Input triode 60hz Noise troubleshooting

[imjonwain]

I need some trouble shooting help. I have a 1980's Sundown Artist that has what appears to be pretty bad 60hz heater noise at the first half of the input triode (this tube is labeled V3 on the schematic). if I remove V3 (really v1) I can max the amps master volume for the gain channel or the clean and the noise level is minimal/acceptable for a ~100w amp. The 60hz noise is loud enough you can't play the amp at any real volume. I can measure a ~160mV P-P 60hz sine at the plate on V3a with my scope and it seems to there at the cathode but I will get a better measurement tonight as it's harder to measure.

-With the V3 tube in and the input wire removed at the PCB the noise is still present so it's not being picked up on the input wire.
-I've replaced all the passives on the first triode (V3) with no change.
-I've tried a couple different tubes with no change. (I only have Sino and JJ 12AX7s but the same tubes work perfectly fine in my other amps)
-I don't think this is a PCB layout issue as lots of these amps were made and I don't seem to find any noise complaints online (Dennis knew what he was doing).
-I tried routing wires differently then the OEM style and OEM and no change.
-I undid some of my circuit alterations and no change.
-Reflowed alot of solder joints
-I replaced all the electrolytics a few years ago but I honestly don't remember if the hum was an issue or not at the time.

Could it possibly be the socket? Maybe some solder ran under a pin and is close to the heater pins picking up noise? I haven't tried to replace the socket yet as I'll have to probably break it to remove it from the PCB.

Thoughts?

Schematic:


Inside of my amp:


Inside of a stock amp for wiring/component reference.

[FUCHSAUDIO]

FWIW: I have serviced and modded my fair share of these. Dennis was a friend and since his passing, I've sorta become the go-to Ampeg/Sundown guy in these parts.

I'd want to make sure it's indeed 60-hz and not 120hz. 120 Hz would/could be filtering and/or a ground loop or ground that shouldn't be contacting the chassis someplace.
Although the filaments are AC and on the board, these amps are pretty quiet, considering the gain they have.

One thought is running the input tube filament on DC heaters. There is a winding for the relays, which has a simple one-diode supply for the relays. It's possible that is a noise source, so I would disconnect it and see if there might be ripple on that supply leaking into the input stage, as the relay is close. The other possibility is to make up a simple bridge (or possibly a doubler) and a 7806 and use it for the input tube filament and the relays.

Have fun.

[pdf64]

Is it heater buzz (mainly high frequencies, little fundamental) or HT hum (all fundamental, no high frequencies)?

[imjonwain]

Thanks for the quick replies! I'll investigate the relay supply and take some measurements of the noise later today then report back. I can say from what I hear it doesn't sound like purely 60hz hum, there's some higher frequencies in there. But the 60hz is what stands out the most volume wise to my ears. I'll see what the scope says to get a better picture.

This amp has been through a lot. I bought it pretty beat up & moldy with no speaker or tubes out of a basement for $100 some years ago.

[pdf64]

As a first step, I suggest you get some 12AX7 that are tested and warrantied for low heater to cathode leakage.
Leaky valves can work fine in circuits with fully bypassed cathodes, so checking them in other amps doesn’t eliminate that issue.
Ideally I’d suggest elevating the heater circuit, but the relay supply makes that rather more involved / somewhat more awkward.

[imjonwain]

It seems based off what I'm seeing it is mainly 60hz with some other noise mixed in? I can definitely hear something higher. Anyway, looking at the schematic and on the scope the relay supply seems to be an AC voltage doubler, not a recitifer. So it's 60hz just elevated voltage wise.

I'm gonna remove the relay supply diode in the morning and see if that changes anything.

Noise at the V3a plate


noise at the V3b Grid with gain maxed.


V3a plate with V3 removed


Heater supply:


Relay supply with the relay "off"


Relay supply with the relay "on"


Noise recording...

noise.mp3

[imjonwain]

Weekend update.
-I removed the relay supply diode & cap. No difference, same noise with V1 installed still.
-The main PS/B+ filter PCB is grounded through the preamp PCB to a single point at the input jack. I tried playing around with moving ground points per some of the more recommended methods but it actually seemed to create other noise issues so I put everything back.
- I picked up some new EHX 7025s to try and no difference in the hum

I assume if it was an issue with the current grounding arrangement I would have the hum even with V1 removed? The gnd wire to the input jack is directly at the V1 Cathode.


I'm starting to think it's just the normal amount of hum for the amp. My other two amps just seem to have very low hum with grounding input jacks and nothing cable connected even with the gain/vol maxed. I did rework them quite a bit though.

[bepone]

connect the amp in the wall directly, with good grounding, not over the extension cable with multi-outlets. remove all consumer from this socket where is the amp powered and try again, if you are lucky, your humm will be gone (often the case)

[FUCHSAUDIO]

Weekend update.
-I removed the relay supply diode & cap. No difference, same noise with V1 installed still.
-The main PS/B+ filter PCB is grounded through the preamp PCB to a single point at the input jack. I tried playing around with moving ground points per some of the more recommended methods but it actually seemed to create other noise issues so I put everything back.
- I picked up some new EHX 7025s to try and no difference in the hum

I assume if it was an issue with the current grounding arrangement I would have the hum even with V1 removed? The gnd wire to the input jack is directly at the V1 Cathode.


I'm starting to think it's just the normal amount of hum for the amp. My other two amps just seem to have very low hum with grounding input jacks and nothing cable connected even with the gain/vol maxed. I did rework them quite a bit though.

You might want to try using a jack isolated from ground and grounding the circuit elsewhere on the chassis. Sometimes input jack grounding is not the quietest means to ground an amp. This is especially true if there is other grounding points in the amp.

[TUBEDUDE]

Steal some voltage to elevate the heaters.
If I was gonna keep it, I'd replace every electrolytic in there.

[imjonwain]

Thanks for the inputs! My apartment is pretty bad noise wise too so I'll try plugging it in elsewhere for kicks. My other amps seem noisier here as well but it's hard to tell what is in my head or not at this point lol.

The input jack isolation/moving the GND is easy enough to try next anyway. I may resort to just elevate the heaters and replace the relays with high voltage ones if nothing else works out.

The electrolytics are all new. I replaced them maybe 6 years ago. I even replaced the preamp filter cap again recently just to make sure that wasn't part of the problem. I plan to keep this as these things sell for so little it doesn't really make sense. My grab and go amp is a reworked Pignose G40V which is basically the same circuit anyway.