[R-390] R-390A front end tubes

[Larry H]

Hi Bob,  You bring up a very good point about problems one can have doing s/n testing at the RF input.  That's why I like the URM-25D, they did a nice job preventing RF leakage from getting out and skewing the measurement (if it is working correctly).  I had a different sig gen that was in a metal box, but it did leak RF a lot.  It was not possible to get accurate readings at low levels with it.

So, just to be sure that my 25D is not leaking, I just tested it again.  It's very easy to do - connect it up to your rx as for normal s/n testing with low output and center the frequency with the rx BFO.  Now disconnect the coax and turn the rx volume up to where you can hear noise.  Rock the rx tuning back and forth a little to determine if any signal is detected.  If there is, your sig gen is leaking and if no signal is detected, it is not leaking.  Now the second part - reconnect the coax and center the rx on the signal with the BFO still on.  Turn the sig gen output all the way down to 0 and look for a signal as before.  If one is detected, the sig gen is leaking through the sig gen output.  This is a problem if the sig gen output level control is working correctly.

Luckily my 25D passes both tests perfectly.  I have no detectable leakage.  My s/n measurements are correct.


Your comment about overloading later stages is why I was careful trying different tubes in the front end and one reason I decided not to use them (except for the 6BZ6 in the 1st IF).

Thanks for your very good comments.
Regards, Larry