[Boatanchors] 0A2 regulation...

[Rodger]

Hi Eugene,

The VR design assumes there will also be current load on the line in
addition to what is "sunk" through the VR tube.  So there should be a
minimum current draw of about 35 mils by the load and the VR tube will
furnish the rest of the current draw needed to reduce the source voltage
from 340 to 150 through 3K.  Effectively, the VR tube can be used in a
circuit which has up to 30 mils of current draw variance by the load (i.e.
transmit to non-transmit) so the ability to use the VR tube is not the total
current needed across the dropping resistor but rather the load can never
vary more than 30 mils without causing a problem.

Your actual frequency variance may be due to a drop in filament voltage
during transmit which can cause frequency changes.  I recall that someone
ended up regulating the heater voltage in their Heathkit SB-300 series
receiver to avoid this and it was common in some of the older boatanchors to
regulate oscillator filament current (NC-400, HRO-50, R-390A, etc.)  Some
tubes are more susceptible than others to this so you might try substituting
different tubes.

Hope this helps!

73, Rodger WQ9E


> -----Original Message-----
> From: boatanchors-bounces at mailman.qth.net [mailto:boatanchors-
> bounces at mailman.qth.net] On Behalf Of Eugene Hertz
> Sent: Tuesday, February 06, 2007 9:09 PM
> To: boatanchors at mailman.qth.net
> Subject: [Boatanchors] 0A2 regulation...
> 
> 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
> 
> 
> 
> 
> 
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