Amp doctors- can you make a diagnosis?

english_bob

Laney Lionheart L20T combo. 

Dull tone, intermittent "farty" distortion, lower than expected volume, crackle when turning some pots. Same on both channels. Started out intermittent but has been getting worse over time.

Had the same valves since I bought it (not new) in 2015, they're all lighting up uniformly with no worrying blue or red bits. Has been used regularly but not heavily.

What do you reckon? Re-valve? Duff pots? Viking funeral? Anything I might be able to fix myself, or does it need to go to amp hospital?

ICBM

Most likely to be a preamp valve, and I think probably V2.

The easiest way to find out is to buy one spare preamp valve - make sure it's a Chinese one (Shuguang, often sold under other brands eg TAD) since Russian-made valves and sometimes JJs can have a problem with failure in the V2 position since it's a cathode follower. Or any old-production Western-made valve will be fine.

Replace V2. If that doesn't fix it, put the original back and try the new valve in V1, then repeat for V3 if necessary. If that still doesn't fix it, it's most likely a tech job since a power valve problem wouldn't make the pots crackle, although it could do the other things.

english_bob

Thanks @ICBM ;;

I've got a spare 12AX7 somewhere- I think it's a Groove Tubes one that came out of my old Cornell. Those are Russian I think..

That would be fine for the diagnostic swapsies though, right? And if it fixes the problem swap in a new Chinese 12AX7?

ICBM

Some Groove Tubes are Chinese. If it's a Russian one I would probably try to avoid putting it in V2 at all - try it in V1 or V3 first, and if that doesn't fix it then use the valve you just took out to test V2.

Chinese 12AX7s have a distinctive double chrome strip on both sides of the plate structure inside.

DJH83004

In the unlikely event that it is not one of the preamp valves as IC has suggested, the L20T is unusual in that the EL84 output valves are a parallel single ended configuration, so one output valve going noisy, partial short. low emissions etc. could affect the overall output, so is definitely worth subbing each one with a known good valve, if not in the preamp section. Otherwise a visit to the amp tech may be required.    

english_bob

Didn't get a chance to look at the amp before I went on holiday last week, but having had a fiddle tonight, it was the TAD E83CC in V1 that was to blame. Swapped in the spare and the amp sounds like my amp again. Yay!

Thanks @ICBM and @DJH83004 !