*** I'm posting this for K0DD.... Reply to him at rbonner@qro.com ***
He's right, there is no reason to do that.
My 77DD amplifiers drove only the cathodes and it only takes a single 1 amp
RFC per tube to keep RF off the B-. (Radio Shack sells a nice 100uh RFC for
the job) The filament just needs to be bypassed with a .01 and a .001 (for
good measure...) on each side to ground...
This only works BELOW 30 MHZ.
Above 30 MHZ there is considerable coupling to the filament and the unit
needs a simple filament choke just like a 3-500... But I wouldn't be in
such a hurry to couple RF to it... JUST Bypass across the thing then and a
choke with caps to ground behind the choke.
Putting RF to this filament is a bad proposition. The cathode is the
emitting element, it is an indirectly heated cathode. But I sure wouldn't
want to risk ANY signal emission off the filament! It wasn't designed for
that. Sure the Cathode encloses the filament, but come on... To reduce AC
hum on signals we center tapped filament transformers and sent B- through
the taps. Why would we purposely unbalance the circuit?
Well I know why.. Henry radio did it with the 4K-Ultra, one of the first
amplifiers to commercially utilize the tube. The mistakes continue to
propagate design after design after design.
There could also be issues with the filament shorting to the cathode. But
personally I'd rather know my tube went bad than having it do weird stuff
without letting me know.
Tom's hot button is resistors and caps isolating grids from grounds... This
was done for generations of amps because Drake did it in the L-4 first,
HAHAHA. This actually destabilizes a grounded grid amp. SO nobody is
perfect
Have at it and don't B+ yourself.
Sincerely,
Bob Bonner KØDD
Bringing Mad Science to Daily Life
Sincerely,
Dr. William J. Schmidt, II K9HZ
Trustee of the North American QRO - Central Division Club - K9ZC
Email: bill@wjschmidt.com
WebPage: www.wjschmidt.com
"If computers get too powerful, we can organize them into a committee; that
will do them in." -- Bradley's Bromide
----- Original Message -----
From: <TexasRF@aol.com>
To: <r@somis.org>; <amps@contesting.com>
Sent: Tuesday, March 01, 2005 11:18 AM
Subject: Re: [Amps] AL-1500 Circuit Diagram
>
> In a message dated 3/1/2005 10:49:36 A.M. Central Standard Time,
> r@somis.org
> writes:
>
> In the AL-1500 diagram, the cathode is connected to one side of the
> heater, C7 couples RF from the tuned-input to the cathode ,and C8
> couples RF from the tuned-input to the other side of the heater. Can
> someone please explain why the heater is driven with RF?
>
> Richard L. Measures, AG6K, 805.386.3734. www.somis.org
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
>
> The most likely reason is to limit the voltage between the heater and
> cathode. If the filament was not tied to the cathode there could be rf
> voltage on
> the order of 90v peak or more between the cathode and filament. I didn't
> see a
> specification for this in the old data sheet I have here. A breakdown test
> could give a hint here.
>
> Also, if the cathode became open circuited for some bizarre reason, like
> the
> series cutoff bias resistor opened up, the cathode voltage could rise to
> near the plate voltage. This would not be good for the cathode to filament
> or
> cathode to grid interface.
>
> Was this a trick question?
>
> 73, K5GW
> _______________________________________________
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> Amps@contesting.com
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>
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