Debugging a nice 60/120 hum in a reverb circuit

[Tom Phillips]

...If I unplug the cable completely (cable from reverb tank out to amp reverb recovery), the hum is about the same. Increases when reverb pot is increased.
No footswitch.
I tuned my open E string down about 4 fret steps to match the tones fundamental. Its pretty low...Just looked it up; the low E is 82hz? No kidding? I would never have thougt the fundamental was that low. Then, the hum fundamental is probably 60.

It appears that the problem is in the reverb recovery circuit and that the hum is 60 Hz. You can leave the reverb tank disconnected while you continue troubleshooting your hum issue since we seem to have ruled out the tank as the cause of the problem.

...Then it can't be 60. Oh boy, where is this hum coming from?...

Yes it could be 60 Hz. Leo just said "Ordinarily the hum fundamental is double the line frequency due to rectification." Not always. I suggest that you re-read the last paragraph in my post #25. Try changing the reverb recovery tube.

[J M Fahey]

Ordinarily the hum fundamental is double the line frequency due to rectification. 120 Hz in US/Canada & other 60 Hz countries, 100 Hz just about everywhere else where 50 Hz line is standard.

Sorry, the Hum FUNDAMENTAL is 60Hz (in USA) 120Hz is its first harmonic, and rather than HUM, is RIPPLE, not the same by a Country mile.

Besides, 60Hz Hum is basically senoidal, may be somewhat buzzy if mains voltage is dirty (usually because of noisy electric motors or gas discharge loads, think Fluorescent lamps or Neon tubes) while Ripple is basically a Sawtooth.

We are talking different frequencies, although one is an exact multiple of the other, very different waveform (which we won´t know unless we scope them) and very different "sonic signature", which we can tell by ear .

For a given amplitude, ripple (what we call power supply hum) is way more audible than 60 Hz hum, which is both deeper and darker, which our ears don´t pick that well

Real hum comes from poor AC returns, is picked inductively "from the air" , capacitively by electrostatic induction in high impedance circuits, while Ripple-Hum usually gets injected by poorly filtered rails *and* again by poor grounding paths.

That´s why I asked for plain Hiss and still mystery Hum scope captures, which in this case will tell a lot.

[mikepukmel]

Great, thanks. My todo list for this weekend:
1) pirate a new 12Ax7 I have earmarked for another amp project (actually have 2 to try) on the recovery side.
2) drag out the scope and probe a) the grid of the reverb recovery, b) just after the 0.003 dc blocking capacitor that is after the reverb recovery tube plate and before the reverb pot.

So, the hum is getting into the circuit in this area?

[pdf64]

Don't scope the grid, the scope probe will likely cause more hum etc; just the output across the speaker / bench load will be fine.
Yes, the hum is likely to be contaminating things around that stage, possibly due to resistance between its signal 0V and the HT 0V.

[Tom Phillips]

Mike,

One additional easy test you can do is ground the grid of the reverb recovery triode and determine if that affects the hum. You could do that with a clip lead or just use the reverb foot switch. This test will tell us if the hum signal is entering through the grid or not. You have made some good progress. The advantage of the troubleshooting approach we are following is that we are closing in on the trouble area by test and measurement rather than jumping right into parts changing. (Except for the tube which won't require soldering work)

Another tip is that you can "borrow" another 12AX7 from the same amp. In this case you could pull the tube from the normal channel pre-amp to try in the reverb recovery stage. That technique is very useful if you need to do emergency repairs at a gig.

Always mark the tubes or otherwise keep track of which is which. We always think we will remember but it's easy to loose track of the original tube positions or the order of the troubleshooting steps performed thus obscuring the clues to the root problem.

Cheers,
Tom

[Chuck H]

Ah! Been at work all day and got home to see Tom asking the question I had. That is, what happens when you ground the grid of the recovery stage. Knowing that will narrow the possible sources further.

So...
The hum is 60Hz
Even with the tank cable unplugged from the amp the hum still manifests analogous to reverb knob setting.

That's actually a lot of info right there. But I'm stumped as to how 60Hz can get into that circuit isolated from the tank and transformer. I can't actually think of anything but the filament circuit. Since the rest of the amp isn't humming that seems odd. Might just be a bad tube.

