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How does a tube's need for bias change with age?

charnich74

Member
2019-11-07 12:45 am
I'm not exactly sure how to phrase this with the proper terminology, but I was wondering if a tube's need for bias changes in a consistent direction with the age of the tube. In other words is there a common trend universal to all tubes? For example if you check bias after one year of use is there a trend to need to correct bias by increasing negative bias voltage, or decrease it? Or does it vary depending on tube type? If you were to set bias on the hot side to begin with would it tend to cool over time? Again apologies if the terminology is wrong or if this is a moronically basic question.

Cheers
 

Jazid

Member
2009-12-15 7:02 pm
In general the valve will get weaker so will need a decreased negative bias to maintain a given anode current. Of course something might go wrong with it and the opposite happens, but that's a fault. Also new and NOS valves may need frequent re-biasing in either direction as they settle in.
 

charnich74

Member
2019-11-07 12:45 am
In general the valve will get weaker so will need a decreased negative bias to maintain a given anode current. Of course something might go wrong with it and the opposite happens, but that's a fault. Also new and NOS valves may need frequent re-biasing in either direction as they settle in.

Thanks for the comment. How long would you let the tube settle in before rechecking bias?
 
Cathode emission drops with age, so for a given bias, the current will drop.
The peak current the cathode will handle also drops.

Many of the last generation of valves had outsize cathodes relative to what they actually needed, which explains how a 25CT3, a tiny noval diode can pass 230m/a, has half the voltage drop c/w the whimpy 6X4, and outlasts it by a large margin.
 

miniman82

Member
2016-12-30 11:53 pm
USA
Short answer is it depends on the tube.

Back in the day major manufacturers (RCA, Stlvania, GE, ect) had good QC and burn in procedures, so the tube was already correctly aged right out of the box and it would be quite stable. Now you have mass produced tubes made eastern bloc countries and China, with next to no QC. I've had brand new tubes redplate and bias incorrectly on a regular basis, so I watch them like a hawk until they make me sleep easy at night.

If you care about your equipment as I do you'll want to do your own verification, because in effect you are the QC department. You're the final say as to whether a given tube will perform correctly, so I always do the following before I put an amplifier or vintage TV into regular rotation:

Perform emission test on a tube tester
Put pairs/quads into an amp I don't care about first, so an arc doesn't take out an irreplaceable output transformer
Check actual bias against plate curves in data sheets, to see if they actually are working the way they're supposed to
Put them into the gear they are destined for, power up and set bias per gear's manual
Observe tube plates in a totally dark room, to make sure they aren't replating at idle (you'd be surprised how many fail this part)
If plates are red, reject and demand a replacement (only exceptions are transmitting tubes, and only if the data sheets say glowing plates are permissible)

If all of that goes well, I put audio through it and give it a torture test at high output levels all the while looking for problems. I'm very demanding of my equipment, I've paid good money for it so I expect a certain level of performance and longevity out of it. If you don't, plug them in and cross your fingers...

And yes, emission will drop with age. I usually redo bias each year, YMMV. My stuff doesn't get much use, but when it does I push it as hard as it was designed to be pushed. It's like car maintenance, take care of it regularly and you'll get good mileage.
 

6A3sUMMER

Member
2016-06-07 6:50 am
I purchase tubes that have been reasonably pre-aged by the manufacturer, and then after crossing the ocean, the local vendor ages them a little more, and then they are retested.

My experience with those tubes has been that the tube current does not significantly change for many many hundreds of hours over a 1 or 2 year period (including the warming and cooling effects of turning them on and off many many times).
I have used both fixed bias, and self bias on these tubes. The current has been stable.

So what tubes are you using?
What are the operating conditions (quiescent current, voltage, plate and screen dissipation; and what kind of signal levels versus max power are they putting out)?

The more you abuse a tube, the quicker it settles in to a non-working condition.

Look at the lifetime of old Hi Fi amplifier tubes that operate for decades, versus a Heavy Metal Rock Guitar amp that exceeds the tube ratings of voltage, current, screen current, and plate current.
Not to mention the hard clipping nature of a run-away concert. And these tubes run Hot,
generally without proper air flow, and then have to cool from that overheated condition.
Metal fatigue?
Cathode Coating fatigue?
 
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Back in the day major manufacturers (RCA, Stlvania, GE, ect) had good QC and burn in procedures, so the tube was already correctly aged right out of the box and it would be quite stable.

If all of that goes well, I put audio through it and give it a torture test at high output levels all the while looking for problems.

