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Old 5th March 2013, 12:21 PM   #1
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Default How can i match power tubes for Transconductance ?

?...without a tubetester and at the right current/impedance point ?

Backround: I am using 2*2 el 34 in my power amp per channel in ultralinear/pushpull. I believe that the so called matched quartetts i bought are not matched for Transconductance, as I have a strong humm which was not there before. I would like now to measure Transconductance of each individual tube using the amp. how can I do this ? Currently all of them are meeting each other through the output transformer windings, right ? How do I know now which one of them is differing from the rest in Transconductance ? Dc-wise they al run at the same current as each tube has an autobias module regulating negative grid voltage per tube to the same current.
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Old 5th March 2013, 01:17 PM   #2
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With out any test gear your only option is power off swapping of tubes until you get the best results with the 4 tubes you have. 12 possible combinations using 4 tubes.
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Old 5th March 2013, 01:30 PM   #3
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Well, i do have a sinus generator and an oscilloscope...but not sure what i will see when four tubes are produe the sinus at the opt...and an arta measurent system to follow with spectrum analyser

Last edited by Blitz; 5th March 2013 at 01:58 PM.
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Old 5th March 2013, 02:34 PM   #4
DF96 is offline DF96  England
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Yes you can. Just set up a test rig with the required voltages to set the required currents, input a small signal to the grid and measure the resultant change in anode current via a small sensing resistor.
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Old 5th March 2013, 03:20 PM   #5
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Could i do this setup with a multimeter measuring current between anode and opt winding ?
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Old 5th March 2013, 03:23 PM   #6
DF96 is offline DF96  England
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No OPT, as that will vary the anode voltage too much so you won't be measuring gm. A decent AC current range is rare on multimeters, but if yours has it then OK.

Last edited by DF96; 5th March 2013 at 03:23 PM. Reason: correction
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Old 5th March 2013, 03:25 PM   #7
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If you place a 1R0 1/2 Watt resistor between each cathode and ground you can them measure the voltage across the resistors, with no signal and bias each valve accordingly.
When you have finished, replace the resistors with links again.
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Old 5th March 2013, 03:39 PM   #8
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Why with no signal ? The whole point is to have an ac signal and meaaure how this changes current, right ? Should the 1 ohm resistor on the anode or cathode ? The bias i know as the auto bias module sets the bias (tentlabs). I have a hameg and fluke multimeter, need to look up their frequency range, but i can use 50hz or 100 hz as testfrequency anyhow as this is the hum frequence..
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Old 5th March 2013, 03:54 PM   #9
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by Blitz View Post
Why with no signal ? The whole point is to have an ac signal and meaaure how this changes current, right ? Should the 1 ohm resistor on the anode or cathode ? The bias i know as the auto bias module sets the bias (tentlabs). I have a hameg and fluke multimeter, need to look up their frequency range, but i can use 50hz or 100 hz as testfrequency anyhow as this is the hum frequence..
You can measure gm fairly accurately without a sine wave, if you have a fixed-biased amp with a 1 ohm series resistor in series with the cathode, and a bias adjustment pot.

Just insert the power valve and set the bias to a suitable point, measure the voltage across the 1 ohm resistor, and calculate the current.
Now adjust the bias voltage by 1V or something, measure the new voltage across the 1 ohm resistor and calculate the new current.
Then calculate gm from the change in bias voltage divided by the change in anode current.

Error is introduced by the output transformer's DC resistance, as well as the screen current, but it should still be pretty accurate, and can certainly be used to match valves.
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Old 5th March 2013, 04:16 PM   #10
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Good advise...never thought this way around, as I thought about this as the DC-conditions and not the AC-Conditions...

...so, with the auto-bias-module of tentlabs.com in place: There I can set the current of all tubes, let's say: 50ma. Each tube would get than the individual right negative grid-voltage to achieve this quiet. current. That is easy for me to measure. So, I could measure of all four this voltage. Than I could lower the current to 35ma or increase it to 65ma four all tubes and measure the neg. grid voltage of each tube.

All of this in the active power amp with all four tubes working into the opt. This should be a correct working setting for matching, right ?
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