Parafeed capacitor values

Paul Joppa · 3772

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Offline Paul Joppa

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on: July 18, 2012, 05:52:51 AM
There is some question about the parafeed capacitor value for the Paramount, so I am putting up this summary. Unlike the Paramour/Stereomour situation, the available iron options are all very similar and call for similar parafeed capacitor values.

First, the voltage rating should be a minimum of 600v. We've never experienced any problems with 600v caps. I recommend a minimum 85 degree C temperature rating; some people have used lower ratings without problems but I still can't recommend doing so.

My rule for choosing parafeed caps is 2*L/(R*R) where L is the plate choke inductance in henries, and R is the output transformer nominal primary impedance in kOhms; the result is in microFarads, uF. Usually a range from half to twice that value will give pretty good results, and I've posted periodically about some considerations that might make one lean towards one end or the other. It is something of a compromise between small-signal bandwidth and power bandwidth.

A much smaller cap will reduce the bass extension. A much larger cap won't hurt anything; it is always at least equal to a series feed transformer with the plate choke's inductance.

Here are some examples:

Paramount, stock iron, 2A3 or 300B: 24 henries, 3Kohms, nominal 5.3uF, range 2.7 to 10.6uF (ships with 3.3uF)

Paramount, Magnequest upgrade iron, 2A3 or 300B: 27 henries, 3Kohms, nominal 6uF, range 3 to 12uF

Paul Joppa


Offline Gerry E.

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Reply #1 on: July 18, 2012, 10:12:03 AM
Thanks for spelling it out.  Especially for those of us who are technically-challenged!

Gerry



Offline Gerry E.

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Reply #2 on: May 26, 2020, 10:24:50 AM
It's always interesting reviving these old threads even if there were only two posts and one of them was mine!  :)

The original owner/builder of my Paramounts had upgraded the parafeed caps to Mundorf Silver/Oil 3.3uf.  I have a chance to upgrade those to even "better" caps at an excellent price (though still expensive).  A couple of questions:

1.  The better caps are 600v and rated for an operating temperature of 80 degrees C.  This is a bit below the 85 degrees PJ recommended above.  Should I be concerned?
2.  I could go with the original 3.3uf value but also have the options to go with 3.9 or 4.7uf for a relatively small additional cost in the grand scheme of things.  Is it worth it?
3.  Given that the Mundorf Silver/Oil is already excellent, is this an exercise in a waste of money?  Note that I have not named the other caps on purpose but just know they are among the highest rated in cap shootouts.

I know questions #2 and #3 are somewhat subjective but what would you do?   Thanks!

Gerry     



Offline Paul Joppa

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Reply #3 on: May 26, 2020, 01:09:33 PM
I have 5.6uF Auricaps in my personal Paramount.

Temperature relates to longevity, and depends on the dielectric material. Keep the caps away from the cathode resistors, they get pretty hot.

Paul Joppa