Supro 1622RT TremoVerb Hum Problem

[Daniel S]

Hello, I'm new here. The title says it all. I bought a reissue amp off of Reverb and it has a low-volume 60 cycle hum with nothing plugged in and all pots set to zero. Hum goes away in standyby mode. Hum is definitely coming through the speaker. Hum changes tone and slightly increases in volume when bass tone knob is dialed up. No change in hum when volume is increased to 10, but there IS a slight increase in the hum volume at between zero to 2, then hums steady. The hum throbs with the tremolo. Hum does not change when guitar is plugged in.

So far,
1) I've taken it to a local tech who said "it's normal" and "it's not too bad" - LOL
2) I purchased a Furman AC-215A Compact Power Conditioner - no change
3) I've tried every wall outlet in my house - no change
4) I purchased and installed a new set of matched triode 12AX7's from The Tube Store " Preferred Series" - House Brand labelled. - no change
5) loosened the tightened the ground in the chassis - no change
6) moved wires and tapped with wooden chopstick - no change
7) Purchased a Strymon El Capistan Tape Delay (Because it made me feel better)

Still it hums. My Epiphone Valve Jr head and cab is dead silent, even when cranked - just a hiss of air.

I can't use this Supro for recording clean as it stands, the hum is low, but audible. I'm open to suggestions of things to try to pinpoint this issue and repair. I'm about ready to move on from this amp, but I really like it otherwise and hope to resolve the hum.

[xtian]

Hi, Daniel! Welcome.

#7. LOL.

Your troubleshooting implies the issue is not in the preamp. So let's focus on what's left. Pull out the phase inverter tube (you know which one that is?) and evaluate the hum. If you still have hum, we know it's after the PI tube. That leaves mismatched power tubes, poor or dying filtering, or PT injecting hum into the OT.

If there's still hum after pulling the PI, also pull the power tubes. Any hum left? If yes, it's PT electromechanical vibration.

[Phil_S]

60Hz hum can also be heater supply bleeding over into the audio. Is this a PCB amp or hand wired? If hand wired, can you post a gut shot showing the tube socket wiring? If PCB, I'm doubtful my armchair assessment holds water.

[xtian]

60Hz hum can also be heater supply bleeding over into the audio. Is this a PCB amp or hand wired? If hand wired, can you post a gut shot showing the tube socket wiring? If PCB, I'm doubtful my armchair assessment holds water.

I think he determined noise is not coming from preamp, so it's doubtful that heater secondary is bleeding into audio path.

[Phil_S]

Yes, I see the logic of that now. It's not the heaters.

[Daniel S]

Hi and thanks for jumping in.

Your troubleshooting implies the issue is not in the preamp. So let's focus on what's left. Pull out the phase inverter tube (you know which one that is?) and evaluate the hum. If you still have hum, we know it's after the PI tube. That leaves mismatched power tubes, poor or dying filtering, or PT injecting hum into the OT.

If there's still hum after pulling the PI, also pull the power tubes. Any hum left? If yes, it's PT electromechanical vibration.

Just to clarify, this is a recent reissue. I'll try to post a photo of the inside of the chassis.

I assume the PI is V1? I wasn't sure which tubes I could pull and still test for hum. So, should I pull 1 tube at a time beginning with V1, turn power back on and check, then if hum remains, turn off, pull next tube in line and keep going?

What about the reverb and tremolo tubes, could they induce the hum as well? if so, I'll need to remove the chassis as these tubes point toward the front of the cab. I'll probably need to set up a small wooden frame to isolate the chassis.

Also, I'm 54 and a career telecom tech. I'm well versed in working with AC and DC power, backplanes, electronic cards, etc. But I admit, I've never gotten into an amp before. I've been watching amp building youtubes, and it seems very familiar. I haven't been able to find a schematic for this one. I really believe this can be solved, at least the technician in me believes this. I troubleshoot audio for a living.

[Daniel S]

If there's still hum after pulling the PI, also pull the power tubes. Any hum left? If yes, it's PT electromechanical vibration.

Honestly, this has been my suspicion all along. I just didn't know how to isolate that to confirm.

[Daniel S]

20190220_193640.jpg

20190220_193612.jpg

[xtian]

What have we got in there? Five 12A?7 tubes and two power tubes? The 12A? next to the power tubes is most likely the PI.

[Daniel S]

Just found this last night. This drawing appears to show one of the preamp tubes on the back!! Weird. I'll pull the chassis tonight.

What I found was that there were (2) two 12AX7's on the front made by EHX, which I ASSUMED were the preamp tubes. Therefore, when I got my matched set of 12AX7's from tubedepot, I swapped them out on the front. There WAS NOT a 12AT7 on the front, not that that matters. But at least I now know I didn't swap my matched pair into the right slots.

Supro.jpg

[Daniel S]

I got the chassis out. I removed the 12AX7 Phase Inverter and 12AX7 Preamp from around back and also disconnected the reverb RCA jacks from the chassis. With the speaker connected, both power tubes in, and I left the 12AT7 reverb driver, 12AX7 reverb gain and 12AX7 tremolo tubes in, it still hums.

btw...I stand corrected from an earlier post of mine, there IS INDEED a 12AT7 tube in the reverb driver socket.

[Daniel S]

I removed the rest of the preamp tubes, leaving just the 2 power tubes installed and speaker connected.

It still hums.

[Phil_S]

That leaves failing or mismatched power tubes, filter caps, or maybe what xtian said about coupling with the OT. Good luck.

[xtian]

Yeah, last step, Daniel: pull out the power tubes also, and evaluate the hum.

[Daniel S]

Pulled the power tubes and left the speaker connected.

It's dead silent.