Recent JMI / VOX AC30 6 TB with idle noise

[GibbyLover]

I bought a 2nd hand JMI AC30 TB. It is a handwired Vox AC30 made with a turretboard made in UK. When testing it I thought it had quite a loud noise level, independent from either volume knobs.
At home I can compare to my own amps and the noise is quite loud, much louder than a 1970 Fender twin, without master volume.
So I tried some things:
- without preamp tubes, the noise is away
- put back only V1 and V2 in and the noise was there
- tried other 12AX7 and it was the same
- the noise changes with the tone knob and with the treble from the TB changes it too (even with the TB tube pulled)
- it sounds like a brrrrrr, not hmmmmm or his
- there is a very light repetitive ticking

While it was open I gently tapped the components to hear if it did something with the sound. I found that tapping the vertical turret board, the below connections of the 3 most right components (next to the on / ofswitch) I heard the tick through the speakers.
Even more strange is that the noice changed drastically when I put the pencil between the most right turretboard (seen from behind) and the cables from the on/off switch. I didn't touch anything, just put it between that board and the cables. Is that normal for this amp?



Anyone have an idea?

[Mick Bailey]

Tapping close to certain components (or components themselves) can sometimes give misleading results. If that component is in either a sensitive or high-gain part of the amp it can act like a microphone. Capacitors are especially microphonic due to their construction, but also resistors - say, the plate resistor in one of the early preamp stages.

Some amps can be marginally stable and a hand or object bought near to parts of the circuit can cause either an existing noise to alter, or create one that wasn't there before. It's sometimes called proximity or hand capacitance. For example, the Fender HR Deluxe III will buzz loudly when the screened back panel is removed and a hand bought close to the control side of the board.

The ticking is probably the vibrato oscillator bleeding through into the preamp. Does your amp have a footswitch, and does this kill the noise?

My starting point with buzz/hum is to isolate the power amp section. You say that removing the preamp tubes kills the noise, but did you run the amp with just the power tubes and phase inverter to make sure the power amp section is OK? Also, does this particular amp have SS or tube rectification?

[GibbyLover]

Hi Mick, thanks for taking your time.

For the ticking, I will have to try to turn off the vibrato with the pedal. Since nothing was plugged in, I didn't notice the trem on or off.

The altering of the noise was with a pencil, but maybe that affects the same as a hand?

Now for the troubleshooting. When I said I removed all preamp tubes, I left the 4x EL84 and GZ34 in. I guess that means the power tubes are OK, or should I try to find replacements?

Here the overview:

[pdf64]

The phase splitter/inverter is the socket closest to the rectifier; try fitting its 12AX7, along with the EL84 and GZ34.

I think that a pencil is a bad thing to use around an amp, as the lead is conductive, leading to potential for a short or electrocution. A chopstick or felt tip pen (with lid in place) are preferred things to use as firkling probes.

[GibbyLover]

I'll try that tomorrow. With V1 + V2 (PI), the noise was there, but probably also part of the preamp section.
The pencil had a broken point, but thanks for the warning, I will think about that! I didn't want to use anything plastic, because it coult melt.

[Mick Bailey]

My favourite probing tool is a melamine chopstick. Thermosetting plastic, so no problems with heat. I have a couple on my bench - one sharpened to a fine point for precision chopsticking.

[GibbyLover]

UPDATE
I tried it with the EL84s, GZ34 and PI. The noise was there. I tried another 12AX7 for the PI, same noise.
Now I should try other EL84 I guess, but also the GZ34?
The light ticking isn't affected by the tremelo pedal, nor the setting vib/trem, nor the speed.

