[Boatanchors] 0A2 regulation...

[Eugene Hertz]

Hello all,

Several of you were very helpful to me on my last round of questions regarding the operation and testing of this tube. 

My problem was whenever I keyed the transmitter the CW frequency could shift 200-300hz in a space of 20 seconds or so. And while I am unlikely to send a 20 second long daaaahhh, I noticed that any word in CW might start and end on noticeably different frequencies. The manual (ce 100v) states that after 5 minutes warm up, frequency stability should be 25Hz.

So here's what I did. I bought a tube extender that allows me to connect a voltmeter to the pins of the tube.  I tested 8 different 0A2 tubes I had under two main conditions "in circuit". First, I varied the input to the transmitter from 110VAC to 120VAC to see which tubes gave better regulation from the house voltage. Second, I measured regulation under key down conditions.

Testing each tube for varying house voltage using a variac resulted in a wide range of regulation, the best being 1VDC and the worst tube being 6.7VDC. According to the tung-sol data for this tube 2v regulation should be acheived with 5-30ma of current. However, Amperex data sheet says 6V with 5-30ma.

I then tested these 8 tubes with 120VAC input and key down into a dummy load (about 90 watts into the load) and measured the variation in regulated voltage.  The range here of the 8 tubes was 0.6V to 4.6V.  

So, I will certainly try the tube that only varied 0.6v when keyed down. I have not taken frequency measurements with that tube yet.

So here's my academic question: The circuit shows a +340V supply line with a 3K Ohm, 20W resistor in series with the anode of the 0A2. the cathode is grounded. At the anode of the 0A2 is then the 150Vdc output line.  Doing some calculation, 340-150=190V across that resistor. Dividing by 3K Ohms gives a current through the tube of 63ma. But the tube is rated for a max of 30ma, so this doesn't make sense.  I then began to think about the non-trivial heat generated by these 0A2 tubes and thought to myself, "self? That heat must mean the tube has some equivalent series resistance" Which would make the 63ma somewhat less.  

How does one properly analyze this circuit? How does one calculate the resistance of the 0A2 to properly calculate the current through it? Perhaps I should calculate I=340/(3000 + Rtube)?

Thanks!
Eugene