Preamp transformer?

[schnarf]

I'm interested in building a preamp. I'm going to start with a pretty simple design, but I know I need a power supply. A lot of the schematics I see need pretty high voltages, for example, one wants a "C" connection of 422V, "D" of 406, and "E" of 402. Can anybody suggest a transformer/power supply that's affordable and will sound good. Alternatively, I have an old broken ADA-MP1, but the power supply from it is just fine. Could that work?

[brett]

Hi
check out what the late Fred did in his Real McTube : "back-to-back" transformers.
If you've got 110 in the wall, you could go 110:24 and 6:110.  Because the latter is being fed 24V, it will put out 440V instead of 110V.
Preamps use minimal current (a mA or two), so even very small transformers would work. (3VA would be fine).
You'll also need 6.3V to power the heaters.  I suggest rectifying and reducing 6.3VAC to about 12.5VDC for the heaters, so that you won't have to worry about heater hum.  Again, the Real McTube can be your guide.  Fred was a clever fellow.
cheers

[R.G.]

If you've got 110 in the wall, you could go 110:24 and 6:110.  Because the latter is being fed 24V, it will put out 440V instead of 110V.

Actually, that won't work. You will saturate the "6v" secondary because you're applying four times the nominal volt-seconds to it each cycle.

You can never safely drive a power transformer winding at a higher voltage than it was intended at the same frequency, or at a lower frequency at the same voltage. Transformer cores load up with magnetic energy proportional to the integral of the volt-seconds applied to them. They get to the max and then saturate.

What you can do is use 120:12ct for the step down and 12:240 for the step up for the high voltage. Most small transformers come with dual primary configurations, so you get two 120v primarys. These you put in parallel or serial to match 120 or 240 in. The 12Vct - or two 6V secondaries in series - run a 6V filament in autotransformer mode, and the 12V runs the 12V side of the step up transformer. The two 120V windings of the step up transformer give you nominally 240Vac.

Full wave rectifying 240Vac gives you 240*1.414 = 339Vdc, which is a fine voltage for driving preamp tubes.

In real life, you get less than that because the smaller the transformer, the larger the open circuit voltage of the secondary is made to account for regulation losses. A "6V" output is only 6V at full load. It might be 7V or 8V at no load. So the input transformer makes 12Vac, and this drives what is perhaps a 14V open circuit winding, losing a couple of voltage of possible drive voltage. Then the step up transformer ratio is less than the ideal 12V:240V, so you get less peak DC, and on top of that you get high regulation losses.

But in this setup, you'll get about 250-300Vdc, which again is plenty for driving preamp tubes well. The 12A?7 series is rated for 300Vdc plate-to-cathode anyway, so this is very near the nominal point where everyone runs them anyway.

[JonFrum]

Have a look:


http://76.162.4.197/MyStore/perlshop.cgi?ACTION=thispage&thispage=parts1.htm&ORDER_ID=644391753

[brett]

Hi
RE:

In real life, you get less than that because the smaller the transformer, the larger the open circuit voltage of the secondary is made to account for regulation losses. A "6V" output is only 6V at full load. It might be 7V or 8V at no load. So the input transformer makes 12Vac, and this drives what is perhaps a 14V open circuit winding, losing a couple of voltage of possible drive voltage. Then the step up transformer ratio is less than the ideal 12V:240V, so you get less peak DC, and on top of that you get high regulation losses.

So it is quite possible to over-shoot the target as well.  In building my Real McTube, I had the experience of a conservative voltage rating and poor regulation on the step-down transformer giving over-voltage by about 30%. The step-up transformer seemed to give closer to its nominal ratio.  My results were: 240:16 (nominally 240:12) and 16:145 (nominally 24:240). 

Would higher voltages saturate small transformers being used back-to-back?  If the transformers were running at double their native voltage (220V instead of 110V), we can calculate the necessary rating of the transformer.  Let's assume that it's providing 250V B+ for a 12AX7 with 2 x 100k plate resistors, and that both plates short circuit to ground, providing only 50k of impedance (this is approximately equal to 2 12AX7s that are running normally).  250V with 50k = 5mA.  So the transformer rating would need to be a tiny 1.25VA.  But, as RG indicates, the transformer is running at double its native voltage, and the flux density is also doubled.  So the VA rating needs to be at least 2.5VA.  That's still very small.  My experience of small transformers is that their VA ratings are quite conservative (typically only 50 to 80% of saturation), but if an extra safety margin was wanted, it could be factored in.

Of course, on any serious power supply, it pays to use the right components, so I'm not suggesting just substituting components willy nilly.  That 4 x increase above the native voltage of the step-up transformer probably was going a bit too far.
cheers