Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: PharmRock on January 16, 2022, 10:55:14 am
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Well I am populating the JTM45 board today, and have been doing some research beforehand as to where to place CC resistors (if anywhere at all). Having read this article from about 20 years ago by R.G. Keen http://www.geofex.com/article_folders/carbon_comp/carboncomp.htm (http://www.geofex.com/article_folders/carbon_comp/carboncomp.htm), it seems that strategic placement of CC's, at the lowest possible wattage necessary, would help "sweeten" (in his words) the distortion from the last gain stage:
"The place to use CC's is where there's big signal - plate resistors, and ideally the stage just before the phase inverter. The phase inverter would otherwise be ideal, with plate resistors carrying the highest signal voltage in the amp, but phase inverters are often enclosed in a feedback loop. The feedback minimizes the distortion the resistor generates."
If this is the case, then it seems in a Bassman/JTM45/Plexi type of circuit, a CC would be useful as the V2a anode resistor, the one typically wired between pins 6 and 1 on V2. What about the cathode follower 100K cathode resistor? It also seems that the more conventional placement of CC's for the PI plate load resistors may not be the best place based on the statement above about the feedback loop with the PI, since Plexi-style amps generally employ NFB.
Any thoughts/suggestions are much appreciated!
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With a JTM45 type amp, I use them for the anode load resistors for the input and 2nd stages.
It’s pointless to use one as the cathode load on the cathode follower, as won’t affect the waveform. Small changes in the value there make no difference, stage gain will be nearly 1 regardless.
Yes, it seems pointless to use them on the phase splitter anode loads, especially on a JTM45, due to its high degree of NFB.
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Thanks PDF...I've now incorporated this into my layout (V1, V2a)
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... The feedback minimizes the distortion the resistor generates."
... What about the cathode follower 100K cathode resistor? ...
The Cathode Follower's cathode resistor results in 100% Negative Feedback within the cathode follower stage. That's why its output is actually smaller than the input.