Effects of too much heater voltage?

[daz]

On preamp tubes, exactly what symptoms will too much heater current lead to in tube failure? In other words, will the tube fail right away or will it just begin to sound bad slowly till it sounds downright horrible? I ask because i have always found preamp tube to last darn near forever. But in my amp with 7.1-7.2v on the heaters there have been several occasions where i thought it wasn't sounding as good as normal and would swap a known good 12ax7 of the same type in the slots only to find one of them usually did sound bad. I've never had that happen to any amp more than maybe once, so either it's just coincidental of maybe the heater voltage IS to blame?

[Steve Conner]

Hi daz

6.3 volt tube heaters have something like a 10% tolerance, so they should take up to 6.9V.

I guess it's not impossible that overvolting the heater could prematurely cook off the mojo. You should be able to measure something wrong, though. If you've killed the emission, the plate voltage on those bad sounding tubes should measure higher and the cathode voltage lower than normal.

I can't see how the tube could fail straight away, unless the heater actually blew. But then you could see that it wasn't glowing and there'd be no sound at all.

[daz]

I'm not saying they fail straight away. Just that i have had a few start to sound bad and i put another in and it then sounds fine. But thats only after a fairly long while i think. I'm thinking this may be why the amp sometimes goes from great to not so great and not my ears having a bad day as often as i thought. So that got me wondering because on other amps i'd generally stick some ax7's in it and i could play it like that for years w/o any of them starting to sound bad. In fact, i can't recall ever hearing a ax7 start sounding bad, only failing in some obvious way like microphonics or hum etc etc.

[Hardtailed]

I can't find it right now, but I remember seeing a graph that showed the expected lifespan of a tube in relation to the filament voltage. The more you went above (or under) the 6.3V +/10%, the quicker the lifespan diminished, the effect being worse when it's overvolted.

I wish I could find it again...

[g1]

That seems pretty high. I would agree it will reduce the lifespan of the tube.
Light bulb filaments are now 130V rather than the 120V they used to be as those don't last very long with modern AC voltage (about 125V where I am).
Not just a filament issue either as I believe the graph Hardtailed mentioned was for overall tube life expectancy.
Also may be more noise. I remember playing with the heater voltage adjustment for a tube mic once. The increase in noise just from 6.3 to 6.8 was quite noticeable.

[m-fine]

The heater filaments wont burn out, but the extra heat can burn off the coating on the cathode and also release trapped gasses which can affect current flow between the elements. It makes sense that the tube would sound bad long before it actually fails.

[R.G.]

I can't find it right now, but I remember seeing a graph that showed the expected lifespan of a tube in relation to the filament voltage. The more you went above (or under) the 6.3V +/10%, the quicker the lifespan diminished, the effect being worse when it's overvolted.

I wish I could find it again...

"Getting the most out of vacuum tubes", Robert Tomer, quoting some work that GE did on its receiving tubes.

For oxide coated cathodes, there is a sweet spot where you get enough emission to meet any conceivable need and keep a healthy space charge cloud shielding the cathode, and not so high that it causes changes which eventually poison/kill the cathode. The nominal operating voltage puts the nominal operating temperature in the middle of the sweet spot. Overvolting the cathode runs it extra hot, cooks off lots of excess electrons, and increases the speed with which the cathode ages, in addition to the outgassing stuff. It can also lead to faster than normal mechanical and evaporative aging of the filament wire itself as it heats up, expands and contracts, and concentrates emission and heat at microscopically thinner spots.

Undervolting retards the mechanical and thermal aging, but cuts down on emission. However, as long as there is enough emission to provide a healthy space charge cloud in the actual use, there is a net gain in life from the slowed-down aging of the filament and cathode. It's when you get so low that the space charge cloud is not maintained that the cathode is no longer shielded by it and deteriorates faster. By cutting emission, you're inducing a "high current" condition even at low currents by removing the space charge cloud with low emissions.

Modest undervolting cuts total emission and gain slightly, but provides a longer and possibly more stable lifetime - when not driven by guitars which have been boosted to 50V signal peaks by outboard boosters, probably.

