Will uncorrect bias setting damage tubes in poweramp?

[blackcom]

I got some new tubes into my Mesa poweramp....after a while the fuse started to go when I turned on the poweramp...then I was told that I haden't adjusted the bias for my new tubes, so I got the bias adjusted...

However, can this have hurt my tubes?

[bluesman714]

Yes it could have, depending on how hard & how long they were played & how hot or cold they were biased. If they still sound good they are probably ok, but you might have taken a little life out of them. If the printing on the tube has changed color, say from white to burnt orange or brown they could be damaged & therefore not be suitable to be put back into service. If they aren't too discolored they could still be ok but might have drifted apart in performance & may no longer be a good match. A good match is important for a push pull amplifier. If you have access to a tube matcher, check to see if they are still a good match....if so go on & play it. If you don't have access to one, then just listen to the amp, if it still sounds ok, then it probably is. In theory, if you replaced your tubes with identical branded & numbered tubes, you shouldn't need to re-bias your amplifier.....in theory!
I always check bias, even with "matched pairs" offered by Groove Tubes & others....just to be sure. You can learn to do this yourself, the peace of mind it affords is well worth the purchase price of a good digital multimeter...the only tool required to check the bias on your tube amplifier.

[Sizzleboy]

Originally Posted by blackcom

I got some new tubes into my Mesa poweramp....after a while the fuse started to go when I turned on the poweramp...then I was told that I haden't adjusted the bias for my new tubes, so I got the bias adjusted...

However, can this have hurt my tubes?

It's been a while since I owned a boogie, but both my Strat 400 and my 50/50 had a fixed resistor for bias, so could not be adjusted without replacing the resistor. You were suposed to order the proper color coded tubes from Boogie. Some tube distributors can match the boogie ratings.

5 dolars worth of Radio shack parts and I was able to install an adjustable pot in my 50/50. I never bothered with the Strat 400.

[ulysses]

Originally Posted by blackcom

However, can this have hurt my tubes?

When you adjust the bias on a tube amp, you're adjusting the idle current through the power tubes. If the bias voltage is too large, the idle current will be small. This results in some unpleasant-sounding nonlinearities in the output, but the tubes will run cool and last forever. If the bias voltage is too small, the idle current will be large. This results in much more power being dissipated in the power tubes. They run hotter, and will deteriorate much more quickly. Way too high an idle current will produce bad sound too, but in general the trick is to find a balance between good tone and reasonable tube life.

If you were pulling enough current to make your fuse blow (assuming the blown fuse wasn't caused at least in part by some other defect in the amp) then your tubes must have been running very hot. Under those conditions, tube life would be measured in hours, or even minutes, instead of months. So yes, you could have damaged your tubes. But the determining factor for whether the tubes are still good should be how they sound. If they still sound good, then don't sweat it.

[blackcom]

Hmmm....my Mesa Rectifier 2:100 poweramp manual actually says the bias cannot be changed because it's "fixed" and that fixed bias is a part of the design.....still mesa are selling 6L6 tubes with different numbers on them....

...i'm really confused now...

[ulysses]

Most amplifiers that have non-adjustable bias have what's known as "cathode bias." In this method, there is a power resistor connected between the cathode and ground. All of the tube's current must pass through this resistor. Ohm's Law tells us that the current through a resistor will cause a voltage drop proportional to both the current and the resistance. Therefore the cathode will sit at a positive voltage relative to ground. Since the grid is held at zero volts DC, the net effect is a negative grid voltage relative to the cathode. A greater negative grid voltage (bias) causes a smaller current to flow in the tube. Because of this, the tube "self biases" and settles into an equilibrium state based on that resistor value. Regardless of the particular tube specimen, the cathode current will remain the same. This is exactly how all of the preamp tubes in almost all guitar amps are biased, and it works very well but isn't necessarily the most linear method of controlling the higher current in a power tube (not that linearity is what you want from a guitar amp). I suspect that this is the bias method used in your non-adjustable amp, but I could be wrong.
"Fixed Bias" on the other hand usually refers to the other popular biasing method, which is to apply a fixed negative DC voltage to the grids, onto which the audio signal is superimposed. Of course this "fixed bias" voltage needs to be adjustable in order to accommodate different tubes. So it's fixed in the sense that it doesn't change while you're playing the guitar.
If Mesa is in fact referring to the non-adjustable Cathode Bias scheme as "fixed bias" then I would say they're using the wrong nomenclature, which is confusing and perhaps misleading. If they are using traditional Fixed Bias and simply neglect to give you an adjustment, so that you must buy their pre-measured tubes, then they shouldn't be bragging about it like it's a feature. But it's also possible they're using some other scheme, perhaps active current regulation or something more sophisticated, that is more "automatic." Considering their reputation for building amps that allow the user a wide range of topological choices, it wouldn't surprise me if they went to lengths to take the guesswork out of amp bias. But I don't know the circuit in question, so I'll quit speculating.

[Sizzleboy]

Originally Posted by ulysses

. If they are using traditional Fixed Bias and simply neglect to give you an adjustment, so that you must buy their pre-measured tubes, then they shouldn't be bragging about it like it's a feature.

This is exactly what they are doing, and have been doing it for decades. This is why I don't like boogie amps, but I like to tinker with different tubes and bias settings. It's really not a bad system for the non techies, saving a trip to the shop for a tube change. Probably cheaper for the end user in that respect. It also adds a saftey factor, as boogies have a good warranty period and a reputation for being very rugged.