Topic: tube biasing

[Tim]

I was wondering how you guys bias the tubes on your amps.

The ASL is my first tube amp so this is all I know. It has a 5 position knob on it with a meter below it. The knob has positions for "off" and then one for each of the 4 el34 tubes. The meter is just like an analog vu meter and is clearly marked (green on the left, red on the right, and a little blank spot in the middle). I just turn the biasing screws until the meter is centered.

is this meter setup significantly different than most tubes amps?

[Lil Kim Jong-Il]

The conrad johnson gear is about the easiest ever to bias.  There is an indication LED and a bias pot for each tube.  All I have to do to bias the tube is turn the pot untill the LED is just below the illumination threshhold. 

[cheshirecat]

Dynaco gear is a little different... you need a VM and need to measure the Bias voltage at a particular pin on an octal socket on the front of the amp.  On the ST-70 there is one pot for each pair of output tubes (so closely matched pairs are essential), you measure the voltage from a particular pin of the octal socket to ground, when you hit 1.56V you are all set (this is based on the voltage that a new D cell battery puts out... right).

[ducati]

That sounds right for your ASL.

For my ARC amp, and every other tube amp I've had in the past, I connect a multimeter to a set of taps (one for each tube) and dial the bias pot (one for each tube) so the reader shows the correct bias (for my ARC that is 65 mV DC).

I prefer the multimeter setup or the built-in analog meter to a bias LED as I feel it's more precise, and I like a quantitative number as to how much bias has drifted since the last time I maintained the amp.

FWIW, it should be mentioned that you should always let your amp warm up for 20-30 minutes with NO SIGNAL to the inputs before biasing.

[Lil Kim Jong-Il]

The bias LEDs are great for convenience sake/ease of use.  But every time I adjust the bias, I wonder if the bias intended for use with the OEM recommended 6550s is really appropriate for the KT90s that I'm running in there right now.

When installing new tubes, shouldn't they be biased initially and then again after warm up?  Is it possible that when replacing tubes the old tubes have drifted enough that new tubes would be operating at an unsafe bias or is this not really an issue?

[ducati]

Ah, you speak of tube rolling...  A hobby in itself  

I have never bothered with setting bias on a brand-new tube until it's warmed up...  Never had any problems. 

Since you have tried this--how much does bias drift from the initial turn on of a brand-new tube to the warmed-up state immediately thereafter?

[Lil Kim Jong-Il]

Since you have tried this--how much does bias drift from the initial turn on of a brand-new tube to the warmed-up state immediately thereafter?

When I installed the 6550s or KT90s initially and set the bias, then again 30min later, there was a very slight adjustment.  Not much but enough change on warm up that the bias LED illuminated.  That was the recommended procedure by CJ.  Both 6550 and KT90 quad sets were from the previous owner so this doesn't really apply to brand new tubes.  Also the amp was already at the correct bias for the 6550s when I recieved it.

I can't recall clearly but I believe that there was not a signifficant difference between old and brand new tubes when I replaced the 6550s in an MV50 that I used to own.  But the old tubes were from the previous owner and I really don't know that they needed to be replaced - I just wanted to try a new set.  I remember that the bias setting procedure was the same and the results were the same: there was a slight adjustment after warmup with the brand new set.

My comment about drift between old and new tubes was a question/speculation.  There is a decent range on the bias pots and I'm sure they designed the range to be safe.  But in general I do not know what happens to tubes if operated at a bias out of that range and how does the bias setting need to change as the tube ages.  I assumed it would drift until it can no longer be biased within the safe range, bit I don't know.  I never had a tube fail so maybe they don't age like that.  I was hoping you guys could tell me.

[cheshirecat]

When I start with a fresh amp (new rebuild or whatever) or different set of tubes is set the pots to the middle then fire the things up wait 30 seconds or so then I measure the bias and adjust accordingly.  Then every 30 minutes or so I recheck and re-adjust for say 4 hours or so.  I usually find over the first 2 or 3 adjustments there is the most drift... On my ST-70 I have had things start at 1V and go to 1.4V (40% increase there) and also down to .70 or so.

If something is clearly wrong with the bias voltage you will get the ever popular 'glowing plates' where the plates of your tubes are glowing red.  When a tube goes bad this is what will happen, you will not be able to bias it properly, the plates will glow red and eventually you will blow either the filament in the tube due to heat and the tube will shut off (like a blown light bulb) or you will blow the fuse on your amp.

