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Q's about Tubes ("JFETs with pilot lights") Answered

Recently I acquired a couple vacuum tubes for $35, and I gave them about 7V filament voltage, and randomly probed around with a 50V 200mA HP lab power supply to see if I can get them to do anything.

I found that the most I could get was 40mA between the heated cathode and the closest gate on the pentodes, and a few mA when connected "properly". I guess I just don't have a high enough voltage. :/ I don't want to mess too much with direct mains, and I don't have an isolation transformer.

I have a bunch of different tube numbers, although they all look the same to me. (some of them have different inners though!) So my questions are:

* Any cool project ideas? I'm thinking FM radio w/ bluetooth tube speaker thing (obviously w/ a separate module) Maybe a nixie display too??? IDK! 

* What do I need to know to get started with messing with them?

* How do I know if I am pushing them too hard or damaging them? I got the anodes glowing, and the current drops proportionally. Bad?

* Most of them are labeled "radio tubes" some probably were in early TVs as well.

* How much current/voltage do these things typically require? Can I do any sort of dark tricks to get them to operate well at or under 48V? I know some tubes were special and can run at 12V due to design.




Discussions

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Josehf Murchison

2 years ago

Get yourself a good vacuum tube substitution guide it will give you the heater voltages and amperage as well as the pinouts. Some tubes have 10 different numbers and some tubes have different pinouts but are basically the same.

There are lots of projects here and on the net just think of vacuum tubs as high voltage transistors.

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rickharris

2 years ago

1. look up the numbers and find out what you have.

2. The heater voltage is usually 6 volts. The anode will be 50 to 200 or more volts (DC) the grids will have a few volts on them.

3. The simplest tube (valve) is a triode. The electron emitter is the cathode and the receiver is the anode the flow of electrons is done by the grid.

SO: If the cathode is at earth potential and the anode made very positive electrons will be attracted across the space towards the anode.

However if the grid is negative this flow will be prevented (because the grid is much closer to the cathode and so has a lot more effect.)

When the grid goes positive this starts electrons flowing from the cathode - the grid is a mesh and the electrons will go through to reach the highly positive anode THUS a small signal on the grid will control a much larger current through the tube between cathode and anode - hence we have amplification.

The grids anode and cathodes of a simple valve have capacitive properties which degrade the quality of the audio. To help with this other grids can be introduced giving pentodes, heptodes and other types of valve.

https://www.google.co.uk/search?q=valve+circuits+a...

http://www.radio-electronics.com/info/data/thermio...

More than enough there to keep you busy - even building a decent power supply will challange your electrical and electronic theory!

tube.jpg
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rickharrisrickharris

Answer 2 years ago

http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

Your anode is glowing because it has no load to dissipate the current into.

Your not going to get anywhere at 48 volts DC unless you can source special low voltage valves. Even portable valve radios used to have a 50 or 90 volt battery in them.

Do a lot of reading - as you already know electricity bites and messing with 100's of volts and significant current isn't something I would usually suggest a newbe does.

If you decide to progress take all possible care - ideally you need an isolated mains supply and an earth free working area - work with one hand in your pocket (no joke) and think twice before you touch anything.

I started learning electronics in the 1960's on valves (tubes) and despite all the safety the RAF drilled into me managed to end up in hospital twice because of electric shock. Both times it wasn't the shock that injured me but 1. falling off a ladder _ AVO meter about 10 feet - fortunately I was unconscious so I fell like a wet sack and no broken bones. 2. I involuntary leaped back and hit my head on the wall behind me 3 days in hospital with concussion + burned finger tips where I touched something that shouldn't have been live.

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-max-rickharris

Answer 2 years ago

Thank you very much for all this great infomation! I will see about making a voltage doubler to get ~240vdc from the 120vac.

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-max-

2 years ago

Does nobody here have any knowledge they can share about tubes!?!?!? ?

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Downunder35m-max-

Answer 2 years ago

I did build some nice tube amps back in the day for friends who did not like digital amps at all.
Your biggest problem is not knowing what you actually have.
Noone really uses them anymore these day but the few that still do often pay top dollar for tubes in perfect working order.
Often russian websites offer a lot of info on tubes that are still in use somewhere - would be a starting point.
I would not mess with them too much until knowing what value you really have hiding in these rotten boxes ;)
If for example you have enough to create an old style tube Theremin of good quality it might fetch close to 500 bucks on the open market ;)

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-max-Downunder35m

Answer 2 years ago

Most of the ones I googled seem to be worth $6 -- $16. The one glowing is a 7B7, and all of the out-of-box ones have clear labels. The couple I took out of boxes have marking on them, but they easily rub / scratch off, so I want to avoid that if possible.

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steveastrouk

2 years ago

Remember they need an HT supply too. The heaters only make the filaments emit electrons, the HT shapes what happens next.

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-max-steveastrouk

Answer 2 years ago

What is an HT supply? I can quite easily whip up a few KV with my new 200V 15A transistors and a TV line transformer.

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-max-

2 years ago

I don't even know how to take them out of the cardboard packaging without the cardboard ripping and falling apart smh.