combining multiple 6.3v taps

[deci belle]

I have a 290v 200mA PT (I know thanks to bob p) from an old stereo amplifier. It has two separate 6.3v taps. One is 2.5A and the other is 2.8A

Can I join both pairs at the pilot lamp with the two 100Ω resistors and use them together for the heaters? I want to build a garden variety tweed something.

[loudthud]

I think you can safely connect the two 6.3V windings in parallel if:

1) You check that the voltage is very close to being the same and
2) You have to get the phase right or the transformer could be damaged or catch on fire.

[deci belle]

Ah! ok, that's cool. Thank you, loudthud! If the 6.3v taps check out very close in output, how then should I determine the phase?

Do I take the bell-end off to check the four 6.3v wires' proximity and pair off the closest ones, or is there a way I can tell with a voltmeter? Like maybe one way will read (-) and the other way (+)?

[loudthud]

I'm making an assumption here that both windings use the same gauge wire. The difference between 2.5 and 2.8 amps is not great enough to require a different gauge wire. And they should have the same number of turns.

Connect together one wire from each tap. Now measure the voltage between the unconnected wires. It should be (about) 12.6V if the phase is wrong or close to zero if the phase is correct.

Note that if you get it wrong when you connect the windings in parallel, the insulation will burn off the wires faster than you can switch the power off. A bulb limiter would be a good thing to use.

[Mick Bailey]

If you've got a dual channel scope you can see the phase relationships.

[deci belle]

ok, loudthud. That's what I'll do— and I will make the bulb limiter, because I should have one anyway.

hi Mick, actually I do have a scope. I'm sure it is easy to use it in this application too …wanna tell me how you'd have me hook up the scope to the PT? I'd like to have a simple test to do with the scope, because I'm not confident at all with it.

[Mick Bailey]

I've just done a hasty setup to illustrate. I've set the scope to chop between traces (make sure channel b is not set to invert), AC coupled, 5ms horizontal and 5v/div vertical (for my particular supply), x1 probes. Trigger normal, no delay.

The first pic shows both supplies in phase. When I reverse the connections of one of the supplies in the second picture you can see them out of phase.

So, with the suppies in-phase you need to mark which connections go to the ground side of the scope (croc clip) and which connect to the probe. These then join in parallel.





Apologies for the fuzzy pics.

[deci belle]

Oh this is great, Mick!! Thank you for the scope tutorial and the examples of in-and-out of phase. Wonderful~ thank you, Mick!!

[deci belle]

I finally got around to putting the PT's 2.5A and 2.8A 6.3v windings on a scope.

It took me a while to figure out that I needed to connect the two separate windings to two separate probes; a probe and its ground clip to the leads of each winding to get waves that Mick described clearly and showed the results of in and out of phase above.

Thank you Mick!

[Diablo]

Why combine the two heater windings in parallel? Couldn't you just hook one of 'em up to the pilot lamp, and have the other one running to only the preamp tubes etc.? Seems that you could reduce some hum by running separate heater supplies to the preamp and power tubes.

[deci belle]

hi Diablo! Since these are separate windings, don't I have to virtually ground both pairs separately to run them separately?

I've usually grounded the heater taps at the pilot light, as you say.

I'm trying to minimize wires and components. Tweed chassis are so tight!

[J M Fahey]

It's safer to use them separate.
Best case you are saving 2 100 ohms resistors, by the way, nit important enough compared top the possible problems.
Even a few mV difference nay cause trouble.

Only way I would parallel windings (and I do that in my own 120/240V primaries) is when I wind them bifilar, go figure.

What worries me a little is that they have close but different specs, why?
I mean, I would understand, say, a 1.5 or 2 A winding fror preamps and , say, a 4 or 5 A one for power tubes, but ... 2.5 and 2.8 A?
REALLY?

I design and wind transformers all day long, so I'm quite aware of the *practical* side of things, besides the pure Math aspect, and simply a 6% difference in nominal diameter makes no sense.

FWIW the diference or "step" between any given AWG size and the next one is roughly 12% in area.

Barely possible but certainly not practical, to make 1 winding and then reset the machine for the next higher or lower AWG size.

Weird.

[deci belle]

Hi Juan! The PT is from a 1959 Stromberg Carlson stereo amp. The 2.5A tap supplies (2) 7408 power tubes, a 7199 and (2) 6EU7. The 2.8A tap powers the same plus an extra 6EU7 (plus more things evidently— but I don't know how to deal with this).

Now I have almost 50 more volts to contend with on the 2.8A tap… I want to actually measure the current on each of the taps. Would you please tell me how to do that?

All the wires are 20ga strand.

[g1]

Current can only be measured when the taps are hooked up to something and doing work. The current would vary depending on how much load you put on the tap. The current ratings on the taps are what it can safely produce. If you overloaded the tap beyond it's rating the transformer would likely overheat and fail.
Not really sure what you are trying to measure. You could load the tap with a power resistor (6.3V at 2.5 amps would be around 16 watts) of whatever value gives the current you want to test at. I guess you could check that the voltage is not dropping too much at the rated max. current of the tap.

[deci belle]

I was just wondering if I could find out what the orange winding's current capacity was. I can't find anything on the number on the PT.