Dropping Voltage B+ - thank you Fender

[lowell]

I just worked on a Twin Amp. I noticed something cool about this circuit's power supply. Fender has incorporated a very easy way of lowering the center tap and screen voltages in this amp. Much simpler than using the amplified zener approach (IMO), and while keeping all preamp stage voltages the same.

First off they've tapped the preamp supply nodes from directly after the rectifier, in parallel w/ the main pi filter - so they are not affected by the power switching. Then they've used a dpdt switch to activate 2 things: lower B+, and bias adjustment. The B+ side of the switch opens the main supply line to the pi filter leaving a diode (CR15) from "between" the main series connected filters. Now that's a great way of splitting the voltage in two! The other side of the switch lowers the negative bias voltage by switching in a larger series dropping resistor.

Just thought I'd share this. Thought it was pretty cool. One more dpdt switch and you can add a tube rectifier and series filter cap in the pi filter. This way you can get rectifier sag and less pi filtering - per old tweed style amps.

[Merlinb]

That is indeed ingenious- thanks for posting!
In case anyone has trouble reading the monster schematic, the circuit in question is shown below. With the switch closed the whole of the winding is used with a bridge rectifier and the output voltage is equal to the peak voltage across the whole winding, and the total capacitance is equal to the two caps in series.
With the switch open it is converted into a two-phase rectifier, so the DC voltage is equal to the peak voltage across one half of the transformer, and only the lower capacitor is used (so you have twice the capacitance on the lower voltage setting).

The traditional arrangement would use a double-throw switch to select the two voltages, but in this case the diode allows the use of a single throw switch, which Fender obviously finds cheaper.

[Gaz]

That's really cool! And thanks for simplifying it, Merlin. Maybe I should scour some newer schematics every once in a while!

[unparalleled]

Funny thing is, unless you show the client the drop in power on an oscilloscope they do not see this as a useful working feature.
I have a Fender 75 in at the moment in which that circuit is working correctly.
The client expects it to cut the volume down substantially at any level.

[lowell]

Are you also adjusting the bias when you use the feature? You should get lower volume w/ much earlier breakup w/ the feature. Another key thing may be to reduce the filtering to loosen the sound up a bit. In my version of this I have 3 filter caps stacked. The bottom cap is 33uf and the switch, in full power mode, shorts the bottom 33uf cap to ground, thus ridding of it. You'll want to adjust your bleed resistors to compensate and acquire half voltage though.

[unparalleled]

Are you also adjusting the bias when you use the feature? You should get lower volume w/ much earlier breakup w/ the feature. Another key thing may be to reduce the filtering to loosen the sound up a bit. In my version of this I have 3 filter caps stacked. The bottom cap is 33uf and the switch, in full power mode, shorts the bottom 33uf cap to ground, thus ridding of it. You'll want to adjust your bleed resistors to compensate and acquire half voltage though.

Yes the switch also adjusts the bias volts.

[Russ]

Can this sort of thing be done with 2-phase full-wave rectifier, the kind with the 2 strings of diodes?

[lowell]

I think so as long as you have the two series stacked filter caps and bleeder resistors. The rectifier sort doesn't matter. If your rectifier is non bridged and the HV winding is center tapped, then it's really just your basic voltage divider. Say you have 2 filter caps in series with 220k bleeder resistors across each one. The 220k resistors are going to split the voltage from the main supply rail to ground. If you adjust those bleeders you can fine tune what you want you "LOW V" voltage to be at. Keep in mind that changing the bleeders will affect how much voltage the filters are seeing, but it won't matter as long as you're keeping them within spec.

Wait a minute I'm not making any sense. Forget everything I wrote above - wouldn't work. Clearly I did not even completely grasp the circuit... thanks to Merlin I get it. You would need to convert your rectifier per the Fender schem, which should be possible. Converting a fullwave bridge rectifier to fullwave non-bridge would not be possible cause of the need for an HV center tap. So you should be fine.

[Merlinb]

Can this sort of thing be done with 2-phase full-wave rectifier, the kind with the 2 strings of diodes?

I think what Lowell was trying to say is a qualified 'no'. If you did, what you would end up with is the normal voltage, or a voltage option which is twice as great, not half as great!

[Patrick M]

It is basically power-scaling with 2 settings. The best part is that it can relativily easily be done on any amp with a SS full-wave rectifier, even on fixed bias amp...