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RE: Dynaco A470 Output Transformers

Posted by strijw426 (A) on August 17, 2014 at 14:36:43

In Reply to: RE: Dynaco A470 Output Transformers posted by strijw426 on August 17, 2014 at 12:16:58:

Mark and Uncelestu;

Thanks to both of you for your help. I found one comment on DIY Tubes forum that mentioned when measuring from plate to plate it was normal to see about 200 to 235 ohms, so measuring from CT that would be about 100 to 117 ohms. So it appears that the transformer that is problematic is the one measuring about 200 ohms from CT to either end.

The same DIY Tubes forum post had a follow-up post that said it was common to see ohm readings CT to either plates or screens vary by 15% to 20% and the unit would be OK. Reason is the winding diameter per turn increases as more layers are built up during construction.

This amp may well be one that was subjected to tubes going into run-away conduction (loss of grid bias) on the right channel. It may also have been in a high humidity environment. There is considerable small pitting of the chassis.

Looks like I will be ordering all new tube sockets throughout, new electrolytics, film capacitors and bias rectifier. A new transformer for one channel will be ordered and later a new stainless steel chassis so it will look decent. The power transformer top bell and the one good OEM transformer's bells will get a rust removal treatment, sanding, priming and repainting to look good again. The PC board is in need of a complete cleaning if not outright replacement. However I may try reusing the OEM PC board first to see how it does.

I now have some used 6CA7 JJ outputs and 7199s to use for a trial that seem to check relatively balanced output although a couple of them are slow to warm up. After I get things operational with a new set of tubes I will decide whether to do any modifications.

Cheers!!

Joe

Edits: 08/17/14

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