Help with hum in Vox AC15H1TV

[straleno]

Picked one of these up last week, and it sounds amazing... except for a fairly loud 60 cycle hum that's audible even with nothing plugged in and both volumes all the way down. True, you can't really hear it when I'm playing, and a quick search reveals I'm not the only Vox owner with this issue, but still. It ain't right.

Couple of clues--the hum is affected by the bass knob on the top boost channel. When I turn it up, the hum gets louder. When I turn it all the way down, the hum diminishes, though you can still hear it.

The hum is worse in pentode mode (vs. triode mode). Again, this is with no instrument plugged in, and the volume knobs all the way down. The volume knobs have no effect on the hum.

Lastly, when I turn off the power, the hum stops right away.

Haven't yet done any tube swapping or other troubleshooting, I wanted to ask first. I've built a couple amp kits, so I'm not a novice, but I'm no expert, either.

Thanks for any suggestions.
John

[loudthud]

Is there a schematic for your amp online anywhere? Does the amp have a standby switch? Does putting the amp in standby mode stop the hum immediately?

[Dano]

My gut feeling is dried up power supply Electrolytic capacitors.

You can check them in circuit with an ESR (equivalent series resistance) meter just make sure the power is turned off and they are discharged.

If you don't have an ESR meter, hmm, a capacitance meter, checking the dc ripple among some things to do.

[straleno]

Hey, thanks for these responses. Dano, dry electrolytics seem unlikely, since the amp is brand new.

Loudthud, the schematic is here: http://www.blueguitar.org/new/schem/..._handwired.pdf
The amp doesn't have a standby, just on/off. When I turn the amp off, the hum stops immediately.

Plus, here's a little update: I did some tube-pulling, and found that the hum went away when I pulled V3 (Sovtek 12AX7WA). I swapped in some known good substitutes--JAN Philips; NOS GE; Groove Tubes "Mullard" style"; JJ ECC83. The JJ--and ONLY the JJ--was quieter, quiet enough that I think I can live with it for now. (I tried several JJ's in this spot--they were ALL quieter than any other choice.) I also put a JJ in V2, for good measure, but the problem clearly is in the vicinity of V3. V3 is part of the top boost channel, though I'll admit I can't read schematics well enough to know just what it does in the circuit.

I also poked around the wiring, but it didn't make a difference. I can see that the preamp socket wiring isn't twisted together, and that the wires are close to the sockets themselves, if that means anything.

Loudthud (or anyone else), I'd love to hear your thoughts on what might be going on, especially if you don't mind taking a quick glance at the schematic.

Also, what could it be about the JJ's that makes them uniquely quieter in this spot?

Thanks!
John

[straleno]

Treating myself to a bump. Anyone else with thoughts? Loudthud, are you around?
Thanks.
J.

[Chuck H]

I think your problem is localized to V3b (second triode of V3). If the problem was in V3a your volume control should effect the hum level. V3b is a cathode follower circuit. That means the cathode is elevated from ground quite a bit compared to the other stages in the amp. Many currently MFG'd 12ax7's don't seem to like a high heater to cathode resistance. Perhaps the JJ is better than the Sovtek for this and that would explain the reduced hum with that tube. All NOS 12ax7's can handle this operation with more aplomb so it may be worth trying a NOS 12ax7 in that position.

Other things to check/try... Check the integrity, polarity and ground for C17. This cap should be decoupling hum from this part of the circuit. You might even try a bigger value cap if all else fails. Try lifting the cathode lead for V3b and pushing all other leads down against the chassis. Give the V3b cathode lead as much space as you can. You could also try elevating the filament supply ctr. You could just lift the grounded ends of R9 and R10 and rejoin them where C16 and R28 meet the power tube cathodes. This will elevate your heater supply by 10 volts or so.

NOTE: For best results when elevating heaters most builders design a voltage divider off the main filter supply and shoot for 30 to 40 volts of elevation. Elevating the heaters would help eliminate some of the problems with a high cathode to heater resistance that some modern 12ax7's can't handle well. I don't know if it would help with hum in your particular circuit, but it usually does with others.

