Bush AC11 Output Transformer

[Zelandeth]

A somewhat depressing start to the restoration of this set. It does appear that the primary of the output transformer is open circuit.

Assuming that it's been cooked by an output stage fault (excuse me while I glare at the grid coupling capacitor on the UL41), and that there's not a readily visible break near one of the terminals I'm guessing that I'll need to track down a replacement. Rewinding it, I suspect is somewhat beyond me at this stage.

I did take a look at the output transformer on RS (item 210-6475), which designed for such a purpose doesn't appear to be suitable. The DC primary impedance of this is 4K ohms, whereas the primary of T1 on the output tranny in this set is only 500 ohms - while I'd not worry too much about a bit of a difference, 3.5K sounds like it might upset the output stage just a bit!

I've got a couple of other sets here which I might be able to borrow a transformer from - but as I've not started them yet, I've not yet got the data...and would rather avoid having to drag the whole lot out of the wardrobe and having to measure output transformer impedances, especially as I realise that even if they match, the ratios could still be absolutely miles out.

Those in a real state and likely only to see restoration if I get REALLY bored and have a lot of time to spare: A Pilot Blue Peter, a Cossor 575.

Also, in somewhat better condition are an Invicta model 32, a Murphey 186, or a Ferranti 146. While I'm planning to get these up and running at some point, I'd happily go about finding an transformer for them in my own time, whereas this set is being repaired for a friend, so takes priority if there's a solution there which will perform well and be reliable.

Looking at the circuit, it doesn't appear that there are any wierd and wonderful feedback taps on this one or anything, so how are the odds of finding a suitable replacement - whether it be something I have laying around here, or whether it be via a post in parts wanted. I'm a bit worried to be honest, as I'm sure I've heard of this problem being quite a common problem on sets like this.

Bit of a shame really, as the set looks to be in really good shape otherwise! Even the rubber wiring seems to be immaculate.

[PJL]

Impedance vs resistance...

The service sheet quotes DC resistance as this is what you should find if you stick an AVO on it. The RS datasheet quotes impedance which is the AC equivalent when the transformer is loaded with the specified speaker.

The recommended impedance can be found on the output valve datasheet http://www.mif.pg.gda.pl/homepages/f...030/u/UL41.pdf which quotes 3K - near enough.

You will have to check your speaker impedance to see if you can make it work.

[paulsherwin]

Don't worry too much about impedence matching. Anything designed for use with a UL41 should work very well, and indeed any scavenged 1950s O/P transformer will probably be 'close enough' regardless of the original output valve. The worst that's likely to happen is you'll lose a bit of volume.

Paul

[Steve_P]

Agreed. Don't get bogged down in figures and formulae. Any near enough O/P transformer will do.

But do everything else first though!

Cheers,

Steve P.

[Herald1360]

Originally Posted by Zelandeth

I did take a look at the output transformer on RS (item 210-6475), which designed for such a purpose doesn't appear to be suitable. The DC primary impedance of this is 4K ohms, whereas the primary of T1 on the output tranny in this set is only 500 ohms - while I'd not worry too much about a bit of a difference, 3.5K sounds like it might upset the output stage just a bit!

Don't confuse the DC resistance of the primary with its signal impedance when the secondary is loaded with a loudspeaker. I can see no reason why the RS transformer wouldn't work OK if it will physically fit, unless there's anything "odd" about the original transformer like separate feedback windings or taps for HT smoothing / hum bucking purposes. Even if there is, there would be a way round it, but it does complicate things.

If it doesn't offer exactly the right impedance it's not too critical- it may just reduce output or increase distortion slightly.

Good luck with therest of the restoration / repairs....

Chris

Oops....

Moral here is... don't go for a cuppa half way through a reply

[Zelandeth]

I remember thinking - "I'm going to run into AC and DC impedances here aren't I?" but thought I was making things too complicated. Yay, I was right to start with!

I'll have a look at the output tranny from the Pilot - it's the set that's in the worst state (cabinet in particular is held together mainly by willpower). Fingers crossed it's not fried of course!

Cheers for the quick responses folks. I'll let you know how it goes.

[Sideband]

The only reservation about using the RS Universal is the maximum DC current. The UL41 is quite a hungry beast and may draw as much as 45mA. The RS transformer is only rated at 40mA. You can reduce the current through the UL41 by increasing the cathode resistor but you can only do this by so much before you start getting distortion. I have used the RS transformer as a temporary sub in a DAC90A with good results but sourced a good transformer for a permanent repair.


Rich.

PS: Ed Dinning can offer a replacement wound transformer. Why not send him a PM?

[Zelandeth]

Right, have just had a look at the one on the Pilot - which conveniently is missing a back, and was visible without having to play a round of Wardrobe Tetris to get to it.

It appears to use a 6V6GT output bottle, and its DC characteristics certainly seem to match those in the Bush - looks like it will even fit on the bracket too.

Looking at the data, it's got a significantly lower rated maximum current though... (either that, or I'm reading the numbers wrong) so the transformer might not be quite up to the task.

[Radio_Dave]

Hi,

Firstly, Here's a useful link to transformer impedance matching that's worth bookmarking http://www.radioremembered.org/outimp.htm

Secondly, If the transformer's primary is O/C you've nothing to loose by trying to find the break. Make a simple winder, remove the secondary and then unwind the primary until the break is found. Check with a meter to make sure there isn't any more breaks then clean, twist together, solder and varnish the break and re-wind.... It's not a perfect fix, but it's worked for me on several occasions

Thirdly, Having said all that, I'm sure I've got a complete, scrap, AC11 in the garage If the output transformer is OK, you can have it

David

[richrussell]

Originally Posted by Radio_Dave

Secondly, If the transformer's primary is O/C you've nothing to loose by trying to find the break. Make a simple winder, remove the secondary and then unwind the primary until the break is found. Check with a meter to make sure there isn't any more breaks then clean, twist together, solder and varnish the break and re-wind.... It's not a perfect fix, but it's worked for me on several occasions

I might give that a go with the transformer from my DAC34 (no doubt identical to the AC11 one), which has an open primary. Trying a second hand DAC90A transformer anyway, but if I can coax some life out of the old one it might be handy to have a spare.

Rich

[paulsherwin]

Originally Posted by Zelandeth

Looking at the data, it's got a significantly lower rated maximum current though... (either that, or I'm reading the numbers wrong) so the transformer might not be quite up to the task.

Just check the operating current, and bump up the cathode resistor a bit if it seems high. It's a good idea to reduce the O/P valve current to around 30mA anyway, since this will increase the life of the increasingly rare UL41 and the reduction in maximum volume will be barely detectable.

Paul