Hi everyone,

I have just picked up an old HMV stereo I'm restoring. It has a dead ultra liner audio output transformer on one channel - Not sure if anyone can assist in either helping me source a replacement, or if I can simply covert it to a standard single ended audio output transformer.

The audio output valves are 6M5, both the current ultra linear transformer have the number 9030322 stamped on the top of them. Each transformer has a blue and yellow lead from the bottom of the transformer and a red lead from the top.

Hope all this makes sense and I look forward to hearing from someone.

Cheers
Scott

Not sure what you mean by ultra linear. Perhaps you could tell us the rest of the valves in it, also the HMV model number, so that a schematic can be matched to it.

The proper meaning of "ultra linear" is to have 2 output valves in each channel in class B mode, with the screen grids fed from separate tappings from the speaker transformer. This means the transformer will have 5 wires on the primary, plus the usual pair going to the speaker on the secondary. Yours doesn't sound like that.

So, we need more information.

I think it might be an N2 chassis. They were used in various early HMV stereo models.

The OP transformer part number matches. I can see how the OP transformers could be mistook for being ultra-linear. They are single ended with one side of the primary going to the plate & the other side to the screen. There is a tap close to the screen end going to the HT.

HMV N2 Service Manual
HMV N2-22
HMV N2-66

Ah yes, I see what you mean. Had not encountered that in consumer equipment before.

It all depends which part has burnt out. If it's the screen side, just connect the screen directly to the HT, problem solved. But if it's the anode side that burnt out, you can do the screen change just mentioned, and then replace the HT-to-anode section with an ordinary 7000 ohm transformer.

That, at least, should work ok, but with a slight increase in distortion.

Or, if you want it as original, you'll need to contact a rewinder service, which some other members know about.

It's a hum cancelling trick. The transformer is balancing the output tube's plate resistance against the resistor between the output tube's plate circuit and the screen grid supply.

This is 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.

Thanks for the info folks - very helpful and appreciated. Monochrome, yes, its a N2-22. The circuit is a great help.

I don't think it's worth rewinding the transformer but I am happy to go with replacing both with 7000 ohm standard transformers - again I'll have to source those too, unless someone here has got a couple I can buy. I'm surprised at them being 7000 ohm though, I thought they were generally 5000 ohm on the primary side.

If I do replace the current ones, is the modification to the circuit going to cause any grief or just a slight hum?

Cheers
Scott

I'm surprised at them being 7000 ohm though

7000 ohms is the manufacturer's spec for the 6M5 in that configuration.

I'll have to source those

Evatco in QLD has a range of universal output transformers. The higher the wattage, the higher the price:

http://www.evatco.com.au/product-category/transformeroutput/

Thanks GTC, so how do I mod the circuit to make it work ok?
Cheers
Scott

PS: Have priced OP transformer through Evatco and they were pricy. Kevin is always a great help though I must say. I guess if I can pick up a couple of used ones, that will suffice.

Seems the circuit, though clever, would increase the transformer saturation - already a problem in class-A amplifiers.

Actually it would reduce the DC core saturation a little, but not enough to be a significant factor. Maybe around 10%.

Ah, because of the steady-state flow in opposite directions?

Thanks for your help folks. I found someone from this site who will rewind the transformer I have for a reasonable price which is great.