Bias?

[lvds335]

Hi,
About 3yrs ago I did a DIY 5E3kit and I'm very happy with it.
Now I would like to do a 5f6a, wich is a little harder to do.
So I gathered a lot of info an I even collected all the stuff I need...I though..
Because looking at one of te chassispictures I noticed a blue rectangle with a screw(?) probably to ajust someting?.
Could this be for the bias?

 

[Paul G.]

I believe that's a selenium rectifier for the bias circuit. It's obsolete technology, and easily replaced with a diode.

 

[rdjones]

Yes, selenium rectifier stack.
Not only are they obsolete, but they are rather hazardous and present a health risk.
I would strongly discourage their use.

 

[Jon Snell]

They were known as Metal Rectifiers, in my younger day.

 

[telemnemonics]

Right, that is not the bias pot but you do need to bias a 5f6a.
Not sure if old ones did the resistor swap method where there is no twisty knob for adjustment?

 

[SoK66]

As mentioned, selenium rectifier. Use a diode. A bias trim pot for a 5F6-A or any Fender that uses the dual 8/150 bias caps goes in series with the schematic value 56k resistor between them. Reduce the value of the resistor accordingly so you have proper sweep for adjustments. You can use fender's usual 10k bias pot but you'd have to drill a hole in the chassis to mount it. I just use a ceramic trim pot.

 

[Ten Over]

Hi,
About 3yrs ago I did a DIY 5E3kit and I'm very happy with it.
Now I would like to do a 5f6a, wich is a little harder to do.
So I gathered a lot of info an I even collected all the stuff I need...I though..
Because looking at one of te chassispictures I noticed a blue rectangle with a screw(?) probably to ajust someting?.
Could this be for the bias?

What power transformer do you have? Does it have a bias tap and if so, what is the voltage?

 

[King Fan]

I'm sure you've seen Rob's 5F6a 'mods' page -- lots of good Bassman info there, and he shows this nice adjustable bias circuit. Notice the diode:

But as @Ten Over notes, the 'front end' resistance there is designed for a PT with a ~50V bias tap. If your PT has no bias tap, Rob also shows how you can borrow high voltage from the rectifier and knock it down. His notes there are helpful for both.

 

[tubedude]

Definitely replace it. When overloaded they emit toxic gasses.

 

[Ten Over]

A couple of suggestions if you want to use a 10k pot. The resistor values may need to be adjusted a little when the actual voltages are known.

 

[lvds335]

What power transformer do you have? Does it have a bias tap and if so, what is the voltage?

The PT is the 290DEX, I read this should be standard for the Bassman.

 

[lvds335]

A couple of suggestions if you want to use a 10k pot. The resistor values may need to be adjusted a little when the actual voltages are known.

View attachment 1082691
View attachment 1082692

Thnxx!! For the effort!
The 1N4007 is for peventing damage after failure I presume

 

[Ten Over]

The PT is the 290DEX, I read this should be standard for the Bassman.

Version #1 should work with the 50V bias tap. I would expect a range from -42V to -55V, about the same as Rob's version.

 

[Ten Over]

The 1N4007 is for peventing damage after failure I presume

The single 1N4007 (D1) is the half-wave rectifier for the bias circuit just like the selenium rectifier was in the original schematic. It changes AC from the transformer into DC for the bias voltage.

 

[lvds335]

The single 1N4007 (D1) is the half-wave rectifier for the bias circuit just like the selenium rectifier was in the original schematic. It changes AC from the transformer into DC for the bias voltage.

Clear!