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maida-regulator-issues-trtcEsvxHIJIazssxzqn.zip (6.36 MB)

I had all sorts of problems with the Maida Regulator. Checked the circuit multiple times, changed the parts, but yet, the incoming B+ will almost a "pass through" to the output, not being regulated at all.

I worked on the same design for 4 different iterations, only with a 25% success rate... Not sure why. The zener does not seem to sustain the voltage drop across the gate and source of the IRFBE20 MOSFET. I've checked the zener, substituted with a string of LEDs, but the result is the same. After voltage rises to a certain level, the bias will drop, the input voltage will pass to output as if the MOSFET was a wire.

I then used a japanese design based on the Maida regulator. The first one worked somewhat ok, then problems with the regulator IC cause I swapped LM317 into the circuit. The LM317 had 1.25v drop across ADJ and OUT pins, the LT1083 had 1.124v instead in that same application, but the LT1083 was more stable. Strange thing is that I reached a stage whereby the regulated output voltage will slowly drop a volt at a time. With the regulator outboard on crocodile clips, it was back to normal. Soldering it back, the regulated output voltage drops. Strange...

Initially wanted to desolder the regulator and move it back to crocodile clips to confirm what I experienced last night, but this time round, I decided to move the 1k R5 resistor that is supposed to be at the regulator's ADJ and OUT pins. With this on crocodile clips, the regulated voltage did not drop! It was this resistor that's the culprit afterall. Somehow in that specific location, there must be some RFI or other effects that cause it to be unstable. And the thing is that application notes suggested this resistor to be placed as close to the regulator pins as possible for stability... Not so in my case.

Well... guess I can now start prototyping for one channel of the circuit. It took so long to get here... And I think I will need to go with a three chassis design. One single chassis with a dual mono power supply that feeds 2 mono circuits. Need more space to accommodate the power supply section.

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psu-maida-regulator-JlguwzrbozyxnjEciqqj.zip (2.79 MB)

This is the CRC filter and Maida circuit. Rectifiers are Cree zero recovery diodes, capacitors are paralleled 120uF 450v Nippon Chemi-con, power transistor is IRF840, regulator is a Linear Tech 1083. Somehow I missed out on the vital voltage setting resistors... Won't be able to test this circuit yet. I'm trying to make the height lower; in case I'm really building it like the Fi amps. ;)

So, here's the scribble on paper. Operating points are pretty standard stuff for 26 and 45. The brief is simple... DC coupled amp, with driver C4S loaded and with LED biasing. Filaments will be DC current regulated. B+ will be regulated by a HV Maida circuit. Tamura F475 is intended to be the output iron, but I may A/B the FT10S that I got from Andix.

Chassis will be a departure from the tower types that I have been building. I'm veering more to the Fi designed ones. Wanted my wife to lug any fi stuff back when she was in New York some time back, but boy, there's a long back log being fulfilled by Don Garber.

Anyways, since the 6T10 project was not successful, I should look for another project that can hopefully reuse the power transformer. The 45 fits the bill as I've already got suitable output transformers and even a pair of RCA 45 tubes. I've also got a stash of 26 driver tubes. This will be my first all DHT amp. Wonder what challenges I will face... Should prototype this on a board first... ;)

Work on this hopefully will start at the end of this month.

ps. an eagle-eyed reader highlighted that my schematic has some mistakes around the cathode resistor for the 45 tube. i've forgotten to stack the driver voltage for the DC arrangement. will update the schematic tonight. thanks nick!