>
>Isn't there also the issue of the current rating of the blocking cap?
>At higher frequencies, the tubes stray C can carry quite a bit RF
>circulating current, thus the tank will draw an equivalent amount
>of circulating current thru the blocking capacitor (in opposite
>phase to the circulating current in the tube C of course). Thus
>I would expect the blocking cap may need to carry significantly
>more RF current than would be implied by the plate impedance
>of the tube if the stray C makes up a significant portion of the
>tune C in the PI network (as in a 6 meter amp for instance).
>
// On the 10m band, the current through the DC blocking C is nothing to
sneeze at with a triode. For a largish tetrode, such as an 8170. Not
only is there the current from the tank through the anode's capacitance
(19 - 21pF) to the adjacent grounded screen-grid, there's the current
through the capacitive voltage-divider in the Breune neutralizing bridge
(6 - 8 pF). Thus, using an 8kV anode supply, the current through the DC
blocking capacitor totals c. 25A-rms at 28MHz -- the heating equivalent
of c. 100A at 60Hz.
cheers, Mike
>----- Original Message -----
>From: "2" <2@vc.net>
>To: "Tom Rauch" <W8JI@contesting.com>; <amps@contesting.com>;
><jstrohm@texas.net>
>Sent: Wednesday, November 28, 2001 5:44 PM
>Subject: Re: [AMPS] Another Stupid Question
>
>
>>
>> >
>> >> What determines the capacitance value for a plate-blocking capacitor?
>> >> By this I mean the capacitor (usually a doorknob cap) between the
>> >> plate choke and anode connection, and the start of the tank circuit /
>> >> pi network.
>> >
>> >Mostly it seems people just guess at what to use, and never think
>> >about why a certain value is required. Most of the sizes used are
>> >actually just plain silly..being far too big in capacitance..and too
>> >low in current rating.
>> >
>> >Technically, all the capacitor has to do is effect the impedance of
>> >the tube anode a minimal amount.
>> >
>> >If the anode load impedance is 3000 ohms (you have to include the
>> >choke, and do this on the lowest frequency), a 300 ohm reactance
>> >blocking capacitor would have minimal effect on the system.
>> >
>> >That means you only need about 300pF blocking C for 160
>> >meters, if the anode system's operating impedance is 3000 ohms.
>> >
>> >I remember laughing at a friend of mine who stuck a 100pF
>> >blocking capacitor (by mistake) in his 3-500Z amplifier for 160.
>> >When he changed the part to 1000pF, nothing happened except
>> >the tuning capacitor slightly changed position. Efficiency and power
>> >out remained the same.
>> >
>> // Amen, Mr. Rauch, Another Ham radio old wives' tale bites the dust.
>>
>> - R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
>> end
>>
>>
>> --
>> FAQ on WWW: http://www.contesting.com/FAQ/amps
>> Submissions: amps@contesting.com
>> Administrative requests: amps-REQUEST@contesting.com
>> Problems: owner-amps@contesting.com
>>
>
>
>--
>FAQ on WWW: http://www.contesting.com/FAQ/amps
>Submissions: amps@contesting.com
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>
>
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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