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Old 19th Dec 2011, 10:29 pm   #1
Miguel Lopez
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Default My first experience with electronic tubes

Hello everyone

I open this thread to ask for help (as ussual) about the misterious world (at least for me) of electronic tubes.

I have always been “transistorised”, and I have a lot of experience using transistors. On th other hand, I have never use valves, despite the fact that when I began to experiment with electronics, there were a lot of valve-made equipment still in use in Cuba, such as Soviet-made TV sets and Chinese-made radio sets. For years, electronic tubes have been surrounded by a mystic curtain, covering them from my eyes. So, that was always a pending subject in my “career” as electronics lover

In recent times I began to read about valves and their ussage. I read a good book from the 50s. Well, I proposed myself to build something with valves before I leave this world (I hope to be here for a long, long, long time, but I’d like to start now because tomorrow never knows) But I still have a huge lack of knowledge about this devices.

The obvious equipment for a begginer should be an audio amplifier. I have search some diagrams and I have recolect some components, but I still have a lot of doubts. The diagram I pretend to implement is attached in this post. The doubts I have are the following:

1.) What should be the DC supply voltaje for this circuit.? (150, 200, 250, 300V)
2.) The first valve in that circuit is a Soviet-made 6Н2П (Dual triode). I don’t have that valve, instead I have several 6Н1П (Dual triode). May I replace in the circuit the 6Н2П by the 6Н1П, taking into account the pinout of each one?
3.) Valves circuit operates at high voltage. Should I electrically isolate the input terminals from the HV circuit in order to prevent electric shock in that points?
4.) Is there any risk for the source signal equipment (DVD player, CD player, Cassete recorder, radio, etc) to be affected by the high voltaje from the valved-circuit. I don’t want to burn out my CD player.
5.) Should I connect the negative terminal of the DC power supply to the chasis, and here I re-ask doubt #3
6.) The valves pinouts that appear in the datasheets are top-viewed (like ICs) or bottom-viewed.
7.) An explanation of the parameters that appear in the datasheets would be fine

Sorry any silly question but as I said before I have a huge lack of knowledge in the electronic tubes world. You could check the 6Н2П & 6Н1П datasheets in the following link. Possibly you must use Google Translator:
http://www.magictubes.ru/sprav/pdf/6n1p.pdf
http://www.magictubes.ru/sprav/pdf/6n2p.pdf

I do not pretend to build a super amplifier, but just a modest one to amplify my CD player and have fun while building it. I really don’t need it because I have a transistorised build by myself several tears ago, but as I said before it is a subject I have pending. Of course I will learn a lot about valves when I finish. Almost all valves that I have are Soviet-made, with some exceptions that are Czeck-made (TESLA). The output valve that I have is the 6П14П (EL84 equivalent).

If anybody thinks that is not the best diagram for a beginner like me, feel free to tell me, and if posible recommend me another diagram. Remember that almost all components I have are used Soviet-made ones

If I finally build this amplifier, maybe I will adapt to it an EM80 magic eye that I have, but that will be for the future, and for further learning.

Any advise most welcomed

Best regards

Miguel
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Old 19th Dec 2011, 10:36 pm   #2
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Default Re: My first experience with electronic tubes

For forum members not familiar with Cyrillic script, the Cyrillic letter "П" is the same as the English letter "P"
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Old 19th Dec 2011, 10:40 pm   #3
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Default Re: My first experience with electronic tubes

Yep, you're right. And the Cyrillic letter "H" is actually the same as the "N" in English.
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Old 19th Dec 2011, 10:43 pm   #4
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Default Re: My first experience with electronic tubes

Спасибо
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Old 19th Dec 2011, 10:48 pm   #5
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Talking Re: My first experience with electronic tubes

You're welcome

PS: Hallowed be Google Translator
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Old 20th Dec 2011, 1:39 am   #6
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Default Re: My first experience with electronic tubes

The circuit looks to be a good simple class A amplifier to practice on.

As for the alternative double triode- even with google translate it's not easy to decide exactly what's going on, since it separates the text from the values in a rather unhelpful way. I would say, simply try it. It won't go bang- valves are quite robust electrically compared with transistors! Once it's running, you can check the various electrode voltages and adjust the anode and cathode resistors if necessary.

Yes the negative side of the HT DC supply is connected to chassis- the circuit shows an isolating transformer, so there's no direct hazard from so doing, except for the HT voltage itself! You will not need to isolate the input of the amplifier from your CD player except if there's any DC coming from that- the grid circuit of the input valve is at 0V (chassis potential) and is inherently isolated from the HT voltage anyway.

DON'T connect either side of the AC supply to chassis!

You can connect the chassis itself to ground if you wish, for extra safety, though depending on the grounding arrangements in the signal source device this may cause hum problems.

