amp gone quiet

[john_the_bass]

Ok so it's a guitar amp, but the same principle applies right?

Saturday night, went to a gig - amp on, no problem, started setting up the other stuff, went to take the amp of standby and it's off. Check the power fuse - blown. No problem, changed it over, plugged in and off we went. Did 2 sets ok, was halfway through the 3rd set and there was something not right with it - very quiet, not a lot of output - plugged it in in the house yesterday and it's almost like there is not much signal getting through.
There was no smoke/fire/burning smells so I am ruling out the transformer and the valves look ok.

I had a half thought that the electricity in the pub might have been a bit suspect as we seemed to have a lot of (pre-tested) kettle leads (or IEC or whatever they're called) not working - blown fuses. The PA amp seemed to be producing a lot of output (although I think we pin pointed the reason for that). I know pubs tend to have a lot of spikes in the power, what with the pumps, freezers, ovens and that kind of thing, but I don't know how that would correlate with my amp not working.

[skankdelvar]

Where's Thunderhead when you need him? In his august absence...failed or failing power tube?

OTOH, could be a manky vibe from boozer crawled into your reverb tank and is sending out a bummer spiritual negativity. :wacko:

[The Funk]

[quote name='skankdelvar' post='315863' date='Oct 27 2008, 03:00 PM']failed or failing power tube?[/quote]

I thought it could have been a failed or failing valve too.

[john_the_bass]

It had a matched quad of new power tubes at the end of august and they all look alright - no one area looks hotter than the other, none of them are glowing blue or anything like that

[Tinman]

Sounds like a valve somewhere. Take the back off, switch the amp on and tap the valves gently with a pencil, you should be able to hear the tapping through your speakers. If one of them rings, sounds microphonic or is silent then it could be the one at fault. Remember, there are high voltages in there. (apologies if I'm teaching you to suck eggs)

[john_the_bass]

Nice one Pete - I take it I need to turn the volume up to check!
I know about the voltages that they can hang onto, mainly the reason why I'm reluctant to start mucking around with one.

[ben604]

My guitarist has a Fender Deluxe combo and had a similar problem although it was a pre-amp tube that was the issue. It was getting unseated when we played at loud volumes and wiggling out of place, leading to a drop in volume and then complete silence...Pushing it back in did the trick initially although it needs a service now as it keeps wiggling out. Try removing and re-installing all the tubes, that might fix it?

[Tinman]

[quote name='john_the_bass' post='315881' date='Oct 27 2008, 03:20 PM']Nice one Pete - I take it I need to turn the volume up to check!
I know about the voltages that they can hang onto, mainly the reason why I'm reluctant to start mucking around with one.[/quote]
Ummm yeah
You shouldn't be in any danger from high voltages, if all you're doing is tapping the valves. If it's anything like my Marshall, all the nasty components will be hidden under some casing, well away from stray fingers

Pete

Edit: It may be worth investing in a surge protector too. I never plug my Trace into anything without that being in the way

Edited October 27, 2008 by Tinman

[john_the_bass]

right you are lads. I hope I don't have to replace the valves - 4 bloody output valves and 4 preamp valves and I'll be at least 20 quid down based on what I earnt on Saturday night!
I only did the outputs in August (stop me if I am repeating myself )

[The Funk]

You wouldn't have to replace all the valves at once! If it's a preamp valve, just swap it for a new one.

[john_the_bass]

ah but if it's an output valve, it'll mean a new quad (or at least you can replace pairs).
It's gone to the tech anyway - I made a snap decision on the way home, rang him up and brought it round. I'm sh*t at stuff like that, I'd rather let somebody else sort it out before I electrocute myself or burn down the house.

[The Funk]

[quote name='john_the_bass' post='316097' date='Oct 27 2008, 07:28 PM']I'm sh*t at stuff like that, I'd rather let somebody else sort it out before I electrocute myself or burn down the house.[/quote]

That's why profressionals exist, right? To keep us from screwing it up?

[umph]

[quote name='john_the_bass' post='316097' date='Oct 27 2008, 07:28 PM']ah but if it's an output valve, it'll mean a new quad (or at least you can replace pairs).
It's gone to the tech anyway - I made a snap decision on the way home, rang him up and brought it round. I'm sh*t at stuff like that, I'd rather let somebody else sort it out before I electrocute myself or burn down the house.[/quote]
can't hurt yourself changing valves unless you decide to do it with a hammer while its turned on or something. Sounds more like it'd be the output transformer to me, i'd take it to a tech.

[The Funk]

[quote name='umph' post='316536' date='Oct 28 2008, 10:56 AM']can't hurt yourself changing valves unless you decide to do it with a hammer while its turned on or something.[/quote]

If they're still hot, you can burn your fingers.

[john_the_bass]

[quote name='umph' post='316536' date='Oct 28 2008, 10:56 AM']can't hurt yourself changing valves unless you decide to do it with a hammer while its turned on or something. Sounds more like it'd be the output transformer to me, i'd take it to a tech.[/quote]

no that's true, i've done valves before

[quote name='The Funk' post='316577' date='Oct 28 2008, 11:39 AM']If they're still hot, you can burn your fingers.[/quote]

and the man speaks the truth.

