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Old 24th April 2007, 01:52 AM   #1
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Default SS power trans for tube experiments

I have a rather large power transformer (weighs 6 lbs with large wire where I can see it) that was apparently intended for solid state. It has 39Vct, 30V and 13V secondaries. Is there any reason that I shouldn't use it for experimental tube circuits (low power) as long as I keep an eye on the core temperature? I am thinking of wiring the 30 and 39v windings in series to get near 100V rectified or using a doubler and then using wire wound resistors to drop the 13v winding for heaters.

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Old 24th April 2007, 02:00 AM   #2
mach1 is offline mach1  Australia
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None whatsoever. However, you will be quite limited in the tubes you can use (a doubler should get you around 160V if your voltages were measured with no load).

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Old 24th April 2007, 02:07 AM   #3
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Bridge rectify the 13 VAC winding with 4X Schottky diodes. Filter with a large 'lytic and follow with a 7812 3 terminal regulator. You get NICE "juice" for small signal tube heaters.

Phase up the other 2 windings and connect them in series. "Full wave" voltage double into large value 'lytics. Follow the doubler stack with a choke and more capacitance. Think "extended" pi section filter.

The max. DC you can pull from the doubler is 0.25X the RMS capability of the composite rectifier winding.
Eli D.
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Old 24th April 2007, 12:30 PM   #4
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Thanks guys. Very helpful!
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Old 24th April 2007, 06:49 PM   #5
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actually, i just finished designing a tube preamp that uses a 12V plate supply. low supply voltages aren't a problem with tubes if you are designing from scratch. with ltspice and the proper models, you can quickly find the optimum plate and cathode loads for low voltage operation. once when i was young i saw a tube breadboarding system that used springs (like the radio shack "300-in-1" kits) instead of fahenstock (sp?) clips..... the springs were much more reliable connectors than the clips(especially once you had more than 1 or 2 wires going to a connection). the breadboard kit came with a set of 1.5v filament tubes, a triode, a dual triode, a dual diode, a tetrode, and a pentode, iirc. the breadboarding kit also came with a 22.5v battery, and a 1.5v battery, and i think a 6v battery for grid bias...... so low voltage operation is nothing new with tubes..... the big difference that you'll find is that the current capability of that trannie will pretty much be wasted, since tubes rarely get over a few hundred mA for current draw (preamp tube plate currents are usually less than 10mA, and output tubes usually less than 100mA.
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Old 25th April 2007, 02:50 AM   #6
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My concern with running at too low of B+ is that linearity might suffer. I looked at the curves for 5687 and 6922 (the two tubes that I am most interested in at the moment) and just eyeballing it, it looks like it will be difficult to get reasonable distortion with less than 100V at the plate and even then the grid bias is only a couple of volts (headroom issue). I suppose choke loading the plate would help but then I just end up buying new iron for the choke instead of the trannie.

It seems like I should have enough transformer here to consider a voltage tripler. Of course that involves more componants also.

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Old 25th April 2007, 03:44 AM   #7
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I once did an experiment with a big SS amp transformer to see if it would work as a tube amp supply.I ran the ~100Vac into a trippler,and loaded it down with a bunch of power held the voltage pretty well under load.I was surprised it worked so well,actually.
I think I used 1N5408's,and some 330uf 400V caps for the experiment.

I say why not,I think it could work well if you pay proper attention to the doubler/trippler stage. Use some big caps,and beefy diodes.
Old SS power transformers can be had for free/cheap,and they have tons of current available to feed a doubler/trippler circuit or whatever else.Plus,they usually an extra winding or two that could be used for filaments,etc.
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