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In Reply to: I agree with Mike - theoretically it will work perfectly.. posted by Allen Wright on March 22, 2007 at 04:45:00:
The output stage will use the 421A tube with a 1250 ohm PP ouput. The PP output can't handle the current I want so I believe the Parafeed chokes will solve my issues. The tubes will be fed about 110v potential on the plate and the cathodes of the two triodes will be trimmed to draw equal voltage and then see a current source before ground. I believe the current source on the joined cathodes would force it into true balanced differential. I also believe the chokes will separate the stage from PS issues. I am thinking about floating the stage if the voltage needs to be higher to make the current sources function. Do you see a potential for oscillation?
Follow Ups:
Everything has a potential for oscillation - but as I have no idea what those tubes are ħI cannot give even a guess.> I believe the current source on the joined cathodes would force it into true balanced differential <
Yes, it will.
> I also believe the chokes will separate the stage from PS issues <
They should.
> I am thinking about floating the stage if the voltage needs to be higher to make the current sources function. <
You may have to - depends on what CCS's you use.
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But you have not answered my question - Tubes age and drift...so how do you intend to maintain identical currents in the pairs of tubes across time?
Regards, Allen
I intend to use the 421A also designated the 5998. This is a dual triode in a single envelope. I intend to use each tube as its own PP pair. It is my hope that the two triodes in the tube will age at the same rate. I will use a dual triode driver stage, preferably direct coupled but possibly transformer coupled.I will have to make periodic measurements on the cathodes to make sure bias voltage doesn't drift but I can see no way, short of using two separate current sources on the plates, to ensure that each tube draws the same current. Although this would seem a way to control both total current and the current of each stage, I have been warned off by others builders who told me I cannot have a current source both on the anode and cathode. I am not sure why this is. I have spoken to other builders who built self adjusting servo circuits but I am not that experienced a designer to craft one. As to the current source, I will have to use one capable of handling the probable 250 ma on the joined cathodes so I may use a a FET with a substantial heat sink. A smaller current source on the driver tube will suffice.
Such series reg tubes drift like crazy and I would STRONGLY recommend you have a 1% one ohm R between each cathode and the CCS and monitor this created cathode to cathode voltage with a sensitive front panel center zero meter- and adjust the drift out on a (almost) daily basis with a front panel accessable pot that varies the respective grid voltages.Putting CCS's in the anodes will NOT control the cathode current very well, if at all, and as others have said - can lead to unworkable situations where one tube goes to max current and the other to zero current and stays locked there until you unbalance the setup enough that it flops the other way!
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