Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: LooseChange on August 16, 2011, 07:56:10 am

Title: Bassman 135 - Power supply questions
Post by: LooseChange on August 16, 2011, 07:56:10 am

On the bench now.  I will post pictures later. This amp is like NOS.

http://www.el34world.com/charts/Schematics/files/fender/BASSMAN_135.pdf (http://www.el34world.com/charts/Schematics/files/fender/BASSMAN_135.pdf)

Bias supply filtering: Seems like it's way overkill with a pi filter with 75uf-80uf on each side?
Curious about the center tapped HV going to the middle of the totem pole of the first filter section. Is this like humbucking??

The owner loves the amp. It has some hum and the hum balance pot has seen a bit of smoke. I am not going to change a thing other than fix the issues including replacing the eCaps.

Thanks!

Title: Re: Bassman 135 - Power supply questions
Post by: sluckey on August 16, 2011, 08:06:01 am

Quote

Curious about the center tapped HV going to the middle of the totem pole of the first filter section. Is this like humbucking??

That connection absolutely insures that the B+ voltage will be split equally across those two 220uF caps. It serves no other purpose that I can see.

Title: Re: Bassman 135 - Power supply questions
Post by: PRR on August 16, 2011, 12:59:55 pm

> Seems like it's way overkill

Yes, and badly drawn also.

Fender may have had problems with poor tube matching, and comparison with transistor amps which (can) have "no hum". The bias-balance is set for minimum idle buzz. Yes, 1.2K 70uFd 2K 80uFd seems extreme, but OTOH in context of the whole amp, the cost is small.

Using BOTH PT CT and balance resistors seems excessive too. But the 285V caps stand-by at 260V. Simple resistors would not be enough (depending on cap leakage unbalance) to avoid something like 230V+290V. And I have never seen a 285V cap, so these are special and perhaps were marginal. OTOH 110uFd charged to 520V is a LOT of potential danger, bleeders would be an excellent idea.

It is either conflicted-committee design or based on some specific Available Parts.

Don't over-think any of the CBS designs.

Title: Re: Bassman 135 - Power supply questions
Post by: LooseChange on August 16, 2011, 01:45:10 pm

Drawn poorly is an understatement.
I've got 47uf caps for the bias which I think is still over kill. And I'm gonna use the standard 220uf/350v for the filters.
There is a 30k/20w which should provide the bleed. Yes?
Thanks!