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 HMV audio OP transformer
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 Return to top of page · Post #: 1 · Written at 10:58:58 AM on 3 October 2021.
Scotty's Gravatar
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 328

Hi folks,
Has anyone got a OP transformer for an HMV N3 chassis.
I had a guy who used to rewind them but he’s retired as no longer doing anything.
Also need a replacement 12” 15ohm dual cone Magnavox speaker if at all possible

Cheers
Scott


 
 Return to top of page · Post #: 2 · Written at 4:04:40 PM on 3 October 2021.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 2018

That's a special tapped primary single ended transformer. Will be VERY hard to find.

You could probably use the Altronics M1120 but you'd have to re-stack the core to get an air gap. Tedious but not difficult.

At least you have a good one in the other channel that you could use to compare and pick the right taps.

Good luck with the speaker!


 
 Return to top of page · Post #: 3 · Written at 9:06:06 PM on 3 October 2021.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4545

I think I have a 15" in the shearing shed stuff, if I can get at it for other peoples household stuff.

Several HMV amps used Ultra linear transformers.


 
 Return to top of page · Post #: 4 · Written at 9:22:14 PM on 3 October 2021.
Scotty's Gravatar
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 328

Thanks Marc, that would be terrific. I can possibly get the transformer rewound if I can’t source a used one but if you can help with the speaker, that would be most appreciated
Cheers
Scott


 
 Return to top of page · Post #: 5 · Written at 10:35:42 PM on 3 October 2021.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 2018

It's not an ultralinear connection Marc. The B+ goes to the tap and a stepped-down antiphase signal is used for negative feedback. Very strange, never seen it done elsewhere.

The HMV engineers used to come up with some weird designs, many of which made it into production. The audio amp in the E2 TV is another example of such weirdness. I'm not saying it doesn't work, mind you. But nested feedback loops always seemed a bit unnecessary to me. The calculations become very complex! I always suspected it was Neville Theile's work.


 
 Return to top of page · Post #: 6 · Written at 7:25:08 AM on 6 October 2021.
Wa2ise's avatar
 Location: Oradell, US
 Member since 2 April 2010
 Member #: 643
 Postcount: 775

This scheme is seen occasionally in American AA5 AM radios. It's a hum bucking scheme.

Ever wonder why some AA5s have a tap on the primary of the output transformer? It's another hum bucking scheme. The usually 1.5K resistor that usually feeds off the rectifier tube B+ is connected to the far end of extra turns on the primary. As far as the hum ripple (interferer E) is concerned, the cap that filters the line that feeds the output tube screen and rest of the set looks like a short to ground. So the hum ripple sees 1.5K to ground, and the output tube's plate resistance looks like 14K to ground on the output transformer's plate lead. The position of the tap is selected such that the amp-turns of one side balance the other side. Thus the hum ripple cancels out so the speaker doesn't hear it. J Stewart made a nice diagram which I pirated above.


 
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