Sunn Studio PA rectifier

[voyager]

Hi everyone, first post here. I have a Sunn Studio PA that was converted to solid state rectifier at some point before I got it. I’d like to convert it back to stock with a tube rectifier, but the wires from the transformer don’t seem to match the schematic. The schematic shows two yellow wires coming from the transformer, connecting to terminals 2 and 8 of the rectifier socket. There are no yellow wires, but there is one orange wire that has been capped off. This is a 3-6PT transformer as shown on the schematic, but my amp is from ‘68, and the schematic is ‘67 so I’m guessing they made some changes.

Terminal 2 of the rectifier tube has solder on it, which confirms that it used to have a wire connected. My guess is that the orange wire connects to terminal 2. If I test the orange wire with my multimeter with the other probe on terminal 8 of the rectifier socket (where the schematic show the other yellow wire going), it registers around 90v when on standby. The schematic seems to show the yellow wires as being 5v, so that makes me less confident in my guess about the orange wire since I’m getting 90v there.

Does anyone know what the stock arrangement would be here? Any help is greatly appreciated!

-Jeff

Schematic: https://schematichell.com/sunn/studiopa.jpg

[Helmholtz]

Pleas post correct schematic.

[voyager]

Sorry, didn’t realize the link didn’t paste correctly. https://schematichell.com/sunn/studiopa.jpg

[Helmholtz]

The missing yellow wires belong to the 5V heater winding for the rectifier.
If you can't find them the PT is probably a replacement that doesn't support a tube rectifier.

You can emulate the effect of a tube rectifier with a 47R to 68R 5W or better 10W series resistor.

[voyager]

Thanks for your help, Helmholtz. Strangely, the transformer is labeled as a Sunn 3-6PT, which is what’s shown in the schematic. I found a thread on a different forum where someone says they have a 3-6PT with a mystery orange wire and no yellow wires - they were trying to restore to tube rectifier but gave up. (link to thread below)

Does anyone have a Sunn 3-6PT with an orange wire that’s actually attached to something?

found thread: https://forum.sunnstillshines.online...?topic=10151.0

-Jeff

[Helmholtz]

Does the orange wire have continuity to any of the PT windings?

[voyager]

Helmholtz,

I just went through each PT lead with my multimeter, and I’m getting continuity readings between the orange wire and both green wires.

-Jeff

[Helmholtz]

Ok now please measure AC voltage from orange wire to ground with amp powered.

[The Dude]

Sounds like possibly an unused CT for the filament winding. I see they have the two 100 ohm artificial center tap resistors installed, so I would just leave it as is regarding the orange wire. It's possible there was a production change and everything you see is original. I don't really see a benefit to adding a tube rectifier to this amp, but that's just my opinion. If it were mine, I'd just leave it as is.

[Helmholtz]

I see the have the two 100 ohm artificial center tap resistors installed,

Where do you see that?

[The Dude]

I circlied them in this picture from post #1.

[Helmholtz]

Thanks.

Strange as the schematic shows a green-yellow wire as heater CT.
Seems there are are 2 completely different PTs.

[The Dude]

Yes, I agree. The transformer in this amp certainly doesn't seem to match the schematic at all.

[voyager]

Helmholtz, from orange to ground I get 5v AC at startup, then it gradually drops to around 350mV as it warms up. I also measured from orange to green, which is steady at 3.2v AC.

The Dude, I agree it could be a production to solid state rectifier. The only things that make me think it was originally tube rectifier are the presence of the tube socket (wouldn’t they have done some other sort of blank-off?), and that there is solder remaining in terminal 2 of the tube socket, where the tube rectifier schematic shows a wire.

Thanks so much for the help!

[The Dude]

My thought is this- and it's pure speculation based partially on the fact that you found a forum where another user sees the same in his amp.

At some point in the production run, there was a transformer swap due to cost, availability, or other. They had to modify/adapt the amp to work with available transformer. As far as the socket? They already had sockets and they work well as terminal strips, so why not use them?

Edit: And, your measurements do indicate that the orange wire is a CT for the filament winding.