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In Reply to: RE: Dynaco mod posted by Eli Duttman on May 08, 2017 at 19:51:38
You can't use a Triad C-17X choke in a Dynaco ST-70 with a stock type chassis. Inside the Dyncao ST-70 chassis, you only have about 1.75 inches of height. The C-17X is about 2.25 inches in height. The C-17X is also to large fit the mount hole spacing for the stock C354 Dynaco choke. The Triad C-24X IS a good substitute for the original Dynaco choke.
Many Dynaco ST-70 users who have never taken off the bottom cover of their amp might be surprised to find a tan colored waxy substance on the inside of the bottom cover directly under the choke. Over the years this "filler material" can and will melt out of those original chokes. When people "restore" an original Dynaco ST-70 they usually replace the quad cap, the selenium rectifier and possibly the driver board. They should also REPLACE THE CHOKE. That choke was cheaply made. The Triad C-24X is a much better made choke. I have used many Triad C-24X chokes in the past 11 years. To my knowledge, not one has failed ...
Bob Latino
Follow Ups:
It would be interesting to know what the C-24X choke measures at lower currents. It's rated 1.0H at 240mA, but as we know (I hope), inductance varies with current, inductors being some of the less "ideal" components out there.
The C-354 at Dynakit Parts is rated 1.75H @ 200mA and 62 Ohms. Triode Electronics claims 1.5H @ 200mA and 55 Ohms for theirs.
I checked the inductance of an old stock C354 choke in a Dynaco MkIII on the bench at the moment. It measures 1.38H with a Q of 7 with no DC current applied. The measurement was taken with an impedance bridge, and it does have provision for measuring inductors with up to 100mA bias current. It is, however, somewhat tricky to set up the test.
Thanks for the specifics, Bob. I was wondering about the available space.
The C-354 is, most definitely, a POS, which should be routinely replaced, like selenium rectifiers, given almost certain long term failure.
FWIW, I have a C-17X on hand, for a project. Triad has kept standards up. The "U" bracket part has a bottom plate. There are no exposed laminations. :> )
Eli D.
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