Advice on power supply

[iccaros]

in current build I need to deliver about 26v for heaters for 26A7GT x2 and I want about 70V B+

I had this in mind, but have two questions, one am I wrong in my thinking, and two what size should C1, C2 and C3 be?



Thanks

[PRR]

26A7GT is rated 50V max..... just saying!

Your plan apparently has no return for the B+, so I can't comment even if I could read the numbers.

[iccaros]

sorry, re-did the drawing to make is easy to read, I am not sure where the return would be, the ideal was from here,  http://en.wikipedia.org/wiki/Voltage_multiplier
and somewhat http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/bargainbin/silvertone1485.pdf

I am wrestling with options.. Most likely making this too hard.

I got 71v from another design, but the datasheet does say 50. So I should just leave this alone at 25.2  or about 35v with full bridge..

[R.G.]

C3 is the output filter cap seen by the amplifier. It's minimum value is set by the needs of the amplifier. C1 and C2 are set by the needs to put charging pulses into C3. In general,  They can be about the size of C3 or slightly smaller in a 60Hz system. Making the pump capacitors too small means it takes a long time to charge up C3 (as measured in cycles of the input power frequency) which is OK in high frequency switchers, not so good in 60 cycle stuff. Making them bigger than C3 isn't cost effective.

Since C1 and C2 are effectively in series with the incoming power, their impedance (1/(2*pi*F*C) acts like a resistance in limiting current flow. So making them large means lower "resistance" in series with the power supply, and better power regulation, lower ripple, etc.

It's OK to set C1 and C2 big enough to get the minimum-to-maximum load sag what you need, and then make C3 big enough to keep ripple down, or as much bigger as you want C3 to suppress ripple more.

[iccaros]

Thank R.G.

Last question, Where do I tap ground?

[PRR]

> Where do I tap ground?

"C3 is the output filter cap seen by the amplifier."

FWIW: the plan drawn above will give about 32 Volts DC.

It may be simpler to use a simple 24VAC winding, FWB, cap, for 35VDC, and a dropping resistor to get 26VDC for the heater.

I do not generally "like" DC for heaters, especially in high-level stages.... it seems a waste of parts.

But in this case the tube WAS designed to run off a 25V battery plate and heater, so it may just work out better.

But there are much larger questions.

Why do you think you want 70V, even 50V, on 26A7, and why two of them?

This seems a complex way to get several watts. The tubes are scarce. The OT is a real oddball (~~1KCT @ 3W).

25L6 or 6V6 will make several Watts simpler with common parts. 6V6 is even available on Saturday night.

Do you have any other tubes in this amp/ How will you heat them?

[iccaros]

To answer the question, I was reading Ax84 and someone suggested 70v on the plates.. but after thinking about it.. I do not see a lot of gain for risking killing the tube. So I will just use a full wave bridge instead

But I saw this doubler and did not quite get how it worked with no "Ground" and it peeked my interest..

I have 12.6v CT (rat shack 2A for $5)  for the preamp tubes, which are 12u7. I could use a dropping resistor, but that is .6 amps each, which is a lot of heat at 6 volts, this is why I wanted to tap off before rectification.

What am I building.. Well I am building a low wattage stereo amp. I have 10 of these tubes I got in 2 sleeves from a Navy sell off at DRMO, so they were cheap, and at low voltage they are a learning point for the kids.. my son is 10 and his younger sister at 6 is interested in what I am doing,

I have OTs so its not a big deal.

But the doubler was interesting, its from Wiki so I may just go back to a this, as I understand it



I just thought it would be cool to try something new.. I do have some 7695 which would work with a 50v and a doubler, I may have to use to investigate..

Thanks for the information its fun to put back in practice what I learned in school 20 years ago, too bad I did not use a lot of this for work.. I may have remembered some of it.

[PRR]

> doubler was interesting, its from Wiki

The numbers are not correct unless the "winding voltage" is specified as Peak. (We usually buy trannies specced in RMS.)

> did not quite get how it worked with no "Ground" and it peeked my interest..

"Piqued"; and floating supplies are perfectly OK, but a supply with no return makes no sense.

> a learning point for the kids..

Then stick to 26V supply. It is hard to get hurt there. 70V really stings, really ought to be protected against finger-contact, and that won't happen when building stuff around kids. The 0.4 Watts available at 26V is enough to show the ideas and also enough to be annoying to parents.

The obvious power supply is a 24VDC wall-wart from an old printer. This avoids all exposed wall-voltage. Pairs of 12V heaters can go in series.

[joegagan]

love the part
"6V6 even available on a saturday night.

reminds me of austin texas. i never saw any other city where even the convenience stores carried guitar strings ( dowtnown austin, 2 AM.)