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Subject: [Amps] ALPHA 77 SX UPDATE
From: Ian White, G3SEK" < (Ian White, G3SEK)
Date: Thu, 22 Aug 2002 08:34:12 +0100
2 wrote:
>>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?

"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
>>>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 ?
I have no idea. The evidence shows only that the gold *did* melt - but 
there is not enough evidence to prove *why*.

>>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.
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.

Say what?

73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'

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