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CR Kalypso Amp with output stage problem- Please help!
CR Kalypso Amp with output stage problem- Please help!
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Old 25th January 2014, 09:11 AM   #1
Andyroid is offline Andyroid
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Default CR Kalypso Amp with output stage problem- Please help!

Hi all, I've been repairing a CR Developments Kalypso stereo 15W push-pull EL84 amp for a friend which caused an EL84 to redplate then blew its mains fuse.

So far I've replaced a leaking filter cap, crackling PI socket, now here's where I'm stuck. In doing some overall checks of continuity etc, I've found that the opposite el84 to the one which redplated has about 69k resistance between its cathode and the 180R cathode resistor. The other three el84s cathodes have about 23R going to the resistors.

This 23R appears to be from a winding on the output transformers which is directly between the cathodes and the resistors. I've never seen a setup like this.

Has anyone seen a problem like this before? would it be safe (and not too detrimental to the sound) to remove the TX winding from this spot? Should I just leave the winding 'open'?

As far as I can tell CR Developments are no longer in business and I have no idea where I would find a replacement TX.

Any help would be greatly appreciated!

Andy
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Old 25th January 2014, 09:59 AM   #2
JonSnell Electronic is online now JonSnell Electronic  United Kingdom
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Location: The Jurassic Coast, England. GB
The winding in the cathode is part of the signal path and provides negative feedback to the valve. This was a novel design in the 50's and 60's by Mullard. It is unusual for the winding to go O/c. Unfortunately it is a requirement of the design.
There are transformer manufacturers who will unwind and then rewind your transformer. I don't know your country but in the UK Majestic Transformers will do it for you. They are in Parkstone, Dorset.
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Old 25th January 2014, 10:17 AM   #3
Andyroid is offline Andyroid
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Thanks, I had a hunch it might provide negative feedback. I'll suggest the rewind option to my friend. Until then do you think simply removing the bypass capacitors would provide enough NF to keep him happy till we arrange the rewind? I'm sure the winding can't be 'crucial' to the safety of the amp. Though I don't have a schematic I'm sure I can make it work like a normal/simple push-pull amp. BTW it's not o/c, just a much bigger resistance (I suppose impedance).

Last edited by Andyroid; 25th January 2014 at 10:19 AM.
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Old 25th January 2014, 11:12 AM   #4
Andyroid is offline Andyroid
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Just had a thought (first time for everything!), would this NFB winding control impedance matching with speakers?

I think this amp can be used with any speaker without actually selecting an impedance. This may be how it's done.
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