[Boatanchors] VR Tube Subs
infomet at embarqmail.com
Thu Jun 5 19:47:05 EDT 2014
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
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
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?
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,
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