Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: MikePR on November 06, 2015, 02:39:59 pm

Title: Fender Blues Deville Bias schematic
Post by: MikePR on November 06, 2015, 02:39:59 pm

Hi Guys:

Been trying to find the answer on the web for 2 hours with no avail. I am doing a Fender blues deville point to point clone without all the extra stuff (channel switching etc.) plus adjustable bias. Want to confirm if my bias schematic is good. Also i do not know what to do with the second Brown wire of the Power Transformer. Any thoughts?

Title: Re: Fender Blues Deville Bias schematic
Post by: sluckey on November 06, 2015, 03:27:53 pm

Measure AC voltage between brown and brown. If it's 50V or higher you can do this...

Title: Re: Fender Blues Deville Bias schematic
Post by: MikePR on November 06, 2015, 06:18:03 pm

If more than 50v what would be the value of cap? If not what would be the other option?

Thx

Title: Re: Fender Blues Deville Bias schematic
Post by: sluckey on November 06, 2015, 06:48:06 pm

I think the voltage will exceed 50VAC. Schematic shows 28VAC from brown to brown/yellow. Bet it's twice that from brown to brown. If you have the amp it will probably take about a minute to verify. Probably even be fine if it's only 45VAC. I just picked 50VAC as a nice round number. Just copy the cap value from the AB763 circuit.

Alternative... Don't even use that winding. Take the AC from the high voltage winding. You'll have to copy the old Ampeg or Marshall bias circuit with an input cap since the HT uses a FWB rectifier.

Title: Re: Fender Blues Deville Bias schematic
Post by: PRR on November 06, 2015, 11:51:37 pm

> confirm if my bias schematic is good.

Looks good to me.

> you can do this...

That should work also. Saves one rectifier. Omits one stage of filtering. DOES work on many-thousand AB763 amplifiers and kin.

> 28VAC from brown to brown/yellow. Bet it's twice that from brown to brown.

You would think. However the opamp side is showing 32.8VDC, which suggests 24V on the other side. 28V+24V is just weird... but Fender has done many weird things since Leo left. And taking the total 24V+28V Brn-Brn connection gives over 70V DC which sure is ample bias (even for 468V on screens).

I like taking ~~50VAC better than messing with the HV to make a LV in reverse polarity. Especially when there is no grounded CT on the HV winding. It can be done. It can be done wrong (boom). It needs a highly-stressed part to drop 9/10 of the voltage, reliably, long-term. To *me*, making small DC from small AC just feels better.