[Drake] Drake 2B Hum

[Glen Zook]

Have you tried a different 6AQ5?  The problem could be a heater to cathode short in the 6AQ5.

Try putting a another electrolytic capacitor from B+1 and B+2 to ground to see if this helps.

Check the ground connection for the speaker.

If there are any machine screws through the chassis holding the tube sockets in place, through terminal strips, etc., tighten every one of these.  Over the years, these machine screws become loose and/or corroded and the ground connections are all made through these.  If the tube sockets, etc., are held in place by rivets, check to make sure that they have not become loose and/or corroded.  If so, you might have to drill them out and replace with machine screws.
 
Glen, K9STH


Website:  http://k9sth.com



On Friday, February 14, 2014 5:02 PM, Bill Stewart <cwopr at embarqmail.com> wrote:
 

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 
  
  

 
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