, it's intermittent and rare.  I hope you get an answer.

What value is your volume pot?  Do you have the schematic exactly as you built it?

Intermittent would be a bummer. Hoping that the variable hum is gone & that yours never returns.

The 1st volume pot was a new 1MA CTS, replaced with a new Alpha of the same value. I have to say the CTS had very little turning resistance, almost sloppy, but not quite.

This build is true to Doug's schematic, with the exceptions of a dwell control, a second bias cap & a dual/series reservoir (w/balance resistors). Initially, I had wanted to put it is a somewhat smaller chassis & cut down the board, so the 100 Ohm heater resistors were jettisoned & are now on separate small terminal. It ended up in a Bassman chassis with lots of room front to back.

I also adapted the layout to Mallory 150s & radial filter caps, but they are wired as per Doug's schematic.

[Lectroid]

Sounds very much like mine, I was able to force it into a Princeton cab by leaving off the trem, so I have a compact little "practice" amp that everyone smirks at until I turn it UP.   

Just in case, I have to ask: did you use shielded cable run to V1a and maybe, V1b?  Can you post closeup pics of your wiring, all sections but especially the input end of the amp?

[PRR]

... hum that would change in loudness with the sweep of the volume control. Slight hum all the way off, ....louder until about 3 o'clock then quiet again wide open. Cell phone app had it's frequency around 465hz,

465/480 is buzz, not hum.

Worst at halfway up (or halfway up the 5-10 side of the dial) is crap getting in on the wiper wire, runs from pot wiper to tube grid. Worst when farthest from both ends. Re-route or shield.

[tdvt]

465/480 is buzz, not hum.

Duly noted

Worst at halfway up (or halfway up the 5-10 side of the dial) is crap getting in on the wiper wire, runs from pot wiper to tube grid. Worst when farthest from both ends. Re-route or shield.

This would be relevant to the current minimal buzz, but the earlier buzz with the perfectly quiet spot at 9 o'clock was/is still puzzling. It went buzz/dead quiet/more buzz/quiet fully open.

The V1a & V1b grids had shielded leads from the beginning, initially running beneath the board, but they are now on top for troubleshooting. I also temp-ed in V3b shielded grid run from a different physical point, directly at the 10pF/3.3M junction, in my efforts to solve the initial issue.

The pics reflect a still work-in-progress, with the shielded leads re-routed, the temp-ed V3 grid wire, temp-ed dropping resistors (until the correct value arrive),etc.

[PRR]

...buzz with the perfectly quiet spot at 9 o'clock was/is still puzzling. It went buzz/dead quiet/more buzz/quiet fully open...

That's not unusual. There are two buzz sources, one before one after the volume pot. Since every other stage is opposite polarity/phase, there will be cancellation. You treat the two buzzes separately. Start from "normal playing setting" and make that clean.