?...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.
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.
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.
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 ?
...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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