[AMRadio] RE: Another receiver update...

[Brett Gazdzinski]

Bacon,
I think you are right on the money.
Looking at the oscillator design I used, the cathode is
above ground for rf, but the filament is bypassed to ground.
(RF choke to ground)

There is another way to do it with a grounded cathode,
I found a good design in the October 1957 QST, but it
would require a lot of changes, the tuning cap
has to come off ground along with the coil.

I am not sure I want to start over with making coils,
trying caps, etc, so I will see how it acts with the NPO 
caps.
As it is, with the glass 6j5 in there, it drifts
about 2 kc from a cold start...
Not unreasonable for a home brew receiver, I just need to 
let it warm up for 10 minutes before its accurate...

Thanks for the tip,
Brett
N2DTS

 
> 
> 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