[Boatanchors] VR Tube Subs

[Wilson]

Doesn't compute.
They are all based the same, but fire at different voltages.
If you put in a lower voltage tube, it MAY run hot, depending upon the 
dropping resistor designed for the original tube.
If you put a higher voltage tube in place of a lower voltage one, it will 
surely fire with no load, but may not be drawing enough current to remain at 
the regulated voltage when load is applied.
Nominal design current is 40 mA, at least on the OD3.  If your circuit goes 
from zero to some load, that current must be a bit less than 40mA or the 
tube will go out.
If your load changes from XmA to YmA, then you can set the idle current to 
40mA when X is applied and have regulation up to X plus a little less than 
40 mA.
In other words, the tube can't be allowed to go out.
This assumes a constant voltage source, which is unlikely.
If your supply voltage drops when the YmA load is applied, the current to 
the VR tube will be reduced and your usable range will be less than 40mA.
It's all Ohms law and easily calculated/measured.
If the circuit was designed for one voltage, why would you sub another VR 
tube?
That said, there are reasons.  A common one is if a VR tube is in a bias 
supply you may want to change the bias level to change the class of 
operation of an amp or compensate for a change of plate voltage.  Or you 
might have a VFO with a voltage regulated supply that's overdriving the next 
stage.  A lower voltage VR tube could reduce the output and maintain 
stability.  Of course you could use attenuation instead, but lowering the 
power level of an oscillator reduces heat and improves stability.
So what's the objective?
WL

-----Original Message----- 
From: Bob Jackson
Sent: Thursday, June 05, 2014 7:25 PM
To: porch.boat ; puck.boat ; qth.boat
Subject: [Boatanchors] VR Tube Subs

Has anyone had good/bad experiences subbing VR tubes, e.g. 0C3 for an 0D3, 
where basing is the same?

Tnx es 73 to all,

Bob AG5X
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