where to find transformer to use

[dsavitsk]

I am thinking about building this circuit (it is a simple 5687 based preamp using a 6x4 rectifier tube for those that don't want to click) as a first tube project. The author calls for a transformer with secondaries of 350-0-350V@100mA, 3.15-0-3.15V@2A, and 3.15-0-3.15V@2A.

I can find plenty with 350 - 0 - 350, 6.3v, and 5v.

What is my best option here? It seems like I can (a) use what I found and parallel the heaters of the 2 tubes, (b) ignore the 5v secondary and add a second small transformer for one of the heaters, (c) get a custom wound transformer, (d) rectify the 5v and run it through a lm317 to get it up to 6.3n, or (e) something else.

As an unrelated question ... will the 350-0-350 rectify to about ~350 or ~700. I have only ever done SS power supplies and I am used to using +/-18V to get 36v rectified. This seems to be something different, though. Any links to for reading about tube rectification?

Thanks,

-d

[alexistheo]

my advice is to put the 6X4 on the 6.3V winding and rectify the 5V for the 5687 heater. it is likely, though, that the windings will all be spec'd for more current than those tubes draw, and your voltages will be high. i'd get some dropping resistors while you're ordering parts.

as for the HV voltage after rectification, it will depend on whether you use cap input or choke input. cap input on a 350-0-350 will be way too high for this circuit. use PSU designer to simulate.

[leadbelly]

The 5687 will also work with 6V heating, it is just that it will draw twice as much current (900 mA). If you buy a xfrmr with at hefty 6.3V winding it will work fine, but you will need to play with the R value of the CRC filter used in that schematic.

Your confusion over why the voltage is rectifying to a lower value than what you would expect is not a specifically a tube issue. Just read up on "full wave" and "half wave" rectification, and center-tapped versus non-tapped secondary xfrmrs.

[Sch3mat1c]

Tubes typically produce from 1x to 1.3x the AC voltage. 300-0-300 FWCT (CT grounded, same as getting +/- voltage from a CT'd transformer and FWB, but skip the negative half) might produce 300VDC, cap-input, near the rectifier's maximum rating (maybe 125mA for a 5Y3, 300mA for 5U4, 50-100mA for 6X4/6X5). This might rise to 350-400V under a lighter load, say 20-100mA, and obviously at zero load, rises to 1.4142x (usually 420-440V from 300-0-300, exciting when the filter cap is only 350V!).
Choke input is in the range 0.8x to 0.9x, much better regulation but needs more input voltage plus a large value choke that can tolerate the voltage put on it.
You can also use SS diodes, which do the exact same thing you're used to - the numbers are just 10 times larger is all. SS tends to place 1.3-1.4 cap input; good regulation, no bulky choke (though additional filtering can be had with a much smaller 0.5 to 5H choke, useful for SE and triode amps with bad PSRR).

I don't see anywhere it says you need to use a CT'd filament. 6.3V is all that's needed for the 6X4 and 5687, and since the 6X4 uses a cathode, it can all be powered off the same winding (though some prefer not to in case of H-K faliure on the rectifier; a crowbar safety could be added in case of this).
The transformer you are eying is monstrously, horrendously oversized for a measly 5687. It's a poor little preamp for pete's sakes! You only need maybe 20mA for this thing, and since signal is nowhere near 80V output, you can drop it to 100-200VDC (though you'll have to change a few resistors).

*Checks* hmm, it would appear people have been busy replying while I type. I got here first!!

Tim

[dsavitsk]

OK, because this is pretty new to me, and because these voltages are high enough to instill fear, I want to make sure I am very clear what is going on before I tackle this project ....

Looking at something like a Hammond 300 series transformer, I can use one set of secondaries for each heater, rectifing the 5v to DC and regulating it up to 6.3 and using the 6.3 AC for the 6x4 heater. That seems pretty straigt forward.

The other part has me a bit confused. From the schematic (the computer drawn one, not the hand drawn one) it looks like I am aiming for about 250V.

Since the PS uses 2 chokes, does this means that I need a little over 300V of rectified DC? If so, would a 150-0-150 be sufficient if I just ignore the center tap. I guess this is a terminology problem --- does 150-0-150 mean the same thing as +/-150 or is it that both sides are +150 but at opposite times? If the second, which I guess seems right, then I guess I need 300-0-300 or so.


Oh, what is H-K failure? I get that ctFW is center tapped full wave, but how about FWB?

Thanks

-d