[mikepukmel]

Chuck, yep, you and Tom both suggested the same. Before I swap tubes I wanted to see if I could "fire" up the old scope and get some readings. Managed to get the "Compact and Light Weight Tek 7613 off the shelf and I think Im close to getting it to do something./

[mikepukmel]

OK Swapped out V1 into reverb recovery spot. Same hum. (left the tube that was there out of the amp, channel 1 is dead quiet). After some back and forth with the old scope, between probing the amp across the speaker, and probing my computer speakers that has a 60hz test tone running, fairly sure its 60hz in the reverb circuit.

Don't have an easy way to get a picture of the trace (would have to drag my camera out), but its not a nice sine wave, its repeatable, sharper peaks on the top than bottom. Really odd looking thing. I will drag the camera out tomorrow and get a photo of the scope trace.

I think the noise is about 200mv p-to-p. I can watch the wave increase and decrease with reverb knob setting, so fairly sure that's what Im looking at. Based on the trace I get with the 0.4v cal square wave.

[mikepukmel]

Don't know if it makes any difference but the speaker is out of phase. The signal is on the - side and ground +.

[mikepukmel]

Chuck: re

.... That's actually a lot of info right there. But I'm stumped as to how 60Hz can get into that circuit isolated from the tank and transformer. I can't actually think of anything but the filament circuit. Since the rest of the amp isn't humming that seems odd. Might just be a bad tube. ...

You haven't seen my wiring!!

[Tom Phillips]

...someone over the past 22 years sitting in a closet gave the "Cal" knobs a twist...

Just turn the Cal knob all the way clockwise until it "clicks." That is the calibrated setting for the scope.

...Vertical Amplifier is set at 20mv/div. There are 9 horizontal lines that span the display. Between each horizontal line there are 5 tick marks. Is the tick mark 20mv?...

No. Each major horizontal line will berepresent 20 mV with that setting if you are using a 1X probe. It will be 200mV if you use a 10X probe.

...The probe says "10 Meg ohm" and 13pf....

That tells us it is a 10X probe.

...There's a little button on the probe. if I push it the trace flattens out, so this must turn on the cap...

No. It shorts the probe tip to the shield. That is so you can determine the ground reference point on the display. The flat line will be the 0V point. You can adjust where that is on the display with the scope's vertical position adjustment.

...So, is this a fixed 10x probe? Read a little, so I think this is a fixed 10x probe. So, all the vertical amp readings read 10x the value...

Yes. They show 10X the value set on the vertical amplifier.

...Set 5mv, then each tick is 50mv? 8 vertical ticks is 8 x 60 = 200mv? Or I could be completely wrong...

To be clear, each major division on the scope face will be 50mV. There are 8 major divisions. Each major division is divided into 5 minor divisions. I am guessing those are what you are calling tick marks. For the settings you describe each minor division will represent 10mV.


The 7313 scope has three plug in bays. The usual configuration is to have one horizontal sweep (Time base) plug in and two vertical plug ins. Don't know what is in your scope.

[Tom Phillips]

Don't know if it makes any difference but the speaker is out of phase. The signal is on the - side and ground +.

Makes no difference regarding your 60Hz hum issue.

Please do the grounded grid test we discussed and report the results.

Chuck: re You haven't seen my wiring!!

It would be helpful if you showed us photos of the wiring.

[mikepukmel]

Makes no difference regarding your 60Hz hum issue.

Please do the grounded grid test we discussed and report the results.


It would be helpful if you showed us photos of the wiring.

Got it, and thanks for all the scope info. I think Its setup pretty close. I will do the grounded grid test tomorrow, out of steam tonight. And, sure, I will get photos of my wiring job and post tomorrow as well. Thanks!

[mikepukmel]

Sorry for one more question, Ithink I had it right: the probe is 10x, so reading 20mv per division the input is actually 200mv, is that correct? At the scope, its 20mv. but back on the on he side ofthe probe, its 200mv. So, I think the added noise is something line 200mv. p to p. (How easy is it to clean the controls, switches and pots, without killing them?)

[Enzo]

A X10 probe shrinks a signal to 1/10 its size. SO a signal that originally filled one grid at 20mv, now needs 200mv to fill that grid.