I'm very demanding of my equipment, I've paid good money for it so I expect a certain level of performance and longevity out of it. If you don't, plug them in and cross your fingers...
Having just gone through a whole pile of more than 100 GEC mil spec brand new valves + many others.
I just don't agree at all with the above...."back in the day" just doesn't cut it.

My experience with NEW, is there is tremendous spread in production....
No aging or "burn in" has ever taken place...
You have a lot of valves in the middle of the curve, some qty weaker, a good quantity stronger, and at the other end, valves which are impossibly strong, so they get hot. Sometimes they will calm down, but if after 15mins it's still not dropping, they go on one side for other duties.

I have seen this with Sylvania 8417, they have NEVER been run, and in some cases run excessively strong...had it also on NEW 70yr old KT8C...never been run ever.....and when in a typical amp runs so strong I can't match it to anything else.

When you start up one of these new stock valves you see a burst of emission to begin with, then they slowly settle.
I reckon at worst it takes 5 mins,- after this point it's just not going to move any more.
The whole ****-eyed story that "burning in takes weeks" and the sound alters is entirely imaginary...(as I can clearly see on a meter) but there, you have to humour people who believe in fairy stories...
 

Jazid

Member
2009-12-15 7:02 pm
Fairy stories are sometimes closer to home than we realise. I have many, many old valves and have tried to keep as many as possible usefully working. My experience is that valves with little use for several decades are often erratic at first, especially those still boxed. The issue is for how long I persevere. I have tried burning in (or out) valves on full emmissions for over 24 hours in hope, yet many remain stubbornly variable and as a technique it is flawed. Maybe outgassing, maybe substandard stock which got shelved, who knows why. Some did come right, some got binned. I'd say 50 hours is sufficient for any valve worth persevering with in order for it to settle down. I wouldn't give a new valve that leeway, it should perform to spec out of the box. But old stock is different. Not one size fits all.
 
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I have another 200 more of the finest to select out over new year.
These were made in 1973, - the best possible imagineable quality GEC-Marconi.
I don't see anything like the phenomenon you describe.

I can assure you the turnaround time of the test sequence (and NO it's not a valve tester it's an amp running at full voltage and proper bias)...is relatively long per valve, because they don't plug in...(no bases).
If I had the slightest doubt the bias would wander after 5mins I would entirely have to change the test sequence, which makes my job at least 10-12hrs longer for that quantity.

So, - as I say, what I see is repeatedly the following...A burst of high emission (say about 10 secs) then it slowly settles, let's give it 5-10mins MAX..most drop into to about 14m/a

To give you some idea of spread, voltage drop over an anode resistor of 49ohms (detection) varies to give idle currents on my test varying from 11m/a (weak) to 19m/a (too strong) a variation of 70%.

Being as I am fitting them as matched quads, you can begin to imagine I don't have huge patience with the argument that what I might be doing would be pointless or stupid.

Once I have found a pair or a quad of them with identical parameters I don't expect this to change, and on tests up to full power over a week, nothing moved at all from the first 5 minute settle down period.....
Fact is running them at proper voltages and loads, 45yrs later they behave much as you would expect.
 

kodabmx

Member
Paid Member
2011-10-31 1:00 am
Toronto
I'm not exactly sure how to phrase this with the proper terminology, but I was wondering if a tube's need for bias changes in a consistent direction with the age of the tube. In other words is there a common trend universal to all tubes? For example if you check bias after one year of use is there a trend to need to correct bias by increasing negative bias voltage, or decrease it? Or does it vary depending on tube type? If you were to set bias on the hot side to begin with would it tend to cool over time? Again apologies if the terminology is wrong or if this is a moronically basic question.

Cheers

I say it all depends on the circuit the tube is being used in. Cathode bias tries to adjust for tube wear and differences in tubes, whereas fixed bias does not.

If you run a tube hard/hot, you'll wear it out faster. In theory, a tube running at like 25% of it's total Pd will last as long as the heater. Some people require perfectly matched tube from a cryogenic treatment plant, others prefer buying a bunch of cheap unpopular tubes and matching them. FWIW I find old US sweep tubes like my 36LW6 take about an hour to "settle" every time I turn the amp on. It has a meter for each tube so I can see the biasing in real time. After about an hour, maybe a little less, when the amp is running at temperature, the bias falls perfectly inline to how it was set but there might be a mismatch of like 40mA within the first minute of power... The tubes are biased at 120mA each though, so the mismatch is "only" 16%. Still, if the design is good, most people wouldn't notice the mismatch as long as they keep it within the class A power envelope. My Soviet 6P45S tubes take far less time, and the 6P43P comes up to bias and stays there in 11 seconds, even never used old stock.