Treat the mains and HT with suitable respect and have some fun......

A few other answers:

HT of 250V would seem reasonable, so you'll need a transformer secondary about 180-200V or so.

Valve pinouts are viewed from underneath, so if you hold the 9-pin valve upside down with the gap in the pins towards you, you count 1-9 clockwise from the pin on the left of the gap. There are some odd other ways of counting on some of the older valve types, but anything made after about 1940 will conform.
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Old 20th Dec 2011, 2:29 am   #7
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Default Re: My first experience with electronic tubes

Quote:
Originally Posted by Miguel Lopez View Post
You're welcome

PS: Hallowed be Google Translator
очень не истинно, а т все

But your English is certainly better than Google's unedited translations, unless they have improved! However, I do still use it myself !

I would agree with earlier replies, by all means earth the HT-, but only if you have an isolating mains transformer.

Best wishes

Pete

P.S.
I will never understand "British" Industry.... At the time my (then) company won a very large Russian contract, I was the only person there who actually spoke Russian, but was was excluded from having anything to do with it !!
I don't think it was in any way politically motivated... just bad management.
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Old 21st Dec 2011, 3:05 pm   #8
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Default Re: My first experience with electronic tubes

Thanks Herald and Pete.

That answers make feel "more safe". In fact, the high voltage was always my first thought (and fear) everytime I interested myself in valves. But since I began to work with microwave ovens some months ago, I thought that 300V is nothing compared to 4kV, so why not?

I have all the neccesary parts to begin to work with my valved-amplifier. Valves, transformers, capacitors, resistors, diodes.....all Soviet-made. But that's going to be for the next year, because we are now in Xmas and that's not a good time for electrocution......sorry, for work too much.

But you have really cleared my doubts (and fears), thanks.

Quote:
Originally Posted by DangerMan
But your English is certainly better than Google's unedited translations, unless they have improved!
Modesty aside, my English is good. I can read and write very well (for a Spanish speaker). I can not speak English so well due to lack of practice, but I do my best. When I really need to use Google Translator is for Russian. The only thing I can do when reading in Russian, it is to replace the Cyrillic letter for Latin ones. Anything else, Saint Google Translator is a good friend.
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Old 22nd Dec 2011, 5:56 pm   #9
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Default Re: My first experience with electronic tubes

I measured the TC-180 power transformer that I have, and I found that I can obtain 260 V DC to supply the amplifier. So, no problem on this .

And now I'm going to ask the 64000 dollars question (or pounds if you prefer).

Is there any way to test the tubes before I start to "cut metal"?

I only have two EL84. I hope that at least one of them be OK. But if none, I won't be so glad when I finish the job and I find that there is no EL84 to amplify the audio signal. Despite this, I think that I will build the amplifier anyway, because I have some other tubes (triodes mostly) and I could build a less powerful amplifier, but it would be better to use the EL84.

Any idea?
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Old 22nd Dec 2011, 9:35 pm   #10
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Default Re: My first experience with electronic tubes

Another tough question:

Is there any software to simulate an audio amplifier using valves?

I always use "Electronics Workbench" to simulate my designs before implement them, but this software does not simulate electron tubes.
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Old 23rd Dec 2011, 9:05 am   #11
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Default Re: My first experience with electronic tubes

Quote:
Originally Posted by Miguel Lopez View Post
Is there any way to test the tubes before I start to "cut metal"?
Yes - the simplest way is just to partially build your amplifier. There's no need for a chassis or anything. Components wired together on the bench is fine. Construct your power supply and check that its output voltage is roughly right. Then you can wire up a socket for an EL84. Just follow the EL84 part of the circuit diagram you have. If you don't have an output transformer, just connect the anode to HT+ (as well as the screen grid). Connect the grid to ground, and use the recommended value of cathode resistor. Connect up the heater. You don't even need a cathode decoupling capacitor for this test. Switch on, and measure the voltage across the cathode resistor when the valve has warmed up. That will give you a good idea of the valve's general health. Check the valve's data sheet for the recommended bias point - I would expect a current in the resistor of about 40mA, normally resulting in a voltage of between 5 and 20 volts (depending on the recommended cathode resistor from the valve's data sheet).

How's that? A valve tester using an HT power supply and one resistor.

Chris
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Old 23rd Dec 2011, 9:10 am   #12
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Default Re: My first experience with electronic tubes

Most circuit simulation software is based on SPICE underneath, and therefore component models are more or less interchangeable. Someone's bound to have made a model for the EL84 -- they're common enough, after all.
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Old 23rd Dec 2011, 12:00 pm   #13
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Default Re: My first experience with electronic tubes

There are Spice models available for many valves here. For simple circuits I'm with Chris, a quick lash-up on the bench is often faster and more informative than drawing and entering parameters in software where the model accuracy is unknown.

