Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jbefumo on February 20, 2016, 10:55:05 am

Title: Hum in POWER section?
Post by: jbefumo on February 20, 2016, 10:55:05 am

Just gutted and rebuilt my KT-120 based single-ended amp in order to implement a refined board layout, shorter wire runs, etc.  It was really quiet before, but now it has a loud hum, and the output is low.  After some basic troubleshooting, I pulled the power tube, and ran a ground and signal jumper from pin 5 of the power tube socket to a practice amp, and ... no hum.  Tried a different output tube and the hum is back. Checked the plate, G2, and cathode voltages and all are where they should be. Before I start shooting in the dark, any suggestions? (it's cathode biased, btw.) Thanks.


Joe

Title: Re: Hum in POWER section?
Post by: shooter on February 20, 2016, 11:23:43 am

Quote

gutted and rebuilt

did you move the iron?
sounds like you have induced hum into the OT?  Maybe from new wire layout?
maybe try swapping for test the primary OT leads?
try putting a nice iron bar tween PT and OT, make any dif?

Title: Re: Hum in POWER section?
Post by: jbefumo on February 20, 2016, 12:10:26 pm

Nope -- didn't touch the iron or the power supply.  Will double check the leads & iron bar trick (never heard that one before.) I'm sure it's something stupid obvious ...

Quote from: shooter on February 20, 2016, 11:23:43 am

Quote

gutted and rebuilt

did you move the iron?
sounds like you have induced hum into the OT?  Maybe from new wire layout?
maybe try swapping for test the primary OT leads?
try putting a nice iron bar tween PT and OT, make any dif?