How And When To Tube Roll...

Aw gee gentlemen, please!

According to my Oxford Dictionary 'argue' commonly indicates an effort to convince another side of your point of view, not necessarily of the truth! And 'bias' appears to be 'slanting' an existing condition to a different 'value' in this case.

Over-bias would thus mean too high a bias (voltage) or vice versa. Regarding quiescent current in semi-conductor circuits ..... but this is a tube thread! So not to discombobulate matters further.

Regarding tube rolling, it is often overlooked (or not understood), that there is a spread in same number tube characteristics. In present times experience shows this to be as large as + and - 15%, and sometimes as high as + and - 30% with the same brand items! (I am talking of batches of 20 - 30 items at a time). That makes it rather meaningless to state a preferred tube brand - what were the exact characteristics of the tube you 'rolled'? Some decade ago there was the then often encountered 'tube rolling' among guitarists. One gentleman was careful to also mention the current drawn together with the subjective experiences had. The difference between tubes causing an audible difference ..... was a factor >two! These were in a voltage amplifying stage; no wonder the sound differed if one contemplates the work point changes in the same circuit!

One would accept that manufacturers would quote electrode voltages using bogey tubes for their data sheets. This, together with component tolerances between models, might account for a sometimes mild horror show of different conditions.

You can 'roll' tubes and/or components in your amplifier to your heart's content; you have paid for it. But let us be practical and sober when glibly claiming that brand A is better than brand B (full stop!) on the basis of one/two-off examples. (I would still moderately assume that designers knew what they were doing when designing a circuit .... )
 
According to my Oxford Dictionary 'argue' commonly indicates an effort to convince another side of your point of view, not necessarily of the truth!
I'm convinced you'll find that seasoned tube amplification designers like Roger Modjeski do understand the truth. :)

If you're not aware of his products, you should be.
 
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The first hi-fi tube rolling I have come across was Kiebert in 1952 showing IM distortion variation between 'rolled' tubes when tweaking a Williamson style amp. That obviously didn't catch on as a frequently used technique, as Kiebert would have been using exotic lab equipment, but nowadays, measuring THD and IM distortion is just so easy with a soundcard setup and software.
https://dalmura.com.au/static/The Williamson Type amplifier brought up to date.pdf
 
It's been a real education, getting this thing. I'm so happy to have the resources of AK at my disposal.

You are lucky to have a built in meter with a place clearly marked to set it. That makes this whole discussion moot, though it has been interesting. I only got involved because I am so tired of the term BIAS being misused. How hard is it to learn a word and use it properly?

Does your manual anywhere state what current is being registered at the designated marking? We only care as it infulences the life of the tubes.
 
Yet your quote said, "Unfortunately the loose language of current audio..."
And, "I would hope the readers of this site have the sophistication..."
Plus, "There are some things you can learn from the internet and some things better to read in a book that was edited by peers in the field. "

So,let's review. My language in audio terms is loose. I don't have proper sophistication. I didn't learn electronics from a proper source.
But you're not trying to argue??? You didn't me much of a chance. Plus, this isn't your thread, but the OP is sidelined now.

I do not believe I singled you out in any way. Your understanding is shared by many tube amplifier owners. However that does not make it correct.

How would you resolve the conversation between a person who says my KT88 (or any of the family) amplifier is biased at 30 mv and another whose is biased at 300 mV for the same tube when in actuallity they are biased at the same level? In both cases the grid voltage (bias) is the same, not 10 times different.

May I ask where you learned about bias?

The OP seems happy with what he has learned. I have happily started another thread just about bias to see where the difficulty lies.
 
I do not believe I singled you out in any way. Your understanding is shared by many tube amplifier owners. However that does not make it correct.

How would you resolve the conversation between a person who says my KT88 (or any of the family) amplifier is biased at 30 mv and another whose is biased at 300 mV for the same tube when in actuallity they are biased at the same level? In both cases the grid voltage (bias) is the same, not 10 times different.

May I ask where you learned about bias?

The OP seems happy with what he has learned. I have happily started another thread just about bias to see where the difficulty lies.
Only a beginner would state the bias as a millivolt reading. So 30 mv or 300mv is dependent on the cathode resistor value chosen.

I have been an Electronic Tech. since 1976 and an Engineering Tech. since 1984. I have worked in Consumer, Industrial, Medical and Military electronics. My study of tube technology started in 2003 so its been 15 years now that I have been learning. During that time, I designed a 6550 SE amp and that's really when i learned about proper biasing of output tubes. I generally set them to the 70% Rule but i like to listen at 75% because there 's a bit more 'fatness' to the Bass region.
I 'm sorry this got crazy. You angered me so much yesterday i could feel it in my stomach. But I think it just came down to semantics. Sorry if my using Bias Current pissed you off. But this is a decent, civil website. We strongly discuss, debate but don't argue.
 
Only a beginner would state the bias as a millivolt reading. So 30 mv or 300mv is dependent on the cathode resistor value chosen.

I have been an Electronic Tech. since 1976 and an Engineering Tech. since 1984. I have worked in Consumer, Industrial, Medical and Military electronics. My study of tube technology started in 2003 so its been 15 years now that I have been learning. During that time, I designed a 6550 SE amp and that's really when i learned about proper biasing of output tubes. I generally set them to the 70% Rule but i like to listen at 75% because there 's a bit more 'fatness' to the Bass region.
I 'm sorry this got crazy. You angered me so much yesterday i could feel it in my stomach. But I think it just came down to semantics. Sorry if my using Bias Current pissed you off. But this is a decent, civil website. We strongly discuss, debate but don't argue.

Thanks for the apology, sorry I upset you. Its nice to know your background.

Thanks also for defining the 70% rule which I was wondering about. So I assume you set the idle current at 70-75% of the manufacturers spec? Don't do it on an RM-9, Im only doing 12.5 watts per tube to get my 10,000 hours and that's enough. I like to set the idle current on a ST-70 to about 75% but then I worry about the voltage on the main filter getting too high.

Since you are skilled in this art I invite you to help me, if you care to, get this matter solved amongst as many tube amp owners as we can. If you know the 100th monkey story when the 100th guy gets it then everyone does.

In simple terms the message is this:

1. We adjust the negative bias on the control grid of the tube to get the idle current into the range we desire (for whatever reason).

2. We are aware that the bias voltage will be different for every tube.

3. Depending on how many bias pots we have we may need matched pairs or quads if there is only one pot per channel. It is always best to have matched tubes for other reasons...

Three is an interesting topic that we can move onto when we get everyone on board. Im not giving up. Come help.


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That's used as a guide for PP amp AB operating points, but not usually for SE amps.
True, i found that out later. But i didn't want the Pd to be at 100%. As it is, the 6550 is dissipating 28w, producing 7 RMS and 12 w peak. This was my first solo amp. design.
 
I'm convinced you'll find that seasoned tube amplification designers like Roger Modjeski do understand the truth. :)

If you're not aware of his products, you should be.
Audiovet is based in South Africa and is a highly respected EE in tube and related audio fields there - as well as here. I think he was using a little humor to lighten up the discussion.
 
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