Mysterious OPT winding tap

[Klimon]

I just dismantled a Grundig Mandello console from the late sixties with ELL80 SE power stage. The opts are very small and have a winding tap I can't identify: I'm measuring 700ohms DCR on the primary winding which has another tap (on the same winding) that adds about 10 ohms DCR. Any ideas what this could be? A special kind of feedback winding???

Thanks! Simon

 

[JohanB]

This is probably a hum balancing tap. The HT+ voltage in these radios has a lot of hum due to a singel capacitor power supply. HT+ is connected to the tap of the OPT and a ellyt-capacitor at the end of the winding is connected to ground. This balance the hum in the transformer like in a push-pull output stage. Saved a few bucks in components for the manufacturer.
This trick was done in most european singelended radios, TV's and small amps in the 50's & 60's

JohanB

 

[llwhtt]

What is pin9 (screen grid) connected to? Could it be an Ultralinear tap?

Craig

 

[Klimon]

This is probably a hum balancing tap

Never heard of it; cost-cutting is the name of the game when you look at the innards so this would make sense

What is pin9 (screen grid) connected to? Could it be an Ultralinear tap?

I broke the circuits boards to remove the opts so it's too late to check. Also thought of UL but wouldn't 700 / 10 ohms seem like a strange proportion?

Cheers,

Simon

 

[mashaffer]

My money is on the hum balance tap. I parted out an RCA console that had these. IIRC the B+ was feed to the tap and the output tube plate was connected to the longer end (higher DCR) while the B+ supply for the tuner/premp was taken from the other end.

 

[llwhtt]

Looks like the UL tap got ruled out. So with a capacitor on the tap and a capacitor on the short end you wind up with a pi filter(CLC) for the everything before the output stage. Never heard of that but sounds logical.

Craig

 

[JohanB]

There is also a power-resistor in series with the short end before the capacitor is connected. The value is usally around 2 KOhms. B+ to the rest of the circuitry is taken from that capacitor. This gives a hum current in the opt to cancel the hum in the speaker.
Maybe also lower hum to the other stages. Someone can make a simulation of this?
Here is a typical schematic diagram of a cheap 50's radio.

I don't know if the resolution is enough, otherwise copy this link.

http://medlem.spray.se/nostalgiteknik/5020s.jpg

An externally hosted image should be here but it was not working when we last tested it.

JohanB

 

[Geek]

I have a few of those transformers here. Yup, hum balance/choke winding.

 

[JohanB]

OOps, my schematic was w i d e !! Sorry, not used to this media and to lazy to read instructions

But without that trick the hum would be tremendous, with a half wave rectifier and 25 uF on B+......

JohanB