Generally, if it's fixed bias, you should check it "once in a while", unless you have a reason to check it like it starts to sound like rubbish. Bonus points for cleaning and using the biasing pots to keep them in good condition. Nothing more annoying than trying to rebias an old and and breaking the damned pot :D

OTOH, there are automatic biasing boards available for a more "set it and forget it" approach. I use several and I'm generally pleased with them.
 
I say it all depends on the circuit the tube is being used in. Cathode bias tries to adjust for tube wear and differences in tubes, whereas fixed bias does not.

In theory, a tube running at like 25% of it's total Pd will last as long as the heater. Some people require perfectly matched tube from a cryogenic treatment plant,
yes except in class B, like you are suggesting audio valves sound terrible.
Class A cathode biased amps make loads of extra distortion, get unstable with certain loads and don't deliver the power, again partly because of screen voltage effective variations.
Ultralinear is also a non starter with cathode bias because the critical screen load is constantly varying.

plus the "cryogenic" nonsense is yet another example of pseudo science.

Why?
Because, if it could be proved to have been useful, the major industrial valve manufacturers like GEC would have done it Eons ago, especially for their monster FM broadcast transmitting stuff.
They had most to gain from it.

my personal opinion:-
Yet another example of junk science being applied, on top of which you can have very little idea of what is really going on in terms of vac inside the component.
Sure as anything I wouldn't expose glass & brittle bakelite bases to extremes of temperature provoking shrinkage/expansion of all the solder joints... I have enough trouble with dry joints in valve bases as it is!

As for running the valves close to limit Pa, you literally don't have any choice there.
The point of least distortion/best sound on 6V6/6L6 and most US beam tetrodes has always been regarded as being just close to the point where it's about to melt, to the point we often wonder how those 6V6 hang in there at all in Guitar amps.

The 7591 is no different, because as I said before it's basically half a 8417,- half the gain, half the anode power, and voltage limited to 450V.

People like Scott, Sherwood et al, knew all this stuff, which is why they run them so hot.
Their previous version of Sherwoods/Scotts used EL84, which is a pentode, but they were short on power so they hammered them to death.
I reckon they were incredibly relieved when Westinghouse-Sylvania and RCA came up with the 7591.

As for biasing:-
(proof for over-driving,- the little known technique to run the 807 - 6L6oid, at as high an anode voltage and Pa as you dare)...
It's one of those undocumented areas, running the 807 around 400V, 300V screen makes a lot of quite nasty sounding 3rd/5th harmonic distortion.
Banging the anode volts up to more like 600, 650, or even 700-800V miraculously gives it an amazingly good sound, where the harmonics start to disappear.
That's because the transconductance curves straighten up.

So, if you want good sound and no cross over distortion you HAVE to run push-pull amps at at least 70% of max idle diss. No choice.

TBH, I find they usually sound even better, even optimal at close to 85%, which makes duff or sub par valves start to glow around hot spots in the anodes. :rolleyes:
 
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Jazid

Member
2009-12-15 7:02 pm
I have another 200 more of the finest to select out over new year.
These were made in 1973, - the best possible imagineable quality GEC-Marconi.
I don't see anything like the phenomenon you describe.



Being as I am fitting them as matched quads, you can begin to imagine I don't have huge patience with the argument that what I might be doing would be pointless or stupid.


Fact is running them at proper voltages and loads, 45yrs later they behave much as you would expect.

Para 1. You are using late stock mil-spec valves with flying leads - 'best possible imaginable quality' . I am coaxing often 80+ year old valves out of their slumber. That is my choice and I report what I find. I imagine you choose the valves you do to avoid such issues, and fair enough. But your findings are conditioned by your choices, mine by mine, our parameters are different and there is no contention except in your mind.

Para 2. No one is accusing you of this, what are you so defensive about? Perhaps try to stick to the point?

Para 3. This is indeed true but does not fit with anything you mention in later posts where you advocate driving output valves extremely hard for distortion and sound quality. Manufacturers service life spec applies to valves driven hard, and it rarely exceeds 10000 hours, for output tubes much less, as you well know.

Edit: Tapatalk has displayed paragraphs weirdly, now para's 1, 2,3 are all referred to as para 1 above...
 
