On May 31, 2006, at 8:19 AM, John T. M. Lyles wrote:
> I suppose Rich is referring to amateur radio
> power amplifiers that he is 'aware' of when this
> was stated:
Not just any amp, only HF ones because VHF amplifiers do not have the
problem since there is only one VHF resonance in the anode circuit
and that's where the amp makes power.
>
>>
>> Every manufacturer that I'm aware of on this planet, except
>> Ameritron, uses a parasite suppressor on their 8877. I'm doubting
>> that everybody else is out of step and only Ameritron got it right.
>> Just how the AL-1500's designer came to the conclusion that 5000-ohms
>> of feedback XC at the tube's max freq rating could not cause
>> oscillation is a mystery.
>>
>> R L MEASURES, AG6K. 805-386-3734
>
> I will provide some other examples which don't,
> to improve this awareness. Several successful
> commercial power amplifiers that I know well do
> not have suppressors between the anode and tank.
> The Broadcast Electronics FM1.5A/B fm transmitter
> which i helped design back in 1983
What freq. range?
> with Bill
> Harness and Geoff Mendenhall in Quincy, IL, ran
> 8877 in class B in a transmission line circuit at
> 1500 watts TPO, and didn't require a suppressor.
> The RF plasma Products HFS-1000G 13.56/27.12/40
> MHz industrial RF generator had none either. I
> converted one of these over to 5 MHz operation in
> October of 2002 ( see archives here for my
> comments then). It worked beautifully and had no
> instabilities. With this 5 MHz amplifier, the
> idling current (Zsac) was set for class AB1
> setting. I ran it into horrible mismatches at 1
> kW output, adding an AGC loop to maintain full
> output even into mismatches, as I was testing
> ferrite cores for a new synchrotron accelerator
> design, and needed consistent voltage across it
> as it swept frequency and tune.
>
> I built a 100 kW class A pulsed amplifier here at
> work in 1998, using a push pull pair of Thales
> TH555A tetrodes, each rated for 200 kW in the HF
> range. Using ferrite cores mounted around the
> socket (basically bolted to the screen ring) this
> amplifier was also stable without drive and
> mistuned.
In my experiences, intermittent VHF parasites are immune to mistuning.
> I originally designed a parasitic
> suppressor using ferrite tiles made by Fair Rite
> company, that sat bolted within the fold of a
> hairpin inductor in the anode to tuning cap
> strap. This worked well on the cold sweeps using
> a network analyzer, in getting rid of a pesky
> high Q peak at 17 MHz. However, under power, the
> amplifier proved that it was not needed, so the
> parts were not installed. The amplifer runs at
> 2800 KHz.
>
> I am not saying that every amplifier can be built
> without suppressors, but the blanket statement
> that every 8877/3CX1500A design has them, is
> unrealistic and also not correct.
Name another Ham, HF-MF 8877 amplifier besides the AL-1500 that uses
no VHF suppressor.
> Layout and
> component selection of the final tuning capacitor
> and tank circuit, as well as input network
> design, affects amplifier sensitivity to
> parasites, as well as the G-BW product of the
> tube, as Rich also refers to - for the 8877
> triode.
>
> Keep up the good work, I rather enjoy discussions
> over arcane topics like parasites (which are the
> bane for every tube power amplifier designer)
> instead of how to wire AC circuits in a ham
> shack. Although, I am about to embark on a new
> garage project - planning to have 220VAC single
> phase outlet in the back for you know what....
Mmmmm. You are interested in pottery and you needl an outlet for a
240V-50A electric kiln?
cheers
>
> 73
> John
> K5PRO
>
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>
R L MEASURES, AG6K. 805-386-3734
r@somis.org
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