The dynamic transconductance test as outlined in the various RCA
receiving tube manuals looks very straight forward and practical.....
What interfering factors would have to be accommodated for to set up and
perform this test with any degree of accuracy?
dynamic transconductance test
Moderators: pompeiisneaks, Colossal
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Andy Le Blanc
- Posts: 2582
- Joined: Sat Dec 22, 2007 1:16 am
- Location: central Maine
dynamic transconductance test
lazymaryamps
-
Andy Le Blanc
- Posts: 2582
- Joined: Sat Dec 22, 2007 1:16 am
- Location: central Maine
Re: dynamic transconductance test
The test works useing less than ideal conditions.... following the basic
schematic and description outlined in the receiving manuals.....
I used an old project amp...... placeing a meter in the plate circuit
and applying one volt at 1khz on the grid of the output tube while reading
results in milliamps and converting to micromhos.....
this said there are issues......
metering........ I got lucky..... my meter is a 1% digital.. I know theres better
but thats what I have.....
voltage drift...... stability is a real issue.... with my signal generator
and test amp need to be isolated or used with a regulated supply
and a good stable signal is required....
accuracy\ calibration...... this is the real hitch...... the test does work...
but what good is it if theres no way to compare it to a known standard
most of these issues look like they can be resolved ......
with the exception of a calibration standard...... what should be used?
where do you source it?
schematic and description outlined in the receiving manuals.....
I used an old project amp...... placeing a meter in the plate circuit
and applying one volt at 1khz on the grid of the output tube while reading
results in milliamps and converting to micromhos.....
this said there are issues......
metering........ I got lucky..... my meter is a 1% digital.. I know theres better
but thats what I have.....
voltage drift...... stability is a real issue.... with my signal generator
and test amp need to be isolated or used with a regulated supply
and a good stable signal is required....
accuracy\ calibration...... this is the real hitch...... the test does work...
but what good is it if theres no way to compare it to a known standard
most of these issues look like they can be resolved ......
with the exception of a calibration standard...... what should be used?
where do you source it?
lazymaryamps
Re: dynamic transconductance test
Create your own standard.
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Andy Le Blanc
- Posts: 2582
- Joined: Sat Dec 22, 2007 1:16 am
- Location: central Maine
Re: dynamic transconductance test
I was thinking that.... but standards are supposed to be verifiable
testable.... valid..... thats why they need to be calibrated for in the first place
Ive found "calibration tubes" for tube testers..... used to be more common
place...... with one certified tube I should be able set up the criteria for a
testing regime...... a valid test....
testable.... valid..... thats why they need to be calibrated for in the first place
Ive found "calibration tubes" for tube testers..... used to be more common
place...... with one certified tube I should be able set up the criteria for a
testing regime...... a valid test....
lazymaryamps
Re: dynamic transconductance test
There was an article in AudioXpress February 2005 on building an opamp and MOSFET circuit to calibrate tube testers for transconductance. I couldn't find it anywhere online but that's not to say it's not out there somewhere. I'll see if I have that issue tomorrow.
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Andy Le Blanc
- Posts: 2582
- Joined: Sat Dec 22, 2007 1:16 am
- Location: central Maine
Re: dynamic transconductance test
I have glass audio and audioxpress thru 2004. might have buy some back issues
like to see that....
like to see that....
lazymaryamps
Re: dynamic transconductance test
I located a copy of the article locally from a friend. It's not quite as I remembered. The author used an IXYS 10M45S FET and basically just added resistance in the source lead to adjust the transconductance and added a little protection. The whole thing is built into a octal plug. You setup your tube tester for a 6SN7 and it should indicate 1500uS. It's somewhat sensitive to the grid signal level which can vary between different tube tester models.
Myself, I would like a more precise circuit with an opamp to force a certain current change. You could probably come up with something that would also work to calibrate testers that use the grid-shift method (see RCA tube manual for explanation).
It really isn't that difficult to precisely measure transconductance. Basically just an AC voltage measurement and an AC current measurement. The trick is in setting the operating point of the tube for the test. Transconductance changes with heater voltage so some type of regulated heater supply is needed.
Myself, I would like a more precise circuit with an opamp to force a certain current change. You could probably come up with something that would also work to calibrate testers that use the grid-shift method (see RCA tube manual for explanation).
It really isn't that difficult to precisely measure transconductance. Basically just an AC voltage measurement and an AC current measurement. The trick is in setting the operating point of the tube for the test. Transconductance changes with heater voltage so some type of regulated heater supply is needed.