Tubes burning up?

irish_admiral · Post by **irish_admiral** » Sat Jul 17, 2004 4:36 pm

OK, I have today replaced my JJ EL84 tubes with some Harma E84Ls (which incidentally sound very nice). However, I can smell a faint whiff of what smells like burning metal above the vent on my combo which wasn't there previously.

The sound is fine. All the power tubes are glowing faintly, so does anyone have any idea what it might be? Is something going to go "kaboom" on me?

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irish_admiral · Post by **irish_admiral** » Sat Jul 17, 2004 6:41 pm

I'll just qualify this...

I put the old power tubes back in, and now the same problem is occurring. Let's rule out the tubes.

The smell seems to be coming from the smaller transformer.

The crackly sound is like the one I get at really cranked levels, only I get it at about 1/4 volume on the amp, and my volume pedal right down. It appears to increase slowly. The tubes ping a little after I have switched my amp from standby-on to full-on.

Is there a loose connection somewhere?

Don't tell me the best way, just tell me the cheapest way...

irish_admiral · Post by **irish_admiral** » Sat Jul 17, 2004 7:08 pm

Ok, just playing around a little further, there is a metallic sort of "fuzz" when I tap the tubes with a pencil whilst on... power and preamp. Sometimes it does this, sometimes it does not.

What does this mean?

Don't tell me the best way, just tell me the cheapest way...

Kurb2004 · Post by **Kurb2004** » Sun Jul 18, 2004 2:40 pm

The ping noise you can hear is just the metal inside the valve Expanding as the valves get hotter. This is not a problem as metal expands when hot and contracts when cooling. The metallic noise is just you preamp valves, thats not a problem either you should hear my soundcity. (now thats a noisy amp if ever there was one) I would get yer output transformer checked to be on the safe side and give all the switches a good clean.

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irish_admiral · Post by **irish_admiral** » Sun Jul 18, 2004 5:21 pm

Yeah, i'm not worried about the pinging. I know that happens.

ALL the tubes also sometimes make a loud metallic buzz/dink noise when I tap them, which they never did before (I'd get a faint clean ding, nothing more). I'm certain that's not healthy. The same sort of metallic buzz/dink is also sometimes present when I play a note on my guitar.

Furthermore, there is a dodgy smell burned metal smell rising from the little transformer, and the amp is suddenly very noisy and crackly when it wasn't this morning.

Don't tell me the best way, just tell me the cheapest way...

irish_admiral · Post by **irish_admiral** » Mon Jul 19, 2004 7:35 pm

Had a chat with Derek at Watford Valves, and he concluded that it's probably the bias resistor that has blown, not the output transformer, and that is the burnt smell (otherwise I wouldn't be getting any sound... or would I?).

Second point to make is that you COULD need to rebias your amp depending on the characteristics of the valve you're replacing. The E84Ls I fitted take a higher plate voltage, so we reckon we burned out the bias resistor. With another EL84, I might not have broken anything.

Don't tell me the best way, just tell me the cheapest way...

Matt_plan-R · Post by **Matt_plan-R** » Mon Jul 19, 2004 7:54 pm

My question is: If I were to call Derek and tell him what tube I do have in my amp, would he be able to tell me if my bias resistor could handle new tubes?

irish_admiral · Post by **irish_admiral** » Mon Jul 19, 2004 8:09 pm

Quite probably. The man is a valve giant. He's matched and supplied valves to most of the big names out there who need 'em. Give him a call, find out for yourself.

Don't tell me the best way, just tell me the cheapest way...

irish_admiral · Post by **irish_admiral** » Sat Aug 07, 2004 8:51 pm

OK, so we worked out that the AD30 doesn't need to be rebiased. And I finally found out what was wrong with my amp 2 weeks and Ã‚Â£75 later...

Problem 1 was down to a duff ECC81 tube. This was causing the increasing hum, and the metallic sounding dinks. Problem 2 was caused by the reverb transformer, giving me some crackling. Replaced this with one taken from a new Hiwatt, and the surrounding caps, and everything is fine again. Incidentally, the reverb sounds a lot nicer with the Hiwatt transformer powering it.

Don't tell me the best way, just tell me the cheapest way...

Tubes burning up?

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