[Drake] Drake 2B Hum

[Bill Stewart]

GA all, 
It's hair pulling time here on ye ole work bench. 
I have a 2B rcvr which has a noticeable hum. When first powering up, 
the hum comes up in volume, then dies back down to a steady hum...not 
blasting but noticeable a couple of feet from the spkr. The vol. 
control does not change it. The can lytic is disconnected and modern lytics 
are in place. I have replaced all the paper bypass caps.  
Here are other things I have done/tried: 
- I clipped in other lytics in place of the first replacement ones....no change. 
- Makes no diff. whether it is in prod. det. or diode position. 
- Disconnected from the det/diode switch at vol. control...no change. 
- Changed out both audio tubes...no change 
- Chd'd coupling cap 6BN8 to 6AQ5, .005...no change. 
- Replaced 6AQ5 plate bypass cap (.001)...no change. 
- Replaced 6AQ5 cathode bypass lytic...it was cracked, but....no change. 
-Changed the 6X4...no change. 
  
The last thing I have done is to remove all tubes, except the 6X4 and 6AQ5. The 
hum is still there and still very noticeable a couple feet from the spkr. 
  
I've probably got tunnel thinking, but it appears to me that the pblm is in the audio 
section. In the 6AQ5 ckt, I have temp. replaced C71, .01 across the audio xfmr; 
the.001 from pin 5 to gnd; & replaced the cathode 10uf lytic. Abt the only thing left 
is the audio xfmr and some resistors. Looks like the only inputs to the 6AQ5 are via 
the screen and B+ lines (with all tubes pulled but 6X4 & 6AQ5). Unfortunately, no 
scope available. 
  
Any suggestions would be very helpful. Tnx... 
73 de Bill K4JYS