Hammond PT question

[rfgordon]

OK, this is probably really dumb, and I should know this, but...

In a new build I'm using a Hammond PT whose 5V recto winding has a center tap. I've never used a PT like this before. If I power on the amp with the CT grounded, the fuse blows, but if I leave it off, all the voltages everywhere are OK.

So, am I supposed to let that CT float? Or am I missing something else?

[Phil_S]

Model number of the tranny? Or post a picture? Or just tell us the colors of all the leads, which hole they emerge from, and what you think they are for.

Anything is possible, but I doubt you've got a CT 5V. If you do, and are running tube rectifier with it, there is no way you can use the CT to ground.

I suspect you have paired two legs that don't belong together.

[rfgordon]

Phil,

I know it sounds like I've mis-wired it, but it does have a 5-ct-5 winding. It's a 272JX.

Anyway, the answer came to me in the shower: of course I CANNOT have a CT on the recto filaments, cuz it makes HT shunt to ground thru one side of the 5V winding!

Doh!! :0

[Phil_S]

So, did you solve the problem of voltages all over the place?

BTW, you work with a really fine singer in Burley Maple. I only had time for Cadillac, but that was plenty to get the idea.

[sportster4eva]

You can use the center tap as your HV lead to your standby sw. or to the first filter cap, unless you've already wired up to pin 8 on the rectifier socket, then just insulate it and let it float.

[David Root]

That's right, it's a fairly common wiring in old hifi stuff that used tube rectification. I have used it once but I'm not sure what if any advantage it may have over the usual wiring.

[rfgordon]

Phil_S, thanks for the compliment for Amber's singing. We just released our second album, called Make Mine a Double.

As for the Hammond PT, I've just let the 5V CT float, and everything is fine. The amp is sounding very nice now. I just need to finish the two-tone snakeskin-black cab.

[LOUDthud]

That's right, it's a fairly common wiring in old hifi stuff that used tube rectification. I have used it once but I'm not sure what if any advantage it may have over the usual wiring.

With certain rectifier tubes like the 5U4 that has no cathode, the 5V center tap provides a balanced point to get B+. Otherwise, there will be some 60Hz component in the 120Hz ripple. But, since there is usually a difference side to side in the resistance of the centertaped B+ winding, there is some imbalance anyway. You can sometimes see this with a scope. Looking at the ripple, every other cycle looks a little different.

With a GZ34, you should just get the B+ from pin 8, the 5V CT will add a 60Hz component. Some people experiment with the phase of the 5V winding, perhaps connecting B+ to pin 2, such that the imbalances cancel each other.

[David Root]

That's interesting, I always wondered why you see old ('50s) amps that take the DC off pin 2 rather than pin 8. Would that be because they were knowingly being cheap on filter caps?