Debugging a nice 60/120 hum in a reverb circuit

[mikepukmel]

When I put the reverb in, I switched two 16uf 475 filter caps for 500v caps but they are 22uf. I took the 'cap can' cover off, checked the wiring, didn't see anything awful.

I also tried turning up the volume on either channel, with no input. I do hear a hum similar to the reverb put hum. Will check ac mv readings on each vol pot.

[mikepukmel]

So I have this old throw away TEK 7613 scope. It came with one set of 10x probes. If I use Ac coupling, can I probe the plates in the preamp? that would be 200v range dc, but curious what AC might be there, and what the trace would read (if it won't do something terrible like sparks or fire)

[Leo_Gnardo]

So I have this old throw away TEK 7613 scope. It came with one set of 10x probes. If I use Ac coupling, can I probe the plates in the preamp? that would be 200v range dc, but curious what AC might be there, and what the trace would read (if it won't do something terrible like sparks or fire)

Should be perfectly safe. One tenth of 200V is 20V hardly a hazard. But with 10x probes the signal you're trying to see will be diminished by 90%, more difficult to see.

All in all I'm wondering, is this hum you're trying to chase down really so bothersome? Scarcely anyone runs their amp with volume controls & reverb cranked all the way up. And we expect to hear some hum when they are cranked up. There's practicality ... and there's unnecessary perfectionism. Where are we going with this? To be honest, I have similar problems here. My home & workshop ambient is so quiet that a normal amount of hum or other noise sometimes initiates a long snark hunt - eventually after excessive time is spent I have to stop myself from going kookoo. They're guitar amps after all. Onstage, one rap on the snare drum and all that noise immediately becomes irrelevant, lost well below the signal/noise ratio.

[Chuck H]

All in all I'm wondering, is this hum you're trying to chase down really so bothersome?

Well... 15mVAC from the reverb recovery passes through the 470k/220k divider. So about 10mV now.?. That's coming out of the mix triode at something like half a volt of noise without a guitar signal. That doesn't even include any noise from the reverb because the recovery triode is grounded.

[Chuck H]

Keep in mind that some of that AC is Johnson noise, or hiss. But 15mV of hash on the plate side of the reverb recovery seems like a lot considering the grid of that stage is grounded.

You're reporting similar readings on the normal channel. Where did you set the volume control and where were you taking that measurement? Obviously the normal channel doesn't have a reverb pot.

The frequency of 60Hz is still confusing me. The last test tells us nothing about frequency though. And like I mentioned, hiss will be part of that 15mV. The test doesn't tell us how much.

[mikepukmel]

Should be perfectly safe. One tenth of 200V is 20V hardly a hazard. But with 10x probes the signal you're trying to see will be diminished by 90%, more difficult to see.

All in all I'm wondering, is this hum you're trying to chase down really so bothersome? Scarcely anyone runs their amp with volume controls & reverb cranked all the way up. And we expect to hear some hum when they are cranked up. There's practicality ... and there's unnecessary perfectionism. Where are we going with this? To be honest, I have similar problems here. My home & workshop ambient is so quiet that a normal amount of hum or other noise sometimes initiates a long snark hunt - eventually after excessive time is spent I have to stop myself from going kookoo. They're guitar amps after all. Onstage, one rap on the snare drum and all that noise immediately becomes irrelevant, lost well below the signal/noise ratio.

Hi Leo,
thanks, Yeah, this hum is just above the annoying range. I never crank the amp. Although I do have a very very small office/practice/music room, the amp is pointing towards my feet. The hum is annoying at about 3 on the reverb knob, similar on the volume. I never noticed the hum that comes from turning the volume way up, never did that before. But even at low volume, the reverb hum is blaah.

[mikepukmel]

Well... 15mVAC from the reverb recovery passes through the 470k/220k divider. So about 10mV now.?. That's coming out of the mix triode at something like half a volt of noise without a guitar signal. That doesn't even include any noise from the reverb because the recovery triode is grounded.

Thanks Chuck. Is 15 to 20mv at this stage in the amp to be expected?

[mikepukmel]

Keep in mind that some of that AC is Johnson noise, or hiss. But 15mV of hash on the plate side of the reverb recovery seems like a lot considering the grid of that stage is grounded.

You're reporting similar readings on the normal channel. Where did you set the volume control and where were you taking that measurement? Obviously the normal channel doesn't have a reverb pot.

The frequency of 60Hz is still confusing me. The last test tells us nothing about frequency though. And like I mentioned, hiss will be part of that 15mV. The test doesn't tell us how much.

Yes, thanks. The johnson noise is tolerable with the channel volume from 1 (min) to about 6 or 7. Above that, the 'hiss' gets to the intolerable point.

