Hi, I am enjoying my little caesar 300b but one of the D3a driver valves flashes bright at start up. Can I use a thermistor on the heater circuit? Two D3A would draw .63A at 6.3v. How do I calculate which thermistor to use? Should I put in a bypass? I know there are better solutions out there but I have very little space to work with and a switch with a thermistor is very compact! Thanks.
I also have similar behaviour tube (long plate ECC82) in one phono preamp..... for decades.
I try to exchange for another tubes ... many good reputation, expensive 12AU7 variants ... but old Tungsram always returned... it sounds better than anything ... and survive everything.
Engineer proverb: what isn't broken, doesn't need to be fixed.
I try to exchange for another tubes ... many good reputation, expensive 12AU7 variants ... but old Tungsram always returned... it sounds better than anything ... and survive everything.
Engineer proverb: what isn't broken, doesn't need to be fixed.
Mastersound, or something else?
Very interesting, like an old Mullard tube.
jeff
It is the one by triode dick, not mastersound. I have maida regulation regulation of the B+ ( thanks Tom Christiansen at Neurochrome) and the 300b heaters are fed off of Tent Labs regulators. It is really lovely. I just don’t want to use up the D3As. Apparently the flash is because small differences in the thickness or coating of the tungsten wire. This is a good video that explains it:
Perhaps a regular resistor is easiest…
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Friend has old CBS/Halcyon tubes and they flash bright at start up. Every start up for the last 4 years. It has to do with the alloy of tungsten in the filament wire, and it's not a problem. If anything, it allows the inrush current to produce photons instead of just heat, which is probably a good thing.
Typically you would use a thermistor on the primary of the transformer. You could also use a limiting resistor on the heaters and have a relay bypass it after a few seconds. This datasheet gives some good info. https://www.amphenol-sensors.com/hu...-325D-Thermometrics-NTC-Inrush-031814-web.pdf
Perhaps your doctor could prescribe some pills for the heart, these jolts can't be good 🙂
Or you can feed the heaters from a current source. Or just do nothing and avert your eyes.
You are right, but I have no space! I used a very slim chassis for this build and don't want to move into another one now. I also want to avoid messing with the primary - everything else in the amp is already soft start. The Carlson tech tips looks like the way to go. I'll just swap different resistors until the surge is tamed. And use a toggle ON(OFF) so I don't leave the resistor in the heater circuit.
The thermistor on the Primary of the PT does help, and it ought to be possible to shoe horn that in onto on-off switch. The body of the ICL needs to be somewhere where it can get hot without causing an issue. I use a CL-90 on other builds I have done and it tames the inrush of a toroid.
Some tubes flash more because some of the heater protrudes below the cathode, so it is just because it is more visible.
I have a TVA-10 where there are 3 double triodes with 12.6V heaters in series on the 36V bias winding, and one of them lights the room up. But, like mentioned above, no ill effects after some years.
Some tubes flash more because some of the heater protrudes below the cathode, so it is just because it is more visible.
I have a TVA-10 where there are 3 double triodes with 12.6V heaters in series on the 36V bias winding, and one of them lights the room up. But, like mentioned above, no ill effects after some years.
Thanks for the tips. There does seem to be a lack of consensus on whether heater flash should be ignored. I have read that it is part of the design to have additional tungsten and acts as a heatsink - and like you say: it is only that the flash is visible on some filaments due to coating differences. The Carlson video above implies that the inrush more on filaments that flash, but then only shows the flashing filament measurements. At least Carlson is taking a scientific approach. In any event, I don't see the harm in providing a soft start to be on the safe side.
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Some older tubes had an A added at the end of their name (6CB6/6CB6A) or other letter that was a reference to controlled warm-up times. Not a thing nowadays. Maybe it was for tubes connected in series, like those All American 5 radios that used the full 120v spread out over the 5 tubes 12-12-12-50-35
Interesting with controlled warm up time suffix. Wonder if it was more a concern to not blow fuses aside from typical slow blow fuses.
Such as older radios or TV's loaded with multiple tubes. Reduce a manufactures worry about fuses blowing even with slow blow.
Wish I had a picture of my old tube organ with 2 banks of 72 keys. Yup tube oscillator for every note. Should have seen the size of that heater transformer.
was actually more than 144 tubes for extra voices. Would have had a heart attack designing that thing for a manufacture and releasing it in the wild.
Absolute insane collection of inductors as well for the oscillators
Thermistor value will be based on transformer VA you'll have to stroll through the manufactures guidelines for far as I remember a math formula and list of recommended products for joule rating expected at switch on. Formula for VA and temp the thermistor needs to have for low or near 0 resistance during normal operation. They basically stay hot for normal operation. If temp rating to high for normal operation. Basically wont get warm enough for low resistance. Motors for Vacuums typically have the thermistor wrapped up inside the motor so guarantee to stay warm. Only need cold at first start of course for high resistance.
Not sure if you had a clogged vacuum start fine, but if you stop halfway through the room, turn it on again and ...pop goes the breaker.
Thermistor already warm so no more soft start.
Such as older radios or TV's loaded with multiple tubes. Reduce a manufactures worry about fuses blowing even with slow blow.
Wish I had a picture of my old tube organ with 2 banks of 72 keys. Yup tube oscillator for every note. Should have seen the size of that heater transformer.
was actually more than 144 tubes for extra voices. Would have had a heart attack designing that thing for a manufacture and releasing it in the wild.
Absolute insane collection of inductors as well for the oscillators
Thermistor value will be based on transformer VA you'll have to stroll through the manufactures guidelines for far as I remember a math formula and list of recommended products for joule rating expected at switch on. Formula for VA and temp the thermistor needs to have for low or near 0 resistance during normal operation. They basically stay hot for normal operation. If temp rating to high for normal operation. Basically wont get warm enough for low resistance. Motors for Vacuums typically have the thermistor wrapped up inside the motor so guarantee to stay warm. Only need cold at first start of course for high resistance.
Not sure if you had a clogged vacuum start fine, but if you stop halfway through the room, turn it on again and ...pop goes the breaker.
Thermistor already warm so no more soft start.
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Flash is normal, but I suppose it can't hurt to slow it down. Since the hot resistance of the heaters is 10 ohms, and the cold resistance is probably like 1 to 2 ohms, I would try a thermistor with a cold resisistance of similar order, like 2 to 5 ohms.
If your raw heater supply is more than 6.3V (which is often the case) then may a simple dropping resistor would work just as well.
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Good point, but I think that is the idea of having a switch or relay the takes the thermistor out of the circuit once it is warmed.
With respect to the Thermometrics (Amphenol) devices -- you have to be mindful of the minimum and maximum current drawn through the device as they don't dissipate a lot of heat.
You can slow-start an LM317 regulator, or just use it in current source mode...dunno about Tent shunt regs
You can slow-start an LM317 regulator, or just use it in current source mode...dunno about Tent shunt regs
I have several 12v tubes that flash when the heaters get turned on. The thing is that it is only a few of that type that do it. One of my 14n7 and three of my cv535 do it but none of the others. They do all play just fine but it does look worrisome.