Beginner question about experimenting with tubes

[I14R10]

For a long time I have had a wish to experiment with vacuum tubes. Mainly with audio amplifier circuits. I am not a complete beginner to electronics. I soldered many circuits, experimented with electrical oscillators, made FM bug transmitters. Blew up (literally) some transistors, fried many more, fried few mobile phones, made Tesla Coil, made antennas for my ham radio, disassembled my radio to clean VFO and successfully put it back... Nothing with tubes except I changed one driver tube in my radio.

How hard is it to make tube work? Meaning, how hard is it to set it up so you can make simple one tube amplifier? I read a bit about that type of amplifiers and I know basics.
How fragile are they? Is it easy to fry them?

 

[betwixt]

Apart from the heater voltage needing to be be fairly accurate, they are almost impossible to fry under normal conditions. The glass might be fragile but electrically they are quite forgiving. They are considerably simpler than transistor circuits but you do have to be careful because of the higher voltages involved. For audio work you will almost certainly need to use an output transformer to match the high output impedance to the low impedance of the loudspeaker.

The simplest audio amplifier, giving maybe 1W of output would be nothing more than a triode, a transformer and two resistors. If you want to reach the dizzy heights of pentode or beam tetrode amplifiers, you need at least one more resistor !


Brian.

 

[I14R10]

Can you recommend me some tube to start with? I found this one is used in few plans http://www.r-type.org/exhib/aai0113.htm, it's EF86 tube. It's pentode, I'm still searching for triode.

 

[betwixt]

The biggest problem you will have is finding a source of tubes, they are now quite difficult to find although there must be thousands of them collecting dust in old warehouses and workshops around the World.

The EF86 is a good tube to work with as a pre-amplifier. It is a small signal tube with special design to minimize microphony rather than a power amplifier. To explain: this dates to before "solid state" and the internal electrodes in all tubes are suspended on wire or mica supports, that makes them very "non-solid state" and prone to vibrations. In audio equipment, one of the problems with tubes was the vibration from the loudspeaker shaking the electrodes and causing feedback - rather like a microphone held close to a loudspeaker would 'howl'. The EF86 was designed with special electrode supports to make it more resistant to vibration for that reason.

Before looking for any particular tubes, tell us what kind of amplifier do you want to build, a power amplifier to drive a loudspeaker or a pre-amplifier to boost sensitivity to small signals?

Brian.

 

[I14R10]

Building power amplifier for loudspeaker sound interesting. I'll start with that. I've seen some tubes with dual grids. I wouldn't want to complicate things with that kind of tubes. Regular tube will be just fine.

 

[schmitt trigger]

Besides what Brian is mentioning, finding the tubes (valves) you actually require to find the actual transformers (power and a pair of output) to work with them.

I would start here:
https://www.hammfg.com/electronics/transformers/classic

My personal advice is to stick with simple triodes for the preamplification, and use beam tetrodes for the output.
There are literally hundreds of schematics on the web, the decision usually boils down on how much you can afford to spend, as tubes and the transformers are expensive.

Personal anecdote:
The very first circuit that I built and worked the first time was a vacuum tube amplifier made from scavenged parts and I knew next to nothing about electronics, yet worked the first time.

 

[c_mitra]

And there was the famous dual-triodes, two triodes in one case, that were push-pull versions.

Basically the valve tubes were potential driven devices, unlike transistors, that are basically current driven systems. One thing was certain: variations amongst the valve tubes were far less compared to the spread in the device parameters of modern transistors.

 

[I14R10]

Is this a good choice? http://www.thetubestore.com/Tubes/12AT7-ECC81-Tube-Types/JJ-ECC81 or this one http://www.r-type.org/exhib/aaa0517.htm

 

[BradtheRad]

In case you decide tubes are expensive, see websites which tell about making homemade tubes. Example:

 

[c_mitra]

I've seen some tubes with dual grids. I wouldn't want to complicate things with that kind of tubes. Regular tube will be just fine.

Perhaps you mean the so called tetrodes and beam tetrodes. The other grid is basically a suppressor grid and the tube works "more or less" like a triode only. If you want power, they are the beasts to go after...

 

[betwixt]

Is this a good choice?

First define what you want the tube to do, like transistors and ICs, they come in many shapes and sizes and are designed for different purposes. If you want a loudspeaker amplifier, you probably want a tube with higher power rating but if you want one as a pre-amplifier you want one designed for high gain and low microphony. For radio work, you almost certainly need a pentode for good results as they are more stable at higher frequencies.

As c-mitra points out, except for some of the more exotic types, in almost all cases you just put a fixed voltage on the remaining electrodes, they still have one input (control grid) and one output (usually anode, sometimes the cathode). The extra electrodes are there to improve linearity, focus the electron beams to improve efficiency or just to screen between the input and output sides.

The picture of the 6SN7 is a bad advert, it is clearly a faulty one! Bear in mind it was designed almost 75 years ago and even before semiconductors took over, it was already superceeded by far better performing tubes.

Just for experimenting, your best bet would be to find an old radio receiver in an antiques shop or a junk sale. It will not only contain a mix of tubes but it will have the power transformer and output transformers in it as well. You can probably buy a complete radio with four or five tubes, plus the other parts for less than the cost of one new tube. Start by getting the radio working, you might be surprised at how well some of them work after restoration.

