The DrP (Damped Resonance Parafeed) capacitor needs to be equal to or larger than the Parafeed capacitor. 1X is quite useful, 5X is usually the point of diminishing returns.
DrP is an option I hope more people come to appreciate.
The DrP resistor is a function of the square root of ( Lprimary/ Cparafeed ) and the ratio of the DrP cap / Parafeed cap. The DrP resistor has little to do with the plate resistance of the tube. The DrP calculator in the excel spreadsheet assumes no help from the DCR of the transformer or the plate resistance of the tube.
Peaking can occur in any lightly loaded LC tank.
…The plate resistance of the tube,
…the DCR of the primary of the transformer,
…the reflected load resistance and
…the core loss of the Parafeed transformer will reduce the Q peaking at low frequencies in a Parafeed.
I would not add series resistance to the primary of the transformer or add resistance to plate of the tube to reduce with ringing because it will
…hurt the damping factor,
…could reduced the gain of the stage and
…cause high frequency roll off.
Parafeed gain peaking only really occurs in very lightly loaded outputs like preamps and driver stages. This peaking can be a good thing, a bad thing or neither. We can add resistance across the output to help damp Parafeed ringing. This resistance can
…Reduce gain,
…make the tube work harder and/or
…change the sound of the tube.
The core loss of the output transformer (and plate choke) help damp the Parafeed LC tank. Unfortunately, I don't have a rule of thumb for the core loss at this point. The core loss at 5Hz can be greatly different from the core loss at 60 and 1000 Hz regardless of the manufacture of the transformer. I’d rather not use core loss for this damping because I want to minimize core loss for sonic reasons.
DrP is a way to add damping with the only draw back I see so far being added cost.
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Thoughts on 1-20 Hz gain peaking issues:
Because the “5 Hz” gain peaking can be excited by B+ and grid conduction, rolling off the gain of previous stages does not completely remove the risk of having problems.
1-20 Hz is a problem area with LPs
1-20 Hz could be a problem with bias recovery from repetitive clipping (either at the drive tube or forward biasing the grid of a tube on the output of the Parafeed stage.) Asymmetrical clipping is usually worse than symmetrical clipping.
1-20 Hz can be a problem with 115V line modulation. I've seen a 3 Hz B+ ripple I can't explain. Line dips and peaks could cause large excursions in the bias point. Plug a 500W lamp in to the same outlet as the amp to check for problems when the lamp is turned on and off.
1-20 Hz could be a problem with motor boating between tube stages.
IF there are more than one gain stage in a system, make sure if there is peaking that the peaking does not occur at the same frequency. Three 15 dB peaks at 5 Hz gives a total system peak of 45 dB. With multiple stages and amps, either split the frequencies where the peaks occur an octave apart or damp the peaking some how.
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Other notes
Because of the DrP resistor in series with the DrP capacitor, the Parafeed capacitor shorts out the sound of the DrP cap at almost all frequencies. This means the DrP cap can be a slightly lower grade than the Parafeed capacitor. I happen to like metal foil capacitors. I could use a metal foil (film and foil) capacitor for the Parafeed capacitor and then use a metalized cap for the DrP cap.
Note: Mike has asked for permission to post the spreadsheet in his magnetics library. After we're done tweaking the spreadsheet, I'm going to let Mike put a copy in his library.
Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.
. . .Oh!. . .Remember: Modifying things voids their warranty.