Quick power tube question...how long without biasing?

[75lespaul]

Say I'm at rehearsal, or worse, in a club, and a power tube goes. In this case, 6l6s (two). If you pop a new tube in the amp, is it safe to run without biasing? If so, for how long. Any kind of rule of thumb? Is it different for different tubes, or the same rules apply to all, in case I pick up an EL-34 amp. Thank you!

 

[the guy who invented fire]

Re: Quick power tube question...how long without biasing?

It all depends...if it doesn't red plate it might run fine and just sound like poop...it might run fine and sound fine and it might not run right...all those things depend on the bias the amp is set at and the tubes you use.

There is no real rule of thumb on this...

 

[75lespaul]

Re: Quick power tube question...how long without biasing?

It all depends...if it doesn't red plate it might run fine and just sound like poop...it might run fine and sound fine and it might not run right...all those things depend on the bias the amp is set at and the tubes you use.

There is no real rule of thumb on this...

I was hoping that wouldn't be the answer, ha ha. I guess the best fix is...backup amp.

 

[the guy who invented fire]

Re: Quick power tube question...how long without biasing?

I was hoping that wouldn't be the answer, ha ha. I guess the best fix is...backup amp.

The best thing to do is to use matched power tubes that come graded, get the amp biased for them and then keep a spare pair (set, quad, etc) of the same grade then you can switch and the bias will still be right but a back up amp is a beautiful thing!

 

[joelfern]

Re: Quick power tube question...how long without biasing?

one useful tip to be prepared for this stuff; is always buy a matched quad when your amp requires 2. use 2 of them bias the amp accordingly and keep the rest 2 handy. so that when any of them blow up during a gig or when you don't have time. Just plug out the old and pop in the new. No biasing required

 

[Gearjoneser]

Re: Quick power tube question...how long without biasing?

Most tubes sound fine if they're within a normal operating bias. The main thing is to look at the tubes for awhile after they've been installed. If the gray plates in the power tubes start turning orange/red, turn it off immediately....it's biased way too hot. If they're glowing normally and the amp sounds fine, don't worry about it.

I think it's a good idea for anyone who uses tube amps to purchase a Bias Tool, which is a socket that goes between the amp and tube, with a lead that goes to your volt/ohm meter. You can get volt/ohm meters at Harbor Freight for $5. Buy the bias tool socket online for around $15.

As long as you're competent at working with a live chassis and learn to not get electrocuted, you should be able to check the bias on your amp. There's a few good youtube tutorials on the subject.

I like Weber's Bias calculator and chart. Look at the bias ranges in the chart. As long as the numbers are in those ranges your amp will work fine. Bookmark this link.
http://www.webervst.com/tubes1/calcbias.htm

 

[Boogie Bill]

Re: Quick power tube question...how long without biasing?

I like fire guy's post. That's the way to do it.

This is essentially one of the benefits of a Mesa amp. If a tube goes bad, just plug in a fresh pair of Mesa tubes and you are good to go.

Bill

 

[75lespaul]

Re: Quick power tube question...how long without biasing?

Thanks for all the great responses. I think I'm ready to try biasing myself, and I saw the biasing socket that Gear mentioned. I'm fifty now, but when I was around twenty three, I bought a Fender Deluxe, plugged it in, unplugged it, then removed the chassis to look at it. I was on lunch at work at the time. Don't know WHAT I touched (with both hands or course) but I got a shock that I never forgot. Weird thing was I got an instant---well, let's just say that I was ready for any woman who came along, lol. If we could invent a controlled electrical shock, maybe we can put the blue pill out of business, haha! Anyway, to get back to the subject, I never touched the inside of an amp again. I remember being soooo tired after the shock, and I almost felt my eyeballs vibrate during. Of course I learned that I was lucky not to have been killed, but it took many years before I could even swap a tube! Actually it was this forum that got me preamp tube swapping again.

Hope you enjoyed my little electrocution story. Thanks again guys.

 

[Dr. Vegetable]

Re: Quick power tube question...how long without biasing?

This is essentially one of the benefits of a Mesa amp. If a tube goes bad, just plug in a fresh pair of Mesa tubes and you are good to go.

I've wondered about this. In the Mesa documentation they make a case that bias adjustments are not necessary because the voltage adjustment is a proxy for setting the current - which would be difficult/dangerous to measure. The statement is something along the lines that once you set the bias voltage, it should not need to change for different tubes. Instead they put all their eggs into the "matched tube" basket.

Is there something different about the Mesa circuit that makes this so, or would this also hold true for other designs? Or is this just marketing speak to get us all to buy genuine Mesa tube sets?

 

[Boogie Bill]

Re: Quick power tube question...how long without biasing?

Say you go buy a set of Groove Tubes' 6L6-GEs, marked in range 7. You have a tech bias the amp for those tubes. Six months later, you're ready for another set of tubes. If you go buy another set of the same tubes, you could simply install them without having to have the amp rebiased. That's essentially what a Mesa owner does.

Now Joe orders a matched set of EL-34s for his Marshall head and they bias at 43.3 MA. His next order he gets a set marked at 36.8 and he has to pay to have it rebiased. His third order comes and needs to be biased at 28.4. Guess what he has to do...yep, needs a rebiasing.

He could have just ordered a set matched to his first set and would have avoided the bench fees, or bought the equipment and learned to do it himself, with the associated risks...as described above.

Most of the major tube vendors can sell you a set of tubes will meet Mesa specs if you order for a Mesa amp. Typically they warranty these tubes for 90 days. Mesa's warranty is 180 days--six months. Mesa is the largest tube user in the world, and they buy their tubes from the same mfgs. the other guys do. They test them, and reject the ones that don't meet their standards. Think about what happens to those rejected tubes. I'm pretty sure they sell them, you can guess who to...

A couple of years ago, I was at a Mesa clinic with Andy Timmons at a local music store. He was using the store's amps to play through, and in the middle of his set, his Stiletto Deuce had one of its EL-34 short. The manager got a new Mesa set of Mesa 34s and popped them in. No biasing. Amp sounded great again. Took less than five minutes. Not many smps you could do that with.

I saw Fleetwood Mac earlier this year. Lindsay Buckingham ROCKED his Tremoverbs! I found out that he installs a brand new set of 6L6s for each show! He doesn't get them for free, either. I wish I had his tube budget!.

Now over time, circuits can drift, even in a Mesa amp. The amp will need to be serviced. Some opt to modify the amp and put in adjustable bias pots, but that kind of defeats the purpose of a sensible system...and will void the warrany.

To me, the fixed, non-adjustable bias of the Mesa amps makes sense. I KNOW it saves me money.

Bill