[ducati]

Definitely always keep an eye out for glowing plates.

In my experience, being off on the bias (even radically so) won't cause tubes to "lock" to that bias as you're afraid, El Barto.  They are operating outside their normal range, tho, and this is hard on the tubes for sure.

I think you're quite right that a manufacturer wouldn't have bias pots on there that have such a range of adjustment that massive problems (like causing the tube to run away) could occur...

BUT, some food for thought: I was reading this morning about the new-ish BAT VK-55 and BAT included auto-biasing.  They claim this is good to (obviously) offset sonic changes with tube wear, but ALSO to lessen wear during line voltage changes and (directly in reference to this discussion) to lessen tube wear upon start-up.

So there--I think you have a better procedure than I do

[BobW]

Definitely always keep an eye out for glowing plates.

Fact !
You should clearly hear distortion at that level as well.
The tube's useful life is going sharply downhill every second the plate glows.

If you need a manual, try posting a request on Audiogon.com or in AudioAsylum.com.
It should have the correct biasing procedure.

[Lil Kim Jong-Il]

I do have all the manuals and follow the manufacturers recommended biasing procedures.  I've had a couple of tube amps, a tube DAC and a couple of tube pre amps over the course of 8 years and honestly I have never had a tube fail.  I've replaced tubes just because I had tubes with unknown hours and wanted to see if new tubes made a difference.  So I really don't have any experience with tube failure modes.

I'll keep an eye out for glowing plates.  Is that in power tubes or small signal tubes too?  I have the power amp cage off but the preamp is enclosed.  So I can't really watch it.

Thanks for all the info guys.  This forum has been really good these last couple of weeks with all the tube gear discussion.

[jk labs]

I was wondering how you guys bias the tubes on your amps.

The ASL is my first tube amp so this is all I know. It has a 5 position knob on it with a meter below it. The knob has positions for "off" and then one for each of the 4 el34 tubes. The meter is just like an analog vu meter and is clearly marked (green on the left, red on the right, and a little blank spot in the middle). I just turn the biasing screws until the meter is centered.

is this meter setup significantly different than most tubes amps?

Yours sounds like a good method. The LED method mentioned was new to me: I would think the sharp LED trip point along with the sensitivity of the human eye would result in very accurate adjustments.

In many older amplifiers the adjustment is done by dial metering but the user must use an external meter (analog dial or digital readout) and clip the test lead sequentially into each tube manually in order to do the measurement.

Lastly some amplifers don't require adjustments - there is circuitry there forcing a preset current through each tube.   

After the amplifer has warmed up, careful adjustment of idle currents will set the operating point for the tubes. With pairwise equal currents there will be no net magnetizing current flowing in the output transformer.  But what when the amp starts passing a signal? To maintain pairwise balance in push-pull amplifiers the tubes should be matched. The first level of matching would be to measure all tubes in the same socket (or with the same bias voltage at least) and see if the DC currents match enough to form pairs of tubes. 

The next level is AC matching.

[jk labs]

My comment about drift between old and new tubes was a question/speculation.  There is a decent range on the bias pots and I'm sure they designed the range to be safe.  But in general I do not know what happens to tubes if operated at a bias out of that range and how does the bias setting need to change as the tube ages.  I assumed it would drift until it can no longer be biased within the safe range, bit I don't know.  I never had a tube fail so maybe they don't age like that.  I was hoping you guys could tell me.

The tube depends on the emission of electrons from the heated cathode.  Through use the cathode simply wears out and becomes less efficient at emitting electrons. On the meter the bias current drops.

Another mechanism for wear is that the glass/metal seals lets air through. The flow of tube current is hampered. 

I think in this same manner possibly every single mechanism behind tube aging lead to a drop in the current.  At least I can't think of one that would increase the current.

Mechanical damage or meltdown on the orher hand ...   

[macdaddy]

Definitely always keep an eye out for glowing plates.

what do glowing plates look like..?

[ducati]

Bright red color on the plates.  Normally you will see a nice even glowing orange color...  If it's bright and red, that's BAD.  Bias is either way off or the tube is going bad fast. 

