Running power tubes too hot

[yyz2112]

Just had my amp serviced for the first time since I bought it used about a year ago. The tech said the output tubes were running too hot and brought the new ones down to 40ma.
I haven't played through it since the service, but before I noticed it had lost sustain. What can I expect now that it's set at a proper bias?

 

[Super Turbo Deluxe Custom]

You should expect it to play like it's supposed to.  Many times if they are running hot, they are less efficient - taking more to do less.  If each tube was running hotter, that's more amps.  Was it blowing fuses yet?

 

[swarfrat]

An improperly set up amp can do a lot of wierd things, but a loss of sustain doesn't really fit the scenario.

 

[yyz2112]

It wasn't blowing fuses, and really the only change I did notice was the loss of sustain.
If this isn't a symptom of mis-bias, what could it be?

 

[Patrick from Davis]

If you had noticed the sustain being lost over the time you had the amp, and the tubes were biased too hot, the tubes might have been dying at an accelerated rate.  When tubes go, they sound fine, til you put a new set in.  Then you notice all of the color and depth that was missing.  Truly, the bias can be set any number of positions, and a lot of times it is done by what sounds good when it is between two values that are calculated as reasonable highs and lows.  Unfortunately, tubes that are biased hot tend to wear faster.  There are others on the board that can probably give a better explanation of the situation.
Patrick

 

[Superlizard]

Just had my amp serviced for the first time since I bought it used about a year ago. The tech said the output tubes were running too hot and brought the new ones down to 40ma.
I haven't played through it since the service, but before I noticed it had lost sustain. What can I expect now that it's set at a proper bias?

40 mA sounds like a right number (for EL34s ?), although it varies depending on the particular type of power t00b (EL34, EL84, KT66, etc)
and the B+ voltage.

Typically what one will do is find the max rating and do approx ~70% of that - but there's no hard and fast rules.

I forget the actual biasing math off the top of my head.

Anyhoo, what one commonly finds with production t00b amps bought in guitar stores are powert00bs that
are biased cold (as in above example, below 70%) from the factory (more t00b longevity at the expense of crappy tone).

Biasing too hot, as mentioned by others, wears out the t00b much quicker and it's possible your old t00bs were
on their last legs (hence lack of sustain).

 

[yyz2112]

6L6s actually

I had noticed the lack of sustain for a little while,  a half dozen gigs or so, then at the very end of a rehearsal session, I got a very fuzzy, raspy, buzzy type of distortion. Put the amp on standby and looked at the tubes which were glowing blue instead of the usual orange/yellow.
Anyhow, was just kind of wondering out loud here. Does a hot bias have any effect on headroom at all?

 

[Cagey]

Depends what you mean by "headroom". If the tubes are biased too hot, they will begin to distort earlier, so you lose "clean" headroom. The overall volume doesn't really change, although at extremes you may lose some overall volume whether it's clean or dirty, as you start feeding a DC component to the the output transformer, which doesn't make it though. Although, it will heat up the transformer.

 

[Patrick from Davis]

Blue/Purple is bad.  Orangey Pink is good.  In general tubes go towards a purplish color when they get gasses in them.  Sometimes you can see a glowing cloud when the notes are hit on the guitar.  Cool looking, not so cool sounding.

The equation for the bias is...  P=IV  Yeah it is just that easy...  P = Power (Watts) or Max Plate Dissipation Wattage, V is the B+ voltage, and I is the bias current.  So for an EL34, P = 25W, at 500 V you rearrange to get 25/500 = bias current of 0.050 amps or 50 mA at 100%.  That drops to 35mA at 70% of Max plate dissipation.  Obviously it depends on the voltage coming out of the rectifier, and the Max plate dissipation wattage of the tube. 

Headroom more of an amp design issue rather than bias issue.  Certainly if it is biased really cold, like Peavey often does, it will not sound as good as it can.  Crossover distortion happens, and that generally is not a reason people get tube amps.  Fun for noisy insano fuzz, but not for well maintained tube amps.  But, headroom is of a product of amp circuitry design.
Patrick

 

[The Norwegian Guy]

My Diezel Herbert suffered from mis-bias for two years. Poppin fuses every once in a while. I started having more fresh fuses in my guitarcase than strings and picks. I lived with that for two years until I had enough. I recently got enough, and drove down to the guitarstore and had them change tubes and set the bias lower. I now got a high-gain amp that is "biased" for optimalized cleans. And I tell you. It plays like never before! I rather have the bias at 60-75 % and have the cleanest clean in town than having an amp that can tear a hous apart just by looking at the the mid cut and the gain knob on channel 3.

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