Vox AC15 Repair

pottolom

My Vox AC15 recently started producing loud sudden staccato buzzing intermittently during use, including when I’m playing and not touching the amp.

Despite replacing power and preamp tubes, the issue hasn't gone away.

I now wonder if it might be the standby switch that's faulty:

• There's a recent similar noisy buzz when turning standby on, different from the usual pop. 
• Holding the switch midway during power transitions reproduces the annoying buzz. 
• Applying slight pressure to the switch's plastic housing generates similar noises.

A couple of questions for you amp repair experts:

1. Could a failing standby switch be responsible for the buzzing issue described above in the first sentence?
2. Where can I find a suitable replacement switch? A photo is provided below. The sticker says Chily 6021 16(6)A 250V T125 and 16A 125VAC 10A 277VAC - but I can't seem to find that exact switch available online in the UK.

ICBM

It could be the switch, but does this model have a valve rectifier? The standby switch is poorly implemented on ones that do, and tends to make the valve arc (then fail).

pottolom

ICBM said:

It could be the switch, but does this model have a valve rectifier? The standby switch is poorly implemented on ones that do, and tends to make the valve arc (then fail).

Solid state I believe. Valve PI, but I’ve tried swapping out that valve.

ICBM

If there’s no rectifier valve, then from the symptom it’s most likely to be the switch itself.

pottolom

ICBM said:

If there’s no rectifier valve, then from the symptom it’s most likely to be the switch itself.

Many thanks! I’ve spotted this US website lists this switch as a replacement: Vox® AC15C1 Power & Standby Switch Upgrade – British Audio

 

I’ve found it here, available in the UK: 641H/2K | APEM Toggle Switch, Panel Mount, On-Off, DPDT, Solder Lug Terminal, 250V ac | RS (rs-online.com)

 

Would that switch work as a replacement? Seems quite expensive, mind you!

ICBM

Looking at the pic, it looks like only half the switch is used. If so, simply swap the wires to the other two terminals, if there’s space and enough wire length to do it. It might be quite a while before the other side fails as well… that will also conclusively identify if it is the switch, before you have to fork out for a new one!

It does look a bit expensive, but you need the correct spec, and everything is now…

pottolom

Thanks! Will have a look tonight and see if that works….

pottolom

ICBM said:

Looking at the pic, it looks like only half the switch is used. If so, simply swap the wires to the other two terminals, if there’s space and enough wire length to do it. It might be quite a while before the other side fails as well… that will also conclusively identify if it is the switch, before you have to fork out for a new one!

It does look a bit expensive, but you need the correct spec, and everything is now…

Wow! Seems to have worked. All symptoms gone, back to normal. Will play it for a few days to be sure, but so far so good.

Is the other side of the switch failing an inevitability? If so, I’ll go ahead and replace now rather than wait for it to happen at a more inopportune moment. But if not, I might just leave it as is and play on.

ICBM

pottolom said:

Is the other side of the switch failing an inevitability? If so, I’ll go ahead and replace now rather than wait for it to happen at a more inopportune moment. But if not, I might just leave it as is and play on.

No, not inevitable.

Ideally, I would connect both sides of the switch in parallel and fit a suppression cap across it - even with one side damaged, that would almost guarantee it doesn’t fail.

pottolom

ICBM said:

pottolom said:

Is the other side of the switch failing an inevitability? If so, I’ll go ahead and replace now rather than wait for it to happen at a more inopportune moment. But if not, I might just leave it as is and play on.

No, not inevitable.

Ideally, I would connect both sides of the switch in parallel and fit a suppression cap across it - even with one side damaged, that would almost guarantee it doesn’t fail.

I’m guessing that this early failure of the switch has been caused by arcing when turning standby on and off (evidenced by the usual ‘pop’ sound before the switch began to fail and a more obvious arcing sound was heard).

I wonder if it’s better just not to use standby at all, and just leave that switch ‘always on’, using the power switch alone to turn the amp on or off? There seem to be quite a few people online advocating that approach. But then, there are also people who say that it *is* important to use standby on an amp with a solid state rectifier, which would include my AC15C1…..

Yorkie

pottolom said:
But then, there are also people who say that it *is* important to use standby on an amp with a solid state rectifier, which would include my AC15C1…..

Hi, I'm glad to read you were able to repair your AC15 (special thanks to @ICBM). Why is it particularly important to use standby on an amp with a SS rectifier? I do use the standby switch on my valve amps, but I've never been 100% clear on the benefits vs turning them on and let them warm up while I tune up. 

ICBM

The standby switch helps reduce power valve wear for two reasons - it reduces the overall time plate current is flowing, when you don’t want to turn the amp off fully (especially important when they’re biased very hot), and it prevents ‘cold stripping’ of the cathodes, which happens with a solid-state rectifier when the power valves are subjected to the full HT before they’ve warmed up. A valve rectifier warms up at about the same rate as the power valves so it stops this. Neither are critical really - overall running time compared to time in active use aren’t that different normally, and cold stripping isn’t usually serious at guitar amp voltages, so it may be best just to not use the standby.