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Tekko 9th April 2005 12:26 PM

Dynaco ST70 clone
Hello all! I have descided to make a replica of the classic Dynaco ST70, i was lucky enough that the OPTs i ordered is the same anode-anode Z as the original st70 OPTs:) Im going to order a few sockets, anode connectors for the EL38:s, two 7199 tubes and a resistor kit. Heres the schematic:

What do you think ?

SY 9th April 2005 01:03 PM

Since you're using different iron and a pentode configuration, you'll have to change the feedback (R15-C5) and compensation (R1-C9). That will take some time with a square wave generator, an oscilloscope, and lots of trim parts. But it's not too difficult if you keep your wits about you and your hands away from nasty voltages (I noticed you don't have any bleeder resistors. Naughty, naughty!).

Also, you'll want to stabilize the screen voltage, either with a regulator (best), zener with cap bypass (OK), or just a bypass cap (barely acceptable). That's the main reason IMO that pentodes have gotten a bad rap- poor screen regulation severely compromises performance.

analog_sa 9th April 2005 01:07 PM

A voltage multiplier?!

Tekko 9th April 2005 01:33 PM

I dident care to draw out bleeders in the schemo. On the amp the output stage will be UL, i just drew it this way as the OPT model doesent have any UL taps. Whats the diff between the dynaco OPT and the hammond OPTs i ordered? The plate to plate resistance (impedance?) Z is the same, 4.3k. Different iron type ?

SY 9th April 2005 01:47 PM

I haven't used the Hammond so can't give you specifics, but transformers vary widely from one type to another. Despite basic impedance (turns ratio) being the same, the frequency response and section to section coupling is likely to be different than original Dynaco. So you'll definitely want to experiment with the compensation and feedback to get the amp stable and clean.

One of the Audio Engineering reprint books from Old Colony has a great article by Norman Crowhurst on how to refit an output transformer- I'd highly recommend that you give it a read.

audiousername 9th April 2005 01:47 PM

Tekko, there's lots more to the OPT than just the impedance ratio. A whole lot of things could be different - the core, the winding geometery, differences in leakage inductance and shunt capacitance....

Tekko 9th April 2005 01:49 PM

hum :-S

SY 9th April 2005 01:54 PM

I found the article I mentioned:

"Electrical Adjustments in Fitting a New Output Transformer," Norman Crowhurst, Audio Anthology Volume 4, p54.

Tekko 9th April 2005 02:02 PM

Hum looks like SY may come in handy sometime in about 25 weeks.

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