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On Gas Reg Tube Operation:

Posted by Thomas Martens on January 4, 2007 at 19:09:09

In Reply to: Re: LF instability always an issue... posted by SteveBrown on January 4, 2007 at 18:15:31:

I suggest you draw a maximum of 20mA THROUGH the gas regs for
good operation, and not to burn them up. Remember: Big filter
cap first THEN the B+ goes through the main dropping resistor (you
need a much bigger one if your VR tubes themselves are drawing
35mA presently) to the VR tubes, THEN on through additional
dropping/isolation resistors for EACH INDIVIDUAL stage/tube to be
powered's (a good 10v of additional drop should be sufficient)
plate load resistors. A small, say 22uF Cap (all you need here)
should be placed at the junction of each INDIVIDUAL above mentioned
small dropping resistor and plate load resistor. This provides
individual MF to HF filtering isolation for each stage, and any
residual LF that gets through to the main VR line will be zapped
to ground by the VR tubes. ABSOLUTELY DO NOT connect ANY capacitance
DIRECTLY to the VR tube rail! only connect after additional dropping
resistors (as mentioned above). Connecting Caps directly to the
VR regulated line can/will CAUSE oscillation!

As you can see, the trick is to calculate everything out so that
you have a high enough voltage supplied by the chosen VR tubes in
order to accommodate the small additional (again, 10V or so usually
fine) dropping resistor to each stage and still have enough voltage
remaining to provide the desired supply voltage to each stages plate
load resistor. It is important to have enough voltage from the PS
supplying the main dropping resistor to the VR tube so that a
MINIMUM of 2k ohms is the value used which allows sufficient total
current to flow to the individual stages AND through the VR
tubes themselves. (10 to 20mA through the VR's is good and safe).
The higher the supply feeding the VR's, and thus the larger the
resistance of the main dropping resistor feeding them, the better the
regulation.

I really hope this helps.

Keep me posted,

-T.M.

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