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.
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.
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,
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,
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.
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.
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?
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.
I would monitor the supply on a meter or scope and see if there is excessive hum or voltage drop out.
The last time I saw symptoms like that it was caused by a rusty fuse / fuseholder. Dirty switch contacts would also qualify.
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 !
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.
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.
Does the sound fade out slowly then come back slowly? A bad connection on a heater will do just that. I agree it's not a capacitor issue, you would more and different problems.
Craig
Craig
Indeed, if there is no hum the caps should be ok. For my previous post I didn't have the chance to look into the schematic, but as you confirm it is cathode biased, I am quite convinced it is indeed the blocking distortion of overloading the output stage. As Morgan Jones describes it "BLocking is an extremealy unpleasant mechanism whereby an amplifier mutes for a short time after a momentary overload" (p190, 3rd edition of Valve Amplifiers). MJ writes mostly about hifi amplifiers, and I can imagine guitar amplifiers are "overdriven" way ofter.
I am posting a link with the description by MJ. If it doesnt work, one can search for page 190 of the 3rd edition of valve amplifiers on books.google.com
Valve Amplifiers - Morgan Jones - Google Boeken
Cheers, Erik
I am posting a link with the description by MJ. If it doesnt work, one can search for page 190 of the 3rd edition of valve amplifiers on books.google.com
Valve Amplifiers - Morgan Jones - Google Boeken
Cheers, Erik
This looks like blocking. When you play real loud, the finals clip and pull more plate current since it's now essentially amplifying a square wave. This leads to more voltage dropped across the cathode bias resistors. More voltage drop, more negative grid bias, less amplification. It could take 30 seconds for that excessive bias voltage to leak off the cathode bypass capacitor(s).
This is very unlikely to be blocking. This is a guitar amp, remember, which use much smaller caps than hi-fi amps, making it virtually impossible for blocking to go one for 30 whole seconds. Also, the amp worked normally for a long time, so I see no reason why gross blocking would suddenly start happening. This sounds like a dry joint or dirty/corroded contact.
I agree with Merlinb, I have worked on hundreds and rebuilt many many Vox AC15s and 30s and have NEVER run across blocking no matter how **** poor the condition they were in. Combo amps have weird and unusual problems that heads don't.
Craig
Craig
I've built and repaired a modest quantity of amps but I've never stumbled upon such strange problem, that's why I'm asking. I'm almost sure this is not bad filter caps and not blocking. Bad joints also have a certain character, and i'm not sure this is the case.
This is not happening all the time, but once in a while (say, once every two weeks) I would play something and suddenly I don't hear myself over the drums. The tone remains the same, just less loud. And then it "recovers". I don't have a better description. The amplifier is usually almost clean, volume about half way up. If I use overdrive it's usually a pedal. It happens with a clean tone too. I do not touch the amp. If I will have a chance to record this I will try to see if there's something special about this, like crackles or timbral changes.
The output stage is four EL84s, cathode biased.
This is not happening all the time, but once in a while (say, once every two weeks) I would play something and suddenly I don't hear myself over the drums. The tone remains the same, just less loud. And then it "recovers". I don't have a better description. The amplifier is usually almost clean, volume about half way up. If I use overdrive it's usually a pedal. It happens with a clean tone too. I do not touch the amp. If I will have a chance to record this I will try to see if there's something special about this, like crackles or timbral changes.
The output stage is four EL84s, cathode biased.
Been racking my brain to remember, but I did encounter similar in an AC30. I remember the cause was odd, and peculiar to the AC30 - however looking at the schematic now I just can't recall, other than it was 'memorable' (!), but now the AC30 doesn't look that remarkable in hindsight.....
What I recall is symptoms were quiescent bias was hot and crept hotter near to the point of red plate, a hum would develop over a period of perhaps 30s after ceasing to play (presumably bias creeping hotter), and symptoms similar to described in the OP. And it would eat rectifiers over a period of weeks.
Might be unrelated, sorry just can't recall further or work it out from schematic now. Really it has to be cathode circuit or coupling cap leakage but something from memory says it was odd......
What I recall is symptoms were quiescent bias was hot and crept hotter near to the point of red plate, a hum would develop over a period of perhaps 30s after ceasing to play (presumably bias creeping hotter), and symptoms similar to described in the OP. And it would eat rectifiers over a period of weeks.
Might be unrelated, sorry just can't recall further or work it out from schematic now. Really it has to be cathode circuit or coupling cap leakage but something from memory says it was odd......
I've had leakage in an EL84 produce similar symptoms. Sometimes I could pop the amp out of the fault condition by playing a loud note again. Sometimes not. Replacing the output valves fixed it.
I've found one recording where it happened and studied the guitar track.
It went like this:
First, at some point I hear some very minor (almost inaudible) cracking noise, like a bad joint / touching wire. Together with this the volume went down, about 30% less loud. About half a minute the volume stayed at this level, then dropped even low, to about 40% of the original volume. Then it started coming back up, which went in uneven steps, like a bad contact being touched, but without any noise. After approximately a minute the volume level restored.
I've noticed no relation between the parts played and the amp behavior. The tone hasn't changed. To my ear sounds like output stage.
It went like this:
First, at some point I hear some very minor (almost inaudible) cracking noise, like a bad joint / touching wire. Together with this the volume went down, about 30% less loud. About half a minute the volume stayed at this level, then dropped even low, to about 40% of the original volume. Then it started coming back up, which went in uneven steps, like a bad contact being touched, but without any noise. After approximately a minute the volume level restored.
I've noticed no relation between the parts played and the amp behavior. The tone hasn't changed. To my ear sounds like output stage.
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