• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Stability question for the gurus

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PRR

PRR

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gorgon53 said:
How about this one from Tung Sol, mu all over the place, would it change your mind?...12AU7...
Click to expand...

100V, 12mA to 1.5mA:

Mu falls from 21 to 14 (1.5:1)
Gm falls from 3,000 to 950 (>3:1)

Which matters more? :rolleyes:

Isn't our JOB as electronic engineers to build "good" performing bridges, generators, or/and audio amplifiers despite not having "perfect" wood, iron, or vacuum tubes? :boggled:
 
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gorgon53

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smoking-amp said:
Woops, that should be Xc = 1/ (2 pi f C ) above

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I've been reading thru the Wolcott White Papers linked earlier and I'm finding some claims that I don't think hold up.
1) the claim that the difference of two differential gain channels cancels odd harmonics.
---- compressive or expansive distortion to the signals is still comp. or expansive when differenced
2) the claim that positive and negative Fdbk loops both reduce in gain when clipping occurs
---- looks to me that only the outer neg. Fdbk loops saturate, leaving an active inner pos. Fdbk

More reading to do. Want to find out how the OTs were made.
Click to expand...

PRR, after my response, inspired by smoking-amps writing about mister Wolcotts paper, the "mu" thing came up, please re-read

For me, "mu" is NOT constant, even it is the least variabel parameter.

If anyone chooses otherwise, it is ok with me, I am done with it:rolleyes:
 
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TonyTecson

TonyTecson

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Joined 2003
when i look at a tube, any tube, i look at its plate dissipation ratings....
then its cathode current limitations...then all else follows...

no one can really say that the tube you are holding now followed the datasheets exactly....you have to accept the fact that it varies...
 
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DF96

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gorgon53 said:
For me the case about "constant mu" is closed now.
Anyway, in audio it may or may not matter, depending on what you want to accomplish.
"constant mu" was only of interrest in respect to mister Wolcotts paper.
Click to expand...
It usually matters in audio because most audio triode circuits work best with constant mu, which good circuit design and appropriate valve choice can approximately deliver.

12AU7 is a poor choice of example, because it is well known that this valve is a long way from being an ideal triode. Over part of its characteristic it has a square law (not 3/2) which is why it can make such a good RF mixer.
 
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gorgon53

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PRR said:
I don't see links to Wolcott in this thread. There is a link to another thread, and a link to Google pictures of amplifiers.
Click to expand...
Sorry I do not have the link anymore.
Inspired by smoking-amps writings about mister Wolcotts amplifier I searched the net and found a swedisch forum where some of mister Wolcotts papers where presented. I took copies and did read through all of the papers presented. So, I have copies but unfortanatly no link.
The text I presented (regarding mister Wolcotts claim regarding mu) is a direct copy taken from mister Wolcotts papers wich I saved on my computer.
The "mu" thing I brougth up can be found under the title:

ULTRA-LINEAR D.C. AMPLIFIER

Henry 0. Wolcott, ,
Single End Push Pull
 
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gorgon53

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DF96 said:
It usually matters in audio because most audio triode circuits work best with constant mu, which good circuit design and appropriate valve choice can approximately deliver.

12AU7 is a poor choice of example, because it is well known that this valve is a long way from being an ideal triode. Over part of its characteristic it has a square law (not 3/2) which is why it can make such a good RF mixer.
Click to expand...

I said: it may or may not because of a variety of things that may influence the coice, uhk, voltage and current capabilities, miller-capacitance, avaiability, price a.s.o.

Also, some may even like the sound of it, or get inventive and put and put the TungSol "vari-mu" to good use.

But you are rigth about the qualyties of the 12AU7, usually it would not be my first choice either, unless I want to do so for some specific reason.

I took the TungSol 12AU7 as an example, not as a first choice.
I took it because it shows mu for the widest range of operation conditions I could find.
 
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nigelwright7557

nigelwright7557

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murtonpikesystems.co.uk
gorgon53 said:
I said: it may or may not because of a variety of things that may influence the coice, uhk, voltage and current capabilities, miller-capacitance, avaiability, price a.s.o.

Also, some may even like the sound of it, or get inventive and put and put the TungSol "vari-mu" to good use.

But you are rigth about the qualyties of the 12AU7, usually it would not be my first choice either, unless I want to do so for some specific reason.

I took the TungSol 12AU7 as an example, not as a first choice.
I took it because it shows mu for the widest range of operation conditions I could find.
Click to expand...

I am quite a big fan of the 12au7. It has come to my rescue in a few designs where the 12ax7 was a bit volatile. The 12au7 has a bit less gain so is easier to tame. I have used it in audio mixers and audio amps with good results.
 
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gorgon53

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nigelwright7557 said:
I am quite a big fan of the 12au7. It has come to my rescue in a few designs where the 12ax7 was a bit volatile. The 12au7 has a bit less gain so is easier to tame. I have used it in audio mixers and audio amps with good results.
Click to expand...
sure, 12AU7 is widely used in audio, and a 12AX7 can only be looked at being "constant mu" at levels you could not use it anyway.
 
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