Brown Tubes?????

[randy warren]

I believe what you are talking about is the quiescent current in an output tube and that some may desire a value of quiescent current that gives an output tube a "flavor" in operation, whatever that is. That 'flavor" is something more desired by guitar players as they do not want linearity in reproduction, but a music generator. As far as linear amplification, moving the quiescent operating point does affect the longevity of the tube, Dave Gillespie has written about that wrt his EFB circuit. How much does it affect the sound? Some would say little or nothing, but I believe that shifting the bias point and thus the quiescent point does affect the overall sound, at least in my experience...

[Stuart Polansky]

Walt and Randy you are saying the same thing. The confusing thing is Walt's use of the absolute value of a number versus considering the polarity. Walt, forgive me, I know what you mean, but it comes across backwards when you say "the lower the voltage the higher the current", etc.

I know that you specifically referenced that it is a negative voltage, thus the statement is true IF we forget about polarity. Just a bit confusing, that's all.

"Randy -- with all due respect -- I have been talking about cathode current. Power tubes in my experience have a "sweet spot" with regards to the cathode current and I have found that it is not the maximum current the tube can tolerate. That said -- with fixed bias you apply a negative voltage to the grid. The higher the voltage the lower the current through the tube. The lower the voltage the higher the current. OK? With cathode bias (preferred by the late Sid Smith) the voltage drop across a cathode resistor (say about 300-ohms as a possible example) creates a positive voltage with respects to the grid potential. The cool thing about cathode bias is that it "tracks" the music flow. Many arguments out there discuss the benefits of both methods. The French company Jadis has used an old G.E. trick using both methods together. A negative voltage on the grid as a starting point together with cathode bias."

Guilty of this myself at times, I gotta think that had you worded it differently, there would be no argument. I think (hope) that most of us know that if a cathode is at or below G1 voltage, the tube will essentially run away. As you stated, it's the differential between the two, and the cathode must be positive with respect to G1.

So, without the "absolute" terms the lower the voltage, the lower the current. The higher the voltage, the greater the current. -32V on a grid (referenced to the cathode), everything else remaining the same, means less current through the cathode than -10V on the grid (G1). -10V is a higher voltage,greater than, -32V.

That's the confusing point. I think.

Stuart

[SoundMods]

A negative voltage in a circuit is a voltage that is more negative in polarity than the ground of the circuit. It can be higher -60 volts DC or lower -40 volts DC. It is not similar to a thermometer where higher is higher than 0-degrees C. and lower is below 0-degrees C. It is not the same. GEEZ guys -- forget the syntax and read up on theory.

[randy warren]

A negative voltage in a circuit is a voltage that is more negative in polarity than the ground of the circuit. It can be higher -60 volts DC or lower -40 volts DC. It is not similar to a thermometer where higher is higher than 0-degrees C. and lower is below 0-degrees C. It is not the same. GEEZ guys -- forget the syntax and read up on theory.

thanks, I'm out...

[SoundMods]

DITTO!

[chris1973]

Wow, I just spent a half hour writing a detailed and helpful reply. Nearing the end, POOF!, it disappeared!

Call me if you get the chance, Chris.

Stuart (410) 919-8750. Tomorrow is fine.

Hey Stuart, Sorry I disappeared on the post. My mom had two surgeries in two months. It's been sort of an ordeal!

Do you still want me to call you?

I really only need a short answer. All this other discussion is way beyond my comprehension!

What I did however, is I measured all the tubes with bias turned all the way down, and I set them all to a couple mV higher than the highest one (260mV). I kept it there for a month or so, and tonight I raised them to 300mV just to see what happened. Some of the musicality disappeared when I went from 500mV to 260mV, but I liked what happened tonight when I went up to 300mV.

As for the brown marks, there is nothing I can do to get rid of them, but if someone has a suggestion about changing the bias to prolong the life of the tubes, then that would be my goal.

Thanks,

Chris

[TubeDriver]

Not sure what causes that but I always assumed it was some impurity that was not trapped by the getter when the tube was produced.

I have seen it in plenty of pentode outputs tubes (EL84, EL34 etc). If I saw a tube with those marks, I just assume "used" and run hot?

Hey, can anyone tell me why my brand new output tubes have brown burn like marks developing on the inside of the glass?

I just bought them about a month or two ago and now they look sort of ugly.

Could I make an adjustment to the Bais that would prevent this from happening? It's set a 500mv where the manufacturer claims the amp performs the best.

Any thoughts?

Chris