6g15 buzz

[oxbow_lake]

Hello again.

I just finished a 6G15-like reverb build, and it works, but it's giving an unpleasant amount of 120Hz hum. It's playable but annoying. Noise seems to be a common problem with this unit, but I am suspicious that there is a problem in my build/design specifically.

Attached are my schematic and layout (my DIYLC skills are not great), as well as photos (I'm not proud) of the build itself. I could not get the light to cooperate with the build shots, so had to take 2 but hopefully you can see clearly enough.

All jacks are isolated, and I tried to arrange the grounds in as close to a true star as possible, with the center being the cluster of turrets at the bottom right of the board, and used a ground lift circuit (which I have never done before). I've confirmed that there is 22R between signal and chassis ground throughout the build. Moving the center tap connection directly to the reservoir cap did not change anything. The heaters are elevated to the 6K6 cathode voltage.

The hum is present regardless of what amp (tube or SS) I connect to, though neither of those amps has a ground lift built in. I do wonder if this is an issue of ground loops between pieces of equipment.

If I ground either the input signal or the return signal from the reverb tank, the unit is dead quiet with the mix/blend knob turned to whichever side is is grounded, and if both are grounded the amp is essentially silent. Shielding/moving/playing with the reverb tank itself does not change anything, and besides the hum is present with the mix set completely dry or with the reverb return grounded.

Thank you as always for your eyeballs and brains.

[pdf64]

Your layout shows sockets whose sleeve lug will be connected to the chassis metalwork, hence shorting out the ground lift network (unless insulating washers are used)?

[R.G.]

[...] it's giving an unpleasant amount of 120Hz hum.[...]

Good on you for separating 60Hz hum from 120Hz hum. Were you able to do this by ear? I always have to scope these things to tell.

120Hz means it's coming from something related to rectification, of course. That means that either
> the rectifier CT return is not at the first filter cap negative (you've tried this, but for good practice, just move it there and leave it)
> the first filter cap is too small or defective, and the actual ripple voltage on it is too high
> rectifier diodes are slamming off, broadcasting squarks of RF into the amp
> a trail of less likely stuff, like choke magnetic field, capacitive coupling from first cap ripple to a grid wire, etc.

All jacks are isolated,

Do they have ceramic RF bleed caps from the jack sleeve lug to the chassis right at the jack? This is not the problem you're seeing but in high RF environments, it will get you.

The hum is present regardless of what amp (tube or SS) I connect to, though neither of those amps has a ground lift built in. I do wonder if this is an issue of ground loops between pieces of equipment.

Probably not. Ground loops with equipment grounding can give 60 Hz, not 120Hz except in very degenerate cases.

If I ground either the input signal or the return signal from the reverb tank, the unit is dead quiet with the mix/blend knob turned to whichever side is is grounded, and if both are grounded the amp is essentially silent. Shielding/moving/playing with the reverb tank itself does not change anything, and besides the hum is present with the mix set completely dry or with the reverb return grounded.

That tells you that the hum signal is getting into >both< inputs. I would be suspicious of the "C" power voltage and the ground wiring to both input tubes to start. Do you have a way to measure just the ripple voltage from the first filter cap negative to the top of the first filter cap, and all of A, B and C voltages?

[Stevem]

Where is the artificial center tap of the heater at the junction of your two 100 ohm resistors getting its needed ground from?

[oxbow_lake]

The 1/4" and RCA jacks have insulating washers to isolate them from the chassis.

For the heater artificial center tap, two 100 ohm resistors are mounted on the 6K6 socket going from each heater pin (2 and 7) to the cathode, (pin 8 ), which sits at about 23V.

I will break out the scope today and confirm that it is in fact 120 Hz rather than 60, and will try to measure the ripple at each capacitor. Useful to know that chassis-to-chassis hum is typically 60 Hz!

Thanks!

[R.G.]

Good point, Steve!!

[Stevem]

If I am not mistaken when you bias up the filament string with positive dc you do not need the two 100 ohm resistors, and in fact you want about 38 to 45 vdc getting to the center tap.

Actually if I was you and with the level of gain that the unit has I would ditch the complication of that DC bias and just place the junction of those two 100 ohm resistors right to ground at the point where your ground is for the output tube cathode .

[B Ingram]

All jacks are isolated ... used a ground lift circuit ...

You might try unbolting both jacks from the chassis, place something insulating (like cloth) between them & chassis, and re-conenct your input/output cabling for a test.

It might be whatever insulation/isolation you tried using didn't actually work. (I prefer plastic jacks in these situations precisely because of my own imperfect isolation attempts)

[oxbow_lake]

Did not realize that about the heater CT. I didn't have a simple way to move the CT to ground, so I tried 1) removing the resistors and just wiring pin 7 to pin 8, and 2) just grounding the heater from pin 7 to signal ground (where the cathode grounds). Neither changed the buzz.

I pulled out the scope and measured some voltages.
At the reservoir cap, 310 V DC with 3.8 Vpp ripple at 120Hz
At the 1st node (A in the schematic): 305 V DC, looks like a mix of 120 and 60 Hz ripple at ~50 and 80 mVpp respectively
2nd node (B): 298 V DC, ~10 mV ripple
3rd node (C): 250 V DC, <5mV ripple

So that all seems okay?

