[Hammarlund] HQ-170A VHF VR tube??

[Craig Roberts]

Thanks to the wise counsel of Pete, Joe and Roy, I'm getting "unconfused" and learning about gas discharge VR tubes.

To recap, briefly: my HQ-170A VHF arrived with a 2K dropping resistor at R53 and an OA2 in the VR tube socket (in other words, the setup for an HQ-180). To bring the radio to "plain Jane" HQ-170A specs I replaced the resistor with a 3K unit and installed an OB2. I was then informed that the HQ-170A VHF called for a 2K resistor with the OB2. I expressed confusion, thinking that the change back to the 2K resistor would fed too much voltage to the pretty little OB2 and blow it to smithereens.  

I have learned, however, that that's not how it works in the eerily glowing world of gas discharge VR tubes. The dropping resistor, it seems, is used as a current limiter -- not a voltage limiter.  With the extra load imposed by the VHF components (a pre-amp and 2-meter converter), the 2K resistor is, indeed, indicated.  I was given a subtle clue to this necessity, but failed to pick up on it: with the incorrect VR components installed, the receiver was pulled slightly off frequency when I activated the VHF pre-amp and converter.  

Bottom line: I should change the resistor back to 2K and leave the OB2 in place.  I misled myself by ASSUMING the voltage on the OB2 would be too high with the 2K resistor, but not actually measuring it. I took a voltage reading with the OA2 in place which would, I have learned, correctly yield greater voltage at the socket (143V, in this case). I also took a reading with the OB2 in place, but with the 3K resistor feeding it. Again, I ASSUMED that the resultant lower voltage reading (104V) was due to the increased value of the droppig resistor, rather than the natural characteristic of the tube.

I'm now guessing that a previous owner installed an OA2 in this receiver because he, like me, misunderstood the operation of the circuit or simply used the VR tube he had handy.

I'll get this right yet.  Many thanks for the very useful education, fellas.  This is why it's so important to understand theory as well as operation.  It's great to be almost 60 years old and still learning neat stuff!

VY 73,

Craig
W3CRR