There was none of the symptoms of a transformer going bang, so I wonder if it is.
Ah well, I'll find out in a week or so.

[skankdelvar]

You don't really need to replace in pairs - just buy one of the same type and get the tech to bias the set accordingly. All this matched pairs stuff is allegedly voodoo invented by Groove Tubes to sell more valves.

[john_the_bass]

I was by the fella that runs a shop selling valves and whatnot that most branded valves are largely made by somebody else or available under a less expensive name. I remember one particular brand of 6l6 or possibly el34 was made by GT and about half the price

[umph]

[quote name='skankdelvar' post='316647' date='Oct 28 2008, 12:54 PM']You don't really need to replace in pairs - just buy one of the same type and get the tech to bias the set accordingly. All this matched pairs stuff is allegedly voodoo invented by Groove Tubes to sell more valves.[/quote]
you match tubes to roughly the same gain so that you can bias them close to each other without one running really hot and the other running really cold

[skankdelvar]

Agreed - as you say - roughly matched. That's my point. Given the usually minimal differential in tolerances between tubes of the same mfr type, matching is much less relevant given the balancing is done in amplifier in the course of the biasing process.

Fair point, [i]too[/i] far out of balance (e.g. using different iterations of 6L6) and you start to lose the humbucking function of the push-pull pair. And sometimes in very powerful valve amps (like SVT's) imbalance can lead to instability.

Biasing isn't strictly about increasing gain. It's the function of the front end of the amp - the pre-amp - to act as one or more gain stages. Here we're talking about the power amp section, where biasing is done to adjust the current that flows through the valve. Bias is a negative voltage that acts as a brake on the main current carrying the amplified signal through the valve (in a manner of speaking).

Put simply, the more bias voltage you apply, the less current flows through the valve. Too much current destroys the valve as it can't dissipate all that heat. Too little bias voltage and it doesn't work right. But that's a process you control when you're biasing.

NB - I'm no expert - just picked this up from my mate Steve. And it still doesn't solve the OP's question. Sorry about that.

[john_the_bass]

First prize to whoever said "output valves"

I rang me techy bloke earlier who said - "output valves". He asked me where I'd got them from and said he didn't think they were much cop to begin with. (I'd thought JJs were supposed to be quite good, but appartently there can be variance between valves of the same brand and how "good" they are!).
The other thing I'd neglected to mention was that I'd not had the amp rebiased, and for good reason. I'd received an email from Laney, when I'd contacted them to check if anything else needed to be done who replied and said "The LC30-II does not require rebiasing after a tube change". Simple - I thought

The techy bloke seems to think that it perhaps should have been looked at anyway. It's only 60 quid to put right, which included a new matched quad of EL84s (that would have cost me £38 otherwise) - although I am a bit pissed off that something's made them fail so quickly (ie in the 3 or 4 times I've used it since I replaced the valves at the end of August.
Something is afoot!

[skankdelvar]

Don't know if the following also applies to the LC range. Cribbed off the web:

[quote]The sockets in the Laney VC30 are wired up to accept only one type of EL84. They use a pin that has no connection on some valves as part of the circuit - however other EL84s do have a connection there - and pop they go when you plug them in. Not too hard to rewire the sockets if you have this problem.

The advice is to get this job done before plugging in any new power valves except those Laney supply.

While we're on the subject, a tech I know pointed out another issue on a VC30 he had in. He'd sorted the wiring thing, but still found that one of the power valves glowed red on the plates however many times he checked the bias. Then he noticed it was very close to the speaker magnet. The magnetic field was helping the current flow, overheating the valve.[/quote]

Edited October 30, 2008 by skankdelvar

[john_the_bass]

That's very interesting - the second part I've heard about before, with the valves being close to the magnet although I was under the impression that was something that had been sorted out on later VC30s. I know the chassis on my LC is different, so I don't think it's affected by that.
I have heard about the first issue (with the pin with no connection) being a problem with certain valves and it (apparently) means you can't use Mullards in an LC/VC - not that I'd really want to.

I originally chose JJ EL84s as they are recommended on the Watford Valves website as a good upgrade (I then bought 'em in Hotrox as I happened to be passing one day), so I was naturally surprised that they would go so quickly. They've been replaced with a set of Edicron "JJ" EL84, which is presumably a clone of the JJ..er..EL84!
Took it to a rehearsal last night - sounds good, hopefully it's all sorted now.
In my original communication to Laney to find out if I needed a rebias when I revalved and was told no, they did tell me that the tubes they recommend are the Chinese Tube Amp Doctor valves - I still have the ones I pulled out and they are Sovteks (I think, need to check). No idea what the craic is with it, I'm just glad it's working.

[skankdelvar]

Glad it's working again...funny about the JJ's, though - generally viewed as a pretty safe buy - as are Laney's. Have used JJ el-34's, 6v6's, 6l6's and 12ax7's, no problems so far.

Will bear all this in mind if I'm allowed to buy another amp, ever..."but it's an [i]VC-30 [/i]and I haven't got one of those....whine, moan"

Edited October 31, 2008 by skankdelvar