Laney cub12r crapping out

midlifecrisis · December 2014

experiencing intermittent loss of volume and distortion, lasts a few minutes then comes back on. Have tried complete set of known working valves, no difference.

I believe its the power stage of the amp as when I take a line out(send) and plug that into a bandit its as clear as a bell. sounds like a problem I had with another amp that turned out to be dirty in and out jacks but I have sprayed and cleaned these. any guidance please

Beexter · December 2014

I'm sure the Cub's were susceptible to problems with some kind of thermal switch. I don't know the technical in's and out's but if you google "Laney Cub thermal switch" it should throw up some pointers.

Sorry if it's a half-arsed answer but hope it helps.

midlifecrisis · December 2014

I read about that but I think it made the amp cut out entirely, and mines the later one which came with the HH speaker.

I wonder if its dirty pots as sometimes when I mess around turning the knobs it comes back again but sometimes not so it may just be coincidense

Voxman · December 2014

Yup, almost certainly the thermal cut out. Happened in mine and Laney fixed it...they are very aware of this problem on earlier cubs. How old is the amp? Regardless of warranty take it back to the store as they have first line responsibility.

midlifecrisis · December 2014

Voxman said:

Yup, almost certainly the thermal cut out. Happened in mine and Laney fixed it...they are very aware of this problem on earlier cubs. How old is the amp? Regardless of warranty take it back to the store as they have first line responsibility.

are you sure its that, as my amp doesn't shut down completely or power light go off, it just goes to a lower volume and distorts a lot?

ICBM · December 2014

That's the failure symptom before they cut out completely. It's a thermistor rather than a thermal switch, so it doesn't switch off, it progressively rises in resistance.

It's bad design. If it's properly specc'ed, an amp should not need a thermal cut-out in the filament circuit.

midlifecrisis · December 2014

its been on a couple of hours now and been ok, but I guess I better get a tech to look at it, I bought it privately used so no dealer to take it back to. what should I tell the tech needs to be done?

ICBM · December 2014

Sorry, I misread your first post. Although the thermistor is a known problem with these amps it won't be the cause here or the preamp output wouldn't still work.

It will be something in the power stage, but if it's not valves you still need to take it to a tech.

midlifecrisis · December 2014

that's what I suspect. the preamp output is fine. but its all been ok for a couple of hours, light leave it on until bedtime (about 1 am) here periodically checking it. Hate the thought of taking it to a tech and them not finding anything wrong as its behaves itself waiting for mw to get it home.

is there a chance it might have been a scratchy pot that's worked itself clean or am I completely barking up the wrong tree?

ICBM · December 2014

I don't think it will be a pot because they're all before the FX send.

midlifecrisis · December 2014

oh yes, except the tone control, I think that's after

ICBM · December 2014

midlifecrisis said:

oh yes, except the tone control, I think that's after

Yes, I had forgotten that - but it's a Vox 'Cut'-type cross-line control, so if it fails the amp will simply go to full brightness. Can't be that...

Grumpyrocker · December 2014

As others have said, probably the thermal thingy. Laney fixed mine under warranty.

midlifecrisis · December 2014

Grumpyrocker said:

As others have said, probably the thermal thingy. Laney fixed mine under warranty.

did your amp cut out completely and go off? mine just loses volume and distorts but carries on.

ICBM · December 2014

Is it always exactly the same amount of volume loss, or does it vary?

If it's always the same, it might be a bad connection causing one power valve to stop working - that will give a quarter of the output power and an oddly distorted sound. It could be a faulty solder joint, a screen resistor, or similar - anything that affects just one power valve. Not the OT probably, since an interrupted plate supply with the screen still running will blow the valve.

Could also be a bias supply problem (maybe with the trimmer) causing the bias to go high (cold) which would make the amp quiet and distorted. The bias supply in these is well designed so a trimmer failure would do that instead of leaving no bias (and blown valves) - although I'd be surprised if it went high enough.

If the volume loss is variable, it's more likely to be a bad connection in the signal path, although that wouldn't explain the distortion really...

midlifecrisis · December 2014

ICBM said:

Is it always exactly the same amount of volume loss, or does it vary?

If it's always the same, it might be a bad connection causing one power valve to stop working - that will give a quarter of the output power and an oddly distorted sound. It could be a faulty solder joint, a screen resistor, or similar - anything that affects just one power valve. Not the OT probably, since an interrupted plate supply with the screen still running will blow the valve.

Could also be a bias supply problem (maybe with the trimmer) causing the bias to go high (cold) which would make the amp quiet and distorted. The bias supply in these is well designed so a trimmer failure would do that instead of leaving no bias (and blown valves) - although I'd be surprised if it went high enough.

If the volume loss is variable, it's more likely to be a bad connection in the signal path, although that wouldn't explain the distortion really...

hard to tell but I think the volume fluctuates a bit, almost like a tremolo but with lots of background noise.it seemed to be if I momentarily pushed the volume to max the sound came back full and clean but It may have been a coincidence as its not doing it now. again I checked the lineout which sounded fine through an active pa speaker so id guess the fault its in the power stage.

impmann · December 2014

Does it have an fx loop? I had a different Laney that did similar and it was a bad socket in the fx loop - if you jumpered it (ie stick a patch cable into the send and back into the return) the symptom went away.

ICBM · December 2014

midlifecrisis said:
if I momentarily pushed the volume to max the sound came back full and clean but It may have been a coincidence as its not doing it now. again I checked the lineout which sounded fine through an active pa speaker so id guess the fault its in the power stage.

That does sound like some sort of bad connection in a current-carrying (rather than just signal) part of the power amp. Quite often if you give it a blast, this sort of thing will almost spot-weld itself where the break is, and fix itself… for a while.

If it starts causing trouble again, don't do that! Get it to a tech while it's still faulty, or it will be much harder to find.

midlifecrisis · December 2014

impmann said:

Does it have an fx loop? I had a different Laney that did similar and it was a bad socket in the fx loop - if you jumpered it (ie stick a patch cable into the send and back into the return) the symptom went away.

yeah, tried that, patching send and return, no difference.

midlifecrisis · December 2014

ICBM said:

midlifecrisis said:
if I momentarily pushed the volume to max the sound came back full and clean but It may have been a coincidence as its not doing it now. again I checked the lineout which sounded fine through an active pa speaker so id guess the fault its in the power stage.

That does sound like some sort of bad connection in a current-carrying (rather than just signal) part of the power amp. Quite often if you give it a blast, this sort of thing will almost spot-weld itself where the break is, and fix itself… for a while.

If it starts causing trouble again, don't do that! Get it to a tech while it's still faulty, or it will be much harder to find.

yeah, I think ive done that a couple of times but obviously I better get it sorted out properly. might have to wait to after Christmas now.

Grumpyrocker · December 2014

midlifecrisis said:

Grumpyrocker said:

As others have said, probably the thermal thingy. Laney fixed mine under warranty.

did your amp cut out completely and go off? mine just loses volume and distorts but carries on.

Mine cut out completely.

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Laney cub12r crapping out

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