I attached scope plot of the reservoir ripple along with the output signal, measured at the output jack. This is weird to me because there doesn't seem to be any significant 60 or 120 Hz signal at the output, but I can clearly hear it through the amplifier. The purple is hard to see on the photo but the voltage range on the scope is maxed out at 2mV/div, and the trace looks pretty flat except for some white noise. I am also attaching an audio clip of the hum/buzz. This is *much* more noise than the guitar just plugged into the amplifier. At this point I don't trust my ears anymore to say if this is 120Hz or just distorted 60Hz.

[oxbow_lake]

All jacks are isolated ... used a ground lift circuit ...

You might try unbolting both jacks from the chassis, place something insulating (like cloth) between them & chassis, and re-conenct your input/output cabling for a test.

It might be whatever insulation/isolation you tried using didn't actually work. (I prefer plastic jacks in these situations precisely because of my own imperfect isolation attempts)

Doesn't hurt to try!

[R.G.]

OK, I squinted through the layout and pictures. Take that wire that runs from the PT center tap over to the ground group on the turrets and move it to the negative terminal of C1 as close to the negative terminal of C as you can solder it. Running it to the board star point like I think I see will give you horrible 120Hz hum. I can doctor the pictures if this doesn't make sense.

[oxbow_lake]

OK, I squinted through the layout and pictures.

You are a brave human being.

Take that wire that runs from the PT center tap over to the ground group on the turrets and move it to the negative terminal of C1 as close to the negative terminal of C as you can solder it. Running it to the board star point like I think I see will give you horrible 120Hz hum. I can doctor the pictures if this doesn't make sense.

Shortly after posting my original message I moved the CT ground point to the bottom of the reservoir cap. The image below shows where I moved it to. Is this what you mean? If so unfortunately the buzz persists. Could the proximity of the negative terminals for the reservoir/filter caps to the hot lug of the IEC connector be contributing to the problem?

If I can't see the hum on the output signal on the scope, is it really likely that it's enough to be audible when fed to the amplifier? Frustrating trying to eliminate a noise that is too small to see on the scope.

It might be whatever insulation/isolation you tried using didn't actually work. (I prefer plastic jacks in these situations precisely because of my own imperfect isolation attempts)

Thinking about this, I figured that if there was a short somewhere then I should not get the 22 ohm resistance between signal ground and the chassis that I do thanks to the ground lift network, right?

[oxbow_lake]

You know, if I put the reverb in the effects loop of my amp rather than into the front of it, the hum is basically unnoticeable even when set to Dick Dale levels. So I may just be hearing normal hum from one tube device being amplified by another tube device. In which case I can live with it, and unless anyone sees any further flaws with my layout I am satisfied and once again supremely appreciative of your help.

[lonote]

As I understand it, the 6G15 has a reputation for potential unpredictable ground loop interaction the the guitar amp.

When I built one, I did the same as you have, using isolated jacks, a single circuit ground point, etc, but also implemented Merlin's Hum Block Loop circuit that connects the reverb-unit circuit ground to the chassis ground. I included that feature right from the beginning, so I never ran it without, but I have no noise issues.

Might be worth a try as it is only a few components & easy to install.

Merlin Hum Blocker2.JPG

[R.G.]

Shortly after posting my original message I moved the CT ground point to the bottom of the reservoir cap. The image below shows where I moved it to. Is this what you mean? If so unfortunately the buzz persists. Could the proximity of the negative terminals for the reservoir/filter caps to the hot lug of the IEC connector be contributing to the problem?

Yes, that's what I meant. Given that it didn't help, it was not the only or even major source of 120Hz hum. Leave it there.
No, the negative terminals of the caps being near the hot lug of the IEC are not likely to be a source of hum. The impedance of that point is quite low, so capacitive coupling to it won't be a big factor.

If I can't see the hum on the output signal on the scope, is it really likely that it's enough to be audible when fed to the amplifier? Frustrating trying to eliminate a noise that is too small to see on the scope.

Well it seems that it is.
The scope trace contains enough hash to be hiding the audible hum. It's possible that the scope is seeing all of the hum, even parts that are high enough frequency to be washed out by the response of the amp. It might eve be that there is RF in that scope trace that gets picked up in the first stage of the amp and down-converted to hum. Stranger things have happened.
You touched on an important point - no hum when you put this unit in the effects loop, hum when it's on the input. A guitar amp's guitar inputs are pretty high gain, and everything you stick into the amp input jack gets amplified up as signal. The input stage of an amplifier dominates its signal to noise (including hum) ratio. Inserting the unit into the input of a guitar amp makes the input of the unit dominate the hum and noise of the two together. The recovery from the reverb tank is another high gain input for hum and noise, as you realized already.

Thinking about this, I figured that if there was a short somewhere then I should not get the 22 ohm resistance between signal ground and the chassis that I do thanks to the ground lift network, right?

A lot depends on exactly where a short is. Shorts between various ground points inside the circuit can cause hum and noise. The one-point connection to the chassis is just a method guaranteed to eliminate the chassis as a method of hum insertion. It doesn't do anything for other hum sources.

If it were me, I would try doing things to mess with hum introduction. For instance, short that 22 ohm resistor. Did the hum change any at all?
Does the reverb tank have both input and output jacks grounded to cable shield? Or is it one that has the nominal cable ground open on either input or output jack? Reverb tanks come in all three variations.
Does moving the balance control from full reverb to full dry change the quality of the hum any? Perhaps moving it quickly end to end? That might make an audible change that could help point to the issues.
If you have another 47uF/high voltage cap, you could try temporarily connecting it across C1 to see if the hum decreases. If so, the ripple voltage is a contributing evil.