Vox AC30 TBX (90's UK) issue.

[Sea Chief]

Hi chaps, [been trying to get MEF for months but n/a.. until today].

Got an AC30/6 TBX.. Marshall-made, Korg one. 1995. Searching for a year+ for a good price.. finally found her a month ago! Nice.

Seems pretty well looked after/ 1 owner, JJ tubes. All working fine too.. until 2 weeks ago. Symptom:

Using either Vib-Trem channel or Bright channel, & to reduce hum to zero, I've done the known trick of turning UP the (unused) Normal vol pot to 1/2. But 2 weeks ago after 20mins playing suddenly a pronounced HUM happens. Turning the Normal vol pot down to 0 & it almost dissapears. But still there.

So it seems something in this Normal circuit, is 'failing'. And consistantly now after ~15 mins or so, gtr plugged into into either of the other channels.

It seems I'm -plagued- by HUM or BUZZ issues. You may recall my Twin Reverb went thru countless tests, & relegated on my bench no cause determined.

I'm praying to goodness.. someone can help. I had a MusicMan 4-10 too (well behaved!) but sold it to help pay for my AC30.

Thanks for reading & for any ideas, Capt

[Jon Snell]

The input sockets have a shorting contact to ground. If it has some tarnish on it, turning the volume control up with nothing plugged in will give you hum.
It is a few years old, check the input sockets.

[Sea Chief]

The input sockets have a shorting contact to ground. If it has some tarnish on it, turning the volume control up with nothing plugged in will give you hum.
It is a few years old, check the input sockets.

Hi Jon.

But the idea is with these, you turn the -unused- volume knob up to half, in order to reduce (a minor) hum. A known trick.

And for 20mins it does exactly what is expected, it's lovely & quiet using/ plugging into either of the other two channels.

It's this 20 minute delay aspect until the 'failure' thing/ HUM starts, exactly consistently with each session, being the bizarre thing.


Thanks, SC

[Sea Chief]

Jon Snell cleaned these two inputs- no hum! Well well you might've solved it Jon.

Not counting my chickens tho, with my amp luck. So will jump back on here IF I get hum numnums again.

Fingers crossed. Appreciated, SC

[Jon Snell]

Hi Jon.

But the idea is with these, you turn the -unused- volume knob up to half, in order to reduce (a minor) hum. A known trick.

And for 20mins it does exactly what is expected, it's lovely & quiet using/ plugging into either of the other two channels.

It's this 20 minute delay aspect until the 'failure' thing/ HUM starts, exactly consistently with each session, being the bizarre thing.


Thanks, SC

The half volume to remove the hum is caused by an earth loop within the amplifier.
Too complicated for me to explain, (dyslexia doesn't help).
I have fixed a number of the Arbiter and Italian built amplifiers with this problem. The earlier UK built amplifers did not suffer from this issue and put it down to wiring errors.

[Sea Chief]

The half volume to remove the hum is caused by an earth loop within the amplifier.
Too complicated for me to explain, (dyslexia doesn't help).
I have fixed a number of the Arbiter and Italian built amplifiers with this problem. The earlier UK built amplifers did not suffer from this issue and put it down to wiring errors.

Hi Jon,

does turning up this unused Normal volume pot to half cause any 'wear' on the tubes? Or is that a stupid question I wonder, if nothing plugged into it.

If nothing's affected by using this ground loop 'Normal Vol pot trick' to rid it, I'm fine as is/ I don't consider it a problem really.

I think I have though seen how to 'fix' it by shifting the odd wire. But I'm not inclined to tinker with her, quite yet. This one btw -is- an earlier '95 one.


Thanks, SC

[Jon Snell]

Hi Jon,

does turning up this unused Normal volume pot to half cause any 'wear' on the tubes? Or is that a stupid question I wonder, if nothing plugged into it.

If nothing's affected by using this ground loop 'Normal Vol pot trick' to rid it, I'm fine as is/ I don't consider it a problem really.

I think I have though seen how to 'fix' it by shifting the odd wire. But I'm not inclined to tinker with her, quite yet. This one btw -is- an earlier '95 one.


Thanks, SC

It won't cause any damage to the valves.

[Sea Chief]

It won't cause any damage to the valves.

Hi Jon,

no but I guess you could say if I turned up whichever channel I'm plugged into, to half, that this also wouldn't cause -damage- to the valves per se.

What I meant was though, say if I were to always play at vol 6 (regardless of channel) IE just over half vol, just into the sweet spot -should I be lucky enough to own a mansion, not worrying about police visits- would this situation put strain on the tubes?

And if so, would my using the unused Normal vol at vol 6, be causing an equal ammount of strain on the valves.. even if I'm not plugged into this channel & am plugged into either other channel at vol 1?


Thanks, SC

[Jon Snell]

Setting the unused channel the half will cause no damage.
Setting your overall volume to half may make it sound too loud but will not affect the valves life, unless you have a really large input signal and drive the amplifier into distortion.

[Sea Chief]

Hi Jon, didn't see this last post of yours here 11 days ago- cheers for that info.

So basically ( forgetting the unused vol knob trick for this question).. any amp's tubes are only into 'wear territory' once they -distort- .. is that right? The distortion being both synonymous of wear, & how the wear itself manifests as.

Am I getting into a whole new avenue of inquiries I wonder, IE what about 'preamp distortion' which must be just the preamp tubes distorting (somehow without the power tubes doing so). Are these getting 'worn' in a similar fashion I wonder.

This is maybe as Neil once said: "Oh no- out of one frying pan... & into another frying pan".

Thanks, SC

[pdf64]

My understanding is that the most significant factors affecting valve wear / aging are (average) cathode current/ anode dissipation being at a high level, especially if above the limits.
Overdriving a preamp stage has little to no effect on its cathode current or anode dissipation. I’m not aware of any credible hypothesis or evidence that overdriving a preamp stage causes increased wear to its valve, ie compared to it idling or being operated within its linear range.
Even with class A output stages, overdriving them probably doesn’t cause much increase in either of the above metrics. Though that’s complicated by class A output stages often being operated close to or above the valve’s anode dissipation limit, so any increase on that may accelerate wear.

However, with class AB output stages such as an AC30, cathode current and anode dissipation will tend to increase as signal level increases, typically significantly so when overdriven. But an AC30 idles its EL84 very hot anyway, usually above the anode dissipation limit, so anytime the darn thing is on you may as well use it as you wish and not be held back by worrying about valve wear. Because whether it’s just idling or being overdriven, the EL84 have little chance of lasting a long time.