On Oct 10, 2005, at 11:32 AM, David G4FTC wrote:
> Ian G/GM3SEK wrote:
>>>
>>> The decoupling ?C? will provide the RF return paths for the input
>>> and output but provide the DC isolation.
>>>
>
>> But only at the risk of RF instability in each amp.
>
> Agreed - it would need some care in the layout etc. However having
> said that remember that the SB-220 series amps use a choke/capacitor
> combination between the grid and the chassis. That's a combination
> which I would have thought would have been a recipie for all manner of
> problems - but it somehow seems to work.
The difference between the grid's resonant freq with the stock
choke/capacitor grounded grid and a direct Cu strap grounded grid is
about 1MHz --- with the RFC/C grounding method being the higher of the
two because the XC in the 600pF tends to cancel some of the XL in the
grid, thereby raising the resonant frequency a bit.
>
> There must be some reasoning why Heathkit used that arangement but I'm
> afraid it's lost on me.
Heath got the idea from Bill Orr. If the RFC had been more
frangible, it would have prevented filament/grid shorts during the
220's intermittent 110MHz parasite. In hindsight, a fusing resistor or
a 0.25A fast-acting fuse would have been a better choice.
>
>
>> I think it can be done by leaving each amp as-is, and adding a series
>> diode from each Ia meter to the common B-minus return. If you trace
>> the current paths, each amplifier's anode current returns via the
>> cathode and its own individual Ia meter. The diodes force each amp's
>> grid current to flow in its own tight local loop.
>
>> It gets more complicated when you also add glitch protection diodes
>> on the B-minus rail and around the meters. Then the diode voltage
>> drops have to be counted much more carefully to make sure the
>> circuits really do stay isolated.
>
> Sounds complicated. My arrangement is realtively simple and adding the
> glitch protection diodes shouldn't upset the isolation. Also the
> "Safety R" is a convenient detection point for PSU shut down in the
> event of an arc (assuming that the arc is from anode to grid).
I have never found evidence of this during a 3-500Z autopsy. The
"big-bang" arc in a 220 typically occurs from the plate (anode) cap of
V2 to the perforated cover above it. The less stentorian arc in a 220
is typically across the Tune-C.
> It would also be necessary to add a spark gap or similiar device
> across the "C" to both protect the "C" and provide a path for the arc
> current.
¿Which C?
>
> 73s
>
> David G4FTC
>
>
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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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