Resonance of PC-parafeed cap-OPT - implications for parafeed cap voltage rating?

[Deke609]

@PJ: I just came across the following statement by 6A3sUMMER on diyAudio that alarmed me:

"If the speaker impedance is high at the same frequency as the resonance of the combination of choke, cap, primary, the voltage can go higher than 2X B+ volts."

Here is a link to his post: https://www.diyaudio.com/forums/tubes-valves/361496-parafeed-se-300b-amp-cheap-2.html#post6373792

Should I or others be worried about this? I use my parafeed amps with 200R headphones connected to nominal 16R, 32R or 64R secondaries, often without a parallel resistor load to hit the nominal secondary impedance.

cheers and MTIA, Derek

[Paul Birkeland]

If you play a 60Hz tone through your amp into your heapdhones, at what AC voltage on the speaker terminals are your headphones painful to listen to?

[Paul Joppa]

1) That resonance is about 8Hz in the Kaiju, very unlikely to match a speaker resonance.

2) This happens with pentodes, not with triodes.

3) The choke/PF cap resonance is well-damped by real-world chokes and/or OPTs

In 20 years of making and selling SET parafeed amps at ~ 400volts power using 600v-rated caps, this issue has never come up. I added that in case you are like me, and don't trust the theory until it's confirmed by experience.

:^)

[Deke609]

Awesome. Many thanks.

cheers, Derek