k7fm wrote:
>The Heath SB-220 used an rf choke to ground the grid, bypassed by 3 x 200 pf
>capacitors. Interestingly, the Drake L4 amplifier used an rf choke to
>ground the grid, bypassed by 3 x 200 pf capacitors. That suggests that one
>manufacturer copied the other - or that both manufacturers copied someone
>else.
>
>It has been stated that Bill Orr came up with the idea, so it is possible
>both companies figured it was magical and copied it without understanding
>it.
It was claimed (here on AMPS a few years ago, if I recall correctly)
that Heath came under heavy pressure from Eimac to use that
capacitor-RFC circuit.
Personally I agree with Joe that all the claimed benefits were nonsense.
It's a very risky thing to try in a multi-band amplifier because we know
that any claims about "series resonance" can only be valid at *one*
frequency, while at some other frequencies the impedance will be
*higher* than could be achieved the normal way, using short copper
straps.
Same with claims about a "grid fuse". If the grid is disconnected from
ground during an arc fault, it will fly up positive and let the arc
through to the cathode. As discussed here a couple of weeks ago,
protection against arc faults can only be done in the HV supply.
Modern-day hams shouldn't try to defend every single thing that was done
in the old days... because some of those ideas weren't too good.
OK, the SB-220s, L-4s are all out there, and they mostly work so they
don't need fixing. But I don't believe this is because of the way the
grids are grounded - rather, they work in spite of that feature.
> However, my 22nd edition of the Radio Handbook (first printing 1981),
>edited by Bill Orr, has a 2 x 3-500 amplifier construction article and the
>grids are grounded directly. If he was the one that suggested it, perhaps
>he was smarter by the time this edition was published.
>
>This is a lucky engineer, when you design something wrong and it still works
>40 years later.
>
That's right: Orr could and did change his own mind... but he couldn't
change what had already been done, based on his earlier judgement.
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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