Ac30 build Hummmmm

[Steelwitch]

Hi! I could use some input on a peavey classic 30 conversion into an AC30. Sonically the amp is basically where I want it but I’m having some issues with preamp noise and possibly bad grounding. Here’s where I’m at:

I’ve localized the hum to the cathode follower. I think the input is noisy as well, but the 60hz hum that I’m hearing gets killed with V1 and V2 pulled. The amp is single channel top boost only so V1a is unused. I was pretty sure that it was a lead dress issue, but after putting in shielded wire at all of the noisiest runs, the problem persists. With the volume control turned down completely, the hum is killed, but there’s signal bleeding through the treble control when turned up, and the bass control lacks bass only when it hits 10. I can hear the 60hz hum changing with the bass control. I’ve checked for miswirings and I can’t find any. I’ll post some pics of the build and feel free to suggest and better ways to go— I did a P2P layout so it’s going to be difficult to spot anything just by looking at it, all that aside I feel as though my layout makes a lot of sense but maybe I’m off the mark here.

Can anyone suggest some tests to determine what might be wrong? I could have sworn that there was a grounding issue behind all of this but I couldn’t find any loops to speak of and my grounding scheme has everything grounded I think. Reading .001 to ground on my bus and the main ground point... with the input jack unswitched, touching the pot shafts or grounding my hands to a guitar affects the buzz level. I would love to get it as quiet as possible so it can be cranked into top boosty goodness!!



Here’s a schematic: https://el34world.com/Hoffman/files/Hoffman_AC30.pdf

And a bunch of pics:
https://ibb.co/LvZFm9t
https://ibb.co/hHNZnqr
https://ibb.co/3BwtsGR
https://ibb.co/wrRNqzy
https://ibb.co/JtQ1pJX
https://ibb.co/1QD1rhb
https://ibb.co/n3K5VLZ
https://ibb.co/S758pDk
https://ibb.co/G7CCkKr
https://ibb.co/87nbps3
https://ibb.co/KGhRmyH

[Steelwitch]

Don’t mind the stray leads, I’m wiring in an effects loop when these issues are worked out

[Chuck H]

Well since you have a PTP rewire of a Peavey Classic 30 that Hoffman layout means nothing. Did you take this mod all the way into the power supply rail? Certainly you didn't copy the AC30 filament supply since the Peavey uses a different voltage.

You may have a ground issue. Having buzz that comes and goes when touching any grounded metal is a good indicator of that. Why does the buzz not go away when you touch your guitar strings then? They SHOULD be grounded to the amp. So touching them should have the same affect as touching the chassis or pot shafts. Try another guitar or try that guitar in another amp and see if the problem is repeatable.

You may have a ground fault on your input jack.

Since the volume control is after the first preamp stage the hum is in the the input circuit, first preamp stage or between that and the volume control. It sounds like the input jack right now. But...

For the record, an actual (not Hoffman) AC30 circuit has the first gain stage cathode fully bypassed. This could also be a source for hum considering that the Peavey uses a different filament wiring than many amps. Though they are DC series wired if I remember correctly. I have to admit that it's hard to tell from the schematic other than the N/C indicators for pin 9 giving a hint. The schematic doesn't show the DC filament circuit.

"With the volume control turned down completely, the hum is killed, but there’s signal bleeding through the treble control when turned up, and the bass control lacks bass only when it hits 10"

Yeah, this is wrong. If you have signal bleeding through with the volume at zero there's some voltage somewhere it shouldn't be. Maybe that floating preamp stage that you aren't using is bleeding on the power supply? But I promise you there are design and/or wiring errors causing this.

[R.G.]

Another source of mysterious hum that simply won't go away is mis-wiring of the center tap of the power transformer. This lead absolutely has to go to the negative terminal of the first power filter cap, and nowhere else. Not wiring it this way, especially wiring it to the chassis ground point or someplace else random almost ensures some level of hum.

There are many others.

[Steelwitch]

Yes, the hoffman layout is pretty irrelevant to this, but it's the schematic I built from which has changed slightly so the EL84's aren't unruly

I should have been more clear when I posted. This is a complete rebuild. I'm using a Hammond 290mx as my PT, basically the only thing original to the amp is the chassis and OT. You'll see in the attached pictures that the heater supply is AC, but I'm not totally sure that's my problem.

The input jack thing has been really getting at me. Is it possible that this chassis just doesn't ground well? My preamp bus is grounded to a switchcraft 12A. Under the faceplate, this chassis is coated so I took some time to sand down the area around the input jack so the sleeve would be making full contact with uncoated steel. My meter claims that it's grounded properly with .001 continuity reading with internal resistance of the meter included. and there's only one other ground point where the entire power section is going to. Aside from this the power supply is pretty dang quiet, any thing that's coming through is just this hum coming through grounded pots I think. I don't think I miswired the input jack. I've tested multiple things like unmounting the input jack and grounding the bus elsewhere with no change. I can't even hear a difference when intentionally making a ground loop. I'm not reading anything even in the MV range. I may post a voltage chart just to see if anyone spots something that doesn't look right.

