Good morning all, I am presently restoring a Philips valve radio model 147. It has a pair of 6M5 valves in push pull mode. The output transformer is marked KOL53, which is open on one side only. The circuit nominates 10000 p-p as the input, and 3.5 ohm as the output. My question is can I configure a 100 volt line transformer to replace it some way. Bowler.

The open circuit would be on the primary, at least that's what I've always found.

A few years ago, as I neared the end of buying things, I bought a Kriesler console. The output transformer had been replaced by a power transformer, using what would normally be the centre-tapped HT winding as the primary for the output valves. Naturally they'd got the wires around the wrong way, but that was easily fixed. One wire was arcing to the chassis too. I can't remember which winding was used for the speaker but I assume the 6.3volt one would do.

Even though it seems that it would be a horrible fit, it actually works quite well, with nice sound.

In your case, the main thing is to get as high impedence (= as many turns) on the primary as you can. If there's not enough the output valves will overheat.

Hi Bowler and Rob, yes of course a power tranny will work as a PP output transformer.

Just in rough terms the ratio between a 6 v filament winding and a 385 PP HT winding is in the region of 60:1
That means the impedance reflected back from a 3.5 ohm speaker to the plate will be about 10,000 ohms or so.
A couple of pentodes would work happily into that.
There are a few other items like inductance and losses and balance and all that stuff that affect frequency response and power transfer but these are valves that have a fair bit of leeway and not much watts.

Even in a single side set up I have used stock items with good effect.
A pair of line transformers correctly phased should work in some way or a pair of 240/6v PTs the same.
Please I don't want any theoretical "experts" telling me this wont work until they actually try it themselves and tell me why it does not work!
As you all know I have wound the odd transformer or two and know a little bit about the subject.

Now there is a little project for me to do when I recover from Covid.
Run a couple of 6v6 into some filment trannys.....hmmmm.

Cheers, Fred.

Hi Bowler,

If you have a cash to splash you could get a Rola 10k CT to 2 ohm isocore type S/H from EBay Valveman 2006 ..for $76

Sometimes the failed winding happens because of corrosion of the fine wire right at the solder termination of the fine wire to the lead wire. You could try peeling away the insulation that covers these solder terminations to inspect closer. Corrosion may have eaten away enough of the copper magnet wire there to make it act like a fuse and blow open. (If the defect is deeper inside the winding you have lost nothing by doing this).

If you do use a power transformer with centertapped high voltage, don't cut off the primary winding lead wires, or the 5V heater wires, just insulate them with tape or such. . (you may find a proper centertapped push-pull output transformer, and you could use it to change out the "power" transformer, and with intact primary leads, put that power transformer to use as a power transformer. Waste not want not...).

Hi All, Thanks for your input. I tried Fred’s solution and it worked a treat. Bowler