I have this model with an O/C output transformer.
There are three taps on the primary.
I would like to know what the purpose of the turn to the 5.6K . R29/R31.

HMV N3 4Y Circuit Diagram

There are two scenarios: Need to wait for a circuit.

Often B+ for the rest of the radio was taken off of the tap, or fed into it and the rest fed off of the bottom. Filtering?

Secondly around the sixties "Ultra-Linear" amps were popular.

Document uploaded.

R29 and R31 supply the output tube screen voltage.
Would not be hard to use a standard output transformer, wired in the conventional way.
JJ

You could do that, however that does destroy what it is being attempted and the sound. This is balanced loading (circa 1937) where this can be a way of providing feedback via the screen such that it balances out to an extent the differences between a Pentode, or a Triode as an amp.

It is actually more complex that it looks. I do have a HMV Record player from the 60's using 6BM8's configured ultra-liner.

Marc

From my web page http://www.wa2ise.com/netcom/Compactron%20'AA3'%20AM%20tube%20radio.html

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 below:

As noted above: This is a clear example of the risks and ramifications of modifying. The way that tapping is applied is not without rhyme & reason and is not simple.

From the point of hum, staggering the AC heater pins can often help, where there is a bit of heater cathode leakage and is often seen. Eg. Octal: heaters 2 & 7. V1, if pin 7 is grounded, on V2 pin 2 is grounded. The thought being based on reversing the phase.

Not suggesting modification.
Just a solution if an original transformer cannot be located or rewound.
And if replaced with a conventional transformer there would be no reason for it to not perform well.
The filtering/ hum balancing not difficult to overcome also.
If it were mine and wished to follow the mod solution I would do both output channels and rewire the power supply and HT distribution.
JJ

One only panics with these things after there is a problem. As said, in the sixties that was popular & I have just taken possession of an interesting pile of … and there may be one, or two in that lot.

Like there is a fairly oxidised & cracked 11-7 cabinet with an intact grill and badge; Even a couple of car radio's one with its cradle. and a Matsushita cradle. Plus a stepdown transformer type & integrity not known or tested.

So even if it has a dud transformer I would put out feelers first.

The designers must have had a basic hum level problem.
Probably a proper choke may have cost too much, so they borrowed part of the output trannies for that choke function.
Cost and space savings tactic. Quite interesting.
I would see nothing wrong in putting in two matched untapped OPTs and adding a separate choke to the power supply and do it correct in principle.
Fred.

I have sent Brad a copy of the 01 Electrogram chassis: I have the whole unit.

I am sure that you will see a whole lot of similar.

It was not uncommon for HMV to have two chokes. There is a radiogram here where the 807 OP tube has its own

Marc

HMV N3 4Y Technical Specification

Re. 'AA-3' radio:
I liked how that GE Compactron had Hor Output + Damper in a single envelope! (used for B+ rectifier & Audio-out)
I think Jack Welch ruined GE by taking the company away from technology.

Document uploaded to Post 11 and original circuit diagram upgraded by Marc.

Yes, it's hum bucking. Both plate and screen supplies are bypassed to audio so it's not an ultra-linear connection.

I'd use conventional transformers, double the B+ filter uF's (not a problem) and leave it at that.

Thanks everyone for the informative answers and diagrams, really appreciated. Humbucking sounds like something I would mutter at a recalcitrant radio but it all makes sense now.