Antek Power Transformers

[imaradiostar]

A small dual primary transformer used backwards from a filament winding would make a great bias transformer, especially if the output grids are driven by mosfets- you'd get a +-160 volt supply if you do it correctly.

There's nothing wrong with adding windings to a toroid! It's pretty easy to do and it allows you to control the voltages.

jamie

[Tage]

All right I like the looks of that, although I will be using the UL taps for this amp so a screen supply wont be needed, but I will be using this set up in a future project.

So the mosfet is used as a buffer to give steady supplied voltage to the screens, and does so as a low impedance source.

I've been looking all over for my copy of Merlins power supply book but I must have left it somewhere strange...

[Tage]

SO for that low impedance mosfet fed screen supply, can I sue the ballancing resistors that are arcross 2 capacitors as my voltage divider? it should be around 1/2 of the HT wich would work well for my application.

[loudthud]

That would be ok except there will be significant ripple on that voltage. About half the ripple on the main B+.

[Tage]

Right so a second Voltage divider with it's own Filter capacitor @ the connection of the 2 resistors. Got it!

I have decided that I like this screen voltage source better than the UL connection I was going to use initially.

[imaradiostar]

You might find you need more screen voltage than 1/2 of plate voltage to reach full power levels with your plate voltage. One of my recent builds used a pot as the top half of the voltage divider so i can adjust the screen voltage anywhere from 1/2 of B+ to all of B+. There is really no reason why you can't have one regulator for the screens and one for the preamp but for many simple designs it's unnecessary. Your call. It'd just be another mosfet and a handful of minor bits.

The cap at the junction of the divider needn't be very big- you can calculate its charge time to select the right value. I'm pretty sure it's t=2*pi*R*C. Since there is such a large amount of series resistance even a .1uf at the junction of a 220k resistor and a 250k linear pot should filter out all the ripple and be almost totally clean while tracking thing plate voltage with a delay of only tiny fractions of a second. I suppose you could use a much larger cap for a "slow turn on" effect but it wouldn't track B+ nearly as well which I see as a disadvantage.

jamie

[Tage]

Sounds good. thanks for the clarification. So I could drop another voltage divider from the B+, maybe separated by a CR filter, to get my pre-amp B+ through another source follower. I like that idea, I can still separate the triode stages with CR filters right just to isolate one from the other.

[Tage]

I finally found my Power Supply design book, and found this mosfet regulator in the series voltage regulators. I'm gonna implement it in this build, although only one, no second one for the preamp as I think it would be unnecessary.

Thanks for the great suggestion Jamie, I thin I will use this in most future builds, in stead of a choke, it's as effective and a lot cheaper...

[imaradiostar]

No problem, happy to help. I have a similar power supply build coming together and I'm planning to feed the screens around 400 volts which, with minimal filtering, will work well for the preamp too. Mine is a cathodyne PI driven by a pentode "loop return" with a preamp that resembles a dual rect dirty channel and a Dr. Z single pentode clean channel. It's gonna be interesting! I'm also planning to implement a mosfet based loop send circuit. The tube count is low- a 6au6 out front, two 12ax7's for the "dirty" channel, and a 6GH8a on loop return and phase inverter duty. No negative feedback so it's got plenty of gain and it's simple. I'm using up junker PT's (voltage is too high, thus the regulator) and OT's but it if works well I'll build one in a real chassis with proper iron. I built it with the thought that it could accept almost any 8 pin bottle- plate voltage is around 550 volts and output transformer impedance is 6k6 so I'm going to probably start with 6L6's and see how it sounds, then go from there.

jamie

[Tage]

Yeah this is going to be used on a bass amp, gonna run a pretty simelar power stage, but probably with some Negative feedback around it just to tame it a bit, as it is intended to be a crunchy bass amp. The pre amp is rather simple, a DC coupled cathode follower running into a FMV tone stack then a parralel triode with adjustable gain on it's bypass cap and on to a 12au7 LTP so I can run it into heavierloads, 56k grid leaks, and a quad of 6550/kt88's

[Tage]

Hey digging up an old thread, but I wanted to inquire about how I calculate the dissipation of the mosfet in the source follower. if I plan to drop 200V do I simply multiply that be my screen+ preamp current to determine the power being dissipated in the mosfet?

[defaced]

Correct. Others will be able to give you an idea of how much, but I suspect that screen current is going to be dynamic, so you will want to add some margin to your calculation.

[Tage]

Yeah I figured it would fluctuate, so I'll just go with a little below the max rating on the data sheets. I have a feeling I'll need a pretty big heat sink

[defaced]

I'm pretty sure you can also parallel MOSFETs on a single node. Or maybe give every screen and the preamp it's own MOSFET. It'll make the circuit much larger and more parts, but distribute the heat dissipation and might make that aspect easier to deal with. You might be able to get away with using the chassis as the heat sink if you go this route.

[Tage]

Yeah chassis as a heat sink always kinda sketched me out. Just because there would be a lot of voltage only separated by a thermal pad and a screw to ground... I suppose I'll see what kinda heat i'm looking at and maybe parallel them up...