Help with hum in Vox AC15H1TV

[Chuck H]

Very good then. If the caps are craps. High ESR can cause hum issues for sure, but IME this can be a layout issue too as some amps use highish ESR caps and still manage to abate hum. I almost mentioned low ESR caps in my above post too. If things aren't absolutely ideal a high ESR cap will introduce hum and a low ESR cap can fix that.

Didn't mean to make implications on anyones chops or experience level... Sorry

Chuck

[unparalleled]

Very good then. If the caps are craps. High ESR can cause hum issues for sure, but IME this can be a layout issue too as some amps use highish ESR caps and still manage to abate hum. I almost mentioned low ESR caps in my above post too. If things aren't absolutely ideal a high ESR cap will introduce hum and a low ESR cap can fix that.

Didn't mean to make implications on anyones chops or experience level... Sorry

Chuck

No need to be sorry, I just wanted to point out that I had already fixed one and how I did it.
You are quite likely more experienced than I am!
Cheers.
Alan.

[01redcobra]

I can't see the grounding being an issue only because the amp is fairly new (and did not have the problem until a few months in use) and has very low mileage. Again, I could be wrong. I have replaced C1 and C2 now, and saw no improvement. That schematic link that Straleno posted is still good last I checked. Guess I'm gonna try the piggyback experiment and see how far I get.

[01redcobra]

I could be grasping at straws here, but should I be seeing any ac voltage on the plates of the EL84's? I was probing with an oscilloscope(which I have very little experience on) and found ac on pin 7 of the EL84's. I checked with a vom and found I have about a half a volt ac there. This amp has a pent/tri switch and I also found that 1/2 volt ac on the two terminals out of the switch for the pentode operation, but only .05-.06 ac on the center(feed) lugs of the switch. I already had tried the piggy back cap on C21 (which should be the filtering for that circuit) and it didn't seem to help. C1 was replaced already as was C2. Help?

[loudthud]

It's not unusual to see AC voltage on the plates of the EL84s. If you measure from one EL84 pin 7 to the other with your voltmeter, you should see close to 0.0 VAC. This amp has a large choke to filter the B+ that should remove most of the hum. There should be much more AC voltage on C1 than on C21. If not, that might indicate a bad choke. Have you verified all the filter cap grounds with an ohm meter?

[01redcobra]

I checked the cap grounds to chassis and all were at .2 ohm. There is .25 vac between the two El84' at pins 7. Talking to the owner, the hum started several months after purchase and he then had the tubes replaced with JJ's and all the output and power trans and choke were replaced with Mercury's about the same time. He said that the hum was a little better after this, but still continued.

[01redcobra]

In moving and probing wires, I found that the hum will diminish or intensify if I move the Ct connection at the Ht fuse.I pulled the fuse holder and checked what I could and did not see a problem. The solder connection is good, it just seems that the fuse to terminal connection is affecting the hum. Cleaned with contact cleaner, but it didn't seem to help. Right now I've got a cable tie to hold it in the position of least hum.

[01redcobra]

Amp still has that hum. Last thing I tried was elevating the filament supply ctr. by lifting the grounded ends of R9 and R10 and rejoining them where C16 and R28 meet the power tube cathodes. This actually seemed to add to the hum. I'm pretty much at wits end with this. Anybody any ideas?

[J M Fahey]

