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In Reply to: RE: OK, where's the distortion, 6DJ8 knockers? posted by Allen Wright on July 05, 2007 at 06:13:57
Interesting, Allen. The only source I have for measurements of 6DJ8 distortion is a Vacuum Tube Valley, Issue 7 article by Eric Barbour. His test conditions were 48 kilohm plate resistor, 250 V B+ and 10 VRMS. He measured 3rd harmonic distortions at between 50% and 100% of the 2nd harmonic distortion. 2nd harmonic measured between 0.037% and 0.235%, 3rd harmonic between 0.027% and 0.215%. I have no reason to doubt either his results or yours, nor do I have an explanation for the apparent discrepancy - different operating conditions perhap?
Follow Ups:
> His test conditions were 48 kilohm plate resistor, 250 V B+ and 10 VRMS <
OK, but how much current was he running - these numbers don't tell us that. It needs current to become linear, the original curves show it to be extremely linear at 10mA and above - I use only 5mA so I'm not at it's best place, but many try and treat it like a 12AX7 and certainly then it's NFG!
Another MAJOR difference, a 2k49 unbypassed cathode R in mine.
Regards, Allen
Don't have any other info on the operating conditions, other than he used a negative grid bias supply with about 1 volt negative bias (range -0.67 to -1.29 V), so I assume that the cathode was tied to ground. Presuably the bias was set to achieve the same operating current for all tubes tested. He did comment that the plate current was significantly less than 10-12 ma but did not give the value. And I forgot to mention that the 10 VRMS was the output signal level.
> I assume that the cathode was tied to ground.
In that case, there was no degenerative feedback. Measured distortion could be expected to be significantly higher than in Allen's tests.
That's reasonable, although the 2nd HD levels at 10VRS out seem to be similar to Allen's tests. However the 3rd HD is significantly higher. Not sure why lack of degenerative feedback should boost the 3rd but leave the 2nd around the same level. From what I've read, feedback tends to lower the low harmonics but add higher harmonics into the mix, albeit at lower total levels.
> feedback tends to lower the low harmonics but add higher harmonics into the mix, albeit at lower total levels. <
NFB can do this, but cathode degeneration linearises the tube curves and also has no time dispersive bad effects. It makes the tube act like a lower transconduction, but vastly more linear tube
Regards, Allen
I understand that you want to make a distinction between loop FB and what happens when one leaves a cathode resistor un-bypassed. But please don't change the language.
"Degenerative Feedback" or "Local Current Feedback" is what this type of feedback has been called for years and years.
RDH4; index
Feedback, cathode-degenerative amplifiers pg 327-330
Feedback, degenerative cathode impedance pg 330
Feedback, current feedback pg 307, 312-313, 315-316, 327-330, 1236
Feedback, degeneration or regeneration at any frequency pg 342
On page 328, fig 7-20 shows a un-bypassed cathode resistor common cathode circuit. The caption reads, "Resistance coupled amplifier with unbypassed cathode bias resistor, giving cathode degeneration with current feedback."
Thank you, Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Then again, Henry often distorts the big picture in favor of isolated and irrelevant minutia. That allows him to justify different conclusions at different times.
Allen, I use "degenerative feedback" mostly because it's universally recognized. I agree that referring to it as simply "degeneration" would be more accurate (and probably less contentious).
Says H.P: "The use of modest cathode resistor will provide degenerative feedback and help to minimize the impact of tube-to-tube transconductance variations."
Sorry, TK, but the quote is correct.
"Degeneration" is just a vintage term that means exactly the same thing
as "negative feedback."
Allen is right that the resistor linearizes the tube and lowers the total
transconductance. It also makes the transconductance less dependent on
the tube characteristics (can you see why?). And it raises the plate
resistance.
There is a delay in the cathode feedback loop, but it's very, very small
and can pretty much be ignored.
I believe I invited you once before to compile a list of my so-called
lies and distortions, and we can go through them one by one and see
which, if any, of your criticisms are valid. The offer still stands.
By the way, that FAQ came from a usenet posting I wrote many, many
years ago when I was still relatively new to tubes. Many people have
copied it to the Web. I think it's mostly correct, but it's mot my best
work.
-Henry
henry, if you'd like to make any changes or addenda, please let me know.
:)
-ken
ps. long time no see. hope you're doing well!
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