Sunn 1000S Hum

[Dr. Fraunkensteen]

I’m trying to wrap up a restoration of 1969/70 Sunn 1000S “transition” amp. A hum that was present that I figured the recap would eliminate is still present. Pulling V1 and V2 has no effect however, pulling the 6AN8A (V3) the hum disappears. This tube appears to share duties as a final gain stage, the mid-boost switch network and the Phase Inverter. Definitely something new for my brain to digest. Other than this one issue the amp sounds fantastic with a matched set of KT88’s with lots of clean headroom.

Any suggestions on how to move forward and eliminate this hum (sounds like 120Hz to me) would be wildly appreciated.

Thanks.

[sluckey]

Replace V3. If no joy, then ground pin 8 of V3. Any joy?

[Dr. Fraunkensteen]

Thanks sluckey.

I have tried swapping V3 with two different examples without joy.
The amp has rotated of the bench temporally but, I'll be back at her soon. I'll be sure to post the result of suggestion #2.

[sluckey]

I have tried swapping V3 with two different examples without joy.

Does that mean you have swapped V3 with two other 6AN8s?

[Dr. Fraunkensteen]

Yes, indeed. 

Thanks for loading up the schematic too. I got excited that it might be in a slightly larger format but, oh well, perhaps that copy is the only one on the interwebs. 

[sluckey]

Grounding pin 8 of V3 will probably tell you if the hum originates before or after the 6AN8. Does the tremolo work?

Here's a copy of the schematic that will fill up a letter size page...

[Dr. Fraunkensteen]

Thanks for the schematic, that's much better!

The amp is back on the bench and yes, grounding pin 8/V3 did cancel the hum. The input section of this amp is very sensitive to ambient noise, so much so, I have to shield that end of the chassis while testing. I'm wondering if what I'm dealing with is actually filament hum as the harness has both signal and heater wires are bundled together. The heater supply wiring is not twisted either. Seems like a perfect hum recipe.

Yes, the tremolo is working and does not appear to influence this condition.

I appreciate your input and am looking forward to your thoughts on my assessment.

[sluckey]

A hum that was present that I figured the recap would eliminate is still present. Pulling V1 and V2 has no effect however, pulling the 6AN8A (V3) the hum disappears.

grounding pin 8/V3 did cancel the hum.

This leaves the reverb recovery circuit and LV power supply. Disconnect the wire from the wiper of the reverb pot. Does the hum go away?

Did you change the two 1000µF LV filter caps located on the board?

I would not suspect the filament wiring. All the Sunns (including my Sceptre) are wired like yours and they don't hum. The sensitive wires are shielded.

[Dr. Fraunkensteen]

A hum that was present that I figured the recap would eliminate is still present. Pulling V1 and V2 has no effect however, pulling the 6AN8A (V3) the hum disappears.

grounding pin 8/V3 did cancel the hum.

This leaves the reverb recovery circuit and LV power supply. Disconnect the wire from the wiper of the reverb pot. Does the hum go away?

Did you change the two 1000µF LV filter caps located on the board?

I would not suspect the filament wiring. All the Sunns (including my Sceptre) are wired like yours and they don't hum. The sensitive wires are shielded.

Disconnecting the wire from the wiper of the reverb pot made no change.

Yes, I did change those caps along with all electrolytic caps. I tested them prior to installation with my Sencor "Z" meter and cross-checked with one of those cheapo component testers.

I'm using a combination of F&T and Sprague-Atom(S.A.), some IC brand and S.A. for the low value (1uf etc) caps and one CE brand can. I must confess that I have not used the "blue" S.A. caps or a CE multi can prior, I've almost always opted for F&T for filtering.

The two 1000uf caps in question I've tried are two different brands I'm not familiar with. One S'Teck(new stock), the other, Tiacon(older stock). In both cases the hum has remained constant.

Please forgive if I've provided too much detail yet, it seemed to me important to do so.

My continued thanks.

[tubeswell]

...yes, grounding pin 8/V3 did cancel the hum. ...

So the hum is coming from somewhere before V3. It could be bad pre-amp filtering or grounding (ground loops) in that area, or perhaps bad lead dress or RF coupling into on a grid, or perhaps a bad pre-amp tube. Try probing* various parts of the preceding circuit (plates, grids etc) with a grounded 600V film cap (say a 100nF cap)


*one end of the cap clipped to the ground return and the other end attached to an insulated probe

[turbofeedus]

Grounding pin 8 of V3 will probably tell you if the hum originates before or after the 6AN8. Does the tremolo work?

Here's a copy of the schematic that will fill up a letter size page...

There's something really aesthetically pleasing about this schematic. Someone could make money putting this on a t-shirt.

[PRR]

> this on a t-shirt.

https://www.vistaprint.com/clothing-bags/t-shirts/anvil-by-gildan-softstyle-r-lightweight-t-shirt

[Dr. Fraunkensteen]

Well, my problem has been identified and corrected but, more about that in a tic....

First off I want to thank slucky and tubeswell for their input. Both of you are very generous with logical suggestions on how to proceed with trouble shooting this issue and, yes tubeswell, I constructed the tool you suggested. Unfortunately, it was not directly able to help identify the issue in this case. However, I will be using that tool going forward, thank you.

The problem you ask? I had installed a 12AX7 in V2. In true chase my tail, forest for the trees fashion, I forgot I had done so. I'm sure all can guess why I tried that experiment. Suffice it to say that things of this nature are required ingredients for crow soup. I simply installed the proper 12AU7 and the hum was essentially gone.

Finally, thanks go out to turbofeedus and PRR. What I perceive as brevity on your part has helped me from completely choking on my embarrassment.

Cheers to all!

[PRR]

#0> Pulling V1 and V2 has no effect
#12> had installed a 12AX7 in V2

I'm baffled. "Pulled" had "no effect" because it is not in the main path and any hum here (why?) could be cut-out by the reverb controls. No shame warranted.

[sluckey]

Pulling V1 and V2 has no effect

The problem you ask? I had installed a 12AX7 in V2.

Not logical. Disconnecting the wiper of the reverb pot should have killed any hum caused by V2. Also, pulling V2 should have killed any hum caused by having a 12AX7 plugged into V2 socket.

[Dr. Fraunkensteen]

While initially troubleshooting, I pulled both V1 and V2 together and separately with the hum remaining. Only when I pulled V3 did the hum go away. I'm wondering now if I have missed something else here. When I was testing the other 6AN8's, I did not pull V1/V2.

In addition, I damaged the original V3 late in the process while experimenting with phase so, it's replaced now. It was directly after that "hold my beer" experiment (not being literal/no beer involved), that I replaced V2 with the proper 12AU7. Viola!, hum barely perceptible as compared to before.

What I failed to do and I'll probably be pulling the chassis (V1/V2 are not easy to access in the head cab) again to confirm, is swapping between the AU7 and the AX7 in V2 to verify if the hum returns. Definitely some holes in my T.S. procedure along the way....

I'm wondering, could there be a coupling situation that was enhanced by the additional gain of the 12AX7? Looking at the construction of the 6AN8, that hypothesis kinda makes sense. Thoughts?