• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Need assistance with an 811a circuit build

Hi guys, newbie here, with a kind of unique need. I am trying to light up a plasma bulb by using an audio signal.

In order to do this I need to place a high voltage audio signal across the electrodes of a Argon or Neon or Helium filled bulb.

I have proved out the concept by using an old Stromberg Carlson tube amp.in order to light up a miniature neon lamp.Please see the videos 1 and 2 provided below.



Also I attached a schematic of this set up. Please see Stromberg Carlson schematic attachment.

I connected a neon lamp across the primary of the audio output transformer and obtained a great signal. But ! now I need to light a larger plasma bulb and so I require a more higher voltage signal. Can I get maybe 2kv audio signal amplitude ?

I have chosen the 811A audio output vac tube for this application because it is spec.d out for +1kv plate voltage supply. If anyone has a better suggestion then I will consider it. I know that the actual AC signal seen at the plate goes higher in amplitude than the supply DC. and so I believe that 1kv supplied should get me a 2 to 3 kv signal at the plate ?

This design I believe can be simple, and straightforward.I have an EICO 377 audio generator which contains a cathode follower output in order to drive the grid. I do however need to know how to arrainge the proper grid drive for the 811 in terms of audio signal and does the dc level matter ?

I can build a power supply to any spec. No questions here.

I can supply the plate voltage through an inductance but I need some help on what to think about this part. Like if I use an audio output transformer then this will load the circuit according to normal spec.' but will limit my supply potential to a maximum of about 1kv.

Can I maybe consider here eliminating the audio transformer and substituting in some form high voltage audio choke ? Does that even exist ? The idea with this build is that I require the highest potential audio signal that is do-able with a vacuum tube.

Next is how would the cathode be wired and do I need to incorporate some form of feedback ?

I have a preliminary sketch of the idea. Please see attachment.

Thanks a bunch,

Dan J


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2019-11-23 10:17 pm
Hi some call them audio chokes. They are filter chokes, Yes there are high voltage types but what's the point of all this?...Do you know where the plate 'is'on an 811?....do you truly realise the danger to hich you are putting yourself, perhaps others, and the effects of caps charged to 1000/1500-2000 volts? I have a new 2000 volt trannie and caps for a power supply I may build for an ART-13 and I am very cautious of the access to the HT, chassis earthing and the engineering in returning the HT to '0'.... Doing it through a short circuit damages caps even at low and elv voltages. At high voltages it's more common to use resistors rather than chokes in pi circuits. Any idea what the collapsing voltage in choke and transformer might reach?

I suppose as an OHS professional I can say..."you may have a sound reason to do what you propose...I can't think of one right now....but it's embarking on a high risk journey".


Paid Member
2003-06-12 7:04 pm
Maine USA
...I need to light a larger plasma bulb and so I require a more higher voltage signal. Can I get maybe 2kv audio signal amplitude ?...

Are you sure you need higher Voltage? What kind of "plasma bulb" is this?

Many plasma phenomena are nearly "constant voltage" over a large range of size/spacing. But a "wider" plasma needs more *current* to cover its higher surface losses.

I didn't know plasma lamps were still a thing. Can you point to one?

> I require the highest potential audio signal that is do-able with a vacuum tube.

There are transmitter modulators scaled to run with 20,000 Volts (and a couple Amps). Be careful what you wish for.
With due respect I smell a VERY DANGEROUS project with not much clue about Electronics, a DEADLY combination.

The Neon bulb across output transformer primary was reasonably safe and cool, but at classroom Physics demonstration level, specially because power involved lighting a neon bulb is measured in less than a milliwatt, if not microwatts, just think power involved inside a neon screwdriver, where (minuscule) current goes through your body and is still "safe".

Scale it higher than Toy/Novelty level and you are building a crude but effective Electric Chair.

You want to light and modulate a gas discharge tube?
Well, there is circuitry dedicated to that, but is nothing like putting said tube across an OT or anything similar.

In principle, you feed them from an audio modulated constant current supply, which initially applies enough voltage to start discharge (maybe using an auxiliary "trigger" electrode) and then voltage drops to a sustaining level which is somewhat lower.

Voltages involved are still high and dangerous , but nothing like what would be needed by using the crude system you imagine.

In any case, we absolutely need to see that gas discharge tube datasheet to have a better understanding.
Any attempt to scale it up enters deadly waters.
Operating a gas discharge tube like the circuits in post # 1 will cause a hot-gas explosion . . .
unless you limit the current to the discharge tube when it ignites

Yes. The audio pulsing current surge "hot gas explosion" is an EMP train. The plasma creates an electric field about it and the idea here is to detect the e-field.

This is a plasma physics experiment and demanding huge caution. Thank you all for your warning about the shock hazard concern here.

Disclaimer: I am not trying to start any trouble or concern here or suggest that people should do this. If a moderator needs to shut down this string then please do so.

Maybe, as JMFahey says, I should stick to smaller and safer parameters with my tests then that is great advice.

There is more to this however. We are also interested to know how 2 signals mix or heterodyne at the plasma. This process is the subject of wide conjecture and no actual lab result.

So, anyhow, I am in the initial phase of brain-storming ideas on how to detect these plasma processes and I think that the help that I am getting from this forum is too keep my circuits lower in wattage for a good safety margin. I am sure that I can accomplish results with this.

Thasnks again guys.