Overhaulin' the ST70

knowledge base for the classic Dynaco ST70

Postby kheper » Tue Nov 13, 2012 9:41 pm

The 70V bias tap will permit the use of KT66s, etc. without rebuilding the bias supply. The 2 x 4A filaments, however, are 2A less than than the PA-060. (4 x KT88 + 2 x 12au7 + 12ax7 = 7.3A filament current) On paper, 8A looks like enough juice, but a surge at start up may stress the transformer. Maybe get a small 6.3V transformer for the driver tubes?
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Re: Overhaulin' the ST70

Postby burnedfingers » Sun Dec 16, 2012 9:33 am

Actually if my memory is correct Geek's Dynamutt II runs 8.75A and I have personally run 8.1A for years without problems. When considering what the dynaco is capable of running you also need to consider that the dynaco was engineered to also run the preamps off the octal sockets on the front of the chassis. This being said the 70 will run the 8A draw you are worried about.


reference this

http://dynaco2forum.forumsmotion.com/t3 ... pabilities
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Re: Overhaulin' the ST70

Postby TerrySmith » Sun Dec 16, 2012 7:41 pm

I have found a couple other options, the Antek 4TK360 toroid and possibly getting a ST-120 PT from Bob Latino.

The toroid would be my first choice if it weren't so ugly! I'm going to figure out a box or some sort of cover to mount it on a '70 chassis.

If I can get the '120 PT from latino I could make a hot rodded '70 possibly. It has a 410-0-410 @ 400ma secondary (I think) so I could use 6550 or KT88 tubes and bias them harder. My power supply can handle 700v so that will be no problem.

I still may go for the Edcor though!
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Re: Overhaulin' the ST70

Postby kheper » Sat Dec 29, 2012 4:06 pm

Here are some transformer covers. But, they are for EIs, not toroids. The larger ones should fit the Antek, but maybe not the chassis?

http://www.goodcomponent.com/transformer%20cover.html
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