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disco

Member
2006-04-17 6:27 pm
Holland
When you start up one of these new stock valves you see a burst of emission to begin with, then they slowly settle. I reckon at worst it takes 5 mins,- after this point it's just not going to move any more.
The whole ****-eyed story that "burning in takes weeks" and the sound alters is entirely imaginary...(as I can clearly see on a meter) but there, you have to humour people who believe in fairy stories...

What I read here and there is the observation a new amplifier gives its best the first hour, to decline furtheron and come back to optimal reproduction within weeks or even months. That effect would come down to settling components which depend on chemical reaction or mechanical construction. In case of old electrolytes I can imagine a change in the oxide coating forming and perhaps with tubes a change in the surface layer of the heater.

Emission of new tubes is understood to be a function of the distance between heater and grid, being not constant with production (less so with frame grid tubes). Perhaps the sag of the coated wolfram wire plays a role with used tubes, but certainly the heater ability to boil off electrons gets exhausted over time.
 
Para 3. This is indeed true but does not fit with anything you mention in later posts where you advocate driving output valves extremely hard for distortion and sound quality.

Manufacturers service life spec applies to valves driven hard, and it rarely exceeds 10000 hours, for output tubes much less, as you well know.
I'm using 60-70 yr old 807s in one of the amps I run (direct coupled AB2).
They are as fresh as the day they were made.

The circuit drives the input up to 20V positive, which means even g1 is conducting a decent bit of current, easy over 10m/a.
On the really big ones I have (125W with 830V HT), they doubled up the driver valves to drive them even harder.
The thing that died on them?
The HT transformers!

Hammering the living daylights out of them doesn't seem to make any difference at all, and I have no idea what they were used for previously, they are all unknown s/h stuff from a batch of 25+.
I culled some, some broke off top caps, and just a few were sub par.

Radio hams used to use them all the time, and caned them for hours at anything up to 50mhz.
I am always suprised by the 807, with it's weeny 0.9A heater, but that and the 0.45A 6V6 were state of the art and had to be carried into battlefields or stuck in bombers.

In the other pair of similar hifi amps, I'm also using 70yr old KT8s.
They have much larger cathodes, stronger heaters, and TBH, I somehow imagine they will get to 100yrs old, still going ridiculously well.

You can drive them to 75W with a square wave and they just glow a bit.
Consumer stuff just melts and blows up.

Fact is, if you use marginal comsumer grade valves you get consumer grade life.
Horses for courses and all that.
It suited the valve makers to get good stock turnover, because replacement valves were very expensive, - exactly the same rule as autoparts.

Look at it New Sensor are dead happy people blowing up 6L6GC with their guitars, it just sells more parts!

In the mil with JAN stuff, they just popped down the stores and pulled out another 30 of them.
Another century, another story.

The US manufacturers stuck their mil grade stuff on steroids when they started making colour TVs.

Fact is the cathodes are so advanced and so enormous on some of them, they will conduct one AMP at 700V.
They even quote an "overload factor" for so many uS without affecting life.
 
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kodabmx

Member
Paid Member
2011-10-31 1:00 am
Toronto
So, if you want good sound and no cross over distortion you HAVE to run push-pull amps at at least 70% of max idle diss. No choice.

TBH, I find they usually sound even better, even optimal at close to 85%, which makes duff or sub par valves start to glow around hot spots in the anodes. :rolleyes:

I find you either need to bias them hot as you describe, or as low as you can while getting rid of the notch. This also helps with GM doubling apparently. Also like 99% of listening is in the first watt. As long as it's biased hot enough to give 1W class A, it's usually good, although I admittedly have mine set up for closer to 15W class A limit (112W total RMS power)...
 
I find you either need to bias them hot as you describe,

This also helps with GM doubling apparently.
Also like 99% of listening is in the first watt. As long as it's biased hot enough to give 1W class A, ... mine set up for closer to 15W class A limit (112W total RMS power)...

I think this first watt is another one of these popular myths and legends.
Fact is, nearly all energy is concentrated in the first 100hz.
(You can FFT the result and it gives the game away)

When you start to look closely, most speakers work very badly in this area, and of course the ear being very insensitive to distortion down there, and at least 10-15dB down in response, makes a nonsense of this 1W rule.

Another thing.
The anode dissipation problem.
Class A area spread wider and wider, the more Pa you have available.
I use typically valves which have 18-19W diss for smaller amps (from small PPP quads), and 35-40W for larger amps with just pairs.

