>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.
>
** Agreed, Ian. However, VHF amplifiers do not have two resonances to
deal with in the anode circuit - wherein the tube exhibits substantial
gain. HF/MF amplifiers uniquely do -- one at the operating frequency,
and one at the VHF resonance formed by the anode-C, the anode leads, the
DC blocker cap's reactance, and the Tune-C.
>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.
>
** According to Eimac's W. B. Foote, it was discovered that the 8877
could oscillate at what appeared to be in the UHF region. According to
Mr. Foote, this resulted in gold evaporation from the grid. [see Fig. 24
on my Web site]
>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.
>
** the grid-resonance of a g-g 3-500Z is in the mid-80MHz range. What
is the grid-resonance of a g-g 8877 ?
- Rich..., 805.386.3734, www.vcnet.com/measures.
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