What happens if you do not connect the secondaries?

[dbishopbliss]

I'm putting together a breadboard and want to test the filaments connections, but I do not have the rest of the circuit ready. 

Is it bad for the transformer if I were to connect the secondaries for the filaments but leave the secondaries for the main power open?

[dstrimbu]

I'm putting together a breadboard and want to test the filaments connections, but I do not have the rest of the circuit ready. 

Is it bad for the transformer if I were to connect the secondaries for the filaments but leave the secondaries for the main power open?

David, as long as that second, secondary winding is not connected to a load, there will be no current induced in that winding.  So it's essentially quiescent; this should pose no problems at all...  kinda like a multi-tap output transformer for loudspeakers, the unused taps sit quietly if they have no loads across them...

-don

[Grainger49]

You can do that, connect the secondaries for the filaments (either in parallel for higher current or in series to add the voltages).  The B+ being open might cause the heater voltage to float up a little.

Background:  With transformers that have multiple secondaries you can add and subtract the voltages on the secondaries by wiring them in series.  This is called boost and buck.  If in phase (there is a "dotted" end of the winding that indicates phase), that is dot to non dotted, they add, if out of phase, that is dot to dot, they subtract.  This can become a whole ball of wax as you can buck or boost the B+ and the heater windings if both are insulated to the same voltage.  Ok, who needs a 250V transformer output plus or minus 6.3V?  But it is possible.

In the FP III build (non-extended) the high voltage (B+) secondaries are paralleled.  That produces the same voltage but delivers twice the current (each delivers up to the rated current).

[dbishopbliss]

Thanks for the replies... you confirmed what I thought.  I was just asking because someone once said (or I thought they said), that I had to have a load on the high voltage secondaries otherwise I would blow up the transformer. 

[JC]

Is your filament supply DC, by any chance?

If so, I strongly recommend you have it actually loaded with the filaments it will be supplying when you test it.  Your transformer will be fine in either case, but an unloaded DC filament supply can end up being considerably higher in Voltage than a loaded one, unless it includes some kind of active Voltage regulation.  Higher enough that the working Voltage on any filter caps might end up higher than you would like.

If the filament supply is just AC, of course, this would not be an issue.

[ironbut]

In a similar vein (or is it so similar?) I'm fooling around with an amp for electrostatics (push-pull, cap coupled to the electrostatic capacitive load). I'm currently using plate resistors on the outputs but of course,  I'm wasting half of that voltage. I'm interested in using chokes on the anodes.
I'm wondering if I could buy a couple of old PP output transformers and just use the primary instead of four chokes?
The primary should be around 4-5k and each of the four output tubes Vp=325, Ip=80mA.

[JC]

Well, a couple of things come to mind: First is whether or not that primary impedance would be the same without the expected load value on the secondary.

Second would be if the primary was wound with the idea that it would handle 80 mA.

[richardl]

put tape over the ends.  Put in tubes and fire it up.  Tubes light up and you are good to go, never have to worry about the heaters again.  Bury your heater wire at the bottom near the chassis plate as you will never have to touch them again.  Voltages will be a bit high as there is no other drain on the PT but, should be good.