Nixie Tube or Neon Lamp? - Exclamation mark vacuum tube

[Tractorfan]

Hi,
I think some valve powered tape recorders and radios used the voltage across the output valve cathode resistor to supply the filament of the DM70.
I've got a couple kicking around here somewhere. No, they're not that exciting really.
Cheers, Pete.

[G6Tanuki]

Yes, O/P-valve-cathode volts was a popular trick.

I also remember at least one 1960s transistor tape-recorder where the HT *and* LT for the DM70 record-level indicator came by courtesy of a couple of extra windings on the erase/bias oscillator coil, which was itself energised by a push-pull pair of AC128 or AC153 transistors.

[SteveCG]

G6Tanuki,

Could that tape recorder have been a Grundig TK1 ?

BTW: It used permanent magnet erase.

[G6Tanuki]

I forget the detail (it was a long time ago!) but don't think it was a Grundig; it certainly didn't look 'european'.

[AC/HL]

It might be uninteresting, but it's usable in portable equipment unlike it's ancestors.

[ScottBouch]

Well, interesting or not, as my first real stab into understanding vacuum tube technologies, I'm learning a lot from it! It looks quite cool, and I'm looking forward to getting it illuminated..

I was inspired recently to discover more by the Birmingham Thinktank - they had a 3 phase mercury vapour rectifier glowing and bubbling away - It was quite an impressive sight to someone form the transistor era!

So I'm interested to learn more about vacuum tube technologies, and this little indicator is my open door.

Cheers, Scott.

[ScottBouch]

I now have a string of 10 x 1Mohm resistors for the Grid connection, as I couldn't find a 10Mohm..

I also have the 470K resistor for the Anode (Plate) connection.

I've got to bodge up an AAA or AA cell holder as I don't have one to hand - and then we'll be away!

Maybe tonight we'll see it light!

Cheers, Scott.

[Herald1360]

A g clamp and a piece of card will bodge up a battery holder. (So will just soldering wires on, but that could dodgy if you're not good a quick joint making).

[ScottBouch]

Great Success!

I got it wired up last night and it worked!

| | | |

Hoorah! My first experiment into vacuum tube tech, and it didn't go bang! I was able to vary the length of the indicator by adjusting the grid voltage between 0V and -7Vdc.

I think it'd make a nice little project to connect up two of these to make a stereo audio indicator box.. perhaps with a 'steam punk' brass and red leather theme! But maybe a project for another day, for now I'm happy to see this light up!

Thanks to all for your help in understanding this little indicator!

Cheers, Scott.

Very nice (despite naysayers) little indicator, most vintage applications have 'shorter is better' (because the AGC gets lower with more signal) however if you do a stereo level indicator a pair would look great as 'longer is more'. Easy to do with modern electronics, -7V is not hard to get.

As an aside, protect the filament from over volts, a shunt regulator of two silicon diodes will do the job very well. Unlike the 'regular' magic eyes I have never seen one of these go dim only failing when the filament goes pop.

[ScottBouch]

Thanks, interesting to know about how to expect this one to snuff it when / if it does! - I think the words I'm looking for are "failure mode"!

Yes, over-doing the filament seems to be a common cause for concern, so I’ll definitely apply some protection as you say! Especially if this unit ever gets built, it will probably end up mains powered, so I'll have to come up with a PSU of sorts, which will pose higher risk of over do-ing it than the AAA cell I used in this test.

I have a couple of Marantz amplifiers that this will probably end up running in conjunction with.. so I may have a poke around inside and see what voltages are already available!

I enjoy speculating, but as I say, a possible project for the future!

Thanks, Scott

[ScottBouch]

Out of curiosity - there has been mention of suitability for portable equipment - what makes a vacuum tube device suitable for portable equipment?

Is it lower voltages, or are they made a bit more robust?

Thanks, Scott

[Tractorfan]

Hi,
It was the low current consumption which enabled these to be used with battery operated equipment, but usually towards the 'posh' end of a manufacturers range.
Cheers, Pete.

[Nickthedentist]

They're also physically small, and I suppose the wire-ended version can never be jolted out of a socket.

N.

[ScottBouch]

Brilliant - thanks chaps!

Learning never stops!

In portable applications, what methods were used to generate the high voltages? Oscillator feeding a transformer? Inductive flyback?

Cheers, Scott

[G6Tanuki]

Usually an oscillator: remember that in a tape-recorder, where these little tubes were used as recording-level indicators, you already have an oscillator running to drive the record/erase heads so it's dead easy to also produce 60 or 80 Volts at a few hundred microamps for the tube "HT".

In portable applications, what methods were used to generate the high voltages?

A big battery in radios.

[paulsherwin]

Portable equipment would typically use a 90V HT battery.

[Radio Wrangler]

By the way, it DID go 'bang'

Old computer people used to 'speak' the exclamation symbol as 'bang'

so a random character string like 1 2 3 F C 4 ! 9 would be spoken as "one, two, three, Eff, Cee, four, bang, nine".

David

[julie_m]

In a transistorised tape recorder with only a low voltage battery supply, there is a bias / erase oscillator (admittedly only running during recording, but you don't need the indicator to light up on playback or wind / rewind) which can be commandeered to produce a milliamp or so of HT for the magic eye indicator (which of course can act as its own rectifier). And if the amplifier construction is germanium PNP, then the chassis will even be positive; so the drive voltage will already be negative.