Biasing and tube life

[jem]

I keep seeing warnings that biasing the output tubes too hot will shorten tube life. I can't however, find any reference as to how much it will shorten the life of the tubes.

Instead of biasing for 70% idle current we bias for say... 80%, how short of life can we expect?

Seems to me if they still will last for a year or two (instead of ten or twenty), that would be fine if it nailed the tone. So are these warnings overstated?

Now that I've thought about it, I wonder what is an acceptable lifetime for most of us? Six months, 1 year, 5 years? Longer?

[mhuss]

New tubes tend not to last real long anyway. As long as your not red-plating, have at it, but I'd test them regularly if you're using it to gig.

--mark

[Andy Le Blanc]

observe the "design-maximum values" in your tube manuals
I have a pair of 6L6 from the early 50's that still in very good shape
but the new tubes dont seem to be the same caliber...
a guitar amp is just about the most abusive use of a tube...
I know full time giggin musicians who can still get 5 to 7 years from
a set of tubes... but they pay attention to warm up and stand by use

[Tubetwang]

i bias my EL34 Winged C matched pair at 45ma.

The only problem with my Express and Liverpool is a little microphony.

[Structo]

OK, I'm kind of new to tube amps, although I have a background in electronics these electron valves tend to baffle me sometimes.
I had a TAD 6L6 go bad and red plate after about 10 hours of use.
It was biased at 33ma.

Could someone clue me in on some terms that are thrown around sometimes?

Cold bias?

Hot bias?

If you set the bias (grid) current low that allows more current to flow through the tube, right?

Thanks

[gearhead]

http://members.shaw.ca/house-of-jim/Htm ... ables.html

Me no expert (these tube things have me baffled too), but hot and cold are the upper and lower safe ranges. As noted at that site, you might want to err on the side of caution with new manuracture tubes.

If you're curioius how tubes do work, read NEETs volume 6:

http://www.phy.davidson.edu/instrumentation/NEETS.htm

Caution: The NEETs refers to actual electron flow, and not current flow. I can't recommend NEETS enough for understanding cathode biasing.

Bottom line, your bias sets up a voltage differential between the cathode (where electrons that boost the signal come from) and the grid, which is where your signal enters the tube. The grid needs to be more negative than the cathode so the electrons don't just flow from cathode to grid; they have to get to the anode(plate) to continue on down the circuit.

Edited to add: The process of biasing is changing the cathode-to-grid voltage differential, but one of the main desired effects is a proper cathode current, which is what you're usually measuring (not the grid current).

[Structo]

OK, so to get my terms right, a hot bias is increasing the plate current and a cold bias is lowering the plate current.

On current production tubes the 70% dissipation rule seems a bit high for these, right?

That's why I was surprised when my TAD 6L6 blew.
I guess it was just a coincidence, because I had changed something on the tone stack and when I powered up, no sound, then I noticed the outboard 6L6 was red plating........
It was biased at around 33ma and according to that table it was around 50%.

[tele_player]

I think you've got to make sure you're bias circuit is correct, and accept that some new tubes just die early.

[Andy Le Blanc]

check out Aiken Amplifications tech section....
the 70% rule is valid... I've seen that in more than one spot
it keeps the valve from crossing the max watts limits when
amp is really pushed..... and you really do have to do the math
its not where you set the milliamps....it where you set dissapation in watts
a value near or at max dissapation is hot .... less is colder
if you figure watts dissapation.... it gives you a number that can be directly
corralated to tube data sheets.... also allows you to repeat a setting if you
find it to be favorable.....

[gearhead]

Appears that those tables do that for you; the math works out. 70% of the tube rated max dissipation is the hot value.

Means you have to measure the plate voltage to get it right. Be careful!

Are you sure you didn't do something when you did the tone stack changes?

[Structo]

Yeah, pretty sure.
I did pull the tubes and hooked up my homemade light bulb current limiter and there was no excessive current being drawn by the circuit.
I got some new JJ's in the mail today so we'll see.

[Phil_S]

While I can appreciate the concern some have expressed about shorter tube life, personally, I wouldn't look for much guidance around this particular issue. The reason I say this is because I think most guys look to squeeze out of the amp the most TONE they can find. Tone is subjective, but clearly linked to bias setting. I agree that running your tubes too hot is a bad idea, but how hot (or cold) is really up to you. Certainly the old 70% at idle is a very useful number and probably a very reasonable starting point. In the end, however, what's the point if the tone is somewhere else? Sometimes you may have to choose between hot bias (shorter tube life) or sacrifice tone. It won't be the case every time, but you will come to that junction sooner or later. So, what's it going to be?

To put a fine point on it, I'm not recommending the red plate method of setting bias. That's where you crank them to the red plate stage and back them off a bit. I think that's just plain silly. That suggests hottest bias = best tone. I don't believe that for a minute. Be sensible and look for the sweet spot, which will be different on every pair of tubes.

[krash]

typically a bias measurement is an evaluation of CATHODE current and not PLATE current.

The plate current can be somewhat less than the cathode current since the cathode also passes electrons to the screen grid.

Remember current really flows from the cathode towards the anode (plate). The electrons all come from the cathode and some end up going into the screen grid (screen current) and the majority end up going into the plate.

[krash]

another note about hot bias = better ... note that the "clipping" stage in a TW amp is intentionally biased very cold.

I am sure there is a lot of mythology surrounding biasing tubes and how it relates to tone. This is maybe a side effect of too many repair techs and too few engineers designing and working on tube amps.

[gearfixer]

OK, so to get my terms right, a hot bias is increasing the plate current and a cold bias is lowering the plate current.

On current production tubes the 70% dissipation rule seems a bit high for these, right?

That's why I was surprised when my TAD 6L6 blew.
I guess it was just a coincidence, because I had changed something on the tone stack and when I powered up, no sound, then I noticed the outboard 6L6 was red plating........
It was biased at around 33ma and according to that table it was around 50%.

Was it a small bottle TAD 6L6? I had a set and mine did the same thing. I would say it is a flaky tube if so.