Baldwin Iron

[wrenchone]

Recently, I got a mess of transformers and such parted from an old Baldwin organ. Included were two massive power transformers (they probably ran whole banks of tubes for the tone generators and dividers), a 7H choke, a bathtub oil cap, one (aw, shucks) output transformer that looks to be 6k with a 16 ohm output, and one autotransformer. I understand the rest of the catch more or less, but can anyone who is conversant with the old Baldwin organs fill me in about the purpose of the autotransformer. It's ungapped, and the primary inductance is very high. It occurs to me that it may be the timing element to a blocking oscillator, but again, maybe not. Does ayone have a clue about it?

[moonbird]

I bought a similar cache awhile back -- I think the autotransfomers were used as phase inverters in many organs.

[wrenchone]

How do you get phase inversion with an autotransformer?

[tubelab.com]

http://www.electra-print.com/docs/6bx7pp0001.pdf

[wrenchone]

That looks to work if you have a 50% tap on the transformer - unfortunately, mine was 17.5V out for 50V in, driven at 300Hz with a sine wave source I have on my bench at work. I use that source so I can get realistic readings for turns ratio and inductance by driving the transformer with a low impedance signal within its bandwidth. Since I have a sensitive Volt/Amp/Wattmeter hooked in series (a Yokogawa WT210) I can also derive primary inductance at a realistic excitation level.
The autotransformer idea as shown is a nice one for a rainy day.

[wrenchone]

Having said that, it would probably be a good idea if I looked at input and output with an oscilloscope to make sure that my sine wave source and DVM are speaking the same language....

[moonbird]

Tubelab --

I have admired this schematic before -- I have a bunch of 6Bx7s. However, I am just too dumb to figure out what impedences the interstage needs be for this application. What is your guess as to the specs for the PSA-2N in this schematic. thx.

[moonbird]

Wow - As I said, I have looked this over before -- but I just noticed that only one leg of the primary is connected in this schem --- will that work or is it 'assumed' that the other leg is grounded. Sorry to butt in a bit here

[moonbird]

Oh -- ok I see it now -- the input is 'into' the 'secondary' (so to speak) -- no 'primary' is required. Also forgot this schem is from the Electra-print Web site. I can get it from them. Sorry for being such a dunce, Tubelab.

[tubelab.com]

Also forgot this schem is from the Electra-print Web site.

Yes the "interstage transformer" has a single center tapped winding that can effectively be used as a phase splitter without a secondary. The "impedance" isn't a factor here, but the transformer that is used must have enough inductance to work at low frequencies, and it should have a low enough winding capacitance to avoid phase shifts at the high frequencies.

I have seen schematics like this dating back to the 1930's when transformers were cheaper than tubes and Hi Fi meant that you could play the AM radio or some 78 RPM vinyl. I would use a vacuum tube phase splitter today (I am partial to the LTP) but Jack is in business to sell transformers. People have built this amplifier using power toroids for the interstage and the OPT. You could use a "real" interstage transformer too.

Look through this thread for several variations on the same idea:

Idea for a 2 tube 6080 PP amp.