Brian.

 

[Warpspeed]

Russia is one country still manufacturing vacuum tubes today.
These are brand new tubes, not something that is sixty years old and possibly damaged, broken, or worn out.

Some of these are direct copies of many familiar US and European part numbers.
Well worth a try, and the price is right. Also some of these are available on e-bay.

https://www.soviet-power.com/product.php?cat=116

 

[I14R10]

Perhaps you mean the so called tetrodes and beam tetrodes. The other grid is basically a suppressor grid and the tube works "more or less" like a triode only. If you want power, they are the beasts to go after...

I meant something like this

[image from www . ibiblio . org /kuphaldt/electricCircuits/Exper/05309.png]

It's like having two triodes in one glass bubble.

First define what you want the tube to do,

I want to experiment with loudspeaker amplifiers. I can't decide on what tube to use. I don't need high amplification, medium amplification will be good. And for price, I would like it to stay undel $20 for a tube.
After all, I'm just experimenting. I just need someone to suggest me what tube to use. As I said, I'm very new to this area and I don't know much.

Just for experimenting, your best bet would be to find an old radio receiver in an antiques shop or a junk sale. It will not only contain a mix of tubes but it will have the power transformer and output transformers in it as well. You can probably buy a complete radio with four or five tubes, plus the other parts for less than the cost of one new tube. Start by getting the radio working, you might be surprised at how well some of them work after restoration.

That's not possible as in Croatia, where I am, there is nothing like that. Bad parts and non-working devices are thrown away.

Russia is one country still manufacturing vacuum tubes today.
These are brand new tubes, not something that is sixty years old and possibly damaged, broken, or worn out.

Some of these are direct copies of many familiar US and European part numbers.
Well worth a try, and the price is right. Also some of these are available on e-bay.

https://www.soviet-power.com/product.php?cat=116

Thanks!

 

[betwixt]

Try with the 12AX7 then. It should be able to produce about 1W of audio from one of the two triodes inside the glass 'bubble'. You can use the other triode as a pre-amplifier or if you want to be adventurous, you can use both triodes in 'push-pull' mode to get a littte more audio power out. To do that you will need a center tapped audio transformer though.

The important thing to remember is most tubes are high voltage but low current devices, exactly the opposite to most transistors and ICs. As loudspeakers are current driven, transistors are usually more suitable, you will need a matching transformer to convert the high anode voltage swing of a tube amplifier to the lower voltage but higher current needed to drive a loudspeaker.

Brian.

 

[c_mitra]

12AX7 was a very popular tube in the good old days. Filaments are always 6.3V (I do not know why) but if you run them at 5.5V, the gain will reduce but the life will be increased very much. I also do not know why the anodes are called plates (it reminds me of dinner). I had the great fortune of seeing the transmitter tube of a radio station (when I was in school; no interviews, please) and it was impressive. For good reasons, it was far away from the regular studio!

 

[I14R10]

I am a bit surprised that tubes can handle so little power. I expected tens of watts even from smallest tubes.

 

[betwixt]

You can get high power from tubes but only from special types, in fact to this day tubes are still used for some special high power applications but the small types you have mentioned so far are for small signal amplification. I think the last high power audio amp I built, probably 45 years ago could deliver about 100W but it ran from a 500V supply and the output tubes ( I think they were type '807' ) glowed red hot if the volume was high for a long time! I have seen many tubes get so hot the glass melted and they imploded! Very high power tubes have water cooled anodes to stop them melting!

Remember that the INPUT power to an amplifier must be higher than it's output power because of inefficiencies. A single 807 with 500V on it's anode is only rated at a maximum current of 125mA so it's input power can be no higher than 500*0.125 = 62.5W.

The 12AX7 in comparison is designed for 250V on it's anode (plate) and a current of 1.2mA which makes an input power of only 0.3W and the manufacturers maximum rating is 1W. Bear in mind that is the INPUT power, the output will be much less.

If you want to produce a few Watts of power in a versatile tube, look for something like the ECL80 or ECL85 which have two tubes in one glass 'bubble', one part is a high gain triode and the other is a high output pentode. They will deliver about 3W fropm the pentode section. There are lots of schematics on the internet but avoid the one on the French web site that shows 4000uF/250V capacitors on the power rail - it looks good but is technically nonsense!

Brian.

 

[I14R10]

This is very interesting. I have a pair of 6146 tubes in my ham radio and they can deliver together 150W of power. So that's one high power tube that I know of. I don't know how it would behave as audio amplifier, but datasheet says 6146 can be used for AF amp.
Alright, enough of this high power dreams . First I need to build preamplifier.

I see here **broken link removed** that ELC85 is very cheap, $1.5. If they work I should buy 10 immediately.

Next I'm searching for a transformer.

 

[Warpspeed]

Once you start to get into serious power, say over 1,000 watts, and especially for a radio transmitter, tubes are much easier to use than semiconductors.

 

[I14R10]

I don't plan to go into RF area. At least not in the near future. All kinds of weird stuff happens in RF. For now I'm staying with audio amplifiers.