On Sun, 11 Apr 1999 17:08:36 "Phil T. (VE3OZZ)" <phil@vaxxine.com>
writes:
>
>At 07:16 AM 4/11/99 -0700, you wrote:
>>
>>In planning my 2 x 3-500z gg amp, I've been looking at schematics of
>similar
>>units, and there are a couple of questions that have arisen:
>>
>>1) Some of them do not ground the grids directly, but do so thru a
>bypassed
>>choke or resistor. Why?
There are several reasons, depending upon the ckt.
Moving the parasitic reasonance is not one of them tho.
The original idea was demonstrated by Collins in the 30L1. The purpose of
the 220pf caps was to cancel out the Xl of the long internal leads of the
811A.
The RFC, when done properly provides a DC path to ground and adds a
little bit of degeneration which enhances stability. It can also act as a
safety fuse during an internal gas arc.
The SB-220 grid RFC is about 24 Ohms DC resistance. The 200pf grid caps
in the 220 were later changed to 115pf as Heathkit engineers looked
closer at the Xl.
The RFC in the TL-922 is only about 4 Ohms and does very little of
anything. Their caps are 220pf and the Japs just copied Collins without
giving any thought to it.
The directly grounded grids and no protection circuitry in the AL-82
makes for expensive damage; a repeat happening according to several
owners.
Other 3-500 amps use various iterations of the above.
73 Carl KM1H
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