[Amps] DC Blocking Capacitor

[Phil Clements]

Amen, "R"! I think that so many guys over the years used whatever they had
in their junk boxes that looked big enough that a proliferation took place.
Sort of like parasitic suppressers...if they worked fine on tube type A they
should work okay on tube type B.

A typical amp running max. smoke from a pair of 3-500Z's or a single 8877
works out to a plate resistance of around 2200 ohms or so. Plugging in the
formula, 5% of this means we need a capacitive reactance in our C/Block of
around 110 ohms @ 1.8 mhz. Your understanding of 200 pf is enough is
correct, as a 200 pf capacitor would have a reactance of around 440 ohms on
160 meters. At a lower plate load impedance, say 1000 ohms, you have even
more headroom. You would have to exceed 8000 ohms plate load impedance to
get even close to exceeding the 5% requirement with a 200 pf cap.

(((73)))
Phil Clements, K5PC






Subject: Re: [Amps] DC Blocking Capacitor

The value of the DC blocking cap is seeming less critical than is 
commonly thought.  As I understand it, in most HF amplifiers, 200pF is 
enough.  However, the I-handling ability become important on the 28MHz 
band since most of the tank circulating-current passes through the DC 
blocker on its way to and from the anode..

On Apr 26, 2005, at 12:26 PM, Bob Easterbrooks wrote:

> Gentleman, What determines the capacitive value of the DC blocking 
> capacitor
> used in a convential shunt fed PI, and can the same rules apply to a 
> simple
> link coupled tank circuit?   I thank you in advance.  Bob, K6JQA