Topic: tube biasing

[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...