Chuck H, I think you're totally right that there's voltage somewhere it shouldn't be, I'd think that the ground being elevated would do something like this. I didn't see this hum in the input stage on my scope. I had originally build a normal channel for this amp that had the same result.

[Steelwitch]

Another source of mysterious hum that simply won't go away is mis-wiring of the center tap of the power transformer. This lead absolutely has to go to the negative terminal of the first power filter cap, and nowhere else. Not wiring it this way, especially wiring it to the chassis ground point or someplace else random almost ensures some level of hum.

There are many others.

That's exactly how my power supply grounding is laid out. I'm not getting any kind of idle hum, just something going on in the preamp.

[Chuck H]

Well it doesn't always take very much to cause an unpleasant hum. Especially when it's in the first stage of the preamp since guitar amps, even vintage designs, are rather high gain WRT amplifiers in general. That you have signal bleed with the volume down is indicative of some sort of voltage moving around relative to signal behind some resistance in that control and it's bleeding through the power supply. I'm not patient enough to look through the PTP images to determine your wiring. I don't think I'm alone in this. But there may be some good news...

Since you're using typical AC filament wiring and floating a triode I'll wager that the hum could be in your first preamp cathode. Maybe there's a ground situation that's less than ideal, but it may not matter. Many guitar amps have gotten away with this historically. If you can fix it without rerouting all your grounds, no problem.

First, I would eliminate daisy chained grounds, Any shared ground points in the early preamp stages should be separated. If you have the first two preamp cathodes and the volume control going to the same node and then to ground, BAD!!! You want THREE GROUND LEADS WITH EACH ONE GOING TO CIRCUIT GROUND ON IT'S OWN.

And try fully bypassing the first gain stage cathode. That .1uf cap won't pass any 60Hz on the cathode to ground. And I'll bet that's where the hum is. Since the original AC30TB circuit uses a fully bypassed first gain stage I don't imagine there's a tonal detriment. And certainly nothing to lose in trying it.

And keep in mind, daisy chained grounds = BAD.

[Mick Bailey]

It also appears from the pics that you have the screened leads grounded at both ends - is this the case?

[Steelwitch]

It also appears from the pics that you have the screened leads grounded at both ends - is this the case?

Not the case! But maybe I should consider grounding them at the bus.

[Steelwitch]

Ok, so there must be a miswiring somewhere. I finally found that when I turn up the volume control, there’s up to .6vAC on the ground bus. Now to find the source.

Fully bypassing the first stage does sound good, but not the source of my problem, although I believe it’s in the input stage now. Is it possible that the heater supply is bleeding into the cathode somewhere?

[Steelwitch]

I tried changing the ground point of V1 cathode but no difference

[pdf64]

Some if I missed it, but have you eliminated a bad tube/s, eg with poor heater-cathode insulation, playing some part in this?

[Chuck H]

Ok, so there must be a miswiring somewhere. I finally found that when I turn up the volume control, there’s up to .6vAC on the ground bus. Now to find the source.

Fully bypassing the first stage does sound good, but not the source of my problem, although I believe it’s in the input stage now. Is it possible that the heater supply is bleeding into the cathode somewhere?

If you fully bypassed the first stage cathode then, since we've already determined the hum source is before the volume control which has only the first gain stage in front of it, you do not have filament AC on the cathode because full capacitor bypass on that circuit should shunt any AC there to 0V (ground). But on that note...

If you're reading .6VAC on the ground bus then it isn't ground. Or more properly 0V. Which seems impossible since I'm sure you have it connected to the chassis and the chassis connected to ground. Those two build elements are pretty hard to mess up. Sooo...

Are you figuring the default reading for your meter probes and subtracting it? You need to press your meter probes together before taking a sensitive, low measurement and then subtract that figure from your reading.

[Steelwitch]

If you fully bypassed the first stage cathode then, since we've already determined the hum source is before the volume control which has only the first gain stage in front of it, you do not have filament AC on the cathode because full capacitor bypass on that circuit should shunt any AC there to 0V (ground). But on that note...

If you're reading .6VAC on the ground bus then it isn't ground. Or more properly 0V. Which seems impossible since I'm sure you have it connected to the chassis and the chassis connected to ground. Those two build elements are pretty hard to mess up. Sooo...

Are you figuring the default reading for your meter probes and subtracting it? You need to press your meter probes together before taking a sensitive, low measurement and then subtract that figure from your reading.

I may have jumped the gun on that reading. I’m not getting it to happen anymore, and may have been a false reading from my meter. Reading .3mv on ground which shouldn’t contribute to any kind of noise.

I can’t seem to find the noise on my scope. It’s dead quiet from the input stage and then the first spot I can see the sh***ness is at the cathode of V2, at the junction of the treble cap and the 56k cathode resistor. If I ground it with my meter right there the hum and buzz is killed. Also if I pull V2 it’s quiet as a mouse. So it’s a bit confusing to me bc it should be before the volume control. I tried another tube and no dice.

[Helmholtz]

If I ground it with my meter

What does this mean? How do you ground with a meter?