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Subject: [Amps] ALPHA 77 SX UPDATE
From: (2)
Date: Wed, 21 Aug 2002 20:33:29 -0700
>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,  

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