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JimB wrote:It is interesting that the ST-70 and Mark III manuals do not mention having speakers or other load connected to the outputs when setting the bias initially. I wonder why, if this is so deadly to the output transformers?

Because they were drooling over the thought of collecting that
$ 7.50 plus the cost of parts for every blown up amplifier that
Railway Express brought them to fix. (see ST-70 manual)

Also back in the day, it was common knowledge to connect speakers.

SS amps don't need such a thing, therefore people are losing their amp tune-up skills

Cheers!

This has probably been covered here somewhere before, but are there any quick and easy tests that can be used to identify transformers that have suffered this kind of damage?

JimB wrote:This has probably been covered here somewhere before, but are there any quick and easy tests that can be used to identify transformers that have suffered this kind of damage?

Yeah, my ST-70 a mate loaned me had suffered from this... the OPT's tend to short out to their secondary from arcing

That's one failure mode...

Cheers!

Speaking of oscillation does anyone have any idea why a set of speakers seem to cause a random onset of it to happen? I have a set of KEF 103.2's that will bring it on with a Citation V amp and it's driving me nuts. They seem to work fine with a ST70.

Sweep the impedance of the speakers. Maybe it's them or maybe's there's something wrong with the amp.

Cheers!