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Dynaco 70 mods

64.203.19.193

Posted by janos on April 4, 2007 at 20:32:30

In Reply to: Re: Bob Latino Dynaco 70 posted by kstagger on April 4, 2007 at 09:57:26:

I agree - every mod suggested makes the st70 more audiofile sounding, but mods to the driver board change the character: you will have a modern amp isnstead of a stereo 70. I found it's better to build an entirely different amp, if you want modern sound: you'll have to rebuild the st70 for that completely, anyway.
If you want st70 sound, change all old caps to the best caps you can find, Mullard EL34s, RCA 7199, and Valvo metal base 5ar4. Will be a surprise. With quality caps alone, it will sound almost as sweet as a stereo 35. And will definitively not run out of juice. Will still not sound as hi tech as an AN amp, but will be on par with any good amp, with the stereo 70 character.
If you have a phenolic board, that is not cracked or burned, keep it, as it has better dynamics than modern boards. It also has a richer, fuller tone. I think the old board is quite a dampener in itself.
I can second lifting the PT with grommets. For one thing, it mechanically dampens, but it also increases the distance from the chassis itself, decreasing the magnetic coupling between PT and chassis - making it even quieter. Also place grommets under the OPTs, to decouple them from the chassis. Another improvement ;).
I have heard tons of mods, and all change the stereo 70 sound. If you can manage, get 2 units, one stock, upgrade with quality parts unchanging the schematics and resistor / cap values, and the other up for mods. You will have fun with the mods; and be amazed at the tone and the musicality of the stock. In addition, you can relieve the stress on the PT with a 120V / 110V isolation step-down transformer.

Good luck,

Janos

PS: replace the selenium rectifier of the bias supply with an UF diode first thing. The selenium r. is prone to malfunction, and is a serious health hazard when it blows... and it will blow, sooner or later.
Also place a 50nF cap accross the power switch, to increase switch life and avoid on/off pops on the speakers.

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