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Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please educate me: Tube testing, the life test, and Grid Leakage
Please educate me: Tube testing, the life test, and Grid Leakage
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Old 15th October 2012, 10:04 PM   #21
Uunderhill is offline Uunderhill  Canada
diyAudio Member
Join Date: Dec 2008
Originally Posted by DF96 View Post
gm and plate resistance are related: Rp = mu / gm

Measure any two of these and you can calculate the third. You could measure Rp by varying Rs in your circuit and seeing how gain varies.
Then, knowing Rp, you can calculate gm from your results.

Seeing this response was a good moment. I think it puts many things together.

for a tube - the small signal parameters are as follows

........................ mu = gm x rp

where, mu is the maximum theoretical gain for a particular tube.
and rp is the plate resistance

for the common cathode amp above, Rp is connected in the circuit, so substitute Av for mu - without a load, the gain is as follows.

........................ Av = gm x rp//Rp

However, a switch could be connected in series with a load resistor - Rs.
Measurements, could be taken - switching the load resistor in and out.

so with a load resistor

....................... Av = gm x rp//Rp//Rs

Last edited by Uunderhill; 15th October 2012 at 10:17 PM.
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Old 16th October 2012, 10:20 AM   #22
DF96 is offline DF96  England
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Join Date: May 2007
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