120Hz hum in Pignose G40V (at the cathode-follower)

[boogieWoogie]

Hi Folks,
I have bad 120Hz hum in a used Pignose G40V I picked up.  This schematic (attached) is nearly identical to a Bassman or early Marshalls.  The noise seems to enter at the cathode-follower just before the tone circuit (see attachment 'cathode-voltage.jpeg').  That cathode-follower has clean B+ voltage (see attachment 'plate-voltage.jpeg'), and the triode prior to it is also clean.  The master volume affects the hum entirely & when I remove the 2nd tube, hum is entirely gone.  I used a free spectrum analyzer app on my phone to plot the noise & confirmed it's 120Hz (the scope photo below shows it too).

What I've tried so far

• All new caps in power supply
• Swapping out all tubes
• Moving ground location of power amp
• Looked for missing ground connections
• Chopsticked all wiring & parts & socket-legs

Any ideas for what to debug next?

Thanks,
BW

[pdf64]

That’s a knotty problem!
What brands have you tried in V2, have you got a Chinese or vintage  12AX7 to try in there?

[boogieWoogie]

Hi pdf64,
The amp came w/ 2 Sovteks & 1 non-branded Chinese 12ax7s.  Moving them around did not help.  After that, I put a brand new Electro-Harmonix 12ax7 in & similarly it did not help.

Thanks,
BW

[shooter]

IF i'm reading the scope right you have ~~.7V AC
That's a big signal, should be able to find it on the PS taps somewhere


The wires to and from the TS/Volume shielded?  if so look for a broke shield

[astronomicum]

Couldn't 162V on the V2B cathode pose issues without an elevated heater?

[sluckey]

Just an observation... There's a 60Hz component riding the bottom of that waveform.

[boogieWoogie]

Thanks for reply Shooter & astronomicum & Sluckey

@shooter
You're reading it right: ~0.7V AC hum

I scoped the power supply taps.  The 1st cap (called Va in schematic, photo 'power-tube-plate-Va.jpg' attached) has ~11V of AC ripple, which seems expected per this Valve Wizard design example (http://www.valvewizard.co.uk/smoothing.html).  The negative power-tube bias voltage has ~0.3V AC ripple (see attached photo).

The only shielding is from jack to V1 (and it's healthy), and from V1a to volume pot (and it's healthy too).

I feel like the next step is to start replacing caps in the tone stack (under the theory that one of them is leaking / oscillating, or I'll discover a missing ground in there somewhere).

@astronomicum
What you say makes sense in theory, but my bandmate has one of these too, and his is much quieter.  (The hum level is unusable, most of us would hear it & say, "something's wrong there".)

@sluckey
Good point, I can hear it too.  I noticed the wavey bottom of that hum and didn't consider that it was 60Hz.

Thanks,
BW

[astronomicum]

A particular design could be prone to issues without every amp experiencing the same problems. Too many variables. IMHO, the 7.2V spec on the heaters could also be a contributing factor especially if they are running even higher than that. The 60 cycle component you are seeing on your scope would be further evidence that part of the issue is heater induced.

[sluckey]

Set your scope to trigger on "line" rather than internal. If the display 120Hz waveform is rock solid, ie, not slowly drifting across the screen, then the signal is definitely derived from the power supply. In that case, you don't need to go chasing some oscillation.

[boogieWoogie]

@sluckey:  cool!  I did not know what AC Line / Line triggering was, but googled it.  Answer: noise stayed perfectly in-place.  So this is definitely noise related to power supply.

@astronomicum: yeah, well put.  I've been interested in elevating heater reference, but want to exhaust other suspects first (cuz their easier, or b/c I've mod'ed them, making them suspect).

It's time I come clean about the mods I've made:

• added humdinger (it helped)
• added bias pot for negative power-tube bias
• selectable bypass cap for V1A cathode (something like 22uF, 100uF, or none)
• selectable treble cap C12 (adds or removes more capacitance in parallel to change treble control response)
• selectable power amp bias: choose cathode or fixed bias
• not a mod, rather stock, but the presence pot is not hooked up exactly as the schematic shows
• replaced every cap in the power supply

Next ideas:
1) I think I'll disconnect the 2 mods close to this V2B (treble cap C12 mod, bypass cap V1A cathode) & see what happens.


