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ST-70 with Edcor Transformers
ST-70 with Edcor Transformers
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Old 12th February 2018, 02:04 PM   #11
20to20 is offline 20to20  United States
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Originally Posted by H713 View Post
I was thinking of building a ST-70 design using an Edcor transformer, mainly the CXPP30-6.6k. A470 transformers are a little more than I'd like to spend
I think if you'll look into using KT-66 instaed of the EL34 you'll have a superb amp. Just the right power and Z for the trannies. Just put the recommended screen grid stoppers in the UL tap leg and it'll sing all day long.

Last edited by 20to20; 12th February 2018 at 02:06 PM.
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Old 12th February 2018, 11:10 PM   #12
audiowize is offline audiowize  United States
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Originally Posted by Eli Duttman View Post
Interesting. Yet if the archives are scanned, info. from a member who successfully executed Williamson style circuitry using Edcor "iron" can be found.
I couldn't get much in the way of a 10kHz square wave out of them. The ringing was pretty extreme, and it was a major compromise between taming the ringing and maintaining the square-ish shape to the square waves.

I've worked with A470 and Z565 clones withoyt running into these issues.
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Old 12th February 2018, 11:38 PM   #13
H713 is offline H713  United States
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Originally Posted by Eli Duttman View Post
Mullard style circuitry, like that previously linked, is an excellent choice. However, EICO's HF89 is (IMO) a better starting point. You will use the Edcor CXPP60-4K and AnTek AS-3T325 "iron" previously linked.
Thanks for the suggestion. Would you recommend this over a Dynaco ST-70 design with Edcor transformers? Yes, I do have good meters and an oscilloscope.
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Old 13th February 2018, 12:34 AM   #14
Eli Duttman is offline Eli Duttman  United States
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I definitely prefer Mullard style to Dyna style. Dyna style uses a "concertina" phase splitter and the peaking tweak could easily fail to work. A "concertina" phase splitter is a tricked out voltage follower, with lots of local current NFB.

IMO, the "budget" O/P tube to use is the ElectroHarmonix (EH) 6CA7. Don't get Eboned. Buy from a reputable dealer, like Jim McShane.

H713, I want you to draw and upload a schematic for the basic B+ PSU. Parallel the 325 VRMS windings of the AnTek power trafo and bridge rectify with 4X Cree C3D03060F Schottky diodes. CLC filter the "raw" B+ with 2 of these caps. and this Edcor choke. Insert a CL-130 inrush current limiter, in the "hot" line, between the O/P of the bridge rectifier and the CLC filter.

Later on, we'll deal with subsidiary B+ voltages/decoupling networks. We'll deal with the negative bias supply later too. Hint, AnTek saves "scratch" again.

BTW, as you are finding out, magnetics are the big ticket items in a tube amp build.
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Old 13th February 2018, 03:02 AM   #15
H713 is offline H713  United States
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Sorry about the hand drawn Schematic. Hopefully I'll have time to draw it up in a proper schematic editor tomorrow.
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Old 13th February 2018, 03:29 AM   #16
kodabmx is offline kodabmx  Canada
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I love it. The CL-130 looks like a resistor playing hockey! Don't forget about Hammond.
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Old 13th February 2018, 03:29 AM   #17
H713 is offline H713  United States
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Where would I buy Hammond for a reasonable price? Everywhere I see Hammond transformers they're about $125 each, and I can buy original A470 transformers for that price.
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Old 13th February 2018, 03:30 AM   #18
kodabmx is offline kodabmx  Canada
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The cheapest source I've found, and I can get there in 30 minutes. They also tend to underrate their transformers. Hammond Push-Pull Tube Output Transformers - (1608 - 1620, 1645 & 1650 Series)
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Old 13th February 2018, 03:33 AM   #19
kodabmx is offline kodabmx  Canada
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The prices are in CAD BTW
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Old 13th February 2018, 03:48 AM   #20
Eli Duttman is offline Eli Duttman  United States
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Originally Posted by H713 View Post
Sorry about the hand drawn Schematic. Hopefully I'll have time to draw it up in a proper schematic editor tomorrow.
That's very neat and fancy/shamancy is not needed. When I draw, hen scratches are produced. There are some minor corrections. The choke's 350 mA. rating is the limiting factor in this setup. 350 mA. is way more than the absolute minimum needed. Reserves of current allow for dynamic headroom and superior bass performance.

The AnTek AS-3T325 has pairs of secondaries. Connect the 325 V. windings in parallel. Remember, when cap. I/P filters are employed, approx. 1/2 of the RMS current is available as DC. Allocate a separate filament winding to each channel.

The filter caps. are 330 μF., not 350 μF.

I'm uploading a sample of my poor handiwork. We will come back to that sort of topology, when we work on negative voltages. Of course, you will build with 100% Schottky diodes. Coincidentally, what you will build also has a low current "tall" rail and a high current "short" rail.
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