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Old 18th Jan 2012, 2:46 pm   #41
Miguel Lopez
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Default Re: My first experience with electronic tubes

Yesterday, I tested my EL84's. They seem to be OK. Following cmjones01 advice, I mounted the valves in a socket previously prepared with the circuit I pretend to build.

Even a third valve that I have with the exhaust tip broken, it's good I think.

Using 230Vcc I measured the following values of plate current:

No.1 38 mA
No.2 35 mA
No.3 (broken tip) 32 mA

Is that enough for the EL84 operation?

I also have two 6П43П valves. They are power pentodes with the same pinout than EL84. I tested them too, but I stop when they reach 85 mA. I think that is too much for a valve like that. I guess they are for a smaller VCC, but I couldn't find any datasheet of them.

What do you think?

PS: The only problem was to remove the valves from the socket, they were really hot. I had to wait almost 10 minutes after each test to remove them.
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Old 19th Jan 2012, 5:26 pm   #42
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Default Re: My first experience with electronic tubes

My EM80 is good too!!!

I tested it yesterday and it worked nicely.

The 6П43П that I mentioned in the last post, are EL86 equivalents. I found it in a russian page.

This is going OK
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Old 23rd Jan 2012, 6:31 pm   #43
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Default Re: My first experience with electronic tubes

I mounted the output stage of the amplifier and tested with a signal from a radio. It works. I'm very happy with this. The output signal was not too strong, because the input signal was weak.

Of course I think that when I mount the pre-amplifier it will work much better.

PD: Is "to mount" a suitable verb for connect all the components according to the diagram in order to make it work.? I have my doubts on that.
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Old 23rd Jan 2012, 8:41 pm   #44
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Default Re: My first experience with electronic tubes

The way I use 'mount, mounted' is for individual components, 'build, built' would have been better for a number of components assembled to make a unit. But as english is such a mixture of languages 'mounted' was fully understood.

P.S. I love your 'strap line' "There's nothing you can do that can't be done."... so true!
 
Old 23rd Jan 2012, 8:59 pm   #45
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Default Re: My first experience with electronic tubes

Quote:
Originally Posted by merlinmaxwell
P.S. I love your 'strap line' "There's nothing you can do that can't be done."... so true!
John Lennon's words.
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Old 23rd Jan 2012, 10:04 pm   #46
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Default Re: My first experience with electronic tubes

I have just built this https://www.vintage-radio.net/forum/...ad.php?t=78559 it works very well, super things these valves.
 
Old 23rd Jan 2012, 10:25 pm   #47
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Default Re: My first experience with electronic tubes

Excelent merlinmaxwell !

Congratulations on a well done job. I almost envy you (in the good way)

I hope that my amplifier looks so good when I finish it.

The cabinet I will use is a Czeck-made one. I have several of this cabinets that can be reduced at will by cutting their pieces properly. Is the same kind of cabinet I used in the equipment I showed in my album.

On the other side I won't let the valves exposed, I don't like that because I don't have enough valves for replacement. And, on the other hand, my wife is a compulsive cleaner, and I want to protect the valves from her duster.

I intend to build a closed cabinet with all component inside. Of course, it will have enough ventilation on top and bottom.
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Old 27th Jan 2012, 2:42 pm   #48
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Exclamation Re: My first experience with electronic tubes

I was testing the bias of the valve 6Н1П in order to use it instead of 6Н2П. I need your opinions regarding the parameter values that I think to use

First stage:
HT=210V
Vp=150V
Vc=3V
Ip=1,66mA
Rp=33K
Rc=1,8K
Rg=470K

Second stage:
HT=210V
Vp=150V
Vc=4V
Ip=1,2mA
Rp=60K
Rc=3,3K
Rg=470K

Everything was by "trial and error" method in part because I don't have any decent chart (Ip vs VP) for the 6Н1П. It seems good to my basic knowledge but I have some doubts:

¿Are those small currents suitable for the amplification?

I think in the first stage there would be no problem, because if I inject a signal of, let's say, 150mVrms, and havin a gain of 15 - 16 (amplification factor for 6Н1П is 33, I think reduce it to a half would be expected for this circuit, any opinion welcome), in the grid of the next stage ther would be 2,25Vrms. This second stage has its cathode at 4V so, I guess there wouldn't be saturation.

But the second stage will amplify this signal, let's say 15 to 16 factor, and I will obtain in the EL84 grid about 33Vrms. If the cathode of EL84 is at 5V it will saturate. I do not want saturation, this is not a guitar amplifier but audio amplifier.

I don't know if I have explained my self well and I hope you had not slept with my wording , but I'd like to know your opinions regardind this. Any help welcome.

Yours

Miguel.
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Old 2nd Feb 2012, 8:42 pm   #49
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Default Re: My first experience with electronic tubes

Reconnecting properly the windings in the TC-180 mains transformer, I obtained 265V HT. That gave a plate voltage of 255V, and a plate current of 49-50 mA. The cathode voltage increased to 6,5V. I guess that with these values the EL84 (6P14P) would amplify a little more.

This is getting better. Let's wait that neighbours don't demand me.
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Old 3rd Feb 2012, 7:46 pm   #50
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Default Re: My first experience with electronic tubes

Excelent!!

The amplifier sounds wery well. It begins to saturate when the sound level is bothering inside the room. I will not reach that level in normal use. I'm very glad with the result. I will try to take some pictures to post them here. It is just an "air mounting" for test but it worked.

