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In Reply to: RE: Tre', What's up with R6? posted by Tre' on May 20, 2017 at 22:37:35
Wouldn't the voltage drop thru r6 provide added negative bias for the upper triode in addition to bias provided by direct coupling from previous stage? The designer's chosen means of fine tuning the bias for that stage.
Follow Ups:
I was thinking one could do all that with R7? But you're probably right.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
As I interpret the designer's intentions, R7 sets the current thru the system while R6 fine tunes the operating point or voltage drop across the upper triode. Probably not a single value of R7 that allowed all design goals to be met.
Would be interesting to know whose circuit this is.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
Speaking of John Broskie, I wonder if he dislikes choke-loaded followers for some reason. I searched through his blogs for an hour or so today and discovered he has never done a write-up on a choke loaded CF. There are several places he delves into theoretical cathode chokes (zero ohm DCR) in explaining follower theory, but no place he discusses a practical, brick and mortar amplifier built using this technique. The closest he comes to even acknowledging this method is in a brief, off-the-cuff discussion 10 years ago of basic follower topologies. In that article, he states the following:
"...the cathode resistor can be replaced by a choke, with relatively high DCR, which will serve as a cathode resistor in DC terms, but function as a constant-current source in AC terms."
Broskie goes on to describe the one unique advantage to this approach, namely that "a choke will allow output voltage swings in excess of the [static] grid-to-cathode voltage. " That's a critically important concept, but one that's easily overlooked because he neglects to flesh out its underlying value. Specifically, this characteristic of the choke's operation allows the cathode signal to swing negative , greatly increasing the signal voltage and current available to drive the load, and it does so without the need for a negative power supply.
I'm actually rather surprised by Broskie's failure to further explore this technique. He otherwise seems to be an avid proponent of cathode followers, and he has devoted considerable space to their pursuit.
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Buy Chinese. Bury freedom.
I asked because I faced precisely this question back when I wanted to implement a CCS for the CF driver stage of my OTL amplifiers. I needed the resistor in the R6 position in order to drop some voltage that would have otherwise been too much plate to cathode voltage for the CF tube above it. (The CF is strung between the upper and lower poles of a positive supply voltage and a negative supply voltage, respectively, for a total of nearly 800V between pos and neg.) I did install a resistor analogous to R6, but I have always wondered whether having a resistance there compromises function of the CCS to linearize performance of the CF.
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