Bassman 135 rectifier

[Leevi]

Why is the rectifier implemented in that way?


The PT has CT but a full wave bridge rectifier is used?  Furthermore the CT is connected
to positive voltage (maybe to lower B+)?


http://el34world.com/charts/Schematics/files/fender/Fender_bassman_135_schem.pdf


/Leevi

[sluckey]

Why is the rectifier implemented in that way? The PT has CT but a full wave bridge rectifier is used?  Furthermore the CT is connected to positive voltage.

That circuit is actually fairly common. Connecting the CT to the junction of the stacked filter caps forces the B+ to split equally across the caps even if the caps are not exactly equal value.

[Leevi]

That circuit is actually fairly common.

Yes, but what is the benefit of the bridge rectifier in this? Same result can be reached by using two diodes
and by connecting the CT to ground?


/Leevi

[sluckey]

That circuit is actually fairly common.

Yes, but what is the benefit of the bridge rectifier in this? Same result can be reached by using two diodes and by connecting the CT to ground?

Not with that same PT. If you replace the bridge in that circuit with a conventional two diode rectifier you will cut the B+ voltage in half.

Probably the biggest benefits of using a bridge is the fact that it puts out twice the voltage of a conventional rectifier and uses the entire HT winding all the time. This means the PT can be built to put out half as much voltage as a PT designed for a conventional rectifier. The lower voltage PT would also probably be wound with larger wire, meaning the current capacity would be higher. It's common to see a bridge in higher power amps that require higher B+ voltage. Putting a center tap on the PT that uses a bridge gives the advantage I already spoke of.

[Leevi]

Ok, Thanks.
I checked the specs of the PT and the secondary is 0-180-360 not  360-0-360. In the latter case the B+ would be huge.


http://www.tubeampdoctor.com/en/shop_Mains_transformers_Mains_Transf_for_Fender/Mains_transf_for_Fender135W_models_Twin_Reverb_Showman_Rev_Bassman_112


/Leevi

[sluckey]

I checked the specs of the PT and the secondary is 0-180-360

Most of us would call that 180-0-180. 

[PRR]

They wanted 500VDC.

At that time, for a new design, the FBW design was a penny cheaper. (As you see, you have to specify the voltages for the FWB instead of the 2-D plan.)

They *also* wanted a high-power 250VDC reference. In this case, just for the stacked-caps, to ensure equal voltage split. However there is a related plan (may be the same PT) which uses the 250V tap for a Low-Power mode. You also see this plan in 600V 6550/8417/KT88 PA amps, which like 600V on plates but just 300V on screens.

This base plan is VERY common in chip opamp work. Except they ground the center tap and take both + and - 15VDC so the opamps can swing both ways.

There's many ways to skin a possum. This way gives two equal size parts, which may be useful.

[Tony Bones]

Though not done with the Bassman 135, I have seen designs with a FWB rectifier that use the CT to derive a distinct power rail at 1/2 the voltage of the B+. Well, the Bassman 135 does this, but it uses the 'middle' rail only for the input capacitors. The designs that I'm thinking of usually needed a lower voltage supply for the screens of the output tubes. Or, the highest B+ is used only for the plates of the output tubes and the half-voltage supply (the CT) is used for everything else. These are high power amps; not your typical 6L6 designs.

I have a half dozen 828 transmitting pentodes that I'll design a circuit for someday. Probably do something like that. I'll also need to come up with 10V @ 6.5A just to light the filaments of a PP pair.