>2 wrote:
>>
>>Another consideration is that 2, 8877s have 0.2pF of feedback-C. Since
>>the 8877 is capable of oscillating) well into the uhf range, this
>>amount of feedback-C is nothing to sneeze at.
>
>Two tubes also have double the input capacitance and half the input
>impedance, so the feedback situation remains the same.
** Ian -- How do you explain that 2, 3-500Z amplifiers have a greater
reputation for squirreliness than 1, 3-500Z amplifiers?
>
>> [intermittent vhf/uhf parasites have a reputation for causing
>>intermittent hv-to-gnd arcs, so lower vhf-Q suppressors might not be a
>>bad idea]
>>
>If the grid is nailed down firmly - VHF-style, using multiple contact
>fingers on the grid ring - then it's questionable whether any VHF
>parasitic suppressors are needed in a single-8877 amp for HF.
** So why did the gold-plating on the 8877's grid boil off in Figure 24
on my Web site ?
>After all,
>the same tube is quite stable in VHF amps, where the input and output
>are both *deliberately* tuned to the same frequency!
In a VHF amplifier, there is a VHF load. In a MF/HF amplifier, a VHF
load is not present in case the amplifier tries to oscillate at the
anode's VHF-resonance.
>The same of course
>applies to the 3CX800 and similar ceramic tubes with a full grid ring.
>
>However, with two tubes in parallel there is also the possibility of
>push-pull VHF resonances, so parasitic suppressors would probably be a
>good idea to kill this mode.
** However, VHF suppressors will melt down at VHF.
...
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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