Hi, I'm on vacation and don't have direct access to measure approximate values.@HerbertR , i tried your method and reached the desired tensions by adding a 47k pot on B+, and only changed the big 5 ohms resistor on the heater (around 7v from factory) to get something closer to 6v . Now i don't have any 120v zener, can i use this PSU "as is" before getting my hands on the correct diode ? I also added additional filtering (1uf 400v) on the B+ rail, and increased the 220nf film cap to a 470nf filtering on the heater, just because i have some good ERO MKT in spare
- but if I'm not mistaken - please correct me - the 120V DC B+ rail of a 251 with 6072 tube and similar should draw about 1.1mA (?). So you need a ~ 100kOhm or so resistor with at least 0.25W then as a mic dummy plugged into the 7-pin connector between the B+ pin and pin #7 ground to pull the value down to 120V. Then you wouldn't need the zener diodes. But: When you activate the power supply and while the tube is heating up, the B+ voltage can briefly reach voltages above 150V and then drops to 120V. Not the best thing for tubes or a sometimes undersized large DC blocking capacitor in the microphone. But for many old circuits this was the standard for such "unregulated" power supplies. I'll be honest... These power supplies sound better in my opinion, more organic as regulated B+ rails, but could be my imagination...
- to lower the heater voltage - it is regulated. First thing: put an insulating disk and an insulating sleeve between the LM7806CV and the heatsink. Otherwise you will get voltage fluctuations between 6.3V DC and 7V DC. Bend the voltage regulator a bit and measure the heater output Voltage and you will see what I mean. All these 180VDC SCT2000 China power supplies have the same issue...! Also: there is no point in changing the large series resistor of 3W or so on the board. This resistor is there to lower the voltage a few volts before "entering" the "fixed" 6V DC voltage regulation. If this resistor is set too high, you are "underfeeding" the regulator with too little voltage. You need at least about 2V more than the 6V to match the voltage of the LM7806 for it to work properly. If you want to underheat the tube a little, perhaps for longer life or to get more harmonics... then I would do it after this "regulated" heater power supply circuit, in an "unregulated" way. You can unsolder the heater wire between the PCB board and the 7-pin connector and solder on an isolated series resistor of 1-3 or whatever...Ohm them to drop the voltage a few hundred mV to 6VDC or whatever you want down.
Then you should be fine and you can drive the Mic for fine tuning . Regards
