ST-120 output transformers

Brinkman · 1 ST-120 output transformers on Sat Mar 28, 2009 11:39 pm

Brinkman

Posts: 13
Join date: 2009-01-01

Hi Bob. I'm gonna throw a bunch of questions your way:

I'm curious as to how the output transformers in your ST-120 kit compare to the outputs of the stock ST-70 (core materials, laminations, bandwidth, winding ratio, UL tap %, etc...).

I'm also wondering if you've ever experimented with mono-blocking them Mk.III-style, and if so, could a Mk.III-compatible driver board be used? What would the requirements be for a pair of power transformers in this configuration? Could the Mk.III chassis be used?

Sorry, but I've been wondering about this for a while now....

Best,
Benjamin

Bob Latino · 2 Re: ST-120 output transformers on Sun Mar 29, 2009 7:00 pm

Bob Latino

Admin

Posts: 249
Join date: 2008-11-27
Location: Massachusetts

Benjamin,

The ST-120 output transformers (and power transformers also) are custom wound for me by a USA company. They are larger than a stock A-470 output transformer as found on the ST-70. They weigh about 2 pounds more than a stock A-470 output transformer and are 3 7/8" tall, 3 1/4" wide and 3 1/8" deep (about 1/2" taller, 3/8" wider and 1/4" deeper than a stock A-470 output transformer). They are interleaved/layer wound with 4200 ohm primaries and 40% ultralinear screen taps and have M6 silicon steel grain oriented laminations. The company that makes them tells me that the cores will not saturated unless they see about 90 watts. A stock A-470 output transformer, as used on the ST-70, will start to saturate the core at 40-45 watts. The bolt pattern on these output transformers will not fit a Mark III chassis.

The ST-120 is basically equivalent to a pair of Dynaco Mark III's with VTA driver boards although the ST-120 runs about 25 volts higher on the plates (500 volts DC on an ST-120 vs. about 475 volts DC on the plates of a Mark III) of the output tubes than the Mark III's. The VTA board on the ST-120 also has proprietary changes to components vs. the VTA driver board on the ST-70. The circuitry of the ST-120 also has a supplementary cap module for extra B+ storage and a set of bypass capacitors whose values were worked out by trial and error with an oscilloscope. The ST-120 also has the convenience of a single chassis and triode/pentode switches.

I have a few customers that use them as monoblocks and a few customers that biamp with them. See photo below showing a pair of ST-120's biamping a pair of fairly inefficient Magnepan MG 3.6R speakers.

Bob

1 ST-120 output transformers on Sat Mar 28, 2009 11:39 pm

Brinkman

2 Re: ST-120 output transformers on Sun Mar 29, 2009 7:00 pm

Bob Latino