Laney Cub 12R - FAULT SOLVED

Voxman

When I switch on, it's fine for a few minutes but then the volume drops and tone starts to get a bit more distorted.  The amp uses 2xEL84's and 3xECC83's.  

Does this suggest a simple valve replacement issue? If so, have no testing gear, but is there a way to easily determine which valve might be the problem?  And does this sound more like an EL84 (power-amp) or ECC83 (pre-amp) issue? 

Thanks folks.

ICBM

It could be either a preamp valve, or the notorious thermistor fault - does it eventually cut out entirely? If so it's likely to be that.

You can check for a faulty preamp valve simply by substituting each one in turn for a known good replacement.

Voxman

ICBM said:

It could be either a preamp valve, or the notorious thermistor fault - does it eventually cut out entirely? If so it's likely to be that.

You can check for a faulty preamp valve simply by substituting each one in turn for a known good replacement.

I was just thinking about the thermistor as the amp had the fault originally & it was sent back to Laney who replaced it.  But I don't think it is the problem because the volume doesn't cut out completely. After playing for 5 minutes the volume drops and goes 'crackly/distorted'. 

I'll try replacing the pre-amps one at a time to see if that has any effect.  

ICBM

Voxman said:

I was just thinking about the thermistor as the amp had the fault originally & it was sent back to Laney who replaced it.  But I don't think it is the problem because the volume doesn't cut out completely. After playing for 5 minutes the volume drops and goes 'crackly/distorted'.

When it does, do the valves look noticeably dim? If so it's still the thermistor.

Voxman

ICBM said:

Voxman said:

I was just thinking about the thermistor as the amp had the fault originally & it was sent back to Laney who replaced it.  But I don't think it is the problem because the volume doesn't cut out completely. After playing for 5 minutes the volume drops and goes 'crackly/distorted'.

When it does, do the valves look noticeably dim? If so it's still the thermistor.

Hi @ICBM ;, no - just checked it in the dark - the valve glow is constant, no change. But the ECC83 valve nearest the EL84's doesn't seem to be glowing much (if at all). Is that normal?  If its a valve I'm just wondering why it would be OK for 3-5 mins - wouldn't it be like that all the time?  

ICBM

Maybe. The easiest way to find out is to replace it and see...

lincolnblue

I had the same thing on my Cub 10. Took it to an amp tech who couldn't find any problems - didn't help that it only happened occasionally. 

midlifecrisis

had the same thing with a cub12r it turned out to be a dodgy solder joint to one of the valve holder terminals.   Must say, think the cubs are lovely sounding amps but over time having owned four of them they have all failed at some time down to bad build quality.

Voxman

midlifecrisis said:

had the same thing with a cub12r it turned out to be a dodgy solder joint to one of the valve holder terminals.   Must say, think the cubs are lovely sounding amps but over time having owned four of them they have all failed at some time down to bad build quality.

That happened to me too @midlifecrisis ;;; after 17 mths, but was fixed by dv247 under their extended warranty. The Cub has been as good as gold for the last 6 years since then, so I cant really complain and to be fair all amps need a bit of tlc now and then. Interestingly, the only one of my amps thats never had any problems is my hybrid modelling/valve 'blue' Valvetronix AD120VTX that I bought new in Jan 2004, nearly 16 years ago, and is the amp I use the most!

Haven't had time to swap the valves in the cub yet though, so I'll start there. If not a dodgy valve I'll need to find a local tech to check it out. Used to have a great guy in Edmonton but I think he may be retired now.

Voxman

I've checked & its not the valves so thermistor is likely.  @ICBM inc VAT what repair costs (North London) should I expect?  Thanks

ICBM

Voxman said:

I've checked & its not the valves so thermistor is likely.  @ICBM inc VAT what repair costs (North London) should I expect?  Thanks

If it is just the thermistor, probably £30-£40 depending on what the tech's minimum charge is. Personally, I would replace it with a fuseholder - with a suitably highly-rated 'last ditch' fuse value that will never blow unless a valve actually completely shorts, very unlikely - or even just bypass it. The thermistors cause endless trouble and I've yet to see any evidence they're really necessary.

midlifecrisis

i thought they did away with the thermistors in the later model, when they stopped supplying rocket speakers and replaced with HH?

Voxman

midlifecrisis said:

i thought they did away with the thermistors in the later model, when they stopped supplying rocket speakers and replaced with HH?

Mine is an original Cub 12R - came with a worse than dreadful Rocket speaker, that I replace with a Vintage 30, and upgraded the tubes to hand selected JJ's. 

Voxman

So took my Cub to Electrohill, Richard checked it over and all fixed. Not the valves, not the thermistor, not the caps or a faulty resistor. Turns out it was oxidation on the valve holders that was degrading the signal, although I don't understand why the tone would be good for a couple of minutes before it 'went.  Anyway, Richard says he's given it a thorough cleaning and it's as good as new. 

Voxman

Picked up my Cub this morning. I got the fault slightly wrong.  It was oxidised connectors on the PCB not the valve-holders. Been playing it for the last hour or so it and its back to its old self.  Lovely valve tone at sensible volumes - I'd almost forgotten what a lovely little amp this is (albeit mine's upgraded with a Vintage 30 and hand selected & tested JJ tubes).