Vox AC30 6TB hum problem; bad power transformer or something else?

[vintagekiki]

Nothing personal

In order for the amplifier worked properly, not to have hum, it is assumed that was previously brought in mint condition (100% correct), which can not be deduced from user posts (first, new tubes, no tubes "as-as" etc ...)

The user writes that he made some modifications related to shielding and grounding. He did not show the performed modifications.
Someone is request upload pics from scope, pics is upload, pics obviously means nothing to anyone, because no one interpreted (comments) upload pics.
The question arises as what we are discussing these 60 posts?

Whenever came to me musicians who didn't know what wanted from amplifier, my answer was "bring me the same amplifier as yours, which works differently." The answer can be assumed.
I’ve come to the conclusion that musicians who “poorly” rule with their instrument, instead of learning to play, they always blame their amplifiers for not having a sound.

Did Hendrix and other greats deal with "hum problem; bad power transformer or something else" or PLUG AND PLAY (guitar)

If Vox, Marshall or any company, produced tens of thousands amplifiers, all the musicians are satisfied, only one is not, wherein is the problem. In amplifier or in musician.

One thing is sure
The factory development engineers did better designed the amplifier than us portal users.

[The Jonald]

Jonald, how was the amplifier set up for that scope shot? Were the pre-amp tubes in or was it only the PI tube and where were the controls set?

All tubes in, everything turned all the way down. I should have pulled everything but the PI, d'oh. I'll do that, scope the PI grids, and also check out the rectifier and reservoir capacitor.

[echoplexi1974]

Update on my Vox ac30, I installed a humdinger and my hum disappeared. I'm a happy camper, the amp is quiet now!

[Helmholtz]

See post #6 .

BTW, as humdinger works, it was heater hum having a 60Hz fundamental - as opposed the power ripple hum (120Hz).

[pdf64]

... One thing is sure
The factory development engineers did better designed the amplifier than us portal users.

Sometimes, engineers have to work to overly tight deadlines; given a bit more time to do the job to a higher standard, the AC30TB may have turned out a bit different.
Back in 1993, when Marshall got the contract from Korg to build a recreation of the 60s AC30, Steve Grindrod recalls

Jim gave me two weeks to design the AC30 for production

[vintagekiki]

Sometimes, engineers have to work to overly tight deadlines

For Vox, Marshall, Fender ... that can't be said

[pdf64]

For Vox, Marshall, Fender ... that can't be said

I disagree.
The C41 and rectifier wiring errors on that version seem to lend weight to Grindrod’s implication, ie that the time allocated to bring that amp to be ready for production was indeed insufficient.

[vintagekiki]

I disagree.
The C41 and rectifier wiring errors on that version seem to lend weight to Grindrod’s implication, ie that the time allocated to bring that amp to be ready for production was indeed insufficient.

I am afraid that there has been a substitution of theses.
One is technical (development) documentation (schematic)
The second is the production documentation (assembly, realization, layouts)
I am not versed in the matter (C41 and rectifier wiring errors) what it is about. Do you have a link that points to schematics.
In general, errors are interpreted as a human factor.

[The Jonald]

I thought since I hadn't done it yet and it takes 2 minutes. I installed a humdinger tied to the cathode of one of the output tubes and was able to dial away the hum.

Now, I hear only a very faint oscillation coming thru the speaker. This is with all preamp tubes pulled except the PI. The amp is otherwise extremely quiet. Does anyone have any ideas what this could be? The only tubes I haven't swapped at this point are the output tubes, they are of indeterminate age and the bias in this amp stock is over max spec, but I have never encountered oscillation from a power tube.

One interesting thing is that now, diming the normal channel is noisy, but diming the brilliant channel is very quiet.

[The Jonald]

In this thread about the same or a similar issue with an AC30, Helmholtz wrote:

A humdinger allows to vary amount and PHASE of heater hum.
Seems the amp has more than one hum source. Also Brilliant and Vibrato channels connect to opposite phase PI inputs. So you need to compromise.

Would this be the case if the amp were grounded well? I know I said I was careful and didn't think I missed anything with the grounding, but it's certainly possible that I did! With the faint oscillation, and the behavior of the normal and brilliant channels, I want to make sure I am not covering up for a defect with the humdinger.

[pdf64]

A recording of the noise and oscillation might be helpful.
Certainly the output valves take a beating, and keeping a good spare set on hand is pretty much essential for an AC30 user.

[The Jonald]

A recording of the noise and oscillation might be helpful.
Certainly the output valves take a beating, and keeping a good spare set on hand is pretty much essential for an AC30 user.

The oscillation is faint but I will try to record it. It sounds like "motorboating" I think is the right term. I don't have a 16 ohm dummy load but I ordered some power resistors to make one and they should be here in a day or two, so I can also employ the scope more effectively then too.

[Helmholtz]

The oscillation is faint but I will try to record it. It sounds like "motorboating" I think is the right term. I don't have a 16 ohm dummy load but I ordered some power resistors to make one and they should be here in a day or two, so I can also employ the scope more effectively then too.

You can take scope pictures with speakers connected.

Is the "motorboating" related to the vibrato frequency?

[The Jonald]

This amp has certainly been a chore. Since my last post, I've had bad tubes, including the 12AU7 which went white; the speaker jack gave up on life; I decided to recap the amp based on age and time invested and found corrosion under C46, the screen reservoir and possibly C45, the main reservoir.

Is the "motorboating" related to the vibrato frequency?

No, and I discovered what it is. The phase inverter is picking up ambient noise. If I lightly shield it with my hand, it goes away. I think I'm just going to put a shield on it, but if anyone has suggestions for something to look at electrically, I'm all ears.

The amp is very close to a good working state now. I'm going to put that shield on and run it through its paces with a guitar. I appreciate everyone who took the time to reply and help me out, I appreciate this community.

[The Jonald]

While testing everything, my speaker cable decided to short out too. I think someone put a curse on this amp. Never had one frustrate me so much.

The noise and hum levels are all very good now. There is one, hopefully final, oddity. On the brilliant channel, diming the treble control seems to cut volume or at least a lot of bass and midrange. I'm not familiar enough with this amp to know if that's normal behavior, can anyone chime in?

edit: Also, plugging into the normal channel and leaving normal volume at 0, with brilliant volume turned up high, I can hear the guitar through the brilliant channel. I believe this is normal for the way these inputs are wired, but my brain is tired.