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Old 27th January 2013, 11:09 AM   #21
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Quote:
Originally Posted by Eli Duttman View Post
Some comments, If I may. The Triode Electronics driver board gets populated with 2X EF86s and a single twin triode. That twin triode may be an ECC99, 12BH7, or 12AU7. I advise installing an ECC99, for its high gm. High gm in designs employing GNFB, like the ST70, provides resistance against slew limiting associated with the HF error correction signal.

As the ST70 uses "fixed" bias and an instant on, SS rectified, C- supply, even the slightest delay in B+ rise is sufficient to protect the O/P tubes against cathode stripping. A NTC inrush current limiting thermistor provides the short delay needed, if either SS diodes or a directly heated vacuum rectifier are/is employed in the B+ circuitry.

IMO, it's perfectly acceptable to SS rectify ST70 B+. You turn the excessively "tall" B+ rail from a liability into an asset, by employing combination bias. "Stand" the cathodes of each O/P tube pair on a 100 Ω/470 μF. network. A very convenient "idle" current test point is available and the original single bias adjustment for each channel is quite satisfactory. Matching requirements are reasonably relaxed. As is always the case, closeness in gm is a must for each pair. However, small differences in cathode current are acceptable.

An additional benefit of SS rectifying the B+ is the cooler running power trafo obtained by holding the value of the 1st filter cap. down to say 10 μF. Replace the overworked C354 choke with a Hammond 159T.
Cathode stripping???

You have got to be kidding.
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Old 3rd February 2013, 02:00 PM   #22
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I finished the kit today and the amp is playing sweet music! Special thanks to SandyG for recommending tubes4hifi. The kit is realy great, the manual is precise and easy to follow. Bob packages all parts in individual bags, which makes the assembly a breeze. And, Bob supported me, when I needed it. I can definitely recommend this kit if you are after a Dynaco ST 70 kit. Being a DIY newbie myself, I can also say that the kit is suitable for beginners. If you can solder and operate a srewdriver you're all set.
Soundwise I like the ST 70 better than the Gainclones that I did before. Thanks again to the community here in helping me make this decision.

Best

Michael

Ps: I have taken lots of photos along the build process and put them in my blog. Should you be interested, check it out. (I payed Wordpress now to remove the ads, so you wont get bothered.... ).
Michael's Project Page

Last edited by zimmer64; 3rd February 2013 at 02:02 PM.
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Old 3rd February 2013, 05:06 PM   #23
SandyG is offline SandyG  United States
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Michael -

Glad you got it going. It's a great amp and sounds very good. I was greatful for how Bob packs all the parts and the instructions and some of the extras you can get.

Enjoy!

Sandy
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Old 3rd February 2013, 07:16 PM   #24
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My opinion: if somebody wants to own Pyramid with golden chassis, I would be glad to know where to order such chassis. :-)
At least, golden mirror behind tubes would look gorgeous.
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Old 4th February 2013, 02:59 AM   #25
SandyG is offline SandyG  United States
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Wavebourn -

Did you post to the wrong thread? I think this might have been for the thread on the 24k 'Special Expensive' Edition of the ST70.

Sandy
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