300B Paramount Hum Pot Not Working [resolved]

[AudioDave]

Just finished up my 300B Paramounts and all resistances and voltages check out on both units.  The only problem I have is the hum pot on one of the amps does not adjust the mV's when I do the hum adjustment.  I've checked all solder joints.  Has anyone else had this problem?
Thanks,
Dave

 

[Paul Joppa]

The likeliest cause is a failed solder joint on one of the 22-ohm resistors connected to the pot.

You can pull the 300B, then measure the resistance between socket terminals 1 and 4 (the fatter pins, which are the filament terminals). It should be about 54 ohms, if everything is wired correctly. Try it with the meter leads reversed as well, since the rectifier diodes might give a false low resistance (depends on the meter).

 

[AudioDave]

I resoldered all the connections to the hum pot and the 4 pin socket.  Resistance between pins 1 and 4 measured 53.1 ohms.  Any thing else to check you would recommend?
I really appreciate the help!
Dave

 

[AudioDave]

I also switched the 300B tubes from the good amp and hum pot still didn't work

 

[Paul Joppa]

It seems to be correctly wired then.

The Paramount has a DC filament power with fairly small ripple - it may not be audible at all unless your speakers are very efficient. How much AC voltage do you have on the output? When you connect the speakers, can you hear it any hum?

If it is below 1mV, and/or you can't tune the pot by ear, then just set the pot to the middle of its range and enjoy the music!

 

[AudioDave]

I think I found the problem!  The bare ground wire from the black speaker terminal was touching the red speaker terminal.  Hum pot is working now and getting 0.7 mV!!  Sorry to bother you on a Saturday night!
Dave

 

[Paul Joppa]

Excellent! I'm glad to hear it's working now.