Yes.
VOXes are notorious for poor filtering, period.
In absolute difference with Fenders and Marshalls (and most everybody else), which use "sequential filtering", meaninf that going from input to output, everu stage gets its power from the previous one, through a new RC filter leg.
Towards the input the ripple becomes nil.
Not so with VOXes, which favor a "Star" filtering ;draw just the filtering stages in one and other case, representing gain stages with little rectangles and you'll see clearly what I mean.
Don't know what where they thinking, even original VOXes from the 50's and 60's were so.
Of course nobody dares to correct that because it would be sacrilege.
We can find among Musicians some the most superstitious people in the world (that's what "Mojo" means).
I would cut the track that goes from the "HT" end of the choke to R1, sat 1/4" gap, scratch both sides of it and solder a 2K2 1W resistor there, and add an external 22uF or 47uF x 350V capacitor to ground. You can repeat the same just before the next branch, be it R2, R22 or R23.
Hum will *dissappear* .
Of course a blue sizzling 1000000 Volt ray will fall on my head, straight from the VOX Heaven, just by suggesting this sacrilege.
You can (silicone or contact cement) glue the new capacitor(s) to chassis and use flexible wire to connect them where needed.
If you can also increase value of filtering capacitors within the board, within space constraints, by all means go for it.
If you see ads with my face and the legend "Wanted dead or alive" , now you know why.
Oh well.
I didn't even suggest the capacitors should be Green Stripe, Script Lettering, Papyrus in White Whale oil, Platinum legs ones. NOS of course, preferably 1936 vintage.

[01redcobra]

There is a turret on the end of the board where the HT line comes from the power supply board and I can add the cap and resistor there without cutting into the board. I can ground the cap at the main chassis ground at this point also. I have nothing to loose but time at this point and I've already well exceeded my time constraints at this point. I just want to get it fixed and return it to the owner. Many lessons have been learned so far. Now to find the 2k2 resistor in my parts bins

[01redcobra]

Well, as hopeful as I was, I added the extra filter stage to the HT line and the hum is still there. This thing has to be haunted.

[J M Fahey]

Sh*t !!! Then it's a grounding problem, the worst to diagnose.
Now we have to start chasing goblins, hoping to find the one.
Since the area around V3 seems to be the source of the problem, let's check there first.
An immediate suspect is that the Ch2 pot, VR4, obviously it can't be grounded to the same point as "BK", the left cathode, or rather, C25 and R36, because it resides on the front panel.
From what you say, putting VR4 on "0" *still* leaves some hum.
Please follow the wire that goes from VR4 cursor to BG (the left grid), unsolder it from the tube socket and place a short wire from BG to the grounded end of C25 and R36.
Straight to C25 negative pad/eyelet. We are testing the *shortest* ground path seen from V3.
Does the hum go down?
If it does not, please lift the left leg of C14 and ground it to R20's grounded pad.
We are trying to cut *every* possible "hum injector" here.
Good luck and post what you find.
PS: is it a PCB amp or eyelet/turret type?
Post some picture of the suspect area.

[01redcobra]

I got some shots of the suspect area, but haven't had a chance to check the things you mentioned. I will attempt to attach the pics here.

[01redcobra]

Grounding the grid BG removes almost all the hum. What's left is livable. But I have already tried a new pot and two conductor shielded cable between the pot and the connections on the board that the original pot uses and it left the hum the same. The pot case is of course grounded to the chassis, but the ground connection from the leg of the pot goes to the main ground buss trace on the board. If I leave the pot unattached to the chassis, the hum is worsened.

[J M Fahey]

Hi.
Well, thanks to the pictures I already have to discard one of the suspects.
The first of those tests are not necessary anymore; what I was going to suggest, that the Volume pot be lifted from its original grounding point (wherever it is) and left "floating", grounding *directly * to V3's cathode capacitor (for that tube, that's the real ground it sees), has already been done.
Now I know hum is not getting into V3 grid either directly nor by poor grounding.
We still have 2 suspects in the list.
Hum might be getting into the inverter through either of its grids.
Solder a short piece of wire across R16, then another across R14. Any change?
Good luck.
The other doubt I have about grounding is that, that PCB/Turret mix they use, shows a wide center track, visibly the audio ground, where cathode components are soldered, and *another* track on top, where apparently the big electrolytic filters are soldered.
Thus cathode ground and filter ground are not the same.
I don't like that.
I would lift the grounded end of C25 and R36, join them "in the air" and ground them to the negative connection of C17.
I would also ground there the volume pot shield.
That way I have all actors in this play grounded to the same place.
If all that fails, I'm out of fresh ideas. Sorry.
Anybody who sees a fault in my reasoning here please say so, "4 eyes see more than two".