I need a bit of help to fine tune my PP EL84 amp - diyAudio
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Old 13th December 2010, 10:59 PM   #1
Trileru is offline Trileru  Romania
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Default I need a bit of help to fine tune my PP EL84 amp

hello people. this is my first time posting here. I got help from my local tube forum but seems no one can help me further so I am going international
I started from a small scheme of only 1 6n1p tube as phase inverter and 2 el84 output tubes and I ended up with a 9 tube stereo amp
Ok, so I opted for a 6n6p preamp stage, each triode for a channel, next stage is the phase inverting stage, one triode from 6n1p I use for driver of the 2 el84 output tubes, and the other triode for the EM84 meter tube. The input tranny is a toroidal 220V in with 3 secondaries: one of 240V with 400mA for the amp itself and 2 of 6.3 each 6A for the filaments. I opted for a DIY scheme that someone helped me with. The output trannys are made custom from a friend, each 15W output 6k6 impedance for 8 ohm speakers. They are very good built, many dudes used his transformers and were very happy. I attached the scheme below with the details. I must note that I am a beginner, this is my first project and it turned out real nice despite my expectations The sound is clear and just perfect. The downside are my speakers. They are bi-amped, no internal crossover. So I had to make a bridge to get full range. Problem is that low is 8 ohm and mid+high are 6ohm. Anyway the output transformer is quite cold, sound is clean so I don't worry. Setup was not so expensive, the whole project cost me about 200$ for the electronic components the transformers and the tubes. The case is yet to come thou. Speakers are from an old Technics system with the bass speaker about 17 cm diameter, no real charm but they sound ok. So I don't worry about any damage, if anything is to happen I don't mind much, it's my first project after all
Anyway, I did some measurements and, if at first with only 6n1p as driver and 2 el84 as output tubes the values I got were pretty perfect now when I added the preamp stage and em84 meters everything is kinda mixed up. I am going for 15W output per channel. The electronic components used are among the cheapest possible here, and it's only what I found in local shops. After I finish the whole thing I'm thinking of ordering some quality coupling capacitors and such. And only after that I will go for the final touch, the actual "house" for everything. I will add some measurements below:

anode of el84 has 270V and a current of 44mA.
cathode has 9V

anode of driver 6n1p has 158V on anode and draws 1.1mA. On cathode there are 40.5V

the anode of preamp 6n6p has 110V with 3,7mA current draw. on cathode I got 5.4V.

pins 7-9 for EM84 have 52V and pin 6 has 147V. it's not at brightest but I can see it. the lines are not touching in the middle, they are 1/3 of total lenght, and I only get movement at high audio level. at normal listening power they almost stand still. This would be one of my problems. The other is the power supply stage. Normally it has been designed for something else but I kept upgrading the scheme and now it isn't delivering enough juice. Not that 400mA wouldn't suffice, the are good for both channels and still got some room. But the voltages are not quite right. Between point E and A I had a choke but since I didn't find any I replaced it with a 150ohm resistor. Anyway I have no hum at all so I considered I don't need one. Now I noticed that I got 313V in the first point after the rectifier bridge so I though I could use that. That's why that's the first contact point, before A. Anyway feel free to recommend any adjustment, but mostly I would like to keep the setup this way. Only for bit adjusting of resistors and such so I get the most out of every tube. As I am aware 6n1p performs best at 200V on anode and 50V on cathode. 6n6p is arround 120-140V on anode, the EM84 needs about 250V before the resistors on 7 9 and 6 pins. And the EL84 does good at about 40-45mA current draw on anode. So ...output stage seems ok but I think I could tweak a bit the preamp and driver. As well I would like to fix the EM84. Can it be adjusted so that it moves the same at low and high volume? Thank you very much for reading my problem, home I get some answers here, seems that my dudez are either too busy or too arogant for my noob problems.

forgot to mention that the preamp/driver stage don't neet 5W resistors but that was later found out. And the cathode resistors for el84 are 177ohm because thats what I actually measured in the series ones that I build to get close to the 180ohm value. I will try to get it closer to 180, that would "almost" make it class A if I'm not mistaken, and going towards 270ohm I go for AB.. At least that's how someone explained to me.
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Last edited by Trileru; 13th December 2010 at 11:10 PM. Reason: forgot attachement
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Old 13th December 2010, 11:08 PM   #2
Trileru is offline Trileru  Romania
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forgot to mention that the preamp/driver stage don't neet 5W resistors but that was later found out. And the cathode resistors for el84 are 177ohm because thats what I actually measured in the series ones that I build to get close to the 180ohm value. I will try to get it closer to 180, that would "almost" make it class A if I'm not mistaken, and going towards 270ohm I go for AB.. At least that's how someone explained to me.
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Old 13th December 2010, 11:31 PM   #3
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Why is there no grid resistor to ground on the input ?
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Old 13th December 2010, 11:44 PM   #4
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wow - sooooooo many things...

How much are you prepared to change?
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Old 13th December 2010, 11:52 PM   #5
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start with this datasheet - get the finals sorted out.
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Old 14th December 2010, 12:25 AM   #6
Trileru is offline Trileru  Romania
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This sounds bad well I'll check it tomorow. I just want to use those tubes transformers and scheme in the most part. Would be kinda awfull to start from scratch again. Anyway it sounds real good and loud with no distortions noticeable by me. I must say that I don't have a deep bass but still, would like to make it better. So, I must change the cathode resistors and anode voltage for el84 right? I would like it in class A or near as possible
I am prepared to change most passive components as long as I get the final output near 15W per channel, and use the em84 meter (or any meter for that matter as long as it's nicer than em84).
Why should I have a resistor from grid to ground?
Does this scheme seem so bad? It sounds nice to me but then again, I don't have much experience in hi-fi I'm also thinking of some FE206 fr speakers for this system. I don't have more than 200$ for the drivers and I wanna build my own enclosures. I'm a diy freak!
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Old 14th December 2010, 02:11 AM   #7
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You have overcomplicated a simple idea imho and yeah, the scheme as built mitigates AGAINST bass... Nothing that cannot be fixed though.
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Old 14th December 2010, 09:20 PM   #8
Trileru is offline Trileru  Romania
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I added a 330k resistor from 6n6p grill to ground. I put back the 150ohm resistor between B and C points, and added an extra 150 ohm between E and A, and I also changed the cathode resistors of el84 from 177 ohm to 147.5 ohm. It's the closest resistor I could get now. I got the following values:

el84 - 265V on anode and 47mA current draw on one and 48mA on the other tube.
6n1p - 216V on anode with 1.63mA current draw and 60V on cathode
6n6p - 127V on anode with 3.4mA current

The voltages from the supply are as follows:
E - 313V
A - 274V
B - 274V
C - 273V
D - 270V

The EