can neglecting to bias a powertube installation damage an amp?

[Jammintree]

I'm looking at buying a used Mesa Boogie Express 5:50 plus. The amp, like all boogies was biased at the factory for 6L6's and at the same value as all boogies of this model. The current owner of the amp put 6V6's in the amp right as soon as he brought it home and never biased the amp. He believes that Mesa's don't need biasing, which I believe is true but only when you buy Mesa power tubes specific to your model which are all choosen to match one another. The amp does have a bias switch, which can be set for 6L6 or EL34's. But to my understanding this guy just swapped out for 6V6's and never did a thing to change the bias.

So the question is: would this harm the amp? Would it perhaps strain the OT or maybe the other way around, the OT would overwhelm the 6v6's?

If there is a chance the amp was harmed, I'll just pass on it. If there's little or no possibility of the amp being damaged, I can get the amp and put the original tubes back in.

What does the peanut gallery think?

 

[seekir]

Not an engineer, but I think biasing hot will damage the tubes rather than the amp? I suppose hot tubes could damage the board and solder joints. I think biasing cold won't hurt anything but the amp's tone. I don't know much about amps that are equipped to accept multiple tube types. I presume the owner changed the bias switch you mention to the 6V6 position after the swap? I guess if the amp sounds good to you and the price isn't too high, it might be a good buy.

 

[Cleeve]

The amp should be fine. The 6v6 draws less heater current and less maximum plate current, so the amp circuit would have been having less stress.
Mesa bias is adjustable just fine, only one must change a soldered-in resistor. There's a good chance that the tubes Mesa used back when the amp was produced was those horrid small-bottle chinese abominations that were about the only newly produced 6l6 available. Those little 6L6 from back then were not robust, so Mesa likely set the bias cold to get them through the warranty period. Cold bias on that old bunk pair of stock 6L6 may have turned out to be just perfect for a good pair of 6V6.

 

[Abu Twangy]

Most of us old-timers didn't bother to bias our amps on changing power tubes 40-50 years ago. Most of us wanted "matched tubes" in the power section but that meant all RCAs. Realistics, or Sylvanias--which means not matched. Occasionally we'd fudge with a bias pot until the amp sounded good.

Power tubes biased too hot don't last as long. The current in the tube is near or above the design parameters. This is more of an issue today where current production tubes are felt to be less durable than the old ones. And cold biasing sounds less musical.

Not biasing your power tubes means that your amp likely won't sound its best. I believe one of the reasons that my amps of the day lost some of their best sonic quality over time was due to unmatched and unbiased tubes as well as drifting of aging tubes.

 

[BobbyZ]

There's a lot of myths with boogie owners biggest one seems to be it'll make them sound good.
(Had to throw a boogie joke in sorry. Couldn't pick on the 8 year old drummer the other night so we picked on the guy with the mark V.)

Mesa's are no diffierent than any other fixed bias amp. Pick a bias point and then use tubes that are in the safe range at that point it's safe. Might not sound the best but most boogies are way over powered for most people anyway. So you never really get the power tubes working anyway.
With the 6V6s they just happend to be in their safe range and didn't melt down that's all.
The other thing about 6V6s is the impedance should be changed to compensate but that can be "close enough" too.

 

[Wally]

JUst an observation, but my concern for the 6V6's in that amp is the plate voltage...and screen voltage. I am not familiar with that amp, but that voltage is an important consideration, imho. AS noted above, the damage---if any---would visit upon the tubes and hopefully not extend the negatives to the transformers. That said....there are amps that run the 6V6 at close to 500vdc on the plates....Jim Kelly FACS amsp, for one.
I would suggest JJ's might be better suited for a high voltage situation. But....when I want a 6V6 amp, I buy a 6V6 amp. When I buy a 6l6 amp, I must have been wanting a 6L6 amp...in particular the one I am buying. I would suggest to the seller that it would be good to hear the amp with 6L6's in it...according to the manufacturer's design. IF the hesitate, offer them less than they are asking..even if you think you like the amp with the 6V6's in it. Why? Because....you will not have heard the amp with the tubes for which it was designed. You might not like the amp with the 'proper' tubes in it. Before talking the deal, it would be of interest to know the B+ in that Mesa amp. I dont' know if you can find that online...Schematic Heaven??? Wherever? Mesa is closed on Fridays.

 

[muchxs]

Neglecting bias throws the amp into cascading failure mode especially if you try something clever like swapping 6V6 for 6L6s. The 6V6 swap usually has the double whammy of off the charts supply voltage and hot bias. Elevated supply voltage and / or hot bias causes screen grid failures which short the tube. We're getting into serious welding when we short the power amp tubes. While we're at it cheapo imports are more likely to short and weld themselves together when run under off the charts conditions.

A shorted tube will generally blow the fuse but don't count on it. A Twin Reverb or any other high powered amp with abundant supply current will hang on for quite a while with a shorted tube. If it keeps running it does fun stuff like torch the screen grid resistors through the circuit board.

Amps that should stop working with the correct fuse continue to work with the incorrect fuse. This is where we blow up expensive transformers instead of buying a nickel and dime fuse.


I should have a cuss bucket for every time a misinformed amp owner thinks they have the correct information. You know how that's supposed to work... every time I cuss I'm supposed to drop a buck into the cuss bucket. By this time each year there should be enough dough in there to take Girlfriend on a cruise.

If I change it to a misinformation bucket and toss a buck in there for every bogus story whether I cuss or not... I'll be able to buy my own cruise line.

 

[telemnemonics]

Just another thought; given the shaky knowledge of the seller and the fact that 6v6 should get a different speaker load from 6L6, plus the multiple impedance selections on the amp, you might ask the seller what impedance speakers he plugged into what impedance output.

If the mismatch was off enough it could damage the OT, even if the tubes were fine.
IDK what could happen or how to tell after the fact, but I would assume that the owner has misused the amp.

 

[Wally]

The first thing I do when I am looking at an amp...for whatever reason...is to pull the fuse.
I have a box of 10-30 watt auto fuses that I have pulled out of amps. I shoudl have kept the solid and perfectly rolled alminum foil 'fues' I pulled out o Vox AC-100.As I was pulling the fuse cap, the owner said the amp had never blown a fuse!!! I showed it to him and said: "I don't think thes will ever blow!> 3 of the 4 EL-34's were shorted out. Randy has passed on. The last I heard form him as he left with the head was that it was going to the Smithsonian for the Beatles exhibit. Maybe so....I simply told him not to try to run the amp...he didni't want ANYTHIng done to it other some new sepaker cables. I did not put a new fuse in it.
Wehn people go outside of designs, I start getting antsyp about buying the thing...unless maybe muchxs had done the work, of course.

Snowing some here today.....it will be moving eastward from here....ya'll are welcome. Bundle up...it was 14 or 16 degrees here in this part of the world this morning. I know you friends to the east need some relief....but it ain't coming soon, it seems.

 

[Johnny Cache]

I think the first thing I'd do would be check the bias and PV. After you know that you'll have your answer. FWIW I don't usually recommend putting 6V6's in an amp built for 6L6's. For no other reason than the tube life.

 

[Jammintree]

Thanks for all the great responses. If the 6V6's were only used for a few short hours of playing at low volumes, what is the chance that any damage was done to the amp?

 

[muchxs]

Thanks for all the great responses. If the 6V6's were only used for a few short hours of playing at low volumes, what is the chance that any damage was done to the amp?

Swapping 6V6s into a "6L6" amp can cause almost immediate damage. Incorrect bias means the idle current is too high. How long you play it like that or how loud you play it almost doesn't matter.

 

[Cleeve]

If the 6v6 the guy used were JJ maybe things aren't such a big deal- I just slapped some of those in my SLM era Reverberocket which was biased for soviet 6L6 Groove Tubes and I barely had to mess with the bias at all. Those are freak 6V6 tubes tho, the JJ.

 

[Jammintree]

I think the 6v6's he used were Tungsols. Now I'm getting nervous because, truth be told I ordered the amp prior to knowing about the tube swap without a bias bit. It should be arriving here by the end of the week. He is going to ship the amp with the stock 6L6's. Upon arrival what can I do to test the functionality of the amp, in addition to the obvious turn it on and see how it sounds routine?

 

[Beachbum]

The amp does have a bias switch, which can be set for 6L6 or EL34's. But to my understanding this guy just swapped out for 6V6's and never did a thing to change the bias.


What does the peanut gallery think?

That comes as a surprise to me. I've never heard of the 550+ having that option. Am I missing something here?

 

[Jammintree]

That comes as a surprise to me. I've never heard of the 550+ having that option. Am I missing something here?

I think that you are correct to be doubtful of the statement. I think that I was confusing the Express plus with many other mesa's that have a two way switch for choosing the bias for 6L6 or EL34's. Regardless the amp has 6v6's put into a 6L6 amp with no bias adjustment.

 

[Beachbum]

I think that you are correct to be doubtful of the statement. I think that I was confusing the Express plus with many other mesa's that have a two way switch for choosing the bias for 6L6 or EL34's. Regardless the amp has 6v6's put into a 6L6 amp with no bias adjustment.

Thanks for clearing that up. I know almost zip about amp electronics but If that's the case, like others have said, I'd check it out long and hard before going with that particular amp. Of course the upside is that at least the seller didn't switch back to 6L6's and not tell you about the 6V6 journey so at least you know where you stand.

 

[Cleeve]

About the only post traumatic stress disorder i can imagine an amp having due to having the weaker 6v6 vs 6L6 types would be due to whatever normal damage happens when a screen grid melts and go boom. Since this is the way one often discovers the correct power tube has failed (flash, pop, fuse blown), amp circuits are hopefully designed to survive a few power tube deaths with only a blown fuse. The occasional screen resistor gets whacked here and there, but that's a routine thing that is easy to remedy cheaply. If the guy took out the still working 6v6 pair and had not made a habit of blowing them routinely, then I believe the amp will have no memories of anything different, heck it may have had less stress with 6v6 than it otherwise would have with the bigger tubes.

When I tried 6v6 in my normally 6L6 ampeg, the 6v6 pair was idling too cold using the bias setting that had the 6L6 pair running right, so my amp circuit was having to do less work at idle with the small tubes.

 

[Jammintree]

Well, live and learn is the lesson of the day.

So after some back and forth emails it seems I may have been mistaken. Apparently the seller had a mesa tech install the tubes and the amp didn't require a bias to make the swap. So, it sounds like the amp should be in excellent working order. The lesson is that email communication and internet communication in general is full of potential miscommunication and in the future a phone call between buyer and seller is the best way to proceed with a long distance sale.