HTH

Chuck

[loudthud]

There certainly seems to be a problem in the area of V3. The only thing I can add is to measure the resistance from either of the heater wires to ground. It should be 50 ohms. R9 and R10 suspect if resistance is wrong. Verify that each side measures about 3.2 VAC to ground.

I haven't heard anything good or bad about EZ81s. It is suspect because of the high heater to cathode voltage. If you have a spare or can borrow one from another amp, I'd try swapping one into the amp.

Since there is no standby switch, you would have to pull the EZ81 out while the power is on. (Use extreme caution!) If the hum stops immediately that points to filter caps or some kind of grounding issue. If the hum fades out, that suggests some kind of problem with the heater supply or a tube.

[olddawg]

Would it be worth the trouble to make an artificial center tap for the filament supply and rewire the filament dress with a better twisted pair as well as swapping in some nos tubes?

[01redcobra]

Hate to bring up an old post, but did anyone ever solve this? I have a Vox handwired here with the exact same problem. I have already redressed the heater wires and still am faced with the same hum. The amp isn't even a year old.

[unparalleled]

I had the same fault with an AC15TVH a few weeks ago.

Change C1, C2, C3, C17 and C18.
I used 33uF 450V axials which I had in stock for all of them, problem solved.
The caps they used for these nice amps were not nice!

[01redcobra]

Sounds like a plan. I already replaced C17 with a 22uf@450 and saw no improvement. I will work my way thru the others and let you know what develops. I'll be using 22uf@450 because that's the closest value I have in stock and they fit physically. Tight working quarters in there. Thanks!

[unparalleled]

Sounds like a plan. I already replaced C17 with a 22uf@450 and saw no improvement. I will work my way thru the others and let you know what develops. I'll be using 22uf@450 because that's the closest value I have in stock and they fit physically. Tight working quarters in there. Thanks!

Tight is the word!
22uF should do the trick.
The caps I used fitted nicely they were NOS Elgen brand which I managed to get from one of my old suppliers who no longer wished to stock these useful parts.
I bought all he had left and I use them in a lot of repairs and builds.

[Chuck H]

Not that those amps won't benefit from more filtering, but...

The issue is probably where things are grounded, not so much the value of what's grounded. I don't know the ground layout for those amps but I'd wager that's where the problem is. If the ground layout is at fault you can increase filter sizes all day and get no where. I'm sorry I've never worked on one (or maybe I'm not, sounds like a PITA) or I might know the solution. A tech will need to have one of those amps on the bench to see what's grounded where and design a fix.

Chuck

[unparalleled]

Not that those amps won't benefit from more filtering, but...

The issue is probably where things are grounded, not so much the value of what's grounded. I don't know the ground layout for those amps but I'd wager that's where the problem is. If the ground layout is at fault you can increase filter sizes all day and get no where. I'm sorry I've never worked on one (or maybe I'm not, sounds like a PITA) or I might know the solution. A tech will need to have one of those amps on the bench to see what's grounded where and design a fix.

Chuck

Hi,
I am a tech. I hope! (1953 vintage)
The caps were rubbish on the one I repaired, the grounding was not a problem and I found the build quality/layout of this model was well thought out.
If I had 10uF caps on hand which were small enough to fit between the turrets I would have stuck to the schematic and used them, however I figured more filtering in the preamp nodes was unlikely to change the characteristics of the amp so I used 33uF.

[unparalleled]

Sounds like a plan. I already replaced C17 with a 22uf@450 and saw no improvement. I will work my way thru the others and let you know what develops. I'll be using 22uf@450 because that's the closest value I have in stock and they fit physically. Tight working quarters in there. Thanks!

I suggest you carefully piggy back a new cap across the existing filters one by one and check for hum cure. WATCH POLARITY!
If you are wary of high voltages you could connect the caps in parallel with the old ones.(amp switched off each time you do the connection)

I would change them anyhow but this test will prove the caps faulty.