My first experience with electronic tubes

[paulsherwin]

As an aside, the transformer turns ratio is not critical in a simple circuit like this. +-50% is likely to be fine.

The lack of a cathode decoupling capacitor introduces negative feedback which will improve the sound quality.

You are right, C7 is there to roll off the high frequency response.

[Miguel Lopez]

Originally Posted by PaulSherwin

The lack of a cathode decoupling capacitor introduces negative feedback which will improve the sound quality.

The improve in sound quality is OK, but I think this also will reduce the gain. Because of the cascade configuration I guess it is no problem if gain reduces a little. The first valve (dual triode) will amplify enough the audio signal. Am I right?

Originally Posted by PaulSherwin

You are right, C7 is there to roll off the high frequency response.

So, I guess this roll off have been considered in the amplifier design and then there is no problem with the frequency response. Am I right? Again?


I will return now to one of my first doubts: the replacement of the 6Н2П valve by the 6Н1П dual triode.

I found that the 6Н2П is equivalent (more or less) to the 12AX7 (ECC83) , and the 6Н1П is equivalent (more or less) to the 6BQ7.

So I'd like that all of you who has experience using valves, tell me something in this matter. Anything will be welcomed

http://en.wikipedia.org/wiki/12AX7
http://drtube.com/datasheets/ecc83-philips1954.pdf
http://drtube.com/datasheets/12ax7-rca1962.pdf
http://www.radiomuseum.org/tubes/tube_6bq7.html
http://www.nj7p.org/Tube4.php?tube=6BQ7

[Herald1360]

Well, the 12AX7 is a hi mu audio triode with ra 62.5K and gm 1.6mA/V and the 6BQ7 is a medium mu RF triode for cascode use with ra 5.9K and gm 6ma/V. It seems to be somewhere between a 12AT7 and a 12AU7 in its characteristics.

Put another way, if you use about 220R cathode resistor and 10K anode resistor with each stage of the 6Н1П it will work OK, but with a lower gain and higher anode current than the 6Н2П in its own configuration. Whether the lower gain matters depends on how much input signal is available.....

It may also be more susceptible to mechanical shock (microphonic) than a valve designed for audio use. Still..... nothing to lose by trying it!

[DangerMan]

Hi Miguel,

C7 will roll off the open loop high frequency response of the "basic" amplifier, but the negative feedback will make sure that the actual response is not rolled off, and is good.
The capacitor is necessary to keep the feedback loop stable. Without it, stray capacitances and the output transformer's characteristics can, or will(!), cause "Nuyquist" stability problems... basically causing the whole thing to act as big oscillator. Because the amplifier has three gain stages and a transformer inside the feedback loop this is a real problem (without some form of "loop compensation" such as this).

Hope this helps

Pete

[DangerMan]

Hi again Miguel,
Everywhere I look at the data of the valves it seems to show different values!
What I would say, is that 6n2 has definitely lower mutual conductance than 6n1.
Nothing seems to suggest that what you have will not work, or cannot be made to work, so it seems a good plan to build with what you have.
As some reassurance, an amplifer meant to use 12AX7 (ECC83) will still work well enough with 12AT7 (ECC81). When it is built (you can do it roughly, just for test purposes) you can measure the anode and cathode voltages to make sure they are within acceptable ranges. I am sure they will be. If not, we can suggest slight alterations.
It might not be exactly what the original circuit intended but it should still work well.

Pete

[Miguel Lopez]

Hello everyone. Happy New Year!!!

Thank Chris and Pete for your answers:

I've been analizing the datasheets (as far as I can) and I noticed what you told me. I must change both cathode and anode resistor in order to replace the 6N2P by the 6N1P, and keep the circuit working. I will build it anyway to try.

I found that the 6N1P has a gain factor of 35 while the 6N2P has that factor around 95, so the gain will be reduced but I think that it still work if the input signal is strong enough. Now another round of questions to you:

1.) I think that I must keep the plate voltage in each triode at around half the HT voltage in order to obtain maximal gain. Is that OK?
2.) What is the purpose of resistor R11 (2K7) between pentode's cathode and filament circuit?
3.) To Chris: What do you mean with mechanical shock (microphonic)?
4.) Is it possible to remove the feedback loop in order to increase the gain with smaller-gain valves.

