Dear all,
I recently acquired two 3B28's to build a two-phase rectifier. I could not find anything on whether it would be possible to run these tubes with a DC filament current.
The manual states that out-of-phase filaments lead to longer tube life compared to in-phase, would that mean that DC filaments would be somewhere in between?
Thanks in advance!
I recently acquired two 3B28's to build a two-phase rectifier. I could not find anything on whether it would be possible to run these tubes with a DC filament current.
The manual states that out-of-phase filaments lead to longer tube life compared to in-phase, would that mean that DC filaments would be somewhere in between?
Thanks in advance!
DC may be better than AC in some cases though it would be a pain to get DC for the filaments. Remember the DC is riding on the rectifier output. Also you need 5 volts 10 amps for the pair.
Having one side of the filament voltage peak at peak cathode current is a the problem. At least DC is an average instead of a peak.
The datasheet mentions quadrature was best meaning the AC filament is at the zero when plate current is highest. This is
possible when dealing with three phase power.
How much voltage are you planning to have for the output?
One other problem is they are a little bit noisy and will get worse with supply voltages below 400 volts. The critical voltage is 50 volts meaning the tube wants 50 volts between plate and cathode before it conducts down to 10 volts. You typically want a choke input supply to avoid switching noise. Also a choke input filter can help kick the tube on when dealing with lower voltages.
I have an 845 amplifier that I may go with 3b28s when I redo the power supply. I have used them before with an 814 SE amplifier.
Having one side of the filament voltage peak at peak cathode current is a the problem. At least DC is an average instead of a peak.
The datasheet mentions quadrature was best meaning the AC filament is at the zero when plate current is highest. This is
possible when dealing with three phase power.
How much voltage are you planning to have for the output?
One other problem is they are a little bit noisy and will get worse with supply voltages below 400 volts. The critical voltage is 50 volts meaning the tube wants 50 volts between plate and cathode before it conducts down to 10 volts. You typically want a choke input supply to avoid switching noise. Also a choke input filter can help kick the tube on when dealing with lower voltages.
I have an 845 amplifier that I may go with 3b28s when I redo the power supply. I have used them before with an 814 SE amplifier.
Hey DavinCarter, could you share the PS schematic of your 814 SE? How much time do you heat the 3B28 before applying HT? Thanks.
I will need to see what is the value of the input choke and draw a schematic. Power transformer is 1100VCT .5A with 120volts on a 125 volt tap.
I can say you want 10 seconds of filament before applying plate voltage. The tube is not happy with the plate voltage immediately on. Tube damage will probably result. 10 seconds is not so bad though.
The 10 second delay can be done with a relay and an R-C delay on the relay coil.
Attaching a picture of the 814s and 3b28s lit.
One of the 814s eventually developed a filament short to grid and I did not buy a replacement. I wanted to change things around anyway. I am now running 845 on a low power SE amp and have not had a problem with daily use for a little more than a year.
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