Vox AC15 handwired 60hz hum

[01redcobra]

I am working on a Vox AC15 handwired head that has a 60hz hum with volume all the way off. The owner says it didn't do it when it was first purchased. I have replaced filter caps, checked grounds and pretty much everything I could think of so far. It is less than a year old, has new JJ tubes in it and was previously unmolested except for having the transformers replaced with Mercury Magnetics and yes they were installed correctly as best as I can tell. The hum will go away if V3 is removed. I have tried several different tubes, but they make no difference. Going to a lower gain tube does lessen it. I have tried replacing coupling caps and many other attempts to alleviate the hum, it still remains. Bringing the bass control up makes the hum louder and turning up the treble will lower the hum volume. I refuse to believe that this is a normal condition for this amp and was wondering if anyone else has experienced this and possibly found the issue. A schematic can be seen at http://www.blueguitar.org/new/schem/vox ... dwired.pdf

Thanks

[Phil_S]

Just some amateur guesses:
120Hz is in range for the bass pot. Not surprised about that.
It there a missing tube shield for V3?
Check for bad grounds and open cathode resistors.
Is C17 leaky?
Did you try lifting the outer leg of the volume pot between c20 and C22?

[quayhog]

Could it possibly be environmental. By that I mean what else is plugged in nearby. Flourescent lights, cell phone chargers, compluter power supplies can all make difficult to troubleshoot noises.

Neon signs at bars can be especially troublesome.

[ampdoc1]

I'd check those components at the chassis ground connection (page 13). These are supposed to minimize hum. Also check all the ground connections to the boards.

[01redcobra]

I'll recheck the grounds, but last I checked they were all good. There are no tube shields on the preamp tubes. And the the cap at C17 was replaced. The amp hums no matter where it's plugged in, including other locations. And no, I didn't lift that leg on the volume control. I've read similar threads on this problem, including the Vox amp forum and no one seems to be able to remedy this. I refuse to believe this is something that can't be eliminated. I have amps from the 50's and 60's that are way quieter at zero volume. Thanks to all that have replied so far. I hope someone can help me with this. I really don't like to admit defeat.

[ampdoc1]

Work back toward the input from V3. Ground the input of the V3 to see if that stops the hum, then to the volume control, etc,...grounding or disconnecting connections until you get to the input. Also check the jacks to make sure one isn't "open".

[01redcobra]

The only way I seem to be rid of the hum is when I probe either of the grids on V4 for voltage. Tried pulling them (the leads) at the grids and hum got louder. Argh.

[01redcobra]

The noise is definitely in the cathode follower stage of V3. Grounding the grid coming from V2 changes the hum but it is still there. Grounding the grid in V4 from V3 takes it away altogether. I have already replaced C24 C23 and C15. Only C19 remains original as I didn't have that value(50pf@500). The only perplexing thing I noticed was that the value for R34 on the schematic shows 680K and the value printed on the board is 220K. Any ideas on any of this?

[rfgordon]

I had an amp I built a couple of years ago come in with nasty hum. It turned out that the cathode resistor on one triode--the V2a--V2b was the cathode follower-- was the culprit. I replaced it and the amp was quiet again.

[01redcobra]

Thanks for the input Rich! I replaced the resistors at R36 and R35 and, alas, the hum is still there. I keep staring at the schematic, hoping that something is going to trigger an Ahah, but I it's just not happening

[01redcobra]

I don't know what I was thinking when I first posted this, but I just realized I said it was a 120hz. It's actually a 60hz hum. Sorry about that.

[01redcobra]

Well, the hum is centralized on the b portion (as per schematic) of V3. I have found that if you ground the grid (pin 7) the hum is increased or at least changed. Pulling V2 does nothing, hum is still present. I am looking for any thoughts on this, as I am running out of ideas. I did notice that Vox is using carbon comp resistors at R25 and R26. Could they be inducing this hum? And I am really unsure if it is 60 or 120HZ. The loudest hum does appear to be around a B on a guitars A string. When I ground the grid on V3 pin 7 it seems to bring in a lower frequency hum along with the initial hum.

[Richie]

if it still hums when V2 is pulled, try replacing one at a time, the preamp tubes before it. I read that you said they were new JJ, but didn't really see that you tried using or completly swapping the tubes out.
Some of the tubes they have out now, cause this loud hum just as you describe. Thinking the tube is new can be a problem. You could rebuild the amp from front to back,and still would have the hum,if the hummy tube was not replaced.

[azatplayer]

I had one of these new and it did the same thing. Great amp, hopefully you can correct what appears a factory issue.

[01redcobra]

Pulling V2 the hum still remains. There is nothing before V2 in that channel of the amp ( the Top boost). Other tubes have already been tried, so early on that I may have forgot to mention it. Azatplayer, I'm going to have to believe that this is a common problem. I've seen numerous posts on the subject, but I've never seen a solution. There must be at least one tech out there that has found the "magic bullet" for this. My next step, I believe is going to be adding extra filtering. This has been the most frustrating problem amp I have ever encountered. I really don't want to do anything to change major circuitry as the amp is only about a year old and it will surely devalue it, but then again, the hum is going to cut the resale value too.