I found that the 6N1P has nothing in common with the datasheets shown in the web. I couldn't obtain the bias point they recommend for that valve. Using a resistance decade box in the cathode and another in the plate, I adjusted the bias trying to reach the values given in the datasheets. This is what I got:

HT=255V
Vp=107V
Ip=2,5mA
Rp=68K
Vc=2,5V
Rc=1K
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Old 9th Feb 2012, 5:51 pm   #51
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Default Re: My first experience with electronic tubes

I mounted a 7,5V Zener diode instead of the 120ohms resistor in the EL85 cathode. I thought that the plate current would decrease but it remains at 50mA (more or less). With the 120 ohms resistor the grid voltage was -6.5V and now is -7.5V.

Is this fixed bias?

I don't like to mix solid state component with valves because as my first project with valves I would like to make it pure-valved, but anyway I'm using silicon rectifiers so, why not.
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Old 13th Feb 2012, 6:02 pm   #52
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Default Re: My first experience with electronic tubes

Althought (saddly) this thread has almost become a monologue, I will keep sharing with you my first experience with valves.

I removed the Zener diode and kept the 120 ohm resistor. A friend told me that if the Zener diode blow out it could take the EL84 with it. That would be very sad for me.

I tested 6N2P (a friend gave one of them) instead of 6N1P. I sincerely prefer the later. The 6N2P (12AX7, ECC83) has a better bass response. It feels like a hammer hitting my ears when the speaker goes forward, but the 6N1P (ECC88) has a flatter frequency response. Despite that 6N1P has lesser bass response it has enough bass for me. I like it and I will keep it in the design. The other thing is that I will supress one of the triode stages. Ony one 6N1P triode provides enough amplification to drive the EL84. So, now I think I'm goint to build a stereo set using a couple of EL84 (one for each channel) and a single 6N1P triode to drive each pentode, then I will only need one 6N1P. The final design will have three valves for amplification (a couple of EL84 and one 6N1P); and the EM80 magic eye.

Anyway I will think in future, and I will build a chassis which will allow me to make future improvement. For the output I will make holes for a couple of octal sockets for each stage. I will put noval sockets for EL84 in that holes that would be easily replaced by octal ones if I get a more powerful valve. Now I will only use one noval but I will pretend to make a push-pull if I get matched EL84) When I'll finish it you will see plenty of empty holes in it.

Hope to take some pictures this week of my "freak" amplifier. (Actually the device I develop to obtain a final design)
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Old 15th Feb 2012, 9:20 pm   #53
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Default Re: My first experience with electronic tubes

Here some pictures I took to the "thing" I have built to test the circuit and to learn a little about valves. Before this, I knew nothing about valves, now I know something at best. It really looks awful, but that is just a test device. , I pretend that the final cabinet and the chassis look much better. I have some experience with cabinets.
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Old 15th Feb 2012, 9:22 pm   #54
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Default Re: My first experience with electronic tubes

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Old 15th Feb 2012, 9:26 pm   #55
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Default Re: My first experience with electronic tubes

I would stick to the cathode resistor for bias, it automatically adjusts the current. Simple and elegant, just like valves. My 3-3 is still going well, I am listening to it now.
 
Old 15th Feb 2012, 10:21 pm   #56
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Default Re: My first experience with electronic tubes

Quote:
Originally Posted by Miguel Lopez View Post
I removed the Zener diode and kept the 120 ohm resistor. A friend told me that if the Zener diode blow out it could take the EL84 with it. That would be very sad for me.
Definitely, though unlikely.

When Zener diodes fail, they tend to fail short-circuit - and this would effectively give zero bias. Then, the EL84 would draw lots of current.

A resistor will also give some compensation for variations between valves - a valve which draws more current will receive more bias, thus reducing current slightly. A Zener diode will give the same bias under all conditions.
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Old 15th Feb 2012, 10:35 pm   #57
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Default Re: My first experience with electronic tubes

Got it. I replaced the Zener diode by the 120 ohms resistor. You can see them in the pictures in the cathode of both EL84's.
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Old 27th Feb 2012, 3:18 pm   #58
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Default Re: My first experience with electronic tubes

Well, now I have four 8 ohms speakers, and I was thinking how to solve the problem of impedances. This is what I did.

I connected a couple of EL84 with their respective output transformers in parallel, supplying them with the same input signal, and then I connect the output windings in series. That will duplicate the output voltage, then I connect two speakers in series (16 ohms) and it worked very good (for my ears).

I will need 4 EL84 (by now I only have three and one in the way) to make a stereo, but I think that they do not need to be matched because they are not in push-pull, they just add one to each other. Yesterday I was listening music with that configuration.

Is that OK, or is it just another Miguelism?
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Old 27th Feb 2012, 3:57 pm   #59
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Default Re: My first experience with electronic tubes

It isn't completely clear what you have done here. Could you draw a simplified circuit diagram / schematic?

Speaker impedence matching is not as critical as many people think. A moderate impedence mismatch will cause the output valve to work less efficiently with less output, but the performace is usually adequate.
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Old 27th Feb 2012, 4:37 pm   #60
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Lightbulb Re: My first experience with electronic tubes

Sorry Paul. I tried to explain what I did to a fellow in Spanish, and he told me the same. Imagine in English. This is what I did.

Let me ask another question. I began to cut metal to built the chassis and the cabinet. Should I include the posibility to mount transformers or even the valves sheets (valves sockets will be in an independent aluminium sheet) on rubber washers?
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