You are missing one significant item in all this discussion. There was a similar discussion in a different forum dating back to 2013 ... but this significant item was mentioned there!
Tube (valve) filaments have a ptc (positive temperature coefficient) ... i.e. as they heat up, the resistance goes UP. So your calculations of the tube whilst cold may be significantly out.
For a typical filament light bulb, at operating temp, the resistance is nearly double the cold resistance!
Now the other vital thing to note is that as soon as you put load on the transformer, it's voltage output will drop.
So, I would connect the tube up directly and measure the voltage ... you are highly unlikely to blow it up! Somewhere, I have an ECC83 (aka 12AX7 ... if you're in the UK, you'll find ECC83 more often and in the US and Canada 12AX7) Haven't used tube devices in YEARS.
Diodes in the filament line is likely to introduce noise as the AC goes across the zero crossing.
Tube (valve) filaments have a ptc (positive temperature coefficient) ... i.e. as they heat up, the resistance goes UP. So your calculations of the tube whilst cold may be significantly out.
For a typical filament light bulb, at operating temp, the resistance is nearly double the cold resistance!
Now the other vital thing to note is that as soon as you put load on the transformer, it's voltage output will drop.
So, I would connect the tube up directly and measure the voltage ... you are highly unlikely to blow it up! Somewhere, I have an ECC83 (aka 12AX7 ... if you're in the UK, you'll find ECC83 more often and in the US and Canada 12AX7) Haven't used tube devices in YEARS.
Diodes in the filament line is likely to introduce noise as the AC goes across the zero crossing.
That’s where I got the 21 ohms from, 6.3V/300mA = 21 Ohms. You are absolutely correct of course, resistance of cold filaments is less than an Ohm!
You could always chop the AC up with a triac circuit 😀
They used a microcontroller in this case... I'm sure it could be modified to work as a dimmer.
Switching low voltage AC with a microcontroller – Arduino, ESP8266, ESP32 & Raspberry Pi stuff
They used a microcontroller in this case... I'm sure it could be modified to work as a dimmer.
Switching low voltage AC with a microcontroller – Arduino, ESP8266, ESP32 & Raspberry Pi stuff
It was probably on the power supply forum but there is a triac and relay based design to soft start large transformers.
When you have a filament secondary that has a little too much voltage under load, then a single series dropping resistor is the simplest soft start for the filaments.
IRMC.
IRMC.
Exactly. The bottom line is no matter what you do heat will be generated as a result. Can't go wrong with a resistor.
> Any idea on the approx. power
Any idea how big they are?
Can you find similar-size resistors with power ratings? Digikey, Farnell, Mouser, Jameco?
Any idea how big they are?
Can you find similar-size resistors with power ratings? Digikey, Farnell, Mouser, Jameco?
They are 60mm x 25mm x 5mm so total surface area of about 4000 mm2 and a volume of about 8000mm3.
It's probably the volume to surface area ratio that determines the rating (volume contains the heat and the surface areas dissipates it), that's a figure of merit of 2mm.
This 5W 1 Ohm resistor is approx. 22mm x 10mm x 10mm, so figure of merit is also about 2mm.
This 10W 1 Ohm resistor is approx. 49mm x 10mm x 10mm, so figure of merit is about 2.25mm.
This 15W 1 Ohm resistor is approx. 49mm x 13mm x 13mm, so figure of merit it about 2.8mm.
So I guess they are only 5W.
It's probably the volume to surface area ratio that determines the rating (volume contains the heat and the surface areas dissipates it), that's a figure of merit of 2mm.
This 5W 1 Ohm resistor is approx. 22mm x 10mm x 10mm, so figure of merit is also about 2mm.
This 10W 1 Ohm resistor is approx. 49mm x 10mm x 10mm, so figure of merit is about 2.25mm.
This 15W 1 Ohm resistor is approx. 49mm x 13mm x 13mm, so figure of merit it about 2.8mm.
So I guess they are only 5W.
Unsure what the level of finicky fuss is all about, unless you're worried the cathode emission will change once you get your design finalized - and the filament right at 6.3.
The tube will easily shrug off a volt over on its filament.
Back in the day of CRT television, they made a device which extended the picture tube performance after cooking for so many years, resulting in low gun emission. It was actually an autotransformer that boosted the filament voltage, "restoring" the picture brightness. A trick the repairman could pull that was cheaper/easier than replacing the big tube. How long it worked for the customer is anyone's ?? - but they sold 'em.
I once came across a 3" oscilloscope with a dull CRT. Brought it to the cal lab at work, who happened to have a CRT "rejuvinator". The guy hooked it up - I saw the cathode get way brighter than normal and while I was looking really close at it, he pressed the rejuve button. Something exploded off the cathode, some kinda rays probably shot through my eyes and brain, and the CRT ran brighter afterward. Apparently the cathode heater filament just laughed it off...
I've deliberately cooked low emission tubes on my tube tester using way higher filament voltages - the control is right there, how easy, why not? They test better at higher filament voltage, but when set back to the proper voltage, they went back to the initial performance. Didnt seem to hurt the tube any; I just couldnt make the performance increase "stick" via some HV filament cooking process.
TLDR; I wouldnt worry about it, nor go to extravagant lengths to correct a 1V over on your tube filament. It wont hurt the tube any.
The tube will easily shrug off a volt over on its filament.
Back in the day of CRT television, they made a device which extended the picture tube performance after cooking for so many years, resulting in low gun emission. It was actually an autotransformer that boosted the filament voltage, "restoring" the picture brightness. A trick the repairman could pull that was cheaper/easier than replacing the big tube. How long it worked for the customer is anyone's ?? - but they sold 'em.
I once came across a 3" oscilloscope with a dull CRT. Brought it to the cal lab at work, who happened to have a CRT "rejuvinator". The guy hooked it up - I saw the cathode get way brighter than normal and while I was looking really close at it, he pressed the rejuve button. Something exploded off the cathode, some kinda rays probably shot through my eyes and brain, and the CRT ran brighter afterward. Apparently the cathode heater filament just laughed it off...
I've deliberately cooked low emission tubes on my tube tester using way higher filament voltages - the control is right there, how easy, why not? They test better at higher filament voltage, but when set back to the proper voltage, they went back to the initial performance. Didnt seem to hurt the tube any; I just couldnt make the performance increase "stick" via some HV filament cooking process.
TLDR; I wouldnt worry about it, nor go to extravagant lengths to correct a 1V over on your tube filament. It wont hurt the tube any.
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I ran 6SL7 at 12V for a week once before I realized the voltage switch was set wrong... They still work.
Oh yeah - just one more story. My father bought a series strung AC line heater stereo amplifier; two 35W4, 50C5 and a 12AX7. When initially turned on the big heaters in the two 35 and 50 volt tubes were way lower resistance than the two little heaters in the 12AX7. I used to watch with such joy as a kid when those 12AX7 heaters would light up white-bright when powered on. I think I even figured out that they didnt do that when I did a "hot restart", though I didnt get why at the time. That tube never died, all the years that we used it.
These days it'd be fun to put my Fluke 125 scope across the filament and see the AC transient that thing took repeatedly. Wouldnt be surprised if there was a couple cycles of all 110V. Too bad that amp is long gone, too expensive on ebay to get back.
These days it'd be fun to put my Fluke 125 scope across the filament and see the AC transient that thing took repeatedly. Wouldnt be surprised if there was a couple cycles of all 110V. Too bad that amp is long gone, too expensive on ebay to get back.
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No, just surface area. (We don't want to "contain" heat, we can't keep it in for very long.)