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Author Topic: Guitar amp power valve reliability  (Read 2590 times)

johnR

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Guitar amp power valve reliability
« on: January 18, 2011, 09:49:01 AM »

It's getting increasingly difficult to find power valves that will sound good and stand up to hard use in a guitar amp. I've been repairing guitar amps for a few decades now, and it seems that every time I find a decent type of EL34, for example, it soon goes out of production. Thirty years ago you could still find new Mullards fairly easily. Twenty years ago the Tesla EL34s were pretty good, and a while later the original Svetlanas were not bad.

Lately I've been using JJ E34Ls which actually sound good to me and to many of my customers. Unfortunately the failure rate has been increasing over the last couple of years. After a spate of early failures in use, I've tightened my testing procedure so that I now give re-valved amps a very rigorous test on the bench before giving them back to the customer. Failure rate in testing is around 20%.

It isn't just EL34s. Last week I tried a set of four Sovtek "military grade re-issue" EL84Ms in an AC30-inspired amp. They lasted long enough to confirm that the bias was correct, but within five minutes the output power had dropped to 25%. All four were drawing excessive grid current and were completely useless.

Searching out NOS valves is not an affordable or practical option for most of my customers. What are the other guitar amp techs here doing to cope with this situation?
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MagnetoSound

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Re: Guitar amp power valve reliability
« Reply #1 on: January 18, 2011, 10:44:31 AM »


It is a conundrum, to be sure. My own experience bears out everything you've said.

Can I assume that you are using a distributor that has a decent returns policy?

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Music can make me get right up out of my chair and start dancing or it can get me so pumped up I have to walk around the block.
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johnR

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Re: Guitar amp power valve reliability
« Reply #2 on: January 18, 2011, 11:20:29 AM »

Dan, I've been covering the cost of bad valves myself up until now, but it's getting unaffordable. I've started putting enough of a markup on them to cover an average 20% failure rate, and most customers are OK with that after it's explained to them.

I've been buying valves from the same couple of places that many amp owners buy them from, as I'm not using large enough quantities to buy from a major distributor. I'm pretty sure that if I sent back every valve that fails during testing my suppliers would quickly become uncooperative.
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