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Re: Berman 76/6SN7 Preamp: B+ voltage too low

Keith, as you know, I have a similar preamp.
Berman's article assumes that a choke input PS will output 90% of the secondary VAC. The correct textbook figure is 80% of one secondary winding.

In your case, you should end up with about 370V, not 400, which is fine, the tubes will last longer and the sound quality will be fine.
In fact, lower voltage with high current sounds better than high voltage & low current, in my experience. The latter sounds slightly bright.

The 20K resistor is correct, it draws enough current (all the time, it's not just a bleeder) for the choke input power supply to regulate.

There are several possible reasons for the large voltage drop across the rectifier:
-Worn out rectifier (fake NOS); try replacing with a new 5Ar4, which has a lower voltage drop.
-The rectifier heater voltage is too low: the tube's internal resistance goes up, voltage drop across the tube goes up. Measure AC V across the heater pins (watch out, you will be measuring a 5V potential difference but the pins sit at 312V!). If the heater voltage lower than 4.8V, your trafo's rectifier winding is probably 1 A, not 3A.
-Another possibility is poor contact between the rectifier tube and the heater pins. Check that the tube pins are shiny and the socket contacts are tight and clean.
-Check all solder joints at the rectifier tube socket for poor or cold solder joints (BTW Wonder solder NEVER gives a cold joint).
-Disconnect the PS from the AC outlet, let the caps bleed and measure DC ohms between each tube pin and its socket (pull the tube half way out to measure ; touch one DMM probe to the tube pin, the other probe to the solder joint under the socket.) You are looking for a fraction of an ohm. If your DMM does not have auto-zero, measure the DMM probe's resistance, then measure the heater contacts. The added resistance should be less than 0.1 ohm.
-The last possibility is that your DMM is uncalibrated or its batteries ran out. Once I spent a whole weekend chasing wrong voltages on the Berman preamp, my DMM had weak batteries.
-Lastly, there is a wiring error in the schematic. if you ever plan to use 5AR4/Gz34 (highly recommended if you like a fast/punchy sound), connect the choke to pin 8, not pin 2. The 5Ar4 has its cathode directly connected to pin 8.

When you start to make the line stage, send me an email. The schematic available on the web has slightly different resistors than Berman's design, it runs the tubes at lower current, a good decision for tube longevity, however I believe you will like better the sound with Berman's values.
Good luck!
Carlos (tube nut from Rio)


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