[AMPS] filament decoupling for RF
km1h@juno.com
km1h@juno.com
Thu, 14 May 1998 21:15:59 EDT
On Thu, 14 May 1998 11:08:02 +0008 "George T. Daughters"
<gdaught6@leland.Stanford.EDU> writes:
>
>hi rich et al.,
>
>rich wrote:
>
>> If the cathode is not at RF-gnd potential, the resultant feedback
>> decreases amplification.
>
>not always! it depends on the phase of the feedback that results.
>in fact, it is very easy to build an oscillator that has the cathode
>away from (RF) ground!
>
>but... rich and i both like the idea of putting a non-inductive
>resistor in the cathode return to ground (in a grid driven tetrode)
>to get some negative feedback at the desired operating frequencies;
>it helps to stabilize the amp; it makes the amp harder to
>overdrive, and it reduces the zero-signal plate dissipation.
I agree with your comments in general George but with the following
caveat.
Rich wrote an article many years ago about modifying the NCL-2000 that
way. Since I was on the original design team and had 3 of them ( still
do) I decided to try it. It was an absolute disaster above
20M....oscillation city.
Svetlana seems to be the only tube company promoting that idea but if you
look at a few of their IMD specs without a resistor I can see why. BTW,
does the Alpha or QRO lift the cathode in their 4CX800 amps? Be cautious
of one thing about some Russian tetrodes...they were never designed for
linear service. Under their original Russian designations they saw
service in phased radar arrays and other military Class C service.
Getting back to the NCL-2000, the amp is only stable as-is due to the
marginal screen bypassing afforded by the Johnson socket. If you add a
full circumference screen bypass capacitor ring THEN Rich's idea will
work but only up to 10M. Tom was the one who steered me to phase shift
problems due to the resistor leads when trying that idea on VHF.
The 8122 and similar ceramic tetrodes are full rated to 500MHz . A
half-assed magazine article can result in destroyed tubes to those not
fully familiar with amplifier details and other "gotcha's".
The point I wish to make is do not believe everything you read in ham
magazines. Repeatability is often very difficult to obtain. ARRL Handbook
articles were famous for this over the years along with never corrected
errors in some classic circuits.
73 Carl KM1H
>furthermore, (depending on the tube) it can make the amplifier more
>linear. (i.e. it can reduce the odd-order IMD products at a given
>output power level.)
>
>73,
>George T. Daughters, K6GT
>
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