Power toggle confusion

[Oddvar R]

I am trying to do the board over again. I just wonder about the fact that you sen two leads from the transformer into to rows of diodes ending in a single point, without any other connection, will there be a DC reading of some significance, or are there other variables that will effect this?

[Oddvar R]

I did measure stripped down, but same low voltage.

[sluckey]

You must connect the PT CT before you can measure any dc voltage at the junction of those diodes.

[Oddvar R]

And that CT goes to ground?

If I put any of the CT's to ground Bulb limiter lights up like Christmas.

If I measure the voltage from one of the AC side oft the rectifier to the end of it, I get 297v. I get the same voltage on the B+3/4/5.

[pompeiisneaks]

With the bulb lighting up like that, it means you've likely got a short to ground somewhere it shouldn't be. The lower voltage at the output of the diodes means it's dropping voltage somewhere via some semi resistive path to ground. Yes the center tap needs to be grounded for the amp to draw current.

You should be measuring DC voltage from ground to that point to get any full voltage readings.

From other points you're getting a voltage drop/difference between those two points.

~Phil

[sluckey]

You must connect the PT CT before you can measure any dc voltage at the junction of those diodes.

[pompeiisneaks]

You must connect the PT CT before you can measure any dc voltage at the junction of those diodes.

He said he connected it and got odd behavior of the bulb limiter going bright. I'm guessing something is shorted on the power rail.

~Phil

[sluckey]

So. I don't trust much of what Oddvar says. I stand by my statement.

[pompeiisneaks]

So. I don't trust much of what Oddvar says. I stand by my statement.

Your statement is fully correct, don't get me wrong ...

~Phil

[Oddvar R]

So. I don't trust much of what Oddvar says. I stand by my statement.

Your statement is fully correct, don't get me wrong ...

~Phil

What?

[Oddvar R]

Could it be that the two 100k resistors to ground will conflict with the CT to ground?

[pompeiisneaks]

Could it be that the two 100k resistors to ground will conflict with the CT to ground?

No, those are just dropper resistors for the circuit. The center tap on the transformer is mandatory for the transformer to work in the first place. That gives each side of the transformer a path to ground so they can conduct. If the center tap is floating, there's no 'closure' to the circuit. the 2 100k resistors (totalling 200k to ground) allow a specific amount of current draw to ground, and create a voltage drop, yes, but once you have a working amp, the path through the tubes etc will be so much less resistance it will 'follow the path of least resistance'.

right now, it's probably one of the few paths' to ground if you don't have tubes in, but it's still just 'part of the circuit'.

~Phil

[pompeiisneaks]

Just look at the schematic for the express:

CenterTap.png

That red circle shows the center tap of the transformer to ground. It's right in the design of the amp, on purpose. no need to question it. Ken knew what he was doing.

~Phil

[pdf64]

Could it be that the two 100k resistors to ground will conflict with the CT to ground?

No.
If you're in doubt about that then just remove them, all they're intended to do is to drain the HT caps down after power is switched off. But as you've got no voltage to drain off, that can't cause a problem, right?
The circuit will work just fine without them.

[sluckey]

Guys, read this statement, especially the part I underlined, bolded, and italicized...

...you sen two leads from the transformer into to rows of diodes ending in a single point, without any other connection, will there be a DC reading of some significance...

The answer to that is there can be no dc reading until the center tap is connected. His center tap is just floating in air.

No doubt he has other problems.