Does this sound like an amp/valve problem to you?

[_tom_]

So I was playing my Laney today and noticed its having a problem with its output and brightness. Sometimes it'll sound bright/punchy/loud then the next minute it'll be a bit quieter and duller. I managed to get a clip showing it. Tone and playing a bit shitee all around but you can hear it change.

http://www.soundclick.com/bands/page_songInfo.cfm?bandID=493678&songID=6984416

So could that be a power amp valve problem or what? I think I had this problem with my Epiphone before so I dont think its the guitar but I'll check to make sure nothings shorting out or anything like that. The valves are quite new, had them for about a month or so I think. The valves are glowing normally, I think.

[hunter]

Phew this is a tough one. Valves would be my best guess. Hopefully the issue appears often, the easier to reproduce, the easier to fix.

I would start replacing preamp tubes one by one. If this won't fix it .... do you still have the old poweramp tubes to try this?

Sometimes relatively new tubes can cr@p out, this happens.

[_tom_]

The only preamp tube I have spare is a 5751 I think, which is lower output than a 12AX7 isnt it?

[Twinfan]

Difficult to say really Tom.

Other than changing valves one by one, as Hunter says, it's a trip to a tech job I think  :(

[hamfist]

Yup,  I can clearly hear at about 0:18 the volume decreases significantly.

I also notice that the gain doesn't seem to be altered much, just the overall volume. That would indicate to me that the issue is post-PI (ie. a power amp problem).
  As has been recommended already, I'd suggest trying new valves. Apparently 95% of valve amp problems are valve-related.   In your case I'd suspect the power valves.  What's the amp BTW. Sounds good !
  If a change of power valves doesn't sort it, then I'd very much advise a visit to the "amp surgeon ".

[_tom_]

The amp is a Laney GH50L through G12H speakers.

I'll go through the preamp valves anyway to see what happens. I cant really afford new power amp ones at the minute, cost me £60 last time to get them and have them fitted and biased :\ Is it safe to take them out, spray the pins and sockets with some contact cleaner then try again or will that damage them? I have some of this - http://www.maplin.co.uk/Module.aspx?ModuleNo=28993

[hamfist]

The amp is a Laney GH50L through G12H speakers.

I'll go through the preamp valves anyway to see what happens. I cant really afford new power amp ones at the minute, cost me £60 last time to get them and have them fitted and biased :\ Is it safe to take them out, spray the pins and sockets with some contact cleaner then try again or will that damage them? I have some of this - http://www.maplin.co.uk/Module.aspx?ModuleNo=28993

Sounds like a very good idea to clean the power tube pins/sockets.  That spray sounds like it will do fine.  Take the tubes out, spray the pins, then put them in and out of the sockets a few times.  Report back !!!

[Fourth Feline]

Hiya Tom,  :)

I once had a very similar problem in the early days of owning a Fender Twin.  The 'Fender' badged valves where just cheap / barely functional Sovteks - and when the output quad of 5881s got hot, they got 'woolier'.  I just had to let them cool down again between sets.  Sure enough, when I replaced the output quad with better quality 6L6 or 7581a  type valves ( +  re-biased ! ) - the problem disappeared and the true splendour of the amp was revealed.

I know the amp maufacturers may have to keep production costs down, but it seems counter productive to sell amps with poor quality valves in, when they can get better ones at trade / bulk buying cost.

The pre amp seems much less fussy, but the output stage very fussy. I do like to put a good pre- amp valve  ( in my case a N.O.S. GEC 5751 ) in the V1 slot .  After V1 - and up to the output stage , swapping and changing made less of a difference : - but the valve choice(s) at the very beginning and end of the signal chain seem crucial to my ears.

I also echo the importance of getting those valve pins / sockets cleaned with aerosol cleaner. It often works a treat !

[Twinfan]

What power valves did you put in Tom?

[_tom_]

They're Svetlana Winged C EL34s from watford valves.

Put the contact cleaner on and pulled them in/out the sockets a few times, seems ok so far. Just not as good as the DSL50 I'm currently in possession of :(

[Twinfan]

Should be OK valves then.  As I've said on your other thread, try swapping the preamp valves with the DSL and see what that does to the tone.

[HTH AMPS]

The volume fading thing certainly sounds like a typical condition of worn output valves, but since they are only a month old I doubt thats your problem.  The fading volume sounds like a smoothing cap in the HT line thats fcuk'ed - had this problem with a plexi that got brought to me last xmas - some new filter caps sorted it right out.

I doubt it's the preamp valves - they tend to go noisy, microphonic or just stop working altogether.

[_tom_]

hmm well I'm not at home right now so I cant try my amp out again til I'm back again (few weeks time) but the contact cleaner seemed to help in the short time I played it afterwards. If it still does it, I guess I need to take it to a tech.

[_tom_]

Right well the contact cleaner hasnt done anything. I got home this weekend but only just got round to having a go on the amp again. Anyway, it started out all mushy like it does when its "broken" but then sometimes it occasionally comes in with what I guess is the proper sound and it sounded awesome, but then went dull/mushy again :( I dont have any 12AX7s to try in there (and I'm back off to uni in a few minutes I think) but will that even be the cause of the problem or is it definitely something a tech needs to have a look at? I'd like to get a full service done by Laney really (or at least someone who knows the amp well), anyone know if they do this, like Marshall do at their factory?

[hamfist]

To quote myself :-

95% of valve amp problems are valve-related. 

If you have not replaced valves yet, then that is still by far the most likely cause of your troubles.