Output transformer for early radios a little help please

[lloydwells]

I have several late 1920s early 1930s German radios these all use a RE134 output valve and were designed to use a separate usually high impedance speaker. I'd like to use a permanent magnet speaker but could use a little help with what I need in terms of an output transformer and where to obtains something suitable . Many thanks

[Ed_Dinning]

Hi Lloyd, a lot of these old speakers were quoted as about 3 to 5K ohm and were connected directly, so not necessarily a perfect match.
If you are using a 3 ohm speaker try and use an old type that may not have had a great frequency response but will be sensitive and give the most sound for the meagre power of the valve. Something of the order of 40:1 should be suitable.

There were also speaker cabinets made with transformers fitted for use with older sets, I may have one of the transformers or a complete cabinet.
At a pinch an old Rediffusion speaker may well work.


Ed

[Herald1360]

There's a small application circuit for the RE134 in Radiomuseum which suggests a load impedance of 12k for the valve, so for a 3R speaker you would require a transformer with a ratio of about 60:1.

https://www.radiomuseum.org/tubes/tube_re134.html

This is within the options offered by the little OEP transformer sold by RS.

https://uk.rs-online.com/web/p/audio...rmers/2106475/

From the valve data in RM, with an anode load of 12k, 250V HT and an anode current of 12mA, you should get around 650mW audio output, enough for plenty of volume from a reasonably sensitive speaker. It's about the same as a typical medium sized tranny would provide and around three times the output of typical valve portables using the likes of DL94.

[John M0GLN]

I was going to start a similar thread as I have a 1930 set which needs a matching transformer so I'm not trying to hijack this thread but to find an answer that 'lloydwells' may also find useful.

With the limited power available from these output valves would the physically smallest transformer with just sufficient power handling be more suitable than a higher quality transformer, designed for say a single ended EL84 output stage, would the latter have higher iron or copper losses using up too much power?

John

[JohnBG8JMB]

I looked up the RS transformer- eye-watering price and not currently available - the suggested alternative is double the price...I don't see paying £38 for a small o/p trans, but I'm mean.

you could sucessfully use a small mains transformer 230:4v for a 3-4Ώ; 230-6V for 8-16Ώ. 3VA should be ample for this application; or 6VA to have something in hand.

John G8JMB

[McMurdo]

Mouser have a selection of 'tube' output transformers but not cheap either.

[PJL]

One of those suitcase battery portables would have an output transformer and speaker that should work.

[paulsherwin]

Any valve output transformer from a scrap class A valve output stage is likely to work acceptably well, even if the turns ratio isn't spot on.

[don200002]

I use an output transformer from a scrap Bush DAC11 on my 1930 Gecophone,
it works well and sounds pretty good, it's driving the center and bass speakers from an old panasonic 5.1 system, 4 ohm total load. good luck, Don.

[lloydwells]

I had a rummage through the parts pile and came up with one from a Murphy early 30s french set it's working quite nicely.

[vinrads]

As mentioned above save the o/p transformers from scrap battery portables , the come in very useful . Mick.