I am wondering if I can make a vacuum tube oscillator for a LED flasher.

The problem is I only have 4 vacuum tubes: 14GT8, 12AU6 and 2 12BA6 tubes. I have a 12 volt DC power supply, 10 volt AC, 20 volt DC, and 5 volt DC.

I know it is most likely impossible to make a flasher with these tubes, so I am wondering if I can combine it with common semiconductors like a 555 timer or a 2N2222 transistor.

I just want a LED flasher/oscillator  circuit that needs the tube(s) to function for fun. Is this possible?

  • \$\begingroup\$ When you look at tube graphs, you see that they are conducting happily with 20V. I think hearing aids and some small electronic devices used 22V and even 12V. If you look at the ideas of sub-threshold-cmos you may be able to go pretty low. You can't get much power as a few mA*20V is not much power - thats why they mostly used 300V. But a few mA is enough for an LED \$\endgroup\$
    – Henry Crun
    Apr 4, 2018 at 4:06
  • \$\begingroup\$ Neon lamp relaxation oscillator would be another candiate, but needs 90V I imagine. Why, since the neon already flashes? Well modern leds are probably much brighter! \$\endgroup\$
    – Henry Crun
    Apr 4, 2018 at 4:11
  • \$\begingroup\$ valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf \$\endgroup\$
    – Henry Crun
    Apr 4, 2018 at 4:17

5 Answers 5


There are several approaches to this problem, but I like to have the tubes as active elements in the flasher.

From the transfer curves provided by @Olin, it seems that there is gain available at 27V, but the total current is limited to less than the typical forward junction current of a LED. It should still be visible.

The simplest circuit would be to put the DC supplies in series to make a 32v B+ supply. The tube heater is connected across the 12V DC supply.

Here is a schematic that may work. It should be a good place to start.

enter image description here

Some notes, the 5v supply is used to assure negative grid voltage when the 555 output is low. The resistor on the output of the 555 (with the missing value) should limit the grid current, and is probably in the 50K range.

Ra, Rb, and C should be set to give you the pulse rate and duty cycle you want. 555 application notes include formulas for calculating these values.

The screen grid and suppressor grid are connected to B+ to encourage a few more electrons to reach the plate.

There is not resistor to limit the LED current. The tube conductance should be a strong enough limiter.

If you want more brightness, use two 6BA6 tubes with the filaments wired in series (to make up the 12V supply) and all other elements wired in parallel.


hmmmm. Speaking in general, there are plenty of tube oscillator circuits which would work nicely for a flasher; however, I'd suggest that a dual triode based multivibrator would be a logical choice. Depending on the current requirements for the LED, you could put one in series with either of the plate resistors. You'd need an LED that only needs a few mA to light so that it will turn on with the limited plate current.

In terms of using the parts you have, I see a few challenges:

  1. you've got 3 pentodes (12au6, 12ba6) and a dual diode + triode (14gt8). (not listed in this question, but in one of your other questions, the 12DT8 is a dual triode and might work).
  2. Tube circuits normally have 300+ Vdc for their plate power supply and your supplies are no where near this voltage level.

While there are solutions to the above challenges, I'm going to assume you will want to make your flasher circuit work with basically only using the parts you have available / described in your question. That is, the tubes, the power supplies, 555 timers, small signal transistors and the assorted R's and C's one would use with these semiconductors.

Thus, my suggestion would be to build an LED flasher using the 555 chip and power it via the 20Vdc power supply connected in series with one of your vacuum tube's heaters. You will need to place a resistor between the heater and ground to assure a minimum current through the heater. Put your LED flasher circuit in parallel with the resistor. I'd suggest something around 80 ohms and greater than 2W. If you don't have any 2W or greater resistors of the right value, you can do series/parallel combinations of lower rated resistors which solve to ~80 ohms. (remember Rseries=R+R+R+R, 1/Rparallel=1/R+1/r+1/R)

While the tube is not functionally active as an oscillator, it does provide the key functional operation of dropping enough voltage to reduce the 20Vdc supply to be within the 555's limits...

Note the opportunities to improve this circuit abound. (It doesn't even have a PIC 10F200! :) )


First let me say I'm glad you are curious about these things. But in reality learning things takes hard work and this could an opportunity for you to learn some interesting stuff. I'd like to encourage that.

Don't be distracted by someone saying "that's old school" no one uses tubes any more. The fact in, the lesson learned in understanding tube operation apply as equally to the operation of MOSFET's. THese lessons learned will stand you in good stead if you want to head in that direction.

I'm sure if you have some fundamental questions people here would love to tell you more. Point you to resources and drop hints. Nothing like getting your hands dirty to learn.

You could just be someone who thinks that Tube's look nice, which is also perfectly fine. But if there is curiosity behind what is actually happening you'll find it worthwhile.

  • 2
    \$\begingroup\$ How about pointing him to some resources in your answer? \$\endgroup\$ Sep 18, 2012 at 13:52
  • \$\begingroup\$ I am interested in how this stuff works and not just to use a vaccum tube. I think it's important to learn how electronics started and I am not one of those kids who build a computer and are satisfied not knowing how it works \$\endgroup\$
    – skyler
    Sep 18, 2012 at 15:11
  • \$\begingroup\$ Yeah, no one does use tubes. I agreed till I had to work on one for the first time. \$\endgroup\$
    – Kortuk
    Sep 19, 2012 at 0:36

Yes, you can make a oscillator for as little as two tubes. Dig around on the net and you should be able to find circuits for this. If not, come back here asking that specifically and maybe we can get you going.

Your 12 VDC power supply will be fine for running the heaters for the 12xxx tubes. However, you will need a high voltage supply of 100-200 V to do meaningful things with tubes. You have to be careful with such things as they can hurt.

I couldn't find anything on the 14GT8 (are you really sure you are reading the numbers right?), but here is the data on the 12AU6:

This is the same as the 6AU6 except for the heater voltage:

Here is the data for the 12BA6:

Which then refers you to the 6BA6 for everthing except the heater voltage:

  • \$\begingroup\$ +1 :-) - just upvoted that and then saw it was yours, which of course makes no difference at all. I was surprised that nobody else had. I suspect you might almost get it going with a few 10's of volts on the Anodes with due care. | Then there here are 6CW4's :-) (which he has not got). \$\endgroup\$
    – Russell McMahon
    Sep 15, 2014 at 8:30
  • \$\begingroup\$ Is it worth pointing out that the maximum allowable heater-cathode voltage difference is 200 V? So one end of the heater supply might need to be attached to the cathode. I do not know about these things, but it came up in a Mr Carlson's lab YouTube video. \$\endgroup\$ Sep 18, 2018 at 15:08

Here's a simple tremolo oscillator circuit for a tube guitar amp. It could be used to pulse an LED...


  • 1
    \$\begingroup\$ Which is just what he does at the last example at the bottom... \$\endgroup\$
    – Henry Crun
    Apr 4, 2018 at 4:18
  • \$\begingroup\$ I completely forgot about that part (I read the page a few years ago). Yes, the LED was to be used as an indicator that the tremolo was operating. Good point. \$\endgroup\$
    – Pete
    Apr 12, 2018 at 18:57
  • 1
    \$\begingroup\$ scored a VFD tube the other day. Now to see if it has enough current gain to make this work... \$\endgroup\$
    – Henry Crun
    Apr 12, 2018 at 19:52

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