[R-390] 60 Hz on Audio - Cause Begins to Reveal Itself

[Jon Schlegel]

Hi All,

After much experimentation,  it's beginning to appear as though a 
large part of the 60 Hz I see on my IF module is related to coupling 
of AC heater power into the signal path within the 
tubes.  Heater-Cathode shorts don't seem to be at issue.  It also may 
not be simple Heater-Cathode leakage either.  I don't have enough 
experience to make this kind of call.  All I can say is that of the 
tubes I have, none showed Heater-Cathode problems when I tested them.

DC Heater power without doubt eliminates the 60 Hz problem so AC 
Heater power is clearly at issue. Generally, what I've observed 
beyond that is that the 60 Hz level stays reasonably low until the IF 
module starts to go into AGC.  Nevertheless, I haven't been able to 
find any 60 Hz on the AGC bus.  Under these conditions then, the 60 
Hz can increase to a level as high as almost 30 dB below a 30% 
modulated tone. It's clearly visible riding on top of the tone as 
shown in the example scope photo.

<http://www.vecnav.com/R-390A_IFmodule/DiodeLoadOut_8kHzMechFilter.jpg>

After going through a lot of effort to eliminate possible heater 
wiring issues; return current in the chassis, heater supply wires in 
wiring harness bundles, apparent sensitivity of mech filters to AC 
heater power, etc, I realized that 60 Hz level was really dependent 
on the specific 6BA6 tube in V502 and V503.  My setup at this point 
had evolved as described and I used it to gauge the sensitivity of 
the 6BA6 tubes that I have on-hand to AC heater power .  All on these 
tubes are old and no doubt well used.  Two checked poor on 
transconductance but the remaining seven checked ok.

Here's the setup.

(1)   IF module run separately from receiver via test fixture and 
regulated B+ supply.

(2)   30% modulated 455 kHz signal injected into V502 Grid via a 
network that simulates the mech filter normally driving the grid. 
Network also allows for  injection of normal AGC voltage into V502 
Grid.  This eliminates the mech filters, switching/grounding 
circuitry and any influence they may have.  Set 455 kHz RF level for 
about negative 5 VDC on the AGC bus.

(3)   DC Heater power supplied to V502 only.  Normal AC heater power 
supplied to all other tubes in module.  This hopefully eliminated any 
60 Hz contribution by the V502 stage.

(4)   V503 socket on IF module used to test each of the nine on-hand 
6BA6 tubes for sensitivity to AC heater power.  Each 6BA6 was plugged 
into V503 socket and then a relative measurement was made of the 60 
Hz level referenced to the 30% modulated tone.


Here's the list of each tube's 60 Hz level relative to the tone.

-59 dB
-62
-43
-42
-39
-33
-42       (Poor Gm)
-38
-44       (Poor Gm)


Seven tube samples are grouped in the -33 to -44 range while two of 
the samples are around -60.  On the average, these two are 20 dB 
better than higher valued group.  (Surprisingly, the best of the 
bunch at -62 has the name "Delco" printed on it!)


*******************
Can Heater/Cathode leakage be blamed on the seven poor performers or 
is something else happening?  Is there some sort of cross-modulation 
thing going on?  A clue here may be that the 60 Hz really only shows 
up when the module goes into AGC despite the AGC bus being 
clean.  Can NOS tubes be had that aren't beat up like all of my samples?

*******************

Thanks for the effort of reading through all this gobbledy goop .  I 
did my best to keep it as short as possible but still make sense (I hope!).

Regards,
Jon