Rectify AC to DC for heating

[woodturner-fran]

Hi all,


There was a thread some time ago I read where a fella had used his 6.3V winding, rectified it, put in I think 2 caps and 1 resistor and ended up with about 6VDC back out for his heater supply. Anyone remember the post or the component values?

Also my xformer has 1 HT winding and 2 6.3V winding. Each of the 6.3V windings supplies half of the valves. Wouldn't it be OK to join these together so that I would only need one recifitication circuit? (assuming there are actually 2 windings and not a CT tapped one?)


Fran

 

[Nature B0y]

There was a thread some time ago I read where a fella had used his 6.3V winding, rectified it, put in I think 2 caps and 1 resistor and ended up with about 6VDC back out for his heater supply. Anyone remember the post or the component values?

I don't recall that post, but there are a number of variables that make the direct use of such a circuit problematic anyway. For example, the value of the resistor is very dependent on the current supplied. Also, the mains voltage level in your area, the losses for your particular transformer and rectifier figure in there as well.

I've done what you are proposing, and I determined the value of the resistor experimentally. I.e., first I guesstimating the value using simple Ohm's Law, 0.7V per rectifier, knowing the current needed, etc. Then I measured the actual voltage, and adjusted the resistor accordingly. It's nice to start out with a resistor that's a bit too big, since you can parallel a lower powered larger valued resistor to get the correct result. With a bit of luck you don't have to unsolder anything.

Also my xformer has 1 HT winding and 2 6.3V winding. Each of the 6.3V windings supplies half of the valves. Wouldn't it be OK to join these together so that I would only need one recifitication circuit? (assuming there are actually 2 windings and not a CT tapped one?)

That would probably be a bad idea. It seems unlikely that the windings could be parallelled without one of the windings hogging more than it's share of current. Are they at least rated for the same current? Even then I personally wouldn't unless the manufacturer said it was ok, but maybe I'm paranoid.

Often multiple windings like that are used to supply power to distinctly different parts of the circuit. For example, does one of the windings run the heaters for the rectifiers? If so, then you can't join them together due to heater-cathode voltage restrictions, even if they were otherwise identical.

 

[woodturner-fran]

Wow, managed to find the thread - here it is!

I have to investigate the 2 windings yet, but from what you are saying it would be better to put in 2 rectifiaction circuits? I can do that without much hassle, I just wondered if it was necessary.

The 2 windings supply exactly half of the valves each, 1 input and 2 power valves (6N1 and 2 x 6P1).

thanks for the help, I'm learing a lot, but the curve is a straight line to the sky. Still waiting for the 2 morgan jones books, should have been here by now!


Fran

 

[G]

Wow, managed to find the thread - here it is!

I have to investigate the 2 windings yet, but from what you are saying it would be better to put in 2 rectifiaction circuits? I can do that without much hassle, I just wondered if it was necessary.

The 2 windings supply exactly half of the valves each, 1 input and 2 power valves (6N1 and 2 x 6P1).

thanks for the help, I'm learing a lot, but the curve is a straight line to the sky. Still waiting for the 2 morgan jones books, should have been here by now!


Fran

Since the total current drawn by the heaters is not going to change it would be fine to parallel the windings and rectify them with one bridge. You still have to calculate the value of the dropping resistor. What is the total current draw of the filaments?

 

[woodturner-fran]

G: 4x 6P1 at 500mA each plus 2 x 6N1 at 600mA each = 3.2A +/- 10%.


So if I'm going to use schotkky diodes I'd want to be getting >4A wouldn't I? I see 3A ones a good bit - would anyone have a RS number for ones with a higher rating?

I know I have a good few 3300uF 35V caps here but I'll have to scavenge around here to find something much bigger than those. I suppose I could always parallel a few of them.

Fran

 

[Nature B0y]

There's always some difference of opinion about parallelling secondaries, but you absolutely *must* get the phase correct if you try hooking them together.

If you look at transformers that are manufactured to be parallelled, you can usually immediately see that the winding geometry is identical for windings that can be parallelled. This is unlikely to be the case for your transformer, and you can verify that by measuring the DC resistance with a ohmmeter for the two 6.3v windings. If they are identical (or very close), then you should have no problems. If the windings are overrated in terms of current delivery by the same percentage amount that the resistance is different, then you are likely to be ok. You could easily Spice a first-order approximation to see roughly how much current-hogging you're likely to get.

Two (identical) bridges allows you to hook them up without regard to phase, and if you make two separate circuits, the dropping resistors will be approximately twice the resistance (and half the power) of the single.

I personally would make (and have made) two separate circuits, as I'm more likely to have those parts lying around.

 

[woodturner-fran]

OK, I'll be taking your advice and making 2 circuits. To be honest it makes life easier for me because I can now use the 3A schottky diodes which I can lay hands on easily and I would have had to buy in heavier duty ones otherwise. I have some 3300uF caps here so I think I'll parallel a few of them to up the capacitance a bit. This will cost peanuts, will be a good exercise for a learner to do and if it helps reduce the hum a bit I'll be a very happy bunny! If it doesn't, I can always take out the circuit and use it in another amp in the future.


thanks again,

Fran