thanks, Phil. Tom Rauch said that a friend inadvertently used a 100pF
D. C. blocker in a homebrew amp, and, when the mistake was discovered
and corrected, it worked no better with 1000pF than with 100pF.
On Apr 26, 2005, at 4:17 PM, Phil Clements wrote:
>
> 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
>
>
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>
>
Rich Measures, 805.386.3734, AG6K, www.somis.org
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