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diy audio and stuff

my diy audio and life

Filed under: LT1083

DC filaments for 300B

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My Vaic302B(300B equivalents) tubes draw 1.8A of filament current (the 1R 5W resistor was later changed to a series of two 0R33 5W resistors). It's quite hot if DC filaments were to be used. In fact, the entire amp is quite hot. Probably need to put in the real aluminum side panels to help dissipate heat rather than the current aluminum-sprayed wooden panels. It's actually quite a nice optical illusion. Most people did not know the panels were wood.

So, I did up a small prototype to see if I can fit the thing into the already cramp enclosure. A capacitor multiplier that provides a theoratical 4.7F of filtering with the help of a TIP142 darlington NPN transistor. The TIP142 has a hfe of 1000. This is followed by a LT1083 current regulator. Two os-con capacitors were used for convenience's sake(I have a bag of 100uF 20V). One after the TIP142, the other after the LT1083. Turns out that the heat sinks may be too small. Buzzing became apparent after around 1hr of audio. It's interesting to note that a hum pot on the filament with DC heating does not yield as much as impact on reducing hum as when it is on AC heating.

Guess it is back to AC heating for this amp, and living with the slight hum.

Maida Regulator issues

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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.

PSU & Maida Regulator

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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. ;)

26 & 45 filament circuit done

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Tested the 45 filament circuit based on the capacitance multiplier. Initially got the wiring of the MJE3055 wrong, but rectified that mistake. Strange that at the LT1083 output, voltage is 2.45v, but at 45 filament, it is 2.28v. It's just a wire in between... Odd.

This perf board is for one stereo channel only. Need to build one more for the other channel when a working prototype of the amp is achieved.

Capacitor Multiplier

Tested the capacitor multiplier and it seems to work! I used some nice parts here for the filament dc circuit. Shindengen 4A Schotty bridge diodes for rectification, followed by some Nichion Fine Gold capacitors that's reused from my very first diy project. The current regulator is a LT1083.

Need to get a few more of those larger heatsinks for my LT1083. The voltage output at the tube is around 1.65v. A little higher than the supposed 1.5v. Should not be an issue.

The same perf board will hold both the circuits for the 26 and 45 tubes. Was doing up the portion for the 45 last night.

The circuit can be found at Rod Elliot's ESP site. I used two MJE3055.

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