Rich wrote:
>>Luck doesn't matter so much here.
>>Tubes like 8877 are inherently good under this perspective and do not
>>require VHF suppressors, other tubes aren't stable and suppprssors must
>>be installed, that's all.
>
>** Not quite, Mauri. The 8877 has about 2/3 as much feedback-C as the
>3-500Z, but it has about 2x as much gain at 100 - 150MHz. 3-500Zs have
>a history of vhf parasite problems, so the 8877 should not be
>absolutely immune to oscillation at vhf/uhf.
The 8877 is well known as a *stable* tube for amplifiers at 50MHz and
144MHz.
With the input and output both tuned to the same frequency, and
typically 15-18dB of on-frequency gain, it's stable. If you provoke it
by removing both the input and output loads and swinging the input and
output tuning, a well-constructed 8877 VHF amp will still not oscillate.
Why? Because the 8877 has a grid ring that makes a very direct,
low-inductance connection to the actual grid inside (a sheet-metal cone
which very effectively shields the input from the output). If you ground
the grid VHF-style, with finger-stock contacts directly on to the grid
ring and fixed directly to the chassis, then the tube will not oscillate
at VHF. This applies equally to an 8877 in an HF or a VHF amplifier.
For glass tubes without a grid ring, such direct grid grounding is not
possible, which is why we have to use Plan B - parasitic suppressors -
to reduce gain at VHF.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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