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
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