Volume drops and then comes back - anyone recognizes the sympthom? - diyAudio
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Old 19th February 2015, 09:35 PM   #1
engels is offline engels  Israel
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Arrow Volume drops and then comes back - anyone recognizes the sympthom?

I've got an old beat-up guitar combo, Laney VC30 which is sort of AC-30 variation. Sometimes when I play loud, I experience volume drops - and after a while (half a minute maybe) the amp "recovers" and gets back to the normal volume.

It's a bit hard to trace the illness since 99% of the time it works fine. I'm not quite sure the loud playing is what causes the volume to drop, but it might be.

Does anyone recognizes the sympthom?
The combo is old (20+), the tubes are old, but it's a workhorse and never broke.
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Old 19th February 2015, 09:46 PM   #2
llwhtt is offline llwhtt  United States
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Make sure all of the heaters are still lit when it craps out.

Craig
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Old 19th February 2015, 10:32 PM   #3
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Hello,

you should check the solder joints and also the tube sockets if the contacts are still clean ( which is hard to believe after 20 years ) and not oxydized.

Also check the heater voltage. Worst case you have to replace the valve itself if the heater filament is faulty ( which can happen ).

Best regards,
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Old 20th February 2015, 09:07 AM   #4
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Yes, I recognise something like it - play loud and the amp goes quiet, and comes back up again shortly after. Though I have only heard it go quiet for a few seconds, it may be the same thing. The problem in this case (my case, that is) was caused by part-time instability. The stages are mostly stable but can be provoked into oscillation.

If the amp is really 20+ years old, and never been re-capped, the cause may be the same too: Degraded electrolytic supply capacitors.

With guitar amps 10 years use can be enough to make renewing them worthwhile, since they often sound so much better.

But there seems to be a weakness in the caps used for preamp stages - 22uF/450V and the like - so that these fail badly enough to lead to oscillation.

If you just want to find the problem, you can usually just add a new cap across the old preamp caps, and check. But renewing them will be worth the trouble.
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Old 20th February 2015, 04:34 PM   #5
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just an idea, but could it be that playing hard drives the autobiased tubes into heavy cut off / blocking? and in a couple seconds the cathode resistors discharge the bypass capacitors?
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Old 20th February 2015, 10:35 PM   #6
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Could be the main power supply electrolytics getting old and losing capacitance.
I would monitor the supply on a meter or scope and see if there is excessive hum or voltage drop out.
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Old 20th February 2015, 11:30 PM   #7
Merlinb is offline Merlinb  United Kingdom
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The last time I saw symptoms like that it was caused by a rusty fuse / fuseholder. Dirty switch contacts would also qualify.
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Old 20th February 2015, 11:33 PM   #8
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Gotta agree with Rod. Sounds like a very low frequency oscillation.
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Old 21st February 2015, 03:40 PM   #9
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Somewhere in the archive of my memory is that AC30s can develop a fault along those lines, where bias changes after playing loud. Can't recall detail, but since OP stage is cathode biased with a common bypassed resistor for all 4 valves suggest looking at the cathode circuit and grid coupling caps - also worth checking on AC30 stock faults. If I recall, the fault tends to result in shortlived rectifiers too, presumably overcurrent. HTH !
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Old 21st February 2015, 06:02 PM   #10
engels is offline engels  Israel
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Default racap

Most of the ideas here are around recapping, and that sounds reasonable. Nothing against recapping, it's healthy and good, but from my experience the first sympthom of an amp need recapping is some sort of hum. In my case the amp is quiet, which means PS caps are still in good shape.

The output stage is cathode biased, but a faulty bypass cathode cap should result in bass frequency rolloff, and again, I do not experience that.

The rectifier in VC30 is solid state, so I'm not sure that's the case too.

I know new caps will improve everything, I'm just not sure the problem of volume drops will disappear, and that's a major problem which renders the amp unrelaiable for gigging, which is critical.
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