[AMRadio] RE: Another receiver update...

[Bob Bruhns]

Hi Brett,

I once read about internal cathode interface resistance in oxide-cathode vacuum tubes.  Evidently the various layers of the cathode can make intermittent internal contact, and this can cause frequency shifting and drift.

Try adding a low value resistor in series with the cathode, maybe a few hundred ohms to 1.5K or so, and see if it helps...  assuming it does not stop oscillation altogether!  I did this with the 6AU6A in my Heathkit VF-1, and it helped.  But I still had to select tubes for best performance.

  Bacon, WA3WDR


Brett Gazdzinski wrote:
> 
> Hello all AM'ers.
> I have been working some more bugs out of the home brew receiver.
> 
> I put the bigger dial onto a shorter front panel, because
> the other dial was just too small.
> I marked the frequencies on it, only to find them off
> after a while.
> 
> I did some checks and the thing was drifting about 10kc
> or more from a cold start.
> 
> So I did some tests....I removed the power from the built in power supply
> with its hot dropping resistors and VR tube, and powered
> it up with the heathkit adjustable voltage power supply remotely.
> I still had the drift.
> 
> I figured the lo tube (6j5) might have something to do with it,
> so I replaced it with a spare.
> Interesting results!
> The warm receiver with the cold 6j5 went back to
> its original frequency, then, after the tube warmed up
> a bit, the frequency went up quickly, almost a jump.
> The original tube did this also....
> 
> I have a glass 6j5 and tried that....no jump!
> Anyone got an idea why a metal tube would act this way?
> 
> So now the warm up drift is about 2kc, and I have ordered
> some NPO caps from mouser to replace the silver mica caps
> in the LO.
> Perhaps that will cut the drift even more.
> 
> I also played around with muting.
> I changed things so the receiver still works when I transmit,
> just the audio output is grounded, and the mixer input is shorted
> to ground through a reed relay.
> This way, I get an IF output for the O scope on both RX and TX.
> 
> Besides the remaining drift, it works very well.
> 
> Brett
> N2DTS
> 
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