[Amps] 8877 Input network testing

[R.Measures]

On Jul 15, 2004, at 6:54 PM, jeff millar wrote:

> I'm restoring an 8877 on 2M and it needs a better match on the input 
> network.  I've designed several input network options and now need to 
> test them.  Here's some questions that I hope someone can answer...
>
>    * According to the data sheet, an 8877 has a Zin of 54 Ohms and 34
>      pF in parallel.  For testing, I've soldered a 51 Ohm resistor from
>      cathode to ground (close enough), drive a bit of power and tune
>      for minimum reflected .  Does this work?

Jeff --- Not as well as driving the cathode since a g-g cathode in not 
the same thing as a fixed resistor -- i. e., the 8877 cathode driving R 
fluctuates from c. 8-ohms on the negative half of the driving cycle to 
well over 100k-ohms on the positive half of the cycle.  So, 54-ohms? is 
just an average.

> Can I expect the input
>      capacitance to remain the same even with the tube off?

Yes

>    * How much additional stray capacitance comes from the tube socket? 
>      The matching network isn't working as expected and I'm not sure 
> why.
>
> thanks in advance...
>
> jeff, wa1hco
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org