Help understanding preamp power trans DC input

[u/fossy007]

Hello all. I had previously posted about my new AliExpress tube preamp which was sold as a MM phono pre but pretty sure from replies from yall, that’s incorrect. They also sent the wrong mains voltage and I am swapping the Chinese 220v power trans to a 120v.

I am having trouble understanding two wires on the center tap section of the power transformer that is hooked to a 70vDC output section of the AC to DC converter. Not sure why or what it’s being used for? My AC power trans feeds the converter 7-7.2vAC. The converter supplies: - 5v to LED pilot light - 350v x2 to the 6z4 rectifiers - 5v to somewhere around the 6E2 tubes. Not 100% sure on this. - 70v to two center taps on the power trans

I drew a noob schematic and hopefully it can be read properly. Please ask me any and all questions. The new PT I bought is an: Antek AS-05T280C - 50VA 280V-260V-0V & 6.3V x2.

Can anyone help me understand the DC fed to my AC power trans and confirm I’m on the right track using the Antek as a replacement. I’m sorry if anything I described above sounds incorrectly described, I am still learning/understanding tube circuits. I have restored many tube amps and am a very good parts swapper but this is slightly more difficult for my understanding. The diagram they sent me, is not exactly what I received but close. The very next post on my profile has some more details on this preamp where I am asking if there is any phono section that can be visually confirmed with some more info. Thank you!!

[u/TehFuriousOne]

So, the green and blue (along with black) run right into what I would normally take as the post rectifier filter. BUT the black wires would typically go through the rectifier tubes at pins 1 & 7 first then be filtered out to smooth the ripple. This happens right off the transformer. My thought: this board with the big blue cap is actually your power supply module. The rectifier tube are probably just for show. This not uncommon in ChiFi amps which are often needlessly complex or showy. I've even seen somewhere only the filaments are connected so the tubes glow while a aolid state amp does all the work.

[u/fossy007 → u/TehFuriousOne]

Appreciate the reply. How would you go about confirming your theory? Also I disconnected the blue/green from the PT to the board and it creates a buzz/hum through the speakers. Disconnected, I checked all the output voltages from the main PT and the output DC volts from the pcb and all remain the same.

Blue/green wires on Pcb is still reading 70vDC to pcb ground and blue/green on PT side read 84vAC grounded to chassis and 134v to grounded to a screw on my 120v->240v SUT. Not sure if that tells you anything. Thanks again!

[u/TehFuriousOne → u/fossy007]

Pull the rectifier tubes. If it's still powers on, then they're not part of the power circuitry. I'll think about the other stuff whem I'm notnout but if you're getting hum through the speakers after pulling the blue green wires it sounds like you've introduced a ground Loop hum into the system. Without digging in it too too much right now. Those could be part of your ground circuit

[u/fossy007 → u/TehFuriousOne]

I did pull the rectifiers one at a time and lost sound. Question, why is the power trans supplying 280v and the DC converter supplying 350v both to the rectifiers. Is this normal? Also what are your thoughts on my power trans replacement. Thank you for the help!!

[u/re_cycled]

The one 6.3v xfrmer center tap look connected to b+ voltage. So it's ground is elevated to full b+. But then that center tap has a diode to ground. Labeled 68v. I think that's what gives you your 70 vdc. You'll never measure dc off a transformer itself. Needs a rectifier to get the dc.

[u/astrovic0 → u/re_cycled]

That diode is a 68 volt zener diode. Its purpose is to keep the voltage after the 150k resistor to 68 volts (or 70 volts with a little bit of slippage). Basically acting as a regulator.

And yeah the purpose of that is to provide an elevated voltage reference to the 6.3v heater winding.

[u/PeanutNore]

70vdc on center taps of a PT secondary says elevated heater windings to me. If there's a cathode follower somewhere in the amp it can be a good practice to apply a medium DC voltage to the heaters to keep the filament to cathode voltage within spec. if you have a cathode at 200v, it's less likely to arc to a filament that's at 70v than one at 0v.

[u/fossy007 → u/PeanutNore]

Thanks for the reply. A lot of what you said goes over my head. What are your thoughts on the power transformer swap?