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RE: Determining the size of inter-stage coupling cap

Posted by Paul Joppa (P) on January 23, 2017 at 14:51:49

In Reply to: Determining the size of inter-stage coupling cap posted by EricSan on January 22, 2017 at 18:24:42:

A few comments:

When one turns on an RC-coupled amp, the coupling cap must acquire a DC voltage. Part of the charge comes through the grid resistor, and part comes by grid current once the cathode is hot enough. This is complicated by the need to charge up the cathode resistor bypass cap at the same time. Passing grid current while the filamentary cathode is warming up can strip the cathode of emissive power. So there is a tradeoff among the power supply, the filament, and the coupling RC time constants. I learned about this from Jac at EML.

When an RC-coupled power tube is over-driven by musical transients (as happens pretty often with flea-power SETs) the coupling cap absorbs the grid current and develops a DC bias shift. The charge bleeds off by the time constant of the cap and the grid-to-ground resistor. This shifting of the operating point is heard as overload recovery. Normally you want this time constant to be short relative to the human ear's ability to hear it. Almost nobody talks about this issue. I try for a time constant around 30mS, which is around 5Hz, only because it seems to have worked for many designs over the decades.

Chokes, including grid chokes, with metal cores do not have a fixed inductive or resistive impedance. The actual values are functions of voltage and frequency. Among other effects, at low frequencies there is a significant resistive component to the choke's impedance. This fortunately prevents the second-order bass resonance from having a large peak. It also makes analysis of the above effects complicated and difficult.

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