6G7A Intermittent Hum

[jazjamr]

 I recently recapped my 1963 6G7A Bandmaster. It has an intermittent hum that I can't trace down. I used MOD electrolytics throughout. The amp was a basket case that I completely rebuilt about 20 years ago. Dead output transformer, crumbling cloth wire, burnt resistors, spray painted black, it was a mess. B+ voltages were too high with modern wall voltages. The chassis was already punched for a rectifier tube, so I added a socket so that I could reduce the B+.  I have spent quite a bit of time trying to eliminate the hum. I am going to order some filter caps to parallel with the ones I changed to see if, possibly one of the new caps is bad. Any suggestions would be appreciated. I have gone as far as to use an oscilloscope with amp idling, but hum is present everywhere except very early in preamp. Thanks to all John Hathaway

[shooter]

https://el34world.com/charts/Schematics/files/Fender/Fender_bandmaster_6g7_schem.pdf


gone as far as to use an oscilloscope with amp idling, but hum is present everywhere except very early in preamp.


I'd start at the beginning, verify the VDC readings you took when it was built
pull PA tubes, verify bias voltage is at the tube side of the sockets and stable

IF so;
put PA tubes back in n re-verify bias voltage
pull all tubes except PI n PA, HUM??

[Esquirefreak]

I've found that the last gain stage/recovery stage of the vibrato channel can be a source of noise. What happens if you pull v5?

/Max

[glass54]

Hi John,
Just an idea. 
I had an old Deluxe 6G3 in for problems. Humm, couldn't fault PS/earthing 
It had a replacement Power Tx and the Bias supply was on a Tx Lug (a NO NO for me  )
As soon as I started to add tension, Hum went away, therefore "dirty" earth connection on Bias.
Regards
Mirek

[tubeswell]

Can you post some hi-res pics of the amp? A good rule of thumb for problems is that it didn’t have the problem before you did the work but now it has the problem, then it was something you did.



How is the quality of your soldering? Did you follow Fender’s layout where some wires are soldered to to chassis? How good was your soldering for those? Etc

[jazjamr]

Thanks for all the replies, B+ w/GZ34  464vdc  V6. P.I. pin1 295v pin6 290v pin3-8 37   V5. pin1-6 330v pin3-8 3.5v  V4.  V4. pin6 262v pin8 66v  V3. pin1 116v pin6 266v, V3 voltages are very low. that's why I thought that perhaps the filter cap was faulty. I have checked all resistors on board, replaced a few that were out of spec. All plate resistors have been replaced. I have pulled and swapped each tube. When I swapped V5 the hum stopped but returned later. Intermittent problems are difficult.

[tubeswell]

Hard to tell from the resolution but the chassis soldering for the current measuring resistors for the output tube cathodes seems to be a bit clumpy, and might have poor ground contact. You could redo those with solder flux paste and a 100-200W flat tipped iron.

[jazjamr]

Sometimes when faced with a problem, it is best to stop for a while and reassess the situation. After reading everyone's suggestions I went back to the work I had done recently. Improper voltages at the filter nodes led me to realize that I had misread the schematic and layout. I had always struggled with the arrangement of the filter nodes. 20 years ago, when I initially restored this amp there were not a lot of resources for the DIY amp tech. Books and Catalogues ordered from the back pages of Guitar Player magazine were my primary sources. I had "Servicing You Own Guitar Amp" and" Dave Funk's Tube Amp Workbook" and a Antique Electronics Supply catalogue. The arrangement of the 6G7A filter nodes is quite different from my first real tube amp, a AA763 Showman. I went back to the schematic and correlated it with the layout drawing. Drew up my own node layout, rewired the cap can, double checked powered it up on the dim bulb and it worked. I had to lower the B+ by putting in a 5R4GB rectifier tube. I believe that I probably had it wired incorrectly for the last 20 years LOL Voltages are now very close to the schematic and layout voltages. NO HUM! The harmonic vibrato in these amps is something very special. I have gigged this amp a few times over the years, but it never sounded right. Sounds Great now. All my sincere thanks to all the folks at the Hoffman Amp Forum. It's so good to have the help of like-minded amp and guitar nuts!