I have only had this happen once--it was an amp I bought used, and one day I noticed the bright glow in the third output tube.  I am not *positive* how long it was glowing, but as soon as I noticed it I shut off the amp and replaced the tube. 

You have to be careful because if the tube is glowing, it's being run very hard...  And if it fails, depending upon your amp you may be in for a repair.

I have never had this happen to a small signal tube in a preamp, only that one power tube.

[twoodruff]

can someone provide a how to on biasing, my new integrated does not come with an auto bias or level meter, any help??

[Frank M]

can someone provide a how to on biasing, my new integrated does not come with an auto bias or level meter, any help??

READ THE DIRECTIONS!  Only the designer can tell you what the bias voltage should be, where to measure it, and how to adjust it.  All I can say is that you'll probably need a voltmeter.  If you weren't given a manual, contact the manufacturer, chances are they'll be very helpful.  Tube amps are not to be dicked-around with if you don't know what's going on inside.  We're talking hundreds of volts, and plenty of current.  If you need to use a screwdriver to adjust pots, use a PLASTIC screwdriver!

The basic approach is to first adust all tubes (probably via trim pots for each tube) by a small amount, then start the process over again.  When you adjust bias on one tube, you're affecting the bias on the rest.

[BobW]

Definitely always keep an eye out for glowing plates.

what do glowing plates look like..?

1st it would be the power tubes 99.9 % of the time. They're the one's that you're adjusting bias on.
2nd use subdued light, at first they glow maroonish-red, and then slowly "run away" to beet-freakin' red.
Guitar amps are much more likely to have that range of bias. Think overdrive, as in Marshall.   

[Tim]

anyone had blue glow on their tubes? my svetlana's had a pretty intense violet color shining out from the middle of the tube through the logo.... very cool looking but it has faded.

[BobW]

anyone had blue glow on their tubes? my svetlana's had a pretty intense violet color shining out from the middle of the tube through the logo.... very cool looking but it has faded.

Yes.  In the good old days, this was a sure sign of a tube going "soft"
Newer chemistry has internal cure for this (sometimes)
There are tubes which have "stuff" in them that makes them glow blue, because they can, and it looks cool.
If it has passed without event, don't worry, be happy.         

[Tim]

everything I've read says that it is not a problem at all

[BobW]

anyone had blue glow on their tubes? my svetlana's had a pretty intense violet color shining out from the middle of the tube through the logo.... very cool looking but it has faded.

Vacuum tubes are never totally empty of gas. And sometimes they leak and so are slowly filled with O2 and N2. This happens with age after many turning on an turning offs: the metal/glass seals struggle to form perfect seals after thermal cycling and different coefficients of thermal expansion (ref water in bottle freezing): leaky paths form.   

You will get a blue glow when electrons strike metal objects or when the electrons bombard and ionize the gas inside the tube.

A strong blue light indicates the presence of excessive gas. Either due too poor vacuum, compromised seals or just plain old age..
Is it bad to have gas inside the envelope? One failure mode is that the oxygen oxidizes the metal parts. You see this in ordinary tungsten lamps: they fail almost instantly if the evacuated glass envelope is punctured.  But tube filaments are not run nearly as hot and will last a lot longer.

Here's a good intro. Stop when you get to the word "getter" and also ask yourself why is there a funny, reflective coating on the inside of some glass envelopes.  These are all about depleting the unwanted gases.

We used-to laugh at the professor who taugh us tube theory. Hell, we knew that fibre optic materiel existed.
We even saw movies of something called a "micro-processor."  The class expected the "micro-processor" to never amount to more than a passing science fiction fad.
But we knew that by 1980 tubes would be history.   

[Frank M]

anyone had blue glow on their tubes? my svetlana's had a pretty intense violet color shining out from the middle of the tube through the logo.... very cool looking but it has faded.

My Sovtek 5881's glow blue... the older they get, the less they glow.  The glow is only occurs on the inside surface of the glass though.  From what I've read, it's energy being released by the electrons when they hit the glass, slow, and release energy.  It's definitely *not* from gas within the tube, if that were the case, the gas itself would be luminous.  For some reason, only certain types of tube have the glow... what kind of tube are you talking about?  The 5881 is a beam-type tube... perhaps that has something to do with it.

