Parallel Rectifiers Balancing Resistors

[rp]

I came across this and checking if I got it right, see my modified Fender 5E6A rectifier drawing. I've heard of doing this with ss diode strings and it's similar to the resistors across series e-caps, but Leo era Fender engineers were pretty sharp and they left it out, so: Does it matter? Just another thing to fail? About what wattage should the resistors be?

If you're stuck with only one winding, but it is capable of powering 2 x 3A rectifier filament... then your option is to simply parallel the two rectifiers, adding a small value of resistance in series with each anode (22-47 ohms) to ensure equal current sharing.

Post#4 - http://www.diyaudio.com/forums/tubes-va ... fiers.html

[martin manning]

You would need four resistors, one at each anode. The idea is the the feed from each end of the PT secondary is split between the rectifiers it feeds by two resistors. The resistors are going to be pretty big since each one is handling 1/4 of the current from the secondary. That's how I'd estimate the wattage, and then use double that value. Notice MESA did not do this on their Dual Rectifier models.

[rp]

Ok, so like this? Yes, seems unimportant considering many of those old Fenders are still cooking. But I guess it's textbook right, so if I ever do a parallel rectifier I might put them in.

[martin manning]

Almost... the resistors connected to the second rectifier's plates go back to the PT secondary winding, not to the first rectifier's plates.

[Stevem]

This set up may interest you.
With this you still get a natural recto sag with a 5AR4 ,and should the tube recto fail you just yank it out , replace the fuse and the amp still plays!

[tubeswell]

Ok, so like this? Yes, seems unimportant considering many of those old Fenders are still cooking. But I guess it's textbook right, so if I ever do a parallel rectifier I might put them in.

If you wire it like this, the 2nd rectifier will get half the vac that the first one is getting because of the 47R/47R voltage divider on each side. For balance, the resistors (and plates) must be in parallel with each other.

[martin manning]

Like this I think...

[rp]

Almost... the resistors connected to the second rectifier's plates go back to the PT secondary winding, not to the first rectifier's plates.

Three strikes? OK Martin, I've now broken new bounds in what can be achieved with cut and paste. I am the king of adulterated Fender schematics. Maybe I should learn a proper schematic program

[martin manning]

Yes, that's it. Doesn't seem to be an issue though. Here's anther way of doing dual rectifiers from a Scott Hi-Fi circuit. Don't know why they chose to parallel the diodes the way they did; still no balance resistors. Note B+ feed from CT.

[Stevem]

Yup, I botched it Martin, thanks!

[rp]

I win the botching, I got the simplest thing in the world wrong, twice!

But I was thinking like the resistors across series caps or the snubber resistor cap networks you see across diode strings sometimes. But, yes I now see it's what you always see recommended in Brit hifi, like from the Mullard circuits book:

The power supply is conventional and uses a Mullard indirectly-heated, full-wave rectifier, type GZ34, in conjunction with a capacitive input filter. The values of the limiting resistors R26 and R27 will depend on the winding resistances of the mains transformer used. Their purpose, when required, is normally one of voltage control only…

You see it recommended but never in any real circuits, guess the resistance in the PT sec is sufficient to protect the rectifier on start up - I assume that's all they are for? Maybe in hifi peeps feel it interferes with the sound.

Here's anther way of doing dual rectifiers from a Scott Hi-Fi circuit.

That Scott set up is pretty clever, looks like it would enable real clean wiring.

[martin manning]

You see it recommended but never in any real circuits, guess the resistance in the PT sec is sufficient to protect the rectifier on start up - I assume that's all they are for? Maybe in hifi peeps feel it interferes with the sound.

Or even in normal use after start-up. Seems like there is enough resistance in the windings that it isn't a problem though.

That Scott set up is pretty clever, looks like it would enable real clean wiring.

Absent the CT on the 5V winding I'd be inclined to take the B+ from the 2-8 connection between the rectifier tubes.

[drew]

I think I recognize that Scott circuit ... and I meant to ask, is adding silicon diodes ala RG Keen's "immortal amplifier" mod a good idea for paralled rectifier tubes, and how do you execute it?

[martin manning]

In the Scott circuit you could put a Si diode in front of each tube rectifier plate, same as for a single. Or, since the plates of each tube rectifier are tied together you could use one Si diode (or two in series) on each side of the secondary winding.