Inrush current on filaments of tube rectifier?
I have been using an old amp of mine that I kept in storage for years. It's a 300B amp with a separate tube rectified supply. No schematics, but it's simple to explain: the filament transformer has five secondaries for the tubes in the amp, the buffer tubes, the 300B's and the rectifiers (5R4). There's a delay for the HV transformer. After the two rectifier tubes I have one 5H coil connected to the filaments of the second tube, then a capacitor bank (450uF), then another 5H coil then another cap bank (450uF). That connects to the O/P transformers on the one hand, and to the buffer 6SN7's tubes (preceded by another 10H coil and 700uF cap bank).
The problem I'm having is with the high inrush current on the filaments when I turn on the amp. Will all tubes I have tried I can see a blue spark inside, about 2 seconds after the amp is on and way before the HV section kicks in. I have tried 5U4's, 5R4wGY (Chatham), 5R4GY (RCA). I have used a pair of the Chatham 5R4WGY for one year with the best sonic results, but after this period the arching inside the tube is getting ugly and I have noticed a decrease in SQ in the last couple of days, which tells me the tubes are going to fail soon.
I have installed a NTC thermistor I had here (Rhopoint SG240) btw the AC and the primary of the filaments transformer, but apparently it's not doing anything, at least I can't tell as the arching is the same. I'm not sure an inrush current limiter can be successfully used here, but that's my best hope as I'm not willing to supply dc to the filaments.
The tubes are expensive and I don't believe this is an acceptable characteristic of the amp, probably the reason I stopped using it many years ago, in spite of great sound.
I thank whoever can help in advance for suggestions.
I have been using an old amp of mine that I kept in storage for years. It's a 300B amp with a separate tube rectified supply. No schematics, but it's simple to explain: the filament transformer has five secondaries for the tubes in the amp, the buffer tubes, the 300B's and the rectifiers (5R4). There's a delay for the HV transformer. After the two rectifier tubes I have one 5H coil connected to the filaments of the second tube, then a capacitor bank (450uF), then another 5H coil then another cap bank (450uF). That connects to the O/P transformers on the one hand, and to the buffer 6SN7's tubes (preceded by another 10H coil and 700uF cap bank).
The problem I'm having is with the high inrush current on the filaments when I turn on the amp. Will all tubes I have tried I can see a blue spark inside, about 2 seconds after the amp is on and way before the HV section kicks in. I have tried 5U4's, 5R4wGY (Chatham), 5R4GY (RCA). I have used a pair of the Chatham 5R4WGY for one year with the best sonic results, but after this period the arching inside the tube is getting ugly and I have noticed a decrease in SQ in the last couple of days, which tells me the tubes are going to fail soon.
I have installed a NTC thermistor I had here (Rhopoint SG240) btw the AC and the primary of the filaments transformer, but apparently it's not doing anything, at least I can't tell as the arching is the same. I'm not sure an inrush current limiter can be successfully used here, but that's my best hope as I'm not willing to supply dc to the filaments.
The tubes are expensive and I don't believe this is an acceptable characteristic of the amp, probably the reason I stopped using it many years ago, in spite of great sound.
I thank whoever can help in advance for suggestions.
Thanks, but I don't see your point. There's obviously an excess of current as per my description of the problem. The question is how to limit the inrush current.
Increasing the inductance or resistance, to limit the current spike. If you get the cathode hot and switch the Ac into the rectifier, then it is this case.
Source: Langfor-Smith's Radiotron chapter 30.
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