New or Old Output Transformer for Hammond Organ Pre-amp?

[Herald1360]

If you're not already aware, VVT do a range of grid coupling transformers in various ratios. Something like their 20something K anode impedance 1:3+3 one would seem to be a good bet. It's about £50.

[PJL]

The transformer is presumably designed to drive very long cables judging by the secondary resistance of 15 ohms and balanced output.

It would be easy to modify a small output transformer by rewinding the secondary which is often on the outside. If you have a sort of working transformer you can get the turns ratio by injecting a small AC signal into the primary and measuring the secondary.

It might still change the 'sound' though

[Herald1360]

Ahhh.... I wonder if the secondary is to drive a 600ohm balanced line?

The circuit of what it drives would be even more interesting, now......

[JonathanP]

Here's one of the tone cabinets from the era that the organ was designed to be connected to, if anyone's interested in how all this works:

We've gone with the Hammond (not any connection to the Hammond organ) 125 output transformer. I just want to get it working for the next 2 years to last through my compulsory music education. This is what my engineer recommended. It'll change the sound slightly, but for now I don't care, as long as it works! I'll get a rewind of the old transformer in a couple of years. He's done an inspection on it and said the damage isn't enormous (hence why the problem only kept coming and going and it was still usable), but the problem is it's quite far into the transformer. That's what will make a repair costly.

[Herald1360]

Thanks for the schematic.

An interesting arrangement- the "output" transformer does do the phase splitting but appears to provide a lowish impedance line drive to feed into the (high impedance) grids of a pair of triodes which each drive half of the output stage. It will probably hum something awful if the input connector were pulled with the power on! I wonder why no grid leaks were fitted? Two resistors can't have been price critical in something as expensive and low production as these organs.

Best wishes for your studies- good to know of some classic kit being used for them.

[Radio Wrangler]

Aha! A cathode-feedback output transformer winding. I wonder how that dates with respsect to the discussions on Quads and 'ultra-linear'?

The schematics use archaic symbols, but 6SN7 6V6 5U4 are not archaic.

All the earlier stages are open-loop. I can see where Hammond didn't want to be compatible with anyone else's amplifiers/speakers.

Even re-winding that transformer will risk changing the sound because disturbed laminations don't insulate to the same degree as when new. If you can determine the alloy used, laminations may still be available and a new transformer could be made.

If the old transformer only flashes over at high voltage, then low voltage measurements will reveal the inductances and turns ratios. So the original need not be unravelled to count the turns.

Usually interstage transformers have disproportionately large cores for the power they carry so they don't contribute much non-linearity, so any 'magical' properties of the beastie probably relate to frequency response, and some R-C networks can shape that as required.

David