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ST-70 with Edcor Transformers
ST-70 with Edcor Transformers
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Old 13th February 2018, 04:08 AM   #21
H713 is offline H713  United States
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So is the AnTek transformer a 650 V center tap then? Is there a datasheet available for it?

Also, how did you calculate the filter cap values?
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Old 13th February 2018, 04:36 AM   #22
Eli Duttman is offline Eli Duttman  United States
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The AnTek AS-3T325 has 2 discrete 325 V. windings and 2 discrete 6.3 V. windings. Sorry if I caused confusion. The 2 rails from 1 CT winding concept will be used, when we get to the subject of negative voltages.

I went "seat of the pants", more or less, in selecting the PSU cap. value. Jim McShane effectively uses 410 μF. in rebuilding a H/K Cit 2. and a Cit. 2 produces a bit less than twice the power of what's being worked on. Plenty of energy storage works well. Keep in mind that the energy stored in a cap. varies with the square of the voltage. The Cit. 2 runs at approx. 485 V. and this project runs at approx. 440 V. So, things should work out just fine.
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Old 13th February 2018, 07:48 AM   #23
Eli Duttman is offline Eli Duttman  United States
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My rough estimate for the LTP "tail" CCS setting is 12 mA./channel = 6 mA./triode.

EICO connects their "tail" resistor to ground. However, a CCS requires volts to operate. We will ensure those operating volts are present by connecting the 10M45S device to a negative rail of approx. 28 V., not ground. Remember the 2 rails/1 CT winding concept. The C- (O/P tube bias) rail is approx. 56 V. Those 2 negative voltages will be "extracted" from an AnTek AN-0120 toroid. 5 Schottky diodes are needed here.

Speaking of toroid power trafos, they are wide bandwidth. Therefore, it is necessary to mount ferrite beads on the primary wires, lest noise riding on the AC mains sneak into the amp's DC rails.
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Old 13th February 2018, 12:29 PM   #24
Eli Duttman is offline Eli Duttman  United States
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5 pieces of this 44 cent item at Mouser will take care of negative rail rectification. Discussion of the rail filters will follow.

As shipping charges can "eat you alive", bundle the orders to the various parts sources. For instance, 2 AnTek toroids have been specified. Order both at the same time.
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Old 13th February 2018, 01:54 PM   #25
20to20 is offline 20to20  United States
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Quote:
Originally Posted by H713 View Post
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.
Your original plan will work perfectly if you just switch to a KT-66 for its more compatible load impedance and power range, suitable for those 30W trannies you want.
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Old 13th February 2018, 02:26 PM   #26
Eli Duttman is offline Eli Duttman  United States
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Incorrect regarding the 30 W. O/P "iron". The Dyna ST-70 employs a GNFB loop. Unless bass extension is severely limited, 30 W. "iron" will have its core saturated.

Also, the 7581 or 6L6GC will draw less heater current than the KT66.
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Old 13th February 2018, 02:54 PM   #27
20to20 is offline 20to20  United States
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No transformer other than an original A470 is going to be a match for the FB loop. If the original plan was to stay under 30W then there is no need for EL34. The biasing would be set accordingly, like any amp, and there would be no saturation concerns. Who care s about the heater current if there is adequate supply?
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Old 13th February 2018, 08:56 PM   #28
H713 is offline H713  United States
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To be perfectly honest, I don't have much preference of 6L6GC vs. EL34. Also, what's the difference between a KT66 and a 6L6GC spec wise? I was under the impression that they were extremely similar.

Part of why I was looking at the 30W transformers is that with a pair of 60W transformers, plus a big power transformer, plus a big chassis to hold it all, etc etc, suddenly this thing is 100 pounds. The extra 20 bucks for the bigger transformers is something I can live with. I'm okay with bumping the output power of this down to 25W. I'd prefer not to go below that. If 50W transformers is what it's going to take, so be it.

Also, regarding heater current, I have on hand a dedicated heater transformer from a (dead) amp I parted out. I also have a Citation II power transformer from that amp as well. No, it was not a Citation II I parted out- it used a Citation II transformer with 2x Heathkit AA-121 output transformers. 4x 6550 tubes. One of the output transformers was fried. Fortunately I knew that when I paid the $0 it cost me

I've attached a schematic of the power supply from that amp (was taped inside the chassis). Not sure if that transformer would be usable. That said, I'm also not exactly sure where the 510 volts comes from. And yes, those are 2900 mfd caps. They're the size of a pop can.

Figure I should make sure that transformer isn't going to work for this before I sell it.

Last edited by wintermute; 22nd February 2018 at 01:37 AM. Reason: attachment removed at H713's request.
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Old 13th February 2018, 09:08 PM   #29
00940 is offline 00940  Belgium
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Originally Posted by H713 View Post
I've attached a schematic of the power supply from that amp (was taped inside the chassis). Not sure if that transformer would be usable. That said, I'm also not exactly sure where the 510 volts comes from. And yes, those are 2900 mfd caps. They're the size of a pop can.
The 510V is coming from a full wave voltage doubler, fed by the 160Vac winding of the main transformer, in serie with the 12.6Vac of the small transformer. It should normally gives you closer to 490V but lightly loaded it might go up to 510V.

See here for more information on doubler: https://www.electronics-tutorials.ws...r-circuit.html
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Old 13th February 2018, 09:26 PM   #30
Eli Duttman is offline Eli Duttman  United States
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100 lbs. is high. A H/K Cit. 2 is 70 lbs and nearly twice the power of the EL34/6CA7/KT77 bunch.

You could use the Cit. 2 power trafo, but its capability would be wasted in this project. Save it for your next amp, a PP KT88 project. Another possibility is sale, as that item has substantial value.

The AnTek AS-3T325 is modest in both cost and weight, while being "everything" a 35 or so WPC stereoblock needs. IMO, you will enjoy full bass extension and the ability to play a Liszt "potboiler" or " head banger" Rock.

As for the 6L6 family tubes, check out the 6L6GC, 7581, and KT66 datasheets.
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