Single ended Class-A tube wear question

[Steve Conner]

I don't see how that follows. The amp only fails when used with Boogie brand rectifier tubes. My first thought would be that the Boogie ones suck but they sell them anyway. Doesn't it make you suspicious that there's actually a warning in the manual to be on the lookout for arcing rectifier tubes?

It's kind of like the bit in the movie "Broken Arrow": "I don't know what's worse- that we just lost a nuclear warhead, or that we lose them often enough that there's a word for it."

[MWJB]

Sounds like something is pulling too much current through the rectifier & blowing it, the Sovtek handles more current so takes it better. Power tube idle current is the prime suspect, as tubes are the things that draw current in an amp, it IS most likely a "tube problem".

[Steve Conner]

That seems logical, but I thought he said that he tried another set of power tubes and it made no difference.

[Jared Purdy]

"Power tube idle current is the prime suspect" Please splain!! The tech at the shop also said that he thought something was pulling too much power. If something is drawing too much power (i.e., more than it is supposed to) how could it be a tube problem? I did replace the power tubes, with a new set of Mesa tubes, at the same time that I replaced the rectifier with the Sovtek. I'm going to try an experiment later this afternoon, I'm going to put the original Mesa power tubes back in with the Sovtek 5Y3. The thing I don't like about the Sovtek is that it rattles in the background. The plates aren't totally tight. And yes, it does make me suspicious that there is a warning in the manual about "lightening in a bottle". I'm going to take the faulty Mesa tube back and exchange it, and try it. Maybe, it is tube related, but the odd thing is, I bought the amp from a differennt dealer than I bought the 2nd Mesa 5Y3. And, I have no idea how long that amp was in the store. Me thinks quite a while, as they haven't replaced it with another, and that was 6 weeks ago.

[MWJB]

The tubes are the only devices that draw current in the amp (unless you had a short, in which case nothing would work & stay not working), if the idle current is set too high, then this stresses the rectifier. There's no point trying other power tubes unless you are trying a pair that you know draw less current...if you just replace the tubes with other tubes that draw the same current the problem will just persist.

[Steve Conner]

Yeah, but don't Mesas have the wonderful "Fixed bias"? That doesn't mean what the rest of us understand by fixed bias, it's Randall-speak for "Non-adjustable bias". They have no bias pot.

So if the tubes are drawing too much current, there's nothing the end user or a tech can twiddle to fix it. Mesa are supposed to supply tubes that are picked to draw the right idle current: the intention being to lock the user into Mesa brand tubes.

[Jared Purdy]

The amp was working fine for the last month though. So, if the idle current is set too high, wouldn't that have caused problems sooner? I have put other power tubes in, but they were also Mesa EL84s. So how does it get fixed?

[MWJB]

You can change the circuit to alter idle current, or change the tubes....a good tube vendor will be able to cherry pick some suitable tubes if supplied with plate voltage & negative grid voltage readings. With a Fender Custom Vibrolux Reverb, or RI Vibroverb, this is the route I'd typically take because Fender (for reasons best known to themselves) stuck the bias range resistor on the control panel PCB, so it's quicker to use bias probes & find tubes that the amp likes.

[Steve Conner]

But this is my whole point. You're locked into Mesa's tubes, but by the same token, any tubes you buy from them should run safely in your amp. If they don't that is Mesa's problem to sort out, not yours.

As MWJB points out, a good tube vendor can give you other tube brands selected to work in a Mesa.

No bias mods on an amp that's still under warranty.

[Jared Purdy]

+1 If it's Mesa's problem, and I bought their amp, then it's my problem! And, if I use other manufacturers' tubes, and anything happens to the amp, I'm up the creek. I think that I am going to have it looked at on Monday when the tech is back in, make sure it is "okay", and then sell it. No more PCB for me, it's my third one this year. Redplate, Carr, Vintage Sounds, Headstrong, they've all got stellar reputations and cost little more than a LSS.

[MWJB]

I don't know how the law works in Canada, but here it's the store that sold you the product's problem? First step, I'd put the old (original) tubes back in the amp, take it back to the store, with any blown Mesa brand tubes, point out that it doesn't work & that you need a full refund/trade in (discounted by the "as new" Mesa price)...well that's how it's supposed to work, not that it does.

This is an issue that can potentially affect ANY amp, some manufacturers fit bias pots so that "qualified personnel" can rebias your amp, but no big manufacturer really wants Joe Bloggs poking about in their amps...not even Carr, the Carr amps I have seen have externally accessible bias pots/meter points.

...but I agree with Steve, Mesa (if everything is done by their book) don't make life easy for customers.

[GregS]

it's the store that sold you the product's problem?

That's what I say too. The product is defective and a big store like Steve's should take it back for a full refund no questions asked. If you sell it yourself you'll probably lose $100's on it. If there's any issues with returning it you might consider going to Long & McQuade for any future purchases. They're very good about stuff like that. They'll even give you a month to decide if you like something or not, and if not you can return it for a full refund.

[Jared Purdy]

I think I'm going to approach Jerry (store manager) tomorrow. I have plunked down thousands' in that store. I'll give the amp tech a chance to look at it, and if I don't like what I hear, I'll ask for a refund. Aside from the Mesa LSS, they have nothing that I am interested in. I want nothing with PCB, and the only PTP amps they carry are the Class 5, and the Fender PTP amps, which are insanely over priced, and not the sound that I am after anyways.

I exchanged the "defective" Mesa 5Y3 (which I bought at Long & McQuade's for the Canadian viewers), they gave me a new one no problem. I took the Sovtek out, and put in the Mesa and played for a few minutes, and it's working - so far, which is what happened last time too. It's on stand-by right now. We'll see what happens, and I'll keep you posted. I want to like this amp, but my limits are close to being reached.

[MWJB]

Fender PTP amps are ball park pricewise, hardly insanely expensive...especially not for a man who is considering a Carr amp anyway ;-) Your aversion to PCB amps is irrational, many are perfectly reliable, I very much doubt that being PCB is your problem...tubes draw current, not circuit boards.

"It's on stand-by right now"...How long do you leave it on standby? You only need standby for the first 30 seconds, or so, after throwing the power switch, then again a few seconds before powering down. Amps don't like being left in standby for long periods. If you're going to leave the amp "off" for more than a couple of minutes, power down completely.

[ThermionicScott]

Going back to the OP (and Mesa's site), I wonder if the TransAtlantic is always fixed bias, and the bias level (and perhaps the plate voltage) is being controlled with each mode. In the "25-watt" mode, B+ is high and idle current is low; in 15-watt mode, B+ is lower and idle current increased enough to keep tubes more-or-less out of cutoff; in 5-watt mode, one tube's bias-feed resistor is "shorted" so it can't amplify the signal coming across its coupling cap, like redelephant posited.

- Scott