Valve power supply - How to size transformer?

[metalsculptor]

Originally Posted by SanderW

But if I have more than one toroidal transformer, can I stack them on top of each other without causing a problem? I assume I can't have a conductive bolt through all of them as that would constitute a one turn winding through all the transformers, but with a non-conductive central shaft this should not be an issue?

Use a normal bolt, stainless steel will have less eddy current loss. Use fibre washers and a sleeve to insulate, unless a full shorted turn is created by the mounting there will not be any problem.

If you use an EI transformer consider getting one with a copper shorting strap around the outside to reduce stray fields and keep it away from any unshielded signal tubes. electrons in vacuum are very sensitive to magnetic fields.

[ryuji]

Originally Posted by metalsculptor

One more thought, If a toroidal HV transformer is available, it is a trivial exercise to wind a few turns of hookup wire around the core to get one or more 5V windings for the rectifier heaters. Saving the expense and bulk of another transformer. Because the primary will be oversized to get the appropriate secondary rating there should be enough surplus capacity to run 20 or so watts of heater.

If you get a custom transformer made then the transformer company will know what size secondary wire to use if you tell them the rectifier type and filter capacitor size. The last time I had a custom transformer made it was reasonably priced YMMV.

This is the transformer I used for my tube rectified 5.1 channel 807 and 12[at]7 based amp, it consumes about 280mA at 400v b+. Adding a 5v wind is trivial, or buy one.
Antek - AS-4T400

[ryuji]

Originally Posted by ryuji

12[at]7

meant to type 12a[t,u]7

Originally Posted by SanderW

But if I have more than one toroidal transformer, can I stack them on top of each other without causing a problem? I assume I can't have a conductive bolt through all of them as that would constitute a one turn winding through all the transformers, but with a non-conductive central shaft this should not be an issue?

toroidial transformers have magnetic fields confined to the insides, but i feel that you may find that you get slight magnetic coupling if you literally place another toroid ontop of the other one purely from having your windings so close to each other

dont think anyone mentioned it, but aside from electrical/magnetic efficiency you obtain from toroid, they do not vibrate like EI's sometimes can (and thus make a audible hum)

[GoatGuy]

First... remember your rectifier tubes will have a voltage drop.

Second, as a good rule-of-thumb, always over specify by at least 20%... it comes in handy at times.

Third, so long as you have faraday shields (copper straps around windings), and outer metal cans, E-I wound transformers are not inferior to toroidal. And cost a bunch less, generally.

Fourth, here would be my specs for the transformer:

Primary - split, for 120/240 operation.
Secondary 1: 5 VAC, 3.5 amp.
Secondary 2: 6.3 VAC, 4 amp (you might want more sections!)
Secondary 3: 300-0-300 VAC, center tap, 500 ma continuous duty.

Now that last part is tricky: each of the halves only is delivering current during half of the 50/60Hz power sine-wave. So, the wire used in the winding only needs to take "250 ma" average. This kind of makes sense from a different perspective: if P = V·A = 300·0.5 = 150 VA, then 150 VA = 600·0.25 too...

Consider the Hammond 372HXP as an almost-perfect fit. See... Hammond Mfg. - Universal Primary - "Classic" Potted Power Transformers for tables regarding these. There is no reason to over-specify. Don't worry about slightly over-tapping the 5 volt line... while there will be additional heating on that secondary, it will be offset by much lower heating in the 6.3 secondary. Everything will come into thermal equilibrium after a 10 minutes of power-up or so...

GoatGuy

[SY]

Originally Posted by GoatGuy

Third, so long as you have faraday shields (copper straps around windings), and outer metal cans, E-I wound transformers are not inferior to toroidal.

They're not inferior anyway- they can have slightly greater radiated field, but the efficient coupling of high frequencies of a toroid is a disadvantage in power transformers. On balance, an EI is a better choice. Toroid power transformers are compact and (these days) cheap, but are at the bottom of the desirability ladder for performance.

[GoatGuy]

SY: I respect your opinion re: the markedly increased frequency response of wound toroids over E-I (or C, or F, or R, or E:E, or C:T) stamped-core designs. And I agree with it/you. With now some 40 years experience [yep, {old} am I], all I can say is: its all about the regulation in the end.

Taking the time (and intellectual interest) in competently designing power supplies that have excellent isolation between the rectification sections, pre-filtering, stage-1 regulation, secondary filtering, stage-2 regulation, and active-circuit compensation should, when done right, become entirely transformer agnostic. In testing, I've actually show this to be the case. I've also intentionally "messed up" the line-drive power, by running EMI noisy rotary drills, AC/DC motors-and-pumps (circulating water at 100 GPM), nasty switching supplies and so on... to get the noisiest, crappiest power I can achieve ... and sure enough - really good regulators isolate all of it, regardless of input transformer.

But, in top-of-chassis mounted tube circuits, I've noted (as another poster hinted) that there can be a remarkable amount of influence from a power-supply transformer on the electron-cloud of both low-signal early-stage tubes, and the power/output tubes. Shield-cans on the tubes is remarkably helpful, if made of Mu metal, but having fully-enclosed cans on the transformers and good Faraday shielding seems to make an equally important (and audible) contribution.

GoatGuy