Dual secondary PT?

[lsmith_33]

Hi, I'd like to design a 200W bass amp. I realize that there are several designs on the net. A lot of these have dual-voltage secondaries, a high voltage secondary winding for the plates, and a lower-voltage secondary for screen grid and preamp voltages. If I wanted to do something like this, could I use a single-secondary high-voltage power transformer like the Hammond 700 series, and a smaller power transformer for the screen grids and preamp voltages?

[VacuumVoodoo]

Yes, absolutely.

[Prairie Dawg]

You may be describing an ultralinear output transformer, which has taps on the primaries at a specific level-memory fails me but it is maybe around 70 per cent. To see how it's done and to get a comparison of what it would take to undo it compare a standard Twin Reverb schematic with an ultralinear 135w Twin Reverb schematic. Of course the power supply is different but we're not looking at that.

[roberto]

Prairie, I think you are confusing OT with PT.
lsmith is talking about one PT to supply plates, and another one with lower voltage and lower current for screens, PI and preamp.
If you supply screens from another PT, you are not going with an UL OT, but you want your power tubes to work in standard pentode mode.

lsmith, of course you can. Take care of voltages and currents needed.

[Firestorm]

Not to plug KO'C, but one of the TUT books (#3 probably) has an SVT discussion that shows three or four different ways to accomplish this. Or you could design around a quad of KT88s and run both plates and screens at about 600V, which should get you your 200W.

[ToneMerc]

Here's a single power transformer method to accomplish your goal in the style of a MusicMan power supply. B1 has around 700V loaded and tapped at the junction of C1 is B2 which is half of B1. In this config the preamp plates are fed via B1 and the screens are fed via the B2 node .



TM

[Prairie Dawg]

Prairie, I think you are confusing OT with PT.
lsmith is talking about one PT to supply plates, and another one with lower voltage and lower current for screens, PI and preamp.
If you supply screens from another PT, you are not going with an UL OT, but you want your power tubes to work in standard pentode mode.

lsmith, of course you can. Take care of voltages and currents needed.

That's true but that's most of what you'd accomplish with an ultralinear output transformer because what is described is why they were used-to run the plates hard and the screens at a lower voltage. Seems to me you could probably figure out the preamp supply with the appropriate dropping resistors off the B+ node after it feeds the output transformer center tap and proceed as usual.

But yeah, more than one or two ways to skin this cat. Depends on the objective and as I've had my face buried in a formerly ultralinear twin I've been thinking about it a lot.

[lsmith_33]

I thought ultralinear output transformers kept the screens at or around plate voltage, since they're basicaly from the same supply. I guess you could do it that way. I guess I'm going to have to invest in the TUT series.

[Firestorm]

Ultralinear OTs do run plates and screens at virtually identical voltages, and so (like the majority of MI amplifiers) require you to select a B+ that doesn't exceed the screens' max voltage rating.

[Kagliostro]

You can use:

A single transformer with more windings

A transformer with only one winding (enough current) and a doubler
(the doubled voltage to plates, the unduble B+ to screens and preamp)

A transformer with an enough high B+ and a SS regulator (mosfet usually) to lower voltage for screens and preamp

And, as you say, also one transformer with adequate voltage for plates and one other with adequate voltage for screens and preamp section

Kagliostro