Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on July 30, 2021, 06:06:29 am

Title: Bassman AB165 bias adjustment
Post by: Leevi on July 30, 2021, 06:06:29 am

What is the basic idea of the bias circuit of AB165?

https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_ab165_schematic.pdf


I have a case where I try to set the bias to 60% of the maximum dissipation i.e. 43mA on 411V plate.
The balance can be found somewhere at 33 mA but if I try to adjust hotter the tubes go unbalanced.
The circuit seems to work in that way and is the idea only to find the balance regardless how much it is?

/Leevi

Title: Re: Bassman AB165 bias adjustment
Post by: sluckey on July 30, 2021, 06:35:21 am

The balance control is meant to adjust bias balance only. The note says adjust for minimum hum (which should occur when the tubes are balanced). There is no provision to adjust the bias hotter or cooler. How are you attempting to do so?

Title: Re: Bassman AB165 bias adjustment
Post by: Leevi on July 30, 2021, 06:54:05 am

Ok, I didn't notice the note. The idea is now clear.


Thanks
Leevi

Title: Re: Bassman AB165 bias adjustment
Post by: AmberB on July 30, 2021, 06:47:02 pm

I had an AB-165 that I used for many years as my small bass amp head.  I had to do some repair work to it, and a cap job, so I modified the bias circuit in it so that I could adjust the bias voltage as well as balance the tubes.  You have to add another pot, which means drilling a hole in the chassis for it, but it's not that complicated to do.
That was long enough ago that I don't remember the particulars in that modification, but I remember having done it.

Title: Re: Bassman AB165 bias adjustment
Post by: Leevi on July 31, 2021, 12:00:42 am

Yes, that is one option or keep it as a one pot control and change the circuit to more common and use matched pairs only. Or  accept it as such.



/Leevi