>>>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?
"Reputation for squirreliness"? Right, so you're looking for a real
in-depth technical analysis here...
If we've learned one thing on this list, it is that there's a huge range
of variability between different amp designs using the same tubes... and
even between different individual amps.
But if forced to give a general answer, I'd guess it is probably because
multiple-tube amps can have more oscillation modes than single-tube, and
also because lead lengths have to be longer.
>>> [intermittent vhf/uhf parasites have a reputation for causing
>>>intermittent hv-to-gnd arcs, so lower vhf-Q suppressors might not be a
>>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 ?
I have no idea. The evidence shows only that the gold *did* melt - but
there is not enough evidence to prove *why*.
>>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
Nope - if the grid is well grounded, a typical VHF GG amplifier is
stable with *no* input or output load, or with any combination of
input/output load and phase.
>>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.
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'