Mick, I'll have to look up these chopsticks, I'm not English speaking by nature )

[Mick Bailey]

OK. Now remove the 12AX7 PI to see if there's any difference - so you have the 4xEL84 + GZ34. If the amp still buzzes then you probably have a bad tube. AC30s can be pretty fussy over the output tubes and if your amp is faithful to the original schematic it will use a shared cathode resistor between all 4 output tubes, which worsens matters if you get one tube hogging the current. Equally, a bad rectifier can put noise onto the supply, but is less common than output tube problems. Anyhow, post back to let us know if removing the PI changes things.

Double checking, but did you also remove the top boost 12AX7 when doing the previous test? I wouldn't be too bothered by the ticking at this stage. Best to get one fault out of the way first and then move on to the trem. As they say, "The man who chases two rabbits catches neither".

[GibbyLover]

OK. Now remove the 12AX7 PI to see if there's any difference - so you have the 4xEL84 + GZ34.
Double checking, but did you also remove the top boost 12AX7 when doing the previous test?".

With the PI pulled, there is no noise.
Indeed, I pulled all preamp tubes, also the top boost at the back.
The ticking is barely audible, but I mention it because it might bring someone to a link between both. It doesn't bother me at all.

I think I'll try a fresh set of EL84's, it's never wasted. I'll wait with the GZ34.

[Mick Bailey]

The output tubes are usually not the problem if the noise disappears with the PI removed. You could have;

Noise on the supply node feeding the PI
Heater noise caused by ground reference fault (such as a failed hum-balance resistor if the heater supply uses them)
Noise coupling through 'lead dress' (the arrangement and routing of interconnecting wires).
A grounding fault
A PI that's sensitive to tube brands or type. 12AX7s can be had with long plates/short plates/spiral heaters/straight heater/low noise or whatever. Some work better in certain positions than others. Sometimes I have to audition a dozen to find one that works well with some amps.

Any further investigation should be carried out with just the rectifier, PI and output tubes.

You could have a bad rectifier - I've had amps that buzz where a rectifier swap has fixed it. With tube amps you should always eliminate the possibility of a bad tube before moving on. I would usually use a scope to see the noise, but a multimeter set to AC can be used to measure ripple on the plates of the PI. Depends on how confident you are working on a powered amp and how safety-aware you are.

Sometimes 'chopsticking' the leads can make a difference to noise. Gently move the interconnecting leads with a chopstick (or similar) to see if there's a better (quieter) arrangement of lead dress.

Do you know if the heater supply has a centre tap connected to ground with your particular amp?

[GibbyLover]

Mick, I'll try your suggestions later. I already tried most things I can with the tubes, see below.

For now I tried new EL84 (+GZ34 + PI) , and like you thought, the noise remains.
I tried another GZ34, also without succes. I noticed the ticking was still there, but a new light repetitive noise came through.
Somewhere I read you can change the noise adding the volume of the Vib/trem channel and behold, that works. With the Vib/trem Volume around half, noise is +-1/2. Higher volume brings in a louder brrrrr

I already tried 2 different brands as PI, I'll try your suggestion and find some others.

[John G]

Hi,
I had similar problems with the same mode (JMI) serial number 4. This was some years ago now, but long story short I eventually redid every solder joint on the tag boards and pots even if they seemed ok.
This did the trick so I concluded that the thing was full of dry solder joints.
BTW, it was one of the best sounding AC30s that I have ever heard.
John

[Mick Bailey]

Good point. Even just one poor joint can give rise to problems. Those particular tag boards have a tendency for the solder to flow preferentially on the tag rather than the component lead and real care has to be taken to make sure the joint is fully 'wetted' during build time.

[GibbyLover]

John, I bought this one, because I was afraid of the work a vintage AC30 might need, then I might have to resolder it whole...
Did you add solder on each joint, or just heat them?
I'll have to try how I can reach all the solders, I guess the TB can be removed. It better sound good afterwards
JMI is not responding to the mails I sent them. It's a shame for such an expensive amp / brand. I had better luck with my Chinese Voxes.

[Mick Bailey]

Given that the procedures so far have reduced the problem to the area of the PI, maybe just take a closer look at that section.