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Old 26th Dec 2011, 6:40 pm   #14
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Default Re: My first experience with electronic tubes

I've read the EL84 datasheet. I found something that I do not understand. The minimal load resistance stated in that datasheet is:

Load Resistance ............................... 4.5K Ω

The output transformer that I have has an ohmic resistance of about 230 ohms.

Is that figure making reference to impedance instead of resistant?.
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Old 26th Dec 2011, 6:44 pm   #15
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Default Re: My first experience with electronic tubes

Impedance and resistance are not the same thing. Resistance figures for transformer windings are only useful as a means of checking for shorted turns.

If your using say an 8 ohm speaker, the impedance ratio will be 4500/8 or 562.5 to 1. The turns ratio of the output transformer will need to be the square root of that or about 23 to 1.
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Old 26th Dec 2011, 8:20 pm   #16
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Default Re: My first experience with electronic tubes

In the particular case of a resistive load on the secondary of a transformer, the primary will also look like a resistor (give or take imperfections inherent in any practical transformer), but only to ac signals and then only within the band of frequencies for which the transformer is designed. Hence the valve spec quoting a resistor value. You could indeed load the valve directly with a resistor, and it would be perfectly happy, but efficiency would be terrible with the large standing DC dissipation due to the anode current heating up the resistor, in addition to any desired ac heating effect.
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Old 26th Dec 2011, 8:52 pm   #17
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Default Re: My first experience with electronic tubes

Resistance is the real part of impedance, so the two terms can often be used interchangeably. In the case of a transformer, resistance may mean the DC resistance of a winding (or the loss resistance) or the real part of the reflected impedance. Impedance usually means the reflected impedance.
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Old 27th Dec 2011, 2:29 pm   #18
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Default Re: My first experience with electronic tubes

Quote:
Originally Posted by Station X
Impedance and resistance are not the same thing.
I know. That's why I asked

Quote:
Originally Posted by Station X
If your using say an 8 ohm speaker
Exactly

Quote:
Originally Posted by Station X
the impedance ratio will be 4500/8 or 562.5 to 1. The turns ratio of the output transformer will need to be the square root of that or about 23 to 1.
The transformer I have is a Soviet TB3-1-9, which have a turn ratio of 2150/58 = 37. I guess this one would be suitable, isn't it? It is the one which is used in audio output of the "Electron 206" TV-set

But I still do not understand the steady-state operation. When the circuit operates without signal (DC operation), the EL84 will only see the 230 ohms resistance od the primary winding. There is no power transfer in this state, but the anode DC current would be flowing throught this winding, producing warming due to Joule's effect.

Quote:
Originally Posted by Herald1360
Hence the valve spec quoting a resistor value. You could indeed load the valve directly with a resistor, and it would be perfectly happy, but efficiency would be terrible with the large standing DC dissipation due to the anode current heating up the resistor, in addition to any desired ac heating effect.
This is exactly what I'm talking about.
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Old 27th Dec 2011, 4:17 pm   #19
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Default Re: My first experience with electronic tubes

Quote:
Originally Posted by Miguel Lopez View Post
But I still do not understand the steady-state operation. When the circuit operates without signal (DC operation), the EL84 will only see the 230 ohms resistance od the primary winding. There is no power transfer in this state, but the anode DC current would be flowing throught this winding, producing warming due to Joule's effect.
Yes, that's true. But the standing anode current of an EL84 at, say, 40mA, will only dissipate (I2R) 0.04 x 0.04 x 230 ohms = 0.368 watts in the transformer. This is too small to worry about, and won't cause the transformer to get noticeably warm.

Where power will be dissipated, though, is at the anode (and screen grid) of the valve. If the anode current is 40mA and the HT voltage 250V, then the anode will be dissipating 10 watts. That's OK because it's designed to do that.

This current is limited not by the resistance of the transformer but by the characteristics of the valve. The impedance seen by the valve's anode only matters when the current starts to change - when an audio signal is present. If that impedance is too low, or too high, the valve won't be a good match to the load and so won't be able to develop its full output power.
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Old 27th Dec 2011, 4:34 pm   #20
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OK, I understand now. The fact is that I've never built a Class A amplifier, not even with transistors, so my huge in-experience on that matter added to my very, very huge in-experience with valves and then you'll understand why I'm asking such questions. Thanks for your answers.

Comparing the diagram I showed in the openning post of this thread with other similar configuration, I noticed two things:

1.) The resistor in the cathode of the output pentode is not bypassed by a capacitor.
2.) There is a 0,01uF capacitor (named C7) in the grid of the pentode, bypassing the resistor in that grid. I guess this would affect the high frecuency response


What about this?
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