Hello everyone.
I'm about to build my first project with a vacuum tube, hence I will want to use a low tension configuration, and for that I have a spare 24V, rated 1A, 24W DC power supply I don't use for anything.
My doubt is about supplying voltage for the filaments: is the wiring the same as for AC? So if I wire them in series I need to connect 12V to one, connect its end to the other one and then this ones end to ground, right?
In particular, and this is the the biggest doubt, will a voltage divider from the 24V supply suffice at providing the needed 12V? If so, say I wanna use two 100k resistors for that, what power rating should they be?
I'm about to build my first project with a vacuum tube, hence I will want to use a low tension configuration, and for that I have a spare 24V, rated 1A, 24W DC power supply I don't use for anything.
My doubt is about supplying voltage for the filaments: is the wiring the same as for AC? So if I wire them in series I need to connect 12V to one, connect its end to the other one and then this ones end to ground, right?
In particular, and this is the the biggest doubt, will a voltage divider from the 24V supply suffice at providing the needed 12V? If so, say I wanna use two 100k resistors for that, what power rating should they be?
Yes, two 12.6V filaments can be connected in series on a 25VDC supply.
No, you cannot use a high impedance resistor voltage divider to run low impedance 12V filaments from a 25V supply.
The 12V filament is effectively an 84R resistor. The divider would have to be made of resistors of a few ohms each.
And a low impedance divider would waste way too much power (10W or 20W).
No, you cannot use a high impedance resistor voltage divider to run low impedance 12V filaments from a 25V supply.
The 12V filament is effectively an 84R resistor. The divider would have to be made of resistors of a few ohms each.
And a low impedance divider would waste way too much power (10W or 20W).
You wouldn't use a voltage divider in this case. If you have a 24V source and a 12.6V heater need, you need to provide a series R between the 24V source and the heater load to drop the 11.4V. To calculate that series R you need to know the current through the heaters (that would also flow through the series R). Assume it is 300mA, the resistor will be 11.4V/0.3A=38ohms. This is known as Ohms Law: R = V/I.
The nearest standard value is 39ohms. Power dissipated in a resistor is I*V so in this case, 0.3A * 11.4V = 3.4Watts. A 5W resistor would do.
If you would have four tubes or two dual triodes needing 25.2V nominal total heater, the 24V source would work with no series R; the small shortfall wouldn't hurt.
Jan
I am assuming based on what you have written that the plate voltage is also 24V? This is rather low for any tube circuit, but in particular for the 12AX7A/ECC83. You might get slightly better results with 12AT7/ECC81, but higher voltages are recommended - 100V minimum.
If you are looking for a single tube project, a UCC88/21DJ8 would be a better fit, wasting less power in a series heater resistor and happier with a 24VDC plate voltage.
Don't overvalue using 1 part you have in any case, tubes=$$$ and you will have to buy multiple things to finish a project.
Don't overvalue using 1 part you have in any case, tubes=$$$ and you will have to buy multiple things to finish a project.
First which tube you will use?
If are two like 12xxxx/ECC83-81-82 you can put two filaments in series connect the pin 4 and 5 and thats'all.
If are two 6922/Ecc88 ( it must use 6,3 volt) and are two the problem exist but you can use a 7812 to lower at 12 volt and put the filments in series
I somehow didn't think about using Ohm's law since I have current and voltage to find the series resistance of the filament... Anyway, thanks! I understand clearly, it's even easier than I thought. The datasheet says 0.15A for series connection and a total of 12V, so Rf = 12/0.15 = 80, I just need another 80 Ohm resistance in series with the 24V supply to get the desired voltage, and dissipating 1.8W. I checked on Amazon, there's a kit of 3Watts resistors with somne 33 and 47 Ohms resistor, so I'm right there with the values too.
Even lower, I want to build a guitar preamp-style circuit to use as a pedal, so there will be a certain resistance between the 24V and the plate. I made as much research as possible and ran some simulations, but at such low voltages I doubt the models are correct. Anyway, if all goes well, I should be able to get a clean boost out of a single triode, I've seen people getting a decent gain out of common cathode configurations powered with a 12V power supply. For a guitar boost, a gain of 10 or 20dB is plenty loud, even more than needed, considering I use a Stratocaster with an average output of 200mVpp, a gain of 10 means 2Vpp which I hope to reach. I just need that the signal doesn't clip harshly, any other added harmonic content (if there will be at such low voltages) is a plus for my application. Anyway, I will consider using other lower mu tubes of the same type, I checked the load lines for the varius 12A*7 tubes and the lowest one (the 12AU7 if I recall correctly) seemed the best behaving ones for 24V supply.
EDIT: Reason why I don't want to build it using HT is because up to know I've only built guitar pedals and so worked with 9 to 12V, I don't have a formal training for working with high voltages and don't want to risk it as of now. I prefer to gather some more practical knowledge before attempting HT projects.
I checked online and read the UCC88 is for UHF application, while I need the tube for audio frequency. Honestly I went for the 12AX7 simply because it's the one that's most used for guitar preamps (along with other various 12A*7 types). Although recently some people are using pencil tubes like the 6205 micro tube for guitar applications
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UCC88 is an ECC88 with a different heater, and you will find countless DIY and commercial audio projects as such. If you are making a guitar preamp, please say so: 1) this is the wrong forum, and 2) it affects what may be a proper response.
Indeed. UCC88 is an ECC88 with a 21V heater iirc.
That would fit your power supply much better.
Jan
That would fit your power supply much better.
Jan
oh I saw a similar question about filament heating (which didn't answer my doubt though) and after reading the name of the section it seemed right to me, sorry if I got the section wrong.
Anyway there have been plenty answers and I've even learned something more which is always great
This might be interesting if you have not seen it before. It also references a PCL82 one tube amplifier.
ECH83 Guitar Preamplifier With Tone Control on 12V DC
ECH83 Guitar Preamplifier With Tone Control on 12V DC