Thanks,
BW

[boogieWoogie]

I've disconnected the preamp from tone-stack, and the hum persists.  The B+ still looks clean, and I've read a bunch about how cathode-followers should have elevated filaments, so I'm thinking @astronomicum might be correct.  (I don't have space in the chassis to do a DC heater conversion, and I'm not sure the transformer has additional current capacity anyway.)

So, I'll elevate the heaters.  Some questions:

• If my V2B cathode is at 162V, what's a good heater elevation voltage? (Valve Wiz says 30-60V, but why not higher?)
• Per Valve Wiz's explanation, the goal is actually to saturate this heater->cathode current flow, so the AC part can't be heard.  I would think that to saturate, you want max potential difference (which is what I have now: 162Vdc - 0Vdc)? Why does elevating heater Vdc cause saturation?

[shooter]

elevated heaters is typically done so you don't exceed the tubes specs.  look at a datasheet, at the filament section, there is a "max" difference between cathode VDC and VAC to heater voltage.  That what you're trying to avoid by elevating heater

[pdf64]

elevated heaters is typically done so you don't exceed the tubes specs.  look at a datasheet, at the filament section, there is a "max" difference between cathode VDC and VAC to heater voltage.  That what you're trying to avoid by elevating heater

It also reduces heater to cathode leakage (diode conduction), thereby mitigating that source of buzzy hum.
Tomer refers to it as biasing the heaters, it’s a technique that’s been around awhile 
P46
http://tubebooks.org/Books/Atwood/Tomer%201960%20Getting%20the%20Most%20Out%20of%20Vacuum%20Tubes.pdf

[boogieWoogie]

Thanks for replies @shooter & @pdf64!

I read @pdf64's link, and my parsing of it is: if you can make the Vdc of the heaters higher than cathode's Vdc, then stray electronics flying off the heater will be attracted back to heater vs. being attracted to cathode, reducing the current flow from heater to cathode.  This makes sense to me.

If my model is correct, then the best voltage for my heaters would be >= 162Vdc (the cathode's bias voltage).   For some reason, I haven't found anyone doing this or recommending going this high.  (Perhaps there's some heater-to-grid max V that would violated in this case).

So, I'll start w/ 80Vdc on the heaters, and see how it sounds.

Thanks all.

PS: @shooter, the 12ax7 Vhk-max = ~ 180V, so even w/o elevated heaters, we're technically in-spec.  However, most docs suggest we'll still enjoy longer tube life if we reduce the Vhk.

[shooter]

I re-purposed my last build as an Audio amp now, it's PP 6V6 cathode biased and uses a cathode follower.
even the cork-sniffers will begrudgingly admit the background "hum" is acceptable.


I don't elevate heaters, my hum voltage at the CF is 12mV NOT 700mV, fix that, you've fixed the problem.
better filtering at the PS, better wire layout and length, all add up to a hum free amp.

[Platefire]

Hi Boog
Got this amp and it was always too noisy for me. You probably want to keep the same gain in the amp, just want to get rid of the noise? I didn't need the extra gain and bypassed the extra gain stage and now my amp is now quite as a mouse and sounds great. Just a thought! Platefire

[boogieWoogie]

Thanks to shooter & platefire for latest comments.

I removed the preamp board & used a bench power supply, very clean, to power the board & tubes.  With even longer wire runs on the bench setup, the noise was much worse, maybe 2x, so I don't think I can reproduce the problem accurately enough to study it outside the chassis.  (I tried DC-rectifying the heaters, and it did not help).  On the bench, elevating the heaters to ~40Vdc reduced the noise @ V2B cathode by 50%.  (Further elevating to ~80Vdc didn't help any more).

I found, w/ the long wire runs on the bench, that the amp is very prone to feedback / oscillation.  Any time I moved an in-phase wire near the tube or another wire of same phasing from earier in the amplification chain, it would oscillate easily: https://giphy.com/gifs/spzKEqxomQ9zPkwGax

Platefire, I found your thread where you explore removing a gain-stage, and will consider applying that mod when i re-install this mess ive made.  Thanks for the tip.

BW