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Re: [Amps] 8877 Input network testing

To: jeff millar <>
Subject: Re: [Amps] 8877 Input network testing
From: R.Measures <>
Date: Sat, 17 Jul 2004 06:00:03 -0700
List-post: <>

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?


* 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.

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