• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

ST-70 with Edcor Transformers

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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.
 
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.
 
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?
 
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 :D

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.
 
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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/blog/voltage-multiplier-circuit.html
 
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. :D 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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Mr. Thorpe used Mullard style circuitry along the very lines I've been talking up. There are a few details to change and nits to pick [of course :D], but that's the basic signal topology to use. Jason successfully connected the LTP "tail" to ground. I want that "tail" connected to a small negative voltage, for "elbow room". Remember, PSUs are at AC ground.

The B+ topology is pretty much what we'll use. As you're finding out, there is very little new under the sun, when it comes to tubed circuitry. IMO, the big advance has been the ease with which constant current techniques can be implemented. Way back when, constant current implementation was such a PITA that nobody bothered. Solid state (SS) items, AKA "sand", frequently make good "spear carriers" for tubes.

Cross linking between "boards" is commonplace. An apology was not necessary.
 
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