[cheshirecat]

My mullard EL-34s do this as well, if you look between the slots in the plates, but from any other angle you're not like "damn, those tubes are glowing blue"  I guess the fact that my amp is about eye level and you can look directly into the tubes is the reason i noticed it at all.

I'm pretty sure this is normal operation.

[Frank M]

My mullard EL-34s do this as well

The EL-34... another beam-type pentode.

[Tim]

the svetlana's I was referring to are also el34's...

[BobW]

  From what I've read, it's energy being released by the electrons when they hit the glass, slow, and release energy.  It's definitely *not* from gas within the tube, if that were the case, the gas itself would be luminous.

The gas is luminous - where it is being struck by the electrons at the correct speed for it to become electroluminesent(sp?)
The electrons are waaaaay too small to see. Even a million of them.
It is energy given off by the gas that we see, excited by the electrons. The cathode electrons are never visible. 
I guess it's really just semantics.  Physics wasn't my bag, anyhow.
I wonder if the glass on the Sovteks is coated with some chemical that gives off the glow, like a fluoresent light.....
A ploy to get you to replace them early ?

Bottom line is don't get overly excited by new tubes showing blue, but NOS tubes might be glowing blue due to approaching failure.
Either way, it ain't over 'til it's over. Play the tubes until you hear the low output or distortion.
Or better yet, run them through a tester, if you can find one.......

And to think that I threw out at least three commercial testers back in the 80s.     

[jk labs]

Here's a good intro.

Stop when you get to the word "getter" and also ask yourself why is there a funny, reflective coating on the inside of some glass envelopes.  These are all about depleting the unwanted gases.

I'm not disputing that the vacuum is low :-)  But whatever gas is left will contribute to spatial lighting effects when bombareded by electrons. And if the light increase in intensity over time, but the bias current and plate voltage do not, then your metallic barium is either spent or buried in thick layers of oxides and nitrides.

[Frank M]

  From what I've read, it's energy being released by the electrons when they hit the glass, slow, and release energy.  It's definitely *not* from gas within the tube, if that were the case, the gas itself would be luminous.

The gas is luminous - where it is being struck by the electrons at the correct speed for it to become electroluminesent(sp?)
The electrons are waaaaay too small to see. Even a million of them.
It is energy given off by the gas that we see, excited by the electrons. The cathode electrons are never visible. 
I guess it's really just semantics.  Physics wasn't my bag, anyhow.
I wonder if the glass on the Sovteks is coated with some chemical that gives off the glow, like a fluoresent light.....
A ploy to get you to replace them early ?

Bottom line is don't get overly excited by new tubes showing blue, but NOS tubes might be glowing blue due to approaching failure.
Either way, it ain't over 'til it's over. Play the tubes until you hear the low output or distortion.
Or better yet, run them through a tester, if you can find one.......

And to think that I threw out at least three commercial testers back in the 80s.     

Regarding the blue glow, here are a few links for my/your/our edification. 


http://www.jacmusic.com/html/articles/blueglow/blueglow.htm

http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm

http://barney.webace.com.au/~electron/tubes/blueglo.htm

[cheshirecat]

Or better yet, run them through a tester, if you can find one.......

I have a hickok 600A, the tubes test very strong still, so I'm pretty sure it is normal.  The entire quad does it not just an individual tube...

[BobW]

Or better yet, run them through a tester, if you can find one.......

I have a hickok 600A, the tubes test very strong still, so I'm pretty sure it is normal.  The entire quad does it not just an individual tube...

Nice !     

[ducati]

Back to biasing...  I just installed a fresh Quad (same tube family, 6550, but different brand) last night and watched the bias during the 20-min warmup:

0 min: 43mv
2 min: 44mv
10 min: 47mv
15 min: 47mv
20 min (ARC's recommended warmup time): 47mv

So in my case it looked like bias changed the most during the first 10 mins, but still only drifted by 10%.  Therefore, I think the idea of biasing them at 0 min and then again after recommended warmup time is a fairly good one...  Bias for my amp is supposed to be 65mv, FWIW.

On the blue glow...  I have never had a power tube that *didn't* glow blue to a varying degree. 

[pfife]

maybe relevant, maybe not, but the only way I can contribute to the convo - but in a guitar tube amp,tubes glowing blue are not a good thing.