Coupling caps size with grid choke?

[DHT112A]

How does one size a coupling capacitor when using a grid choke?

 

[Gluca]

I thinks it depedens on the tube Rp and its load as well. What load are you thinking at?

Ciao
Gianluca

 

[DHT112A]

What I have is a 45 SET parafeed with Magnequest anode choke and output transformer. The 45 is driven by a 5687 loaded by Gary Pimm’s pentode CCS somewhere around Rp of 2500ohms.

 

[lndm]

How does one size a coupling capacitor when using a grid choke?

With great care as you are creating a second order high pass filter. You need to be aware of the resonance frequency and the Q of the resonance, as this can potentially cause a peak in the response.

Personally, I like to size my cap by simulating, in order to anticipate this issue and then measure afterwards to be sure.

 

[kevinkr]

INDM is absolutely correct in my experience, I helped someone design a similar amplifier a few years ago, and the capacitor required to put the resonance below the audio band was quite large - on the order of 1uF or so.

Simulating your specific circuit in LTspice or similar will help you to make the right trade offs. Inserting a small resistor (typically a couple of K) in series with the choke to reduce the Q at low frequencies may help to tame the subsonic resonance. You can also do the same with the cap at the expense of slightly increasing the driver source impedance.

I didn't bother to de Q as described above and just placed the resonance well below 10Hz, with the particular choke boost was about 6dB @ 5Hz IIRC. Note that this could be a problem with LP playback - this system plays FM, SACD and CD primarily so it has not been.

Don't forget the dcr in the choke model as this is significant at low frequencies in terms of resonant circuit Q.