"Stacking" power transformer secondaries?

[vaughn]

I searched and found one reference to doing this but I need more information...

I just picked up a Tektronix 533 scope that I am parting out. Lots of beautiful ceramic terminal strips and other parts, including a huge transformer.

I found a reference on Pete Millett's website about this transformer and how by stacking the secondaries one could achieve higher voltages. I understand that to achieve higher voltages one would connect the secondaries in series, and to achieve higher current one would connect them in parallel.

My questions are;

1. Connecting two secondaries in series, for instance 205V @ 170Ma and 130V @ 130 Ma, I would have 335V. How would I calculate the current that would be available with this arrangement?

2. Connecting two secondaries in parallel to achieve higher current, for instance the same two secondaries as above, would achieve an available current of 300ma. How would I calculate the available voltage? Also, how do I determine the phase of the secondaries to ensure I have them properly connected?

Thanks for your help!

[TheGimp]

When you stack them in series you get the lower current rating of the two.

You can't stack them in parallel effectivly with such a difference in voltage ratings unless you are willing to accept the lower voltage, and drop the higher voltage down with some type of pass device.

[HollowState]

Originally Posted by vaughn

My questions are;

1. Connecting two secondaries in series, for instance 205V @ 170Ma and 130V @ 130 Ma, I would have 335V. How would I calculate the current that would be available with this arrangement?

2. Connecting two secondaries in parallel to achieve higher current, for instance the same two secondaries as above, would achieve an available current of 300ma. How would I calculate the available voltage? Also, how do I determine the phase of the secondaries to ensure I have them properly connected?

1. In series you are limited to the lower current of the two, which in this case is 130mA.

2. You cannot connect these in parallel because they are of different voltage and would overheat and damage the transformer. To parallel windings, they must each be the same exact voltage. The currents would then be additive.

You are correct in your concerns about phasing. The surest way would be to use a dual trace oscilloscope. Place the two channels, with equal gain, in the "add" position and connect each winding to a channel. If the trace amplitude increases, they're in phase. If it decreases, they're antiphase.

A second way would be to put a 60 watt light bulb in series with the primary. Then parallel your chosen secondaries. If they're in phase, the lamp will not light or increase any very dim brightness. Out of phase and it will light brightly.

[vaughn]

Thanks for the responses...

One more question; Can I connect more than two sets of secondaries in series to achieve higher voltage? I think I understand that for current I will be limited to the lowest value.

[Wavebourn]

You may connect as many secondaries in series as isolation permits.
One trick that I use is to rectify voltages, then connect outputs from rectifiers. Such a way cheaper diodes and capacitors may be used instead of higher voltage ones that are more expensive and less available.

[HollowState]

The answer is yes in both cases.

[wa2ise]

If you were thinking of paralleling the heater windings to get more current, you could split up the load instead. That is, connect some of the tubes' heaters to one winding of one transformer, and the rest of the other winding of the other transformer.

As for the high voltage windings, one could dedicate the heavier current winding for the output stage B+, and the lighter one for the preamps, drivers and screen grid supplies of an audio amplifier.

[vaughn]

Yes, this is what I am thinking...

I'm not even sure what I am going to build with this yet, I just want to be aware of my options.

I do need a phono stage, and this would be overkill for that purpose, but workable.

[Tom Bavis]

Be aware that Tektronix (and HP) transformers are quite conservatively rated. I would measure the resistance of a winding and estimate its capacity based on a 2-3% voltage drop (5% or more for a smaller transformer) - i.e. a 17 Ohm 170 V winding could supply 0.2A RMS or more.

[vaughn]

One more question (I think)

Proper phase of the secondaries is important when connecting in parallel, is it also important when connecting in series? Also would this be important if I rectify first and then connect in series as Wavebourn suggests? I guess that is two questions...