Two Rectifier Circuits

[RWood]

I'm converting a Conn to a Bassman 6G6 single-channel.

The Conn has a massive PT and has two separate rectifiers: a 5Y3 and a 5U4. Each has its' own filament leads and its' own HT leads. The two share a CT.

Unloaded readings on the pins are 385 - 385VAC for the 5Y3 socket and 312-312VAC for the 5U4 socket.

For test purposes, since I just finished up the filtering and choke work, I took the B+ off the lower voltage secondary but used a 5AR4, which is what was called for in the 6G6 circuit. With only the one tube installed, I got 421VDC. Tomorrow I'll take the B+ off the other socket to see how much higher, and closer to the 450 target, I'll get.

Anyway, I started to think about using both of the rectifiers, possibly a pair of 5AR4s. Looking at the tweed circuits that used two rectifiers (5E6, 5D8, etc) I see that there was one filament secondary and one HT secondary, and that they were simply paralleled, with the B+ coming of both tubes' pin 8s.

Can I do this with the Conn? Or am I asking for trouble because of the completely separate secondary sets? If so could I use one set of filament + one set of HT, parallel a la Fender and disconnect the unused sets? If I did I'd be drawing much more off of the set that's being used, but nothing off the set that wasn't.

Ideas?

[Steve Conner]

The transformer has a separate HT winding for each of the rectifiers. So before you go paralleling the rectifiers up, you need to find out whether the windings have the same voltage and current rating.

If they don't, then the rectifier on the higher rated winding will do most of the work and the other one will just be along for the ride. And your measured results suggest that they don't.

I'm guessing the Conn was an organ amp, and one rectifier was for the power amp itself, while the other one powered all the tubes in the organ. The PT would be massive because tube electronic organs contained a lot of tubes. This massiveness doesn't necessarily translate into massive power available for your power amp.

[Enzo]

50 little tubes all using a milliamp or two doesn;t add up to much power for your B+, but their heaters do, and that is most likely why the PT is large.


When he said the HV windings shared a CT, I got the impression the lower voltage leads were taps off the higher volatge winding.

[Steve Conner]

When he said the HV windings shared a CT, I got the impression the lower voltage leads were taps off the higher volatge winding.

Oh, well if they're taps, it should be fine to just connect both rectifier tubes to whichever set of taps gives you the desired B+ voltage.

The reason is that, if they're taps, all of the winding has to be wound with wire thick enough to take the total current draw. If they were separate windings joined only at the CT, then the high voltage one could be thick wire and the lower voltage one thin. A few minutes with an ohmmeter could get you the answer.

[RWood]

I appreciate the responses.

There is continuity from the one socket's HT connection to those of the other tube: about 33Ω from pin 4 to the corresponding pin 4 and the same from pin 6 to the other pin 6. Then about 3Ω from either pin 4 to either pin 6.

Is this showing that the higher voltage of the two is the full HT winding, and that the lower is tapped off the same winding and just has fewer turns?

And if that's the case, as you said, I can take whichever set I want (but not both) and parallel the two tubes. I presume I would leave each filament set intact, or would I disconnect the one I am not using and jumper over to the ones I am ? Likewise, is it okay to disconnect the HT set I am not using, shrink wrap each lead a couple times and coil it up, out of the way?

Btw, with a 5AR4 in the higher voltage socket but no other tubes, I get 520VDC. I assume this will drop some when I load in two 6L6s and three 12AX7s. And yes, this amp did have the multitude of 12AU7s but it also had four 12V6GTs, too, for two additional power amplifiers. I won't be using the 12V6s.

Thanks,

RWood

[Steve Conner]

Is this showing that the higher voltage of the two is the full HT winding, and that the lower is tapped off the same winding and just has fewer turns?

It would show that if the readings were the other way round: 3 ohms between corresponding plate pins on the two rectifier tubes, and 33 ohms between the two plate pins on the same rectifier tube. (Though to be pedantic, you should measure 6 ohms less between plate pins on the lower voltage rectifier socket.)

The way round you have it, I've no idea what it shows. So I'm going to assume that you got the readings switched.

You have two 5V filament windings, so I'd use one for each rectifier, unless you suspect that one of them might not have enough current capability for the kind of rectifier you want to use.

Yes, you can stow the unused HT taps out of the way somewhere. You're lucky, in that you don't have to use the higher voltage taps. If the finished amp's B+ turns out too high, just hook up the lower voltage ones instead.

It's interesting that the extra power tubes were 12V6s and not 6V6s. How many heater windings does the transformer have? Maybe they're all 12.6V. Were the 6L6s' heaters hooked up in series?

[RWood]

That makes sense, and although I think I got the readings right, with leads to each tube the same color, it could be that the assembler swapped them on one of the tubes and wired them reversed with respect to the leads on the other rectifier. With no reason to measure the impedance at that time I can see why it would not have mattered.

It's interesting that the extra power tubes were 12V6s and not 6V6s. How many heater windings does the transformer have? Maybe they're all 12.6V. Were the 6L6s' heaters hooked up in series?

Aside from the two 5V windings there is one 12.6V filament winding with a center tap. Each 6L6 gets one leg of 6.3V to its pin 2; the pin 7s are grounded. The 12AX7s are wired to 12.6V, too, with nothing on pin 9. It's funky but it seems to work.

I applied B+ to the power tubes last night after building the bias supply; got 480V on the plates and set the cathode current for each tube around 40mA. I am a little concerned that the screens are also around 480V as the only drop to the screens in the 6G6 Bassman circuit is the 470Ω resistor connected to each. Screens come in after the choke, btw.

You're right - it's nice to have two choices for B+. The first one is a little low, and it being easier to drop voltage than add it, I'll probably work with the higher set, maybe put a zener on the center tap and see about lowering it by 30V. If you have other thoughts I'd like to hear them.

Thanks,

RWood