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In Reply to: RE: Approx. Cost to Rebuild a Dynaco ST70 and PAS posted by Charlie8521 on January 20, 2016 at 06:50:37
David Gillespie and George Ronnenkamp wrote a detailed analysis of the Dynaco ST70 amp (Dave) and the PAS preamp (George). It is definitely worth your while to read these articles before starting.
Eli, I would be interested in your method of replacing the GZ34 with ss diodes for the ST70.
Follow Ups:
"Eli, I would be interested in your method of replacing the GZ34 with ss diodes for the ST70."
Use 2X Cree C4D02120A 2 A./1200 PIV Schottky diodes. Insert a CL90 inrush current limiter between the rectifying diodes and the PSU filter. Use only enough capacitance in the 1st position of the CLC filter to keep the B+ rail voltage up. 10 to 15 μF. should be quite ample. Unless the available space demon stirs things up, a 600 WVDC film part is best here. As stated many times, the OEM C354 choke is a POS. Squeeze a 300 mA./2.5 H. Hammond 159T in. Pile the energy storage up in the reservoir position.
The rail voltage will come in higher than OEM and that fact must be dealt with. Adjust the resistance value in the decoupling network to bring things into line for the small signal circuitry. Last and definitely not least, switch the O/P tube pairs to combination bias. "Stand" each pair on a shared 1% tolerance 100 Ω/470 μF. network. The OEM single bias trim pot./channel is OK. Notice the very convenient "idle" current test point. O/P tube matching requirements are slightly relaxed. As is always the case, gm must be close, but a small amount of cathode current imbalance is OK. The shared RC network will correct small differences in cathode current.
Eli D.
Great minds think alike. I posted the same advice probably at the same time you did.
There's also a great thread detailing Dave's ST70 reports along with questions and answers over at Audiokarma.
http://audiokarma.org/forums/index.php?threads/dynaco-st-70-base-line-testing.578485/
Looks like you beat me by 4 min and 2 sec although I didn't see you response when I replied. The great minds are Dave and George as these articles demonstrate. Am anxiously waiting for the PAS power supply and suggested PAS mods articles.
Agreed.
I've done mods to a Fisher 400 and two Dynaco ST70's that Dave had a hand in and both turned out fantastic and were huge learning opportunities. It's amazing the work that he and George put into the articles. They're so well written and informative.
I'm looking forward to the final PAS articles also. I even have a old PAS 2 ready for whatever is recommended.
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