Mosfet B+ reducer in Voltage Doubler

[half_smith]

Where to place a Mosfet B+ reducer in a Voltage Doubler circuit?

• Japanese Univox (Lafayette) U-320 - designed with a voltage doubler and meant to run 7591's
  Someone earlier changed the transformer and rewired it for 6L6GC's
  Owner wants it returned to 7591's
  The new transformer is putting out 268vac across the secondaries, so running through the voltage doubler gets pretty high.
  New OT is costly and may require fitting...
  Mosfet zener to reduce B+ sounds good but where exactly does it go in the voltage doubler circuit.

Did I mark the correct points?
*EDIT here's the schematic MM marked


Thanks!

[martin manning]

Did I mark the correct points?

You're almost half-right. There isn't a single DC ground return path at ground potential as in a two-phase or FWB rectifier circuit. There are two returns, one at ground and the other at B+/2, so that FET's tab must be insulated from the chassis. Two FET circuits are required (one at the negative terminal of each of the stacked caps), and you would reduce the Zeners to half of the desired voltage drop. I don't recall ever seeing B+ reducers applied to a voltage doubler supply. I think I Would just use a dropping resistor and another filter ahead of the plate and screen feeds to knock the voltage down.

[Stevem]

I think it’s best if the owner wants a trouble free amp is to get a new PT.

Mosfets can introduce a fair amount of noise if not mounted far enough away from gain stages and or grid wires.

A common stand up type for a hot rod deluxe isn’t too expensive and a search might find a remove original one from a amp that got a upgrade.

[Astronomicum]

I've seen mention of this scenario but couldn't find the thread again

https://ampgarage.com/forum/viewtopic.php?t=27035

[R.G.]

I think it’s best if the owner wants a trouble free amp is to get a new PT.

Mosfets can introduce a fair amount of noise if not mounted far enough away from gain stages and or grid wires.

A common stand up type for a hot rod deluxe isn’t too expensive and a search might find a remove original one from a amp that got a upgrade.

Good advice, Steve. In general, it's easier to make simple things more reliable. A single, properly sized PT would be simple and probably more reliable.

MOSFETs are a two edged sword, like so many "amazing" things. They have huge input impedance, high voltage and current capabilities, and very high frequency response, any of which can get you into trouble if you don't mange them properly. But if you do manage them properly, they can solve many problems.

@half-smith: you have two easy choices of places to put a MOSFET amplified zener. The issues are how to get the heat out of it, and whether you need two of them.

A MOSFET in the rectifier return offers easy(-ier) heat-sinking if you can just bolt the drain of an N-channel device to the chassis, forcing it to be at DC ground. Your power transformer doubler etc. makes this difficult to do in the easy-peasey way that FWB or FWCT setups use.

You could put a MOSFET zener either in the B+ high side of the final filter caps or in the low side. The high side is simple, but requires a heat sink that floats on top of B+.

The low side means that you could move the circuit "ground" to the drain/(+) end of the MOSFET zener, and tie the source/(-) end of the MOSFET zener to the negative end of the capacitors you have filtering the power supply now. The rectifier setup you have now is only "grounded" because you have the negative end tied to the point you have designated as circuit ground. You could stick the MOSFET zener between the negative end of the rectifier setup and your circuit ground and then the DC voltage to the circuit would be lower by the MOSFET zener voltage. But you would also likely need another filter cap between B+ and circuit ground to act as a reservoir cap for AC signal reasons. This is more complicated, but would allow an N-channel MOSFET to have its drain on the chassis/ground for heat sinking.

[martin manning]

The low side means that you could move the circuit "ground" to the drain/(+) end of the MOSFET zener, and tie the source/(-) end of the MOSFET zener to the negative end of the capacitors you have filtering the power supply now.

Ah yes (and Doh!), a single reducer could be placed there, as shown below. A pair of them as I suggested above also works. I'm not really a fan of these thing since they seem prone to failure.

[Astronomicum]

• Japanese Univox (Lafayette) U-320 - designed with a voltage doubler and meant to run 7591's
  Someone earlier changed the transformer and rewired it for 6L6GC's
  Owner wants it returned to 7591's
  The new transformer is putting out 268vac across the secondaries, so running through the voltage doubler gets pretty high.
  New PT is costly and may require fitting...

There are two versions of the U320: the U320A (Lafayette), and the U320APB (Univox). The Univox uses a 780VCT PT.

https://el34world.com/charts/Schematics ... 320APB.pdf

I have a PT for the Univox version which I would be willing to sell or trade for the 250V PT currently installed.

[R.G.]

[...] I'm not really a fan of these thing since they seem prone to failure.

I think it's more like they are intolerant of not thinking things through before applying them or of being sloppy about mounting and wiring them. I put that under the "proper management" topic. The user simply has to know the rms and peak currents that they will carry, as well as the peak and average voltages across them, the heat they will produce; and provide enough heat sinking to keep the junctions down to about 175C. At least a nod to stopping RF oscillation and amplification is a good idea.

They will not withstand a few seconds of short circuiting or spikes of over +-20V or so let through to their gate.

In return, you can adjust voltage as you like, with resistors instead of going back to the transformer catalog for a new one, or burning off the excess as heat.

It's a tradeoff.

[half_smith]

thanks for the replies!

I'm not going to spend much money on this amp, so if we're not happy with the power zener approach, I may just lobby the owner to stick with 6L6's

You're almost half-right.

I am half-smith lol, the other half is dyslexic and dim.

you would also likely need another filter cap between B+ and circuit ground to act as a reservoir cap for AC signal reasons.

RG, hello maestro, I've seen you describe this additional filter cap (in another thread somewhere, again I could not find specifically)
While I'm sure your explanation is clear, I am still unsure where to install this in my schematic...
will someone please mark the schematic with this additional filter cap?

[martin manning]

I believe RG is referring to adding a filter as shown below, perhaps without the dropping resistor. However, if you do add the 390 ohm resistor you can drop 50V. Use a 10W resistor, and it will dissipate about 5W, same as the MOSFET would have to do. Simple, reliable, and cheap.

[half_smith]

thanks Martin!

Is this the whole picture then?

[half_smith]

... Use a 10W resistor, and it will dissipate about 5W, same as the MOSFET would have to do. Simple, reliable, and cheap.

I thought the reason folks were recommending the Mosfet/zener was because a dropping resistor will react badly to fluctuating current and sag and suck tone and inevitably melt?

[martin manning]

Yes, but how much voltage are you looking to drop? I’m suggesting you don’t need the FET.

[half_smith]

Yes, but how much voltage are you looking to drop?

Somewhere beween 60-80v ?

[martin manning]

I think a resistor could do that, and I think a 390 would get you down to the schematic voltage. BTW, do you have a link to the full schematic?