Output transformer advice wanted.

[Neil Purling]

I need the ratio to provide 3W max into 4 ohms out of a 25A6G. Is that around 50:1?
I was thinking of using a mains transformer, but what type?
I am not sure if the RS Universal gets anywhere near.
The laminations of audio transformers are an 'E' around the core with 'I' laminations over them. Perhaps with a paper tape shim gap as well.
Audio transformers also seem to have the primary & secondary windings in layers wheras a power transformer does not.
What, if anything is likely to be noticeable with a single output stage, which is part of a LW/MW radio.

This has probably been discussed before. If someone wants to direct be to relevant discussions.......

[Neil Purling]

Oops. I meant "Output transformer advice wanted.

[ms660]

35:1 possibly.

EDIT: Based on this:https://frank.pocnet.net/sheets/127/2/25A6.pdf

And using this:http://www.maxmcarter.com/classecalcs/tratiocalc.html

Lawrence.

[Station X]

Sounds about right.

The load impedance is 5000 ohms. The speaker impedance is 4 ohms.

The impedance ratio is 5000/4 or 1250 to 1.

The turns ratio is the square root of this ie 35 to 1.

Don't forget to check the current and power rating of the transformer you use.

[ukcol]

Hi Neil,

Has the loudspeaker in your set been replaced with a modern type? As far as I know 4 ohms wasn't a standard speaker impedance when the 25A6G was in its prime. Having said that, as has been said many times before on this forum, optimum load matching is not a critical factor, quite a degree of miss-match is generally tolerated.

There are a lot of transformers about designed to match the popular EL84 (5.2k ohms) into 3 ohms, you should be able to source one of those relatively easily. A transformer designed for the EL84 should be good for 48mA of anode current which exceeds that required for the 25A6G.

[Neil Purling]

I have an EL84 transformer although it is bigger than the original transformer which sat on the back of a field coil speaker which was defective. It is too big to mount to the speaker basket.
The only way would be to use some aluminium sheet as a bracket that was bolted to unused mounting holes on the rim of the speaker.

[Herald1360]

Originally Posted by Neil Purling

I need the ratio to provide 3W max into 4 ohms out of a 25A6G. Is that around 50:1?
I was thinking of using a mains transformer, but what type?

You'd probably get away with a 12VA 6V secondary mains transformer, but the HF response would be poor and core saturation effects would also muddy things somewhat.

[k_yller]

Originally Posted by Neil Purling

I have an EL84 transformer although it is bigger than the original transformer which sat on the back of a field coil speaker which was defective. It is too big to mount to the speaker basket.
The only way would be to use some aluminium sheet as a bracket that was bolted to unused mounting holes on the rim of the speaker.

What size you need? (distance between holes in bracket)

[Neil Purling]

What are the ratios offered by the old RS Universal o/p transformer?
It looks like the best I can do for a 4 ohm speaker is tags 1&4 on the primary then B&D on the secondary.
For a 8 ohm speaker a better selection looks like 1-3 then A-D.
The oldest speaker I have is a 4 ohm 1950's Celestion driver from a Pye portable, which is being re-used in another set with a 25A6 output bottle.

[Station X]

The data is in post #6 in this thread:-

https://www.vintage-radio.net/forum/...ead.php?t=8633