Okay, I'll buy this one. But I have a question. Wouldn't the low pass
filters in the radio have some effect on reducing higher frequency
energy from getting at the PA?
Martin
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
On Behalf Of rlm
Sent: 18 March 2003 20:17
To: Tom Rauch; AMPS
Subject: Re: [Amps] 813 No parasitics
>
>> Ok, you asked Skip; but let me suggest or better saying, ask, could
>> be the 30 MHz highest frequency for full ratings?, or maybe at lower
>> ratings you could have parasitics also?.
>
>As a general rule the most difficult to stabilize tubes are tubes with
>low upper frequency limits. The grid and anode leads inside such tubes
>are usually thin and long.
>
The 811A & 572B being examples. On the other hand, G-G triodes with a
low-L grid that have substantial VHF gain can also become squirrely in
HF
amplifiers. The 3cx1500A7, 3cx3000A7 and the 3cx6000A7 are examples,
This week, I heard about a 3cx6000A7 amplifier project that was being
driven directly by a FT-1000D. The amplifier made a bang. At the same
instant, the finals shorted in the 1000D. The lesson: a G-G tuned input
circuit not only acts as a low-pass filter to keep VHF harmonics out of
the amplifier, it acts to keep VHF energy from the amplifier from
travelling back into the radio and causing a problem.
-- Amplifier engineers would do well to pay attention when the
technical specification sheets say "Maximum Ratings for Amplifier and
Oscillator Service".
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
end
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