Stacked DC sources for multiple HT rails

[airguitarist]

Hi,

I'm building a 5w version of a slo using el91 as output tubes in push pull. I'm using junk-box parts and want to retain the pre-amp HT voltages, the power-amp will be lower ~ 250v.

I have a transformer rated at 100ma for 250v ac output, and was thinking of using this to provide the HT rail for the EL91's - I would need to drop some voltage for the EL91's , and then to stack a second floating dc voltage from a 6Va transformer with 100v ac output on top of the rectified output of the 100ma transformer to provide a second HT jacked up to around 500v dc or so to provide 15ma supply for the pre-amp tubes.

As the pre-amp tubes draw less current, I assume it's OK to stack a second lower current dc source on top of the power amp rail to supply the pre-amp?

The pre-amp will be drawing ~ 15ma at ~ 460v dc after smoothing, the power amp ~ 50ma at 250v dc after smoothing.

Can anyone see any issues with this approach? Are their any issues with possible failure modes if the jacked up supply isn't earthed directly and is instead connected to earth via the secondary of the first transformer?


Thanks,

Jonathan.

[PRR]

It could work. But you need to draw it out to check.

[airguitarist]

Hi,

Sorry for the delay in getting back to you, also I apologize for the untidiness of the attached - drew it out in a rush as had to collect the kids from school.

I have two transformers each are 115V+115V primary to 125v @ 100ma secondary.

My thoughts were to float the rectified voltages on top of one another while running them back to back.

Please see attached. Would this work?

I need a low B+ for the power tubes and a high B+ for the pre-amp, and am trying to use the parts I have.

Thanks,

Jonathan

[kagliostro]

I was wondering if the red connection (see my draw) is correct or if that resistor and cap must be connected to ground


I ended thinking you are correct and that way is possible to use a lower voltage e-cap





Franco

[DummyLoad]

something like this... use 350V caps for all except the 420V nodes cap - use 500V.

one of the bleeders is arguably redundant.

--Pete

[airguitarist]

Thanks for redrawing Franco. Yes, that's what I had in mind. Just a little unsure of what will happen under load?

[airguitarist]

Hi Pete,

Yes wasn't sure about the bleeders, but thought I might need something to help balance the currents / voltages in the 2 ht supplies. Also I'll be feeding both anode and screens of the power tubes from the choke filter, otherwise the choke draws too little current to work effectively. Alternatively I might dispense with the choke altogether.

[shooter]

one of the bleeders is arguably redundant.

One of my worse pokes from DC came from a PS that had a mechanically broke bleeder, redundant is good    (NOT assuming is even better :)

[kagliostro]

I remembered where I've seen such a solution to use lower voltage (only   400V) e-caps on a 2500V PS

http://www.chambonino.com/construct/constwire2.html



Franco

[airguitarist]

Wow! That's some power supply Franco!

Thinking about it, I'm sure I've seen similar in some old tektronix scopes.

I think the guy who designed that amp is based in Nottingham, uk where I live?

Thanks for the link 🙂

[kagliostro]

I think the guy who designed that amp is based in Nottingham, uk where I live?

As far as I can know, yes

Some years ago I had a bit of correspondance with him, the guy that did the re-drawing of the schematic (Giulio) is a friend of mine

Franco

[airguitarist]

Ok, turns out the transformers couldn't provide enough current, so am going to have a go at winding some.

Scrap cores stripped out after a couple of days soaking in petrol.