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Old 4th June 2009, 05:29 PM   #1
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Question How do I tell whether a different replacement output transformer will be ok?

Hi
This is probably a simple question for those who have experience working with valve amps. I've not got a lot of knowledge in this area but am trying to repair a Leslie hammond organ amp model 610.

2 of the output transformers have failed (2 channels out of 4) The amp is a 4 channel push-pull design with 2x 7189 valves (EL84?) on each channel.

I have an old Armstrong A20 valve amp (uses same valves) I am thinking of using the output transformers from this to repair the Leslie. I'm not sure how I know whether they will work.

I've compared the primary DC resistances and the Leslie transformer on one of the working channels reads 460 ohms from each winding to the centre tap. The Armstrong transformer reads 85 ohms to the centre tap. Because of that difference I'm hesitant to try it!!!

Any advice here would be much appreciated because I've been unable to find replacement Leslie transformers to repair the amp with.

Many thanks, Jim
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Old 4th June 2009, 06:00 PM   #2
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The short answer is probably not.

The right way to do this is find out the specifications of the transformer and have 2 custom wound. They should be able to re-wind the damaged ones, or at least determine how to replace it.

The other thing that might help is a schematic.

If the amps are both tube and use the same rectifier tube, you may be close. If they use SS diodes, there are too many possibilities to make even a good guess.

HTH

Doug
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Old 4th June 2009, 07:01 PM   #3
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486 Ohms is very high for a 7189 transformer; 85 Ohms is very low. In the first case, the transformer will dissipate more than 1.5W in the winding resistance with 40 mA per tube. The second seems more typical of a 60W transformer than a 15W one... (edit: I found a schematic of the A20 - are you measuring end-to-end? This has screen taps on the primary - 5 leads.)

What's important is the impedance ratio. 7189s will generally use 8K to 10K primary with the secondary matching your speaker impedance. You can measure the ratio using an AC signal (50-60 Hz will do) - the impedance ratio is the turns ratio squared. So if you apply 1V to an 8 Ohm secondary and measure 31V across the primary, the ratio is 31:1. The impedance ratio is 961:1, or 7688 Ohms to 8 Ohms.

It IS possible to use a 4K:4 Ohm transformer as an 8K:8 Ohm, but the low frequency response will suffer. Perhaps the Leslie can do without the last low octave...
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Old 4th June 2009, 07:07 PM   #4
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Hi,
Many thanks for the reples both. I did suspect that there was more involved! I've been reading about testing output transformers and working out the impedance ratios to get the primary impedance.

If I do that test for both transformers, how close does the result have to be (+/-) roughly for it to be ok match?

Also, I have schematics for both amps. The Armstrong uses a rectifier valve and the Leslie uses diodes (I have had to rebuild most of the power supply using new caps and bridge rectifiers but thats all working ok)

Amp I'm repairing:
http://www.digitallake.co.uk/610.gif

Amp I'm wanting to use transformers from:
http://www.digitallake.co.uk/a20.gif

Cheers, Jim
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Old 4th June 2009, 07:39 PM   #5
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Thanks for the schematics.

One quick look tells me they are very different transformers.
The will in no way be interchangeable.

The Transformer you are trying to replace has 2 windings, a non-center taped winding hooked to a full wave bridge, and a center tapped full wave that is stacked on the 300 V to get 400v total.

your 2 choices are to get this transformer custom wound or use multiple transformers to supply the correct voltages.

You could make some educated guesses with Duncan amp tools PSUD2, but ultimately measuring a good transformer with a signal generator and a true RMS voltmeter is your best bet.

Doug
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Old 4th June 2009, 08:01 PM   #6
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Hi Doug, thanks but it's the output transformers I'm trying to substitute not the mains transformer......I know I wouldn't be able to use the mains transformer they are too different.
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Old 4th June 2009, 09:51 PM   #7
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Sorry

My bad.

Doug
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