Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on January 08, 2021, 01:26:12 pm

Title: Biasing/hum issue, Fender Bassman AB165
Post by: Diverted on January 08, 2021, 01:26:12 pm

Hi all,

Scratching my head trying to figure out a bias issue I'm having with a Fender Bassman AB165:

https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_ab165.pdf

I've recapped the power supply with fresh electrolytics and new dropping resistors, and a new bias capacitor. I also cut the grounds on V5 and V6 cathodes and put in 1 ohm resistors to ease biasing. I've checked grounds, looked for cold solder joints, cleaned the pots etc.

All voltages appear to be on spec except for a glaring issue with V5. I can't bias it any hotter than about 5.7ma. I have reversed the tubes and also swapped in a fresh close pair of 6L6s, but the issue stays at the V5 socket.

Voltages:

Bias voltage: -46

V5
Plate: 428
Screen grid: 417
Cathode: 5.7ma

V6
Plate: 426
Screen grid: 424
Cathode: 31ma

I swapped out the coupling cap to V5 plate in case there was an issue with it; but the issue remains, regardless of the cap so I put the original back int.
The amp plays but there is also a pretty good hum. Any idea why one of the two output tubes would be running so cold?

Edit: Forgot to add that each output tube has new 470 ohm screen grid resistors and 1.5K grid stoppers. The 470s were looking a little heat-damaged.

Title: Re: Biasing/hum issue, Fender Bassman AB165
Post by: sluckey on January 08, 2021, 01:35:00 pm

I would suspect the OT. Measure the dc resistance fron each plate lead to the center tap. What have you.

Title: Re: Biasing/hum issue, Fender Bassman AB165
Post by: Diverted on January 08, 2021, 01:36:19 pm

They're pretty close so I didn't suspect the OT. I checked a while ago and don't have the numbers in front of me but the bad socket had about 47 ohms and the good one around 55. I'll get exact numbers.

Title: Re: Biasing/hum issue, Fender Bassman AB165
Post by: sluckey on January 08, 2021, 01:38:34 pm

What voltages do you measure on pin 5 of each output tube?

Title: Re: Biasing/hum issue, Fender Bassman AB165
Post by: Diverted on January 08, 2021, 01:51:38 pm

I bumped up the AC voltage to 123ac to get real-world numbers. V5 is biasing a little closer to normal now for whatever reason, but is still low. I switched tubes' positions and the low bias still stays at V5. It also looks like screen grid on V5 is low; it should be equal to the plate voltage according to the schematic. Here's what I got

Bias voltage: -48.9

V5
3: 455
4: 425 (should be 455 per schematic)
5: -46.7
8: 24.8
DCR, plate to CT: 46.5

V6
3: 455
4: 455
5: -48.8
8: 33.3
DCR, plate to CT: 45

Title: Re: Biasing/hum issue, Fender Bassman AB165
Post by: Diverted on January 08, 2021, 02:29:41 pm

Found the problem. Stupid stupid error on my part.
In the V5 screen grid position I had mistakenly put in a 27K resistor. Don't know how I ended up putting those resistors in the 470 ohm drawer. But I didn't check it and just put it in. Took it out, put a 470 in there and the V4 voltage is back to where it should be, pretty much equal to the plate.
The hum issue has reduced too.
Sorry to ask questions before figuring out the user error issue.

Thanks!