Confusing voltages on hi-fi 6.3v windings

[deci belle]

http://www.audiophool.com/Schem_A/ASR-220C.pdf

Mick Bailey provided the link. The schematic is at the bottom of the page; PT at bottom right corner.


The 290-0-290v PT cam from this amplifier. It has two pairs of 6.3v leads. Two are green and two are orange (schematic says green and gray).

I have labeled the leads Orange/Black+Orange and Green/Yellow+Green.

When I ground the 2ndary CT to the PT+AC GND, I get the following values with the VOM's blk lead grounded:

Or/Blk_ _ _ 41v (42v with CT floating)
Or_ _ _ _ _4.5v (9.7v)

Gr_ _ _ _ _2.2v (8.6v)
Gr/Yel_ _ _5.4v (11v)

When I float the PT 2ndary CT, I get these values with the VOM's blk lead grounded (values placed above for easy comparison):

Or/Blk_ _ _ (42v)
Or_ _ _ _ _(9.7v)

Gr_ _ _ _ _(8.6v)
Gr/Yel_ _ _ (11v)

When I measure each colored pair of leads together I get:

Or/Blk><Or_ _55.7v

Gr><Gr/Yel_ _7v


The schematic shows things going on with the 6.3v taps that I don't understand. I hope to join these 6.3v taps in phase for use in a Tweed Twin head. Their outputs are similar: 2.5A and 2.8A. But I don't understand the voltage readings I'm getting.

Will someone please tell me what's going on with these taps?

I hope I can still be able to join these taps to keep things simple in this crowded chassis.

[g1]

Forget about the CT, it is only for the HT winding (reds). Don't measure to ground, measure between the greens and between the oranges. None of the heater windings have any reference to ground.

[eschertron]

It sure looks to me like the orange leads are a bias voltage?

[g1]

It sure looks to me like the orange leads are a bias voltage?

Agree, also the color doesn't match the schematic, orange instead of gray.
Could this transformer be from a different version of the amp? Is it marked 161000-232 ?

[deci belle]

Ya, g-one— thank you for clarifying the way to measure the taps. Also, my PT doesn't have that number.

My PT says TR-13-2 10005121

That's exactly what I thought too, escherton. Makes sense to me. But the orange taps did go to the heaters, just the same.

I believe my amp was a 220b. There's that 10Ω 5w resistor coming off one of the legs of the gray tap (on the 220c schematic).

Not sure how to make this work…

[g1]

So the orange taps were going to heaters but are measuring 55VAC ? Were those heaters working when it was wired that way? Was the amp working?

[deci belle]

Yes, g-one.

It was manufactured that way as per the schematic which I don't understand. I provided the link at the top. On the schematic, it says the gray taps (I have orange and green wires) do other things too like go to a aux lamp socket and that winding also has a 10Ω 5w resistor on it too.

Yes, it was designed that way by Stromberg Carlson. No, I never turned it on— I just stripped it for the iron, cuz it had a 5v tap.

I don't know how to make that 55v winding work for a heater, much less a heater AND bias tap. Can I do that?

[g1]

How do the 2 heater windings compare resistance wise? If the orange was connected to 6.3V heaters it must be bad. If it was 55V it would have fried 6.3V heaters.
Try putting a resistor across the orange winding and see what it's voltage does. Maybe start with 1K. If the voltage holds up, it will be about 3watts so don't leave it on long.

[deci belle]

Yay! Thank you, g-one. When I stripped the amp I didn't notice any evidence of damage or burnt parts.

The green winding has .2Ω resistance and the orange winding has 11.2Ω.

When I put 120vac on the PT and have a 1k 3w resistor across the orange winding it goes from 55.6v down to 54.9v

What can you tell from this?

[eschertron]

Yay! Thank you, g-one. When I stripped the amp I didn't notice any evidence of damage or burnt parts.

The green winding has .2Ω resistance and the orange winding has 11.2Ω.

When I put 120vac on the PT and have a 1k 3w resistor across the orange winding it goes from 55.6v down to 54.9v

What can you tell from this?

I took a look at the schematic provided and the gray wires do indeed provide 6.3v to heaters, etc. HOWEVER
From your experiment I can only say that the orange wires are not 6.3vac, so unless it was dropping across a bunch of heaters in series (NOT like the schematic) it was not working as a heater tap.
You mentioned above that the part numbers didn't match. I still have a sneaking suspicion that the PT had been swapped out and didn't work properly after that. Or that all the heaters were moved over to the first tap. Either way, the orange wires are not a voltage source that you'll be likely to use for heaters,
unless you want DC heaters (and can drop a lot of voltage);
or put a 4:1 dropping tranny on there to get ~13 to 14 volts and provide balanced AC to your preamp.

...and yes, it's providing 55ma of current, and the resistor is dissipating 3W

[deci belle]

Thank you for looking over the schematic, escherton! Ya, well, that might be expected as I got the amp from a (Los Alamos) pal of the friendly neighborhood mad scientist. haha!!

I wish I could find out how to determine the current rating of the green 6.3v tap. I can't find anything out from the numbers on the PT: TR-13-2 1005121.

Do you know how to find this?

[deci belle]

A transformer factory said they'd test it to find out the specs for me, so I'm going to send it to them!