(in my case the small pairs used in matched quads have a stupidly MASSIVE heater and cathode.. 0.635A for a valve the size of your finger, and a brief check, dropping bias momentarily to zero....conducts more than 1/4 of an amp at 500V).
That's the great part about industrial voltage stabiliser valves, rather than consumer grade stuff.

If you work it out, that's effectively a composite 18W valve - conservative book figure -with a 1.3A heater....almost the power of a KT66 in a size no bigger than a large felt tip marker!
+
There's a smart reason for using PPP, especially with small valves.
It's got a lot to do with increasing transconductance & advantages in the low powered output end.
It also tends to hide aging away quite well, although I find you have to be much more careful with oscillation.

If you look at the width of that "class A" operating area, it's clear the larger amps, sitting with a pair of valves at 27-30W idle are going to be able to deliver a lot more linear class A power than something smaller....as much as 22-25W in fact, before moving into the class AB area.

I saw just a few people using 6V6 in PPP.
They are quite suprising run that way.
The miniature versions like 6BW6 take up no space and have low heater requirements.

If you listen to lots of Organ music, try running in that 'first watt".
It doesn't work!
 
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kodabmx

Member
Paid Member
2011-10-31 1:00 am
Toronto
The fact is, my speakers are 95db 1W-1m... This is excessive SPL for most purposes. This is what I mean by the first watt of power. Also, the logarithmic response of the ear means you need 10 times the power to realize 2 times the "volume".

Anode dissipation is just a number. As long as the cathode can provide the current, the glass is kept within specs (cooler the glass, the longer the tube life, apparently), and the plates don't incandesce (red plate), you're good. That's why I can run 36LW6 tubes at 35W Pd...

This is an interesting book: http://www.tubebooks.org/Books/Atwood/Tomer 1960 Getting the Most Out of Vacuum Tubes.pdf

I've build PPPP 6P1P (Stupendous amplifier on here), and my monoblocs are PPP 6P45S @ 35W each. I rarely ever use anything other than Triode connection, so oscillation rarely is an issue, and if it is, a 10R-27R power resistor on the plate of each tube solves it.

If you can't enjoy organ music in the first watt, you're speakers aren't sensitive enough IMHO. FWIW, with my Energy RC70s and my monoblocs, I can max the scale on my SPL meter, however, apparently the "Sound meter do not follow a A or C curve and readings will be too low at 1000Hz and above" It's designed for measuring HVAC noise.
 
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Actually anodes can incandesce no problem in high power tubes - normally nickel alloys are used for such anodes and they keep mechanically strong at red and orange heat. In small tubes the glass is too close to the anodes to allow red-plating without damage.

The physics of electron tubes means the bias ought to be fairly stable, as the voltage ratios between electrodes should be principally a function of the geometry of the electrodes. However the actual current does depend on the electron cloud density (aka space charge), which is affected by emissivity and thus temperature of the cathode.

As I understand it most receiving tubes (that's anything except high power RF transmitting tubes really) are always designed to have lots of emission to spare so that as the cathode degrades it makes less difference to performance or bias point.
 
Actually anodes can incandesce no problem in high power tubes - normally nickel alloys are used for such anodes* and they keep mechanically strong at red and orange heat.

In small tubes the glass is too close to the anodes to allow red-plating without damage.
Hence why 807s can be caned to death.*
They don't have the same structures, metals nor the smaller size of the smaller 6L6, never mind the Russki 6P3S-E or the 7591.

People forget also, industrial valves,- especially transmitter variants have special silica glass qualities that have no worries at all running at 250C.
(I think the STC 5B254M has this).
It's about half the size of EL34, but will eat one for breakfast in the longevity stakes.

I have industrial valves that will char cardboard, but they don't red plate, and they are tough as old boots.
The only way I could kill them was overdoing a square wave test at full chat, which melted the screens....so, well, now I know.

After that I knew they were damaged because the internals had distorted enough to make a section of the anode glow, where it didn't before.

The fact is, my speakers are 95db 1W-1m**... This is excessive SPL for most purposes.
If you can't enjoy organ music in the first watt, you're speakers aren't sensitive enough ... and my monoblocs, I can max the scale on my SPL meter, however...

This I doubt very much, but you can dream.
Nobody I know would try to reproduce a 25HZ organ note at 1m, it's impossible.
The wavelength is many 10s of feet long, so that's nonsense.

If you think you can get 95dB at 1W, down there, then that is purely imaginary at the low frequencies I am talking about.
Oh yes 400hz-1khz maybe, but not at 22-35hz.
For that, even with really CLASSY speakers you need serious power.
 
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