[Amps] Grid resonance

[Richard Measures]

On Apr 7, 2005, at 11:57 PM, Steve Thompson wrote:

> Steve Thompson wrote:
>
> Sorry, should have said this was on a 8877.
>
>> A had a few minutes before leaving the office yesterday, so I fired
>> up the network analyser. I made some rough, not precision,
>> measurements and, as these were done on a cold tube lying on the
>> bench I'm uncertain as to how useful they are.

The 8877 needs to be installed in a socket on a flat metal sheet - with 
the grid grounded via an Eimac grid-grounding collet.   The needed 
measurement is to find the freq. where grid-Z is maximal.

>>
>> I measured the impedance between the grid ring and one cathode pin.
>> At low frequencies it's capacitive. As the frequency increases the
>> inbuilt inductance comes into play and series resonance occurs at
>> around 250-270MHz. The impedance then goes inductive until parallel
>> resonance at about 1.7GHz.
>>
>> I also looked at feedthrough, feeding into the cathode from a 50 ohm
>> source and measuring into a 50 ohm load at the anode. From 1-2000MHz
>> the feedthrough
>> is around -60dB except for around 250MHz and 1.7GHz where it comes up
>> to around -45dB.
>>
>> Steve
>>
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Rich Measures, 805.386.3734, AG6K, www.somis.org