Re
"You're reporting similar readings on the normal channel. Where did you set the volume control and where were you taking that measurement? Obviously the normal channel doesn't have a reverb pot. "

Yeah, sorry for my convoluted writing style:
- Original issue I reported in the thread: Volume channel 1 = 1, and no instrument plugged in. Volume on channel 2 = below 3 or 4, with my guitar plugged in, turn the reverb up to about 3, no more than 4, hear an annoying hum. Turn reverb back down, don't notice the hum.

- Later on (after going through the tests you suggested) I put the reverb on 1, then turned the volume pot to 10. Also, did the same with channel 1 (channel 2 vol = 1, reverb = 1), turned vol pot to 10. On both cases, I could hear a louder hum than the reverb knob only test with volume at 1.

Going through the workflows you suggested, I found that the amp does have hum, just that it comes up later on the volume pot setting, that I don't ever use.

Chuck, did you see the scope trace above? I could not quite understand what it means. Its got sharp peaks that are 60hz, but it looks like some kind of odd partially rectified thing. Peaks, then a smoother bump in the middle.

Could I have hooked up the Immortal Mod 'solid state rectifiers on the input of the GZ34' wrong, or could this be causing some of the problem?

Should I get a scope trace on the + side of the main PS capacitors, with AC coupling on the scope? Any chance the AC is laeking in from there, some problem with the capacitors? Or maybe a bad GZ34?

[mikepukmel]

If I have AC coupling turned on on the scope, what is the safest DC voltage I can probe with this old 7613 scope? For the AC coupling setting, they must have some kind of capacitor switched in to filter out the DC, but the switch, the cap and other components have some voltage rating. I tried looking in the manual could not find it right away.

[mikepukmel]

oOoch. Maybe bad GZ34 out of the box?

https://www.thegearpage.net/board/in...e-mod.1529042/

See post #3 from Mr Robbins:
"If a valve diode does go bad, then hopefully you hear some added hum, and then get the amp serviced, otherwise you may end blowing the first filter cap from higher ripple in the long term."

[Chuck H]

Well to be perfectly honest Mike, I'm having some trouble following along at this point. I don't have an interpretation of the scope trace you posted. I'm afraid no one else does either because the context is a little confusing and there's probably not a precedent reference. That is, what would a scope trace look like for that measurement on another amp? I'm trying to digest info about the extra power supply node and now the added diodes. There have been a couple of things that MAY be part of the issue, but haven't been fully defined and I'm starting to think there may yet be other deviations not disclosed. Not to mention that you are now reporting hum in operating conditions other than the original condition that we worked hard to isolate for. Basically tossing every premise we had out the window.

60Hz could be a harmonic artifact of 120Hz caused by improper implementation of the diodes on the rectifier, I dunno. Otherwise 60Hz has to be inducted (into???) via the power transformer EMF or coming through the filament circuit.

You can always remove the extra diodes to rule that out. The amp doesn't need them so it's safe. If they aren't the problem then put them back in if you like.

I actually don't use a scope much for chasing hum or buzz issues. I rely on my ears to tell me the frequency and start with those possible noise sources, tracing back through the associated circuits examining lead dress, grounding scheme and any filtering/decoupling. So if you do indeed have 60Hz hum there are limited possibilities for where it's coming from.

[Leo_Gnardo]

Maybe bad GZ34 out of the box?

It's been known to happen. Also a common misteak to wire the filter bank to pin 2 instead of 8, imposing a 5V AC wave on top of the rectified voltage. Who knows, that kind of error might have been made inside the tube but I can't say I've ever seen it. How about subbing in another GZ34, or for that matter tube rectifier of any variety, or even a solid state replacement just to see if the hum level drops. FWIW our man Stan - not heard anything of late but he was a valuable MEFster - recommended Ruby's 5AR4C as top of the pops for currently made GZ34/5AR4 types.

[nosaj]

If I have AC coupling turned on on the scope, what is the safest DC voltage I can probe with this old 7613 scope? For the AC coupling setting, they must have some kind of capacitor switched in to filter out the DC, but the switch, the cap and other components have some voltage rating. I tried looking in the manual could not find it right away.

I have used a very low value 600v capacitor attached to my scope probe to probe higher than 300v. Film cap with one end fishhooked and the other end done in a wrap fashion so probe clips on it. Then using the fishhook on whatever I want to probe.

nosaj

[nosaj]

It's been known to happen. Also a common misteak to wire the filter bank to pin 2 instead of 8, imposing a 5V AC wave on top of the rectified voltage. Who knows, that kind of error might have been made inside the tube but I can't say I've ever seen it. How about subbing in another GZ34, or for that matter tube rectifier of any variety, or even a solid state replacement just to see if the hum level drops. FWIW our man Stan - not heard anything of late but he was a valuable MEFster - recommended Ruby's 5AR4C as top of the pops for currently made GZ34/5AR4 types.

Have to put that one in my bank of common mistake to not overlook.
nosaj

[Enzo]

How exactly would that work? The 5v is AC, but has no reference to ground. The B+ is referenced to ground, so how would the 5vAC add and subtract from B+?