[Miguel Lopez]

Another silly question from a pagan guy (I mean, a transistorised guy will always be a pagan in Valve's World )

What is that coefficient expressed in mA/V? What does it mean?

[ppppenguin]

Originally Posted by Miguel Lopez

What is that coefficient expressed in mA/V? What does it mean?

Mutual conductance, often written as gm. For a valve it means that if you vary the grid voltage by 1V the anode current will vary by a number of mA. So if gm = 10mA/V varying the grid voltage by 1V will cause the anode current to change by 10mA.

[Miguel Lopez]

Thanks Jeffrey.

Is it better to supply the heater with DC or AC??

I've seen some diagrams using rectified and filtered supply for the filaments.

[ppppenguin]

All these valves are designed for heating with AC. To minimise hum, use twisted pairs of wires to connect the heaters to the transformer. If hum is a problem then there are several possible remedies before going to the trouble of DC heating.

[Miguel Lopez]

OK.

And what about resistor R11 (2K7) in the circuit? What is its purpose?

[Miguel Lopez]

Reading about valved amplifiers, I found several times the words "matched pair".

I do not pretend to build a push-pull amplifier. I have not enough parts to do it. But if my Class A amplifier works properly, possibly I'd try to expand it to an stereo amplifier.

In such case, should I require a matched pair of EL84 for both outputs?

[ppppenguin]

Originally Posted by Miguel Lopez

In such case, should I require a matched pair of EL84 for both outputs?

No. Matched pairs are only needed for the pair of valves in a push-pull output. Some designs can cope with mismatch better than others.

[Miguel Lopez]

I pretend to begin this week with my project. My first step will be to assemble the DC power supply because I don't have high DC voltage in my bench in order to begin the tests.

In some diagrams that I have seen, I notice that the diodes in the bridge rectifier are often paralleled by 1000pF capacitors. I have never used this configuration in transistor projects.

What is the purpose of those caps?

They reduce the switching noise. When a diode stops conducting it happens quite quickly generating RF noise at quite a large bandwidth up to several 10's of megahertz. This trick is used in many good power supplies. Transistors use a much lower voltage and the effect is much smaller.

[Herald1360]

The 1000pF (or whatever value) capacitors across the diodes suppress radio frequency noise generated by the reverse recovery of the diodes. Some charge is stored in the diode junction when it's forward biassed, this results in a short fast turn off reverse current pulse as the charge is swept out by reverse voltage. This pulse can excite ringing in wiring etc which can be conducted or radiated to parts of the circuit powered by the rectifier or local radios etc causing interference.

PS I hope you intend to start your project, it would be a pity for it to be imaginary

[Miguel Lopez]

OK

I will include them. I have several 1000pF(400V) that are suitable for this purpose.

[Miguel Lopez]

I pretend to introduce a choke between two capacitors for the power supply. Looking the circuit I understand that the 6K2 resistor (R13) belongs to the amplifier, and not to the power supply itself, so, my intention is to put the choke before resistor (R13).

@Chris .aka Herald1360 (There are a lot of "Chris" in this forum)
Don't worry friend. I pretend to build this amplifier. It is my project for this year, but still it is a hobby, not a need, so there are several reasons for me to advance slowly.

1.) This is my first experience with electron tubes. Everything I know nowadays about such devices, I have learned in the past two months.
2.) I don't have enough parts for replacement, so I don't want to destroy those I have because of my lack of knowledge. Learn before build.
3.) I don't have much time to spend in this project. I work on electronics when I finish my job, basically at night; and still I need to build some other devices in order to earn enough money for living.
4.) I'm very cautious with high voltage. Electrocution is not a nice experience, and it is permanent.
5.) There's no need to go fast

Don't worry. When I finish this amplifier I will post the pictures of it.

And I have not finished to ask questions about this circuit. I will be bothering a few more time here.

PS: Some of my finished projects are shown in this link:
https://www.vintage-radio.net/forum/...hp?albumid=377

PS2: Right now, I don't have any amplifier to listen to the music. I use the TV set as amplifier.

Some of my finished projects are shown in this link

With a portfolio like that you will have no problem with valves!

[Miguel Lopez]

I'll try to open a new album, with the pictures of my valved amplifier since the beginning.