Many conservatively designed tube radios worked for a lifetime of several decades with no change in tubes needed as a result of conservative design. We can still do that - but probably won't.

[daz]

Well, i swapped out the ax7's and nothing changed. Amp is sounding pretty bad even tho i recently did a tweak that made a great improvment. Happy as i can remember being that day, next day i was bumming as the amp sounded WORSe than it has in a very long time. BUT, i took out the new EH EL34's from tube depot that have maybe 15-20 hours on them at bedroom levels and stuck the old set of the same brand/model back in and WHAM....that great tone is back ! I checked the bias on the new tubes and they were right about 40 Ma where i adjusted them last. (have dual bias setup) The old tubes were off a bit but sounded great. biased them up to 40 Ma (i prefer the sound when matched ! Am I the only one?) And even better. Do you think tube depot will consider a return? If not that will likely be my last tubes from him because the last 2 set drifted so far apart i had to take the closest two of the four and use the others as emergency spares. Not to sure about that place !

[R.G.]

Light bulb filaments are now 130V rather than the 120V they used to be as those don't last very long with modern AC voltage (about 125V where I am).

I remember my open-eyed stare when I was first informed that incandescent lamp filament life varies inversely with the *thirteenth power* of the applied voltage. Even minor overvoltage cuts incandescent lamp life like crazy, because they're already run so close the the edge of the material's capability.

[Hardtailed]

Do you think tube depot will consider a return? If not that will likely be my last tubes from him because the last 2 set drifted so far apart i had to take the closest two of the four and use the others as emergency spares. Not to sure about that place !

I've had a problem with EL34s drifting after a few months, but mine were JJs from AES (tubesandmore). I actually bought my last tubes from Tube Depot because of that.

My amp still sounds great though, but I have to go very high on one tube to get the other high enough to make the crossover distortion almost invisible (I don't have a dual bias setup...)

I know AES know offers burned in JJs. Wonder if that help.

If anything, the only time I bought EH EL34s (from neither of those places), they blew something in my JCM2000 within a few days and I had to bring it back to Marshall to fix it. I've never had the courage to try them again ever since. It's been JJs since, and apart from the drifting on that one pair, it's been a love story!

[daz]

I've had two other pairs and never had a problem like that. Even in this amp where i've used one of them for the last year. I only bought a new set to have spares and put the old set in the kit foe emergency. Of course now they're back as my main tubes till i get another set. But i contacted tube depot and they have a 90 day warranty so it should be good. I only tried JJ 34's once years ago. probably 10 or thereabouts. Don't know if they've changed, but at that time they sounded very lifeless. the top end was just dead. I'd like to try them again but i don't want to chance the same thing. I'm sure they changed them since then tho, or at least i think i've heard that somewhere.

[barfoden]

I've had two other pairs and never had a problem like that. Even in this amp where i've used one of them for the last year. I only bought a new set to have spares and put the old set in the kit foe emergency. Of course now they're back as my main tubes till i get another set. But i contacted tube depot and they have a 90 day warranty so it should be good. I only tried JJ 34's once years ago. probably 10 or thereabouts. Don't know if they've changed, but at that time they sounded very lifeless. the top end was just dead. I'd like to try them again but i don't want to chance the same thing. I'm sure they changed them since then tho, or at least i think i've heard that somewhere.

I just returned a cheap but wonderfull little guitar amp head as it was running way too hot on the heaters. Those poor EL84 tubes and the single 12ax7 was running at 7.3V to 7.5V DC (DC filtered supply). Also the idle dissipation was really high on the EL84 tubes, which I do not know if that is a result of the high heater voltage increasing the gm.
But the bias resistor is also very low at 91 ohm, when the amp operates its EL84 at 328V plate and 281V screen.
I could tell that the tubes were stressed.. The sound was way to aggresive and broke up early on the dial, granted the preamp does not have a ton of gain.

[Gingertube]

This is a copy of a post I made on another forum, haven't edited to be more relevant etc.


The way a cathode works is that emission increases exponentially with temperature towards a limit, such that at absolute zero (zero Kelvin), no electrons are emitted; at room temperature, a few escape. At glowing temperatures, there are enough electrons being "boiled off" to make use of. Due to the exponential curve, you gain very little emission by increasing temperature past where it really gets going.

That means you get no performance benefit from running heaters at higher than specified voltage, only lifetime reduction.

I still can't get a sensible consistent answer about running tubes at lower than recommended heater voltages. Many say tube life is reduced, many other say the opposite.
I would suggest that you get longer life, lower transconductance and perveance (that is lower current capacity). Many highend HiFi Preamps run heaters at lower than recommended voltage. This means that the tubes do not meet full spec when new but maintain that "not quite new spec" way longer than the expected lifetime when operated at full heater voltage

Broskie had a few things to say about heater voltage here:
http://www.tubecad.com/july2000/
If you keep clicking "NEXT" to the 10th page there is a graph of tube transconductance vs lifetime at 5.04V, 6.30V and 7.56V heater supply.
Direct Link http://www.tubecad.com/july2000/page10.html
The 6.3 and 7.56 volt curves show the transconductance falling off by 25% fairy quickly over the first 1000 hrs of operation with the 7.56 curve falling off at a higher rate after that. The 5.04 volt curve starts at that 25% low point and basically stays at that point way out to over 5000 hours. Passed the 1000 hour point the 7.56V and 6.3V curves have actually fallen below the 5V curve.
The old tube guys recommended changing a tube when transconductance dropped below 70% of new value but that was because circuits were designed to operate with tubes down to 70% of nominal value - so 70% of new transconductance was considerd to be the end of life point.
If you design the circuits to work with a lower transconductance to start with then it seems you can get VERY extended tube life by running at lower voltage.

Some guys say that you get reduced tube life at low heater voltage and talk about things like cathode poisoning. I think this just applies to big transmitter tubes but possibly to audio output tubes too.

I would therefore not hesitate to run preamp tubes at lower voltage but would perhaps try to keep output tubes near nominal voltage.

Cheers,
Ian

[Chuck H]

Excellent Ian! I think you wrote that post here on another thread. not another forum. Also from another thread here:

A 12AX7 and its ilk can run for 20,000-50,000 power on hours before failing either by not enough emission, too low gain, or outright failure. Eight hours a day, 270 days a year is 9.2 years for 20k power on hours. If Acheson was on it, you're running them ((7.1/6.3)-1.0)*100 = 12.7% hot. If you lose 7% of life for each % hot, you're losing 89% of their possible life. I'm sure that Acheson was quoting a "median" number from his experience, but it's pretty gruesome.

At the time a few guys were cracking books for the info so I assume this is based on some scholarly information. Maybe the whole story isn't accounted for in R.G.'s post but it's clearly bad enough that it shouldn't be ignored. And there's A LOT of amps out there with 7+ volts on the filaments. Pretty much anything built before guitar amp companies started using 120V primaries and many amps built after that based on vintage designs that didn't correct for modern mains voltage plus any amp built with a Hammond PT up until about last year because the 260/270 Hammond iron all had 115V primaries right up until then. Add to that the popularity of NOS tubes and then consider how much use time of that limited resource has been flushed down the crapper.

To address Daz's Q about the sound, someone in that thread did mention that high heater voltage seemed to make the tubes a little higher in gain, so, maybe similar to what you might get with higher plate volts. Also in that same thread it was mentioned that the tonal difference from lowering the filament voltage isn't really significant until you get quite low. Like 5V. Regardless of the potential for some tiny tonal shift I would reduce the voltage to normal or slightly low and never consider that circuit again WRT it's affect on tone. Focusing instead on the audio path.

[daz]

Well, I've been meaning to do this but i need to get a pair of .1 ohm resistors as stated in a link above. When i do, can anyone tell me what a definite safe wattage is on my 2xel34, 3x12ax7 amp with a 4A heater winding?

Hmmm....just realized i have a bunch of .22 5 watters. If i use 2 in parallel on each leg, that would be .11 ohm. Will that be close enough and running them parallel should double wattage to 10, no?