Laney cub 12r ongoing problem

midlifecrisis

This amp has been back to two techs but as it was an intermittent occasional fault neither could find or replicate the fault.
After playing for a length of time, the amp would sometimes lose volume and distort. Please dont tell me its the known thyristor problem as its not. the amp still produces sound but distorted. sometimes wiggling the amp fixes it , maybe a dry joint somewhere, sometimes it doesnt. Today i tried it again and when it started distorting i switched off, removed the first power valve and switched on again. it actually sounded ok.  I switched off, replaced the first power valve and removed the 2nd valve, this time it was very distorted. I then tried swapping the valves in each position. the trouble is its still intermittent but it does seem to me the valves are working ok but something about the first position is causing a problem.  I saw @voxman had a problem with one of his valve sockets so im guessing i may have the same fault. my soldering skills are not great (more due to bad eyesight) and it looks as though it involves removing and disconnecting the pcb to get to the sockets so i think its back off to maybe a different tech. just wondering if im on to the right idea with this or is it unlikely

ICBM

It does sound like you’re on the right track, yes - something related to just one of the power valves.

If it isn’t to do with the valve socket it could still possible be a problem with the OT. I had a LC15 for repair a while back which did something similar and turned out to be an intermittent partial short on one side of the OT winding - it ran fine with only the other valve in, and most of the time with the one on the faulty side, but would intermittently drop in volume and sound very distorted.

midlifecrisis

i assume a new output transformer is quite expensive? maybe il have to find another tech,  @ICBM wish you were nearer.

midlifecrisis

Is it bad practise or likely to cause damage running on one power valve? i presume it wont be as powerful but other than that?

ICBM

It’s not a great idea even with a fixed-bias amp - which this is - other than just for fault-finding, so I probably wouldn’t.

midlifecrisis

ICBM said:

It’s not a great idea even with a fixed-bias amp - which this is - other than just for fault-finding, so I probably wouldn’t.

ok, advice heeded.

DJH83004

I think by plugging / unplugging the valves you are disturbing a joint on the PCB which does happen, you could use the reverse end of a slim (insulated!) terminal driver and put pressure on the PCB at different points to see if you can replicate the fault. Other than that its off to your local amp tech I'm afraid. 

midlifecrisis

DJH83004 said:

I think by plugging / unplugging the valves you are disturbing a joint on the PCB which does happen, you could use the reverse end of a slim (insulated!) terminal driver and put pressure on the PCB at different points to see if you can replicate the fault. Other than that its off to your local amp tech I'm afraid. 

yes, not wanting to electrocute myself il seek out a man in  brown overalls. 

Voxman

Had what seemed to be the same problem on my cub 12r...it was a faulty connection on one of the EL84 ceramic tube holders.