Bare Knuckle Pickups Forum
Forum Ringside => Tech => Topic started by: _tom_ on October 18, 2008, 03:46:40 PM
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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.
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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.
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The only preamp tube I have spare is a 5751 I think, which is lower output than a 12AX7 isnt it?
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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 :(
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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 ".
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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
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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 !!!
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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 !
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What power valves did you put in Tom?
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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 :(
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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.
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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.
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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.
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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?
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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.
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ok, cheers. I ordered some new EI Elites but apparenty their factory is down for retooling so I couldnt get any in time for the weekend I was back :( I just dont want to spend £40 or so on a whole set of preamp valves only to find out theyre not the cause. I have a 5751 (70% the gain of a 12AX7) at home that I can give a go, should I just try it in each position and see what happens? Only thing is, because its a different valve and output, I dont really have a point of reference as I dont know what to expect, if you know what I mean. Its annoiyng living away from home, I cant get this all sorted easily :(
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I have a 5751 (70% the gain of a 12AX7) at home that I can give a go, should I just try it in each position and see what happens?
That would be perfect. There's no need to replace all the pre-amp valves. Just replace one at a time with the 5751.
A 5751 is very similar to a 12AX7 really. All you might notice is a slight reduction in gain with the 5751 instead of a 12ax7.
However, if your problem is a faulty 12ax7 then when you do replace the faulty valve with your 5751, your amp's nasty symptom will disappear !
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I'd save up for a full set of tubes. If they fix the problem, great! If they don't, you have a backup set for when you need them next.
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Looks like the 5751 for V1 fixed it. I plugged in expecting dull mushiness but it was a lot tighter and clearer :) Played for about half an hour and it seems to have kept up the good sound for that, so I think its fine now. Hopefully it stays this way. The 5751 sounds really good btw, a lot brighter than the TAD stock V1 though.
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Looks like the 5751 for V1 fixed it. I plugged in expecting dull mushiness but it was a lot tighter and clearer :) Played for about half an hour and it seems to have kept up the good sound for that, so I think its fine now. Hopefully it stays this way. The 5751 sounds really good btw, a lot brighter than the TAD stock V1 though.
Glad to hear you improved things Tom.
Sounds like you might have had a bit of a duff V1 valve. It would be interesting to swap a known good 12ax7 with your 5751 in V1.
BTW, a 5751 in the PI can be a really amp-tightening move with some 3-4 gain stage amps.
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Glad it's sorted with the pre amp valve. FWIW... Another cause for this problem can be dirty/ oxidised jacks on the effects loop. I've had the volume fluctuations on a couple of amps over the years and at first I couldn't understand why it could be the case as I never used to use the loop. But a couple of times it turned out to be exactly that.... so keep the loop contacts nice and clean too whether you use them or not.
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Parasitic oscillation can cause amps to seemingly randomly distort/ change volume, and can be a real problem in high gain amps. Reducing the gain can often solve the problem, and this might be why a 5751 seems to have "cured" the amp.
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For $%&#s sake, its back again :lol: The 5751 still sounds way better than the TAD 12AX7 in there, and the effect is less noticeable, but its still there every so often. Didnt start until about half an hour into playing today.
edit - d1dsj, I have the fx loop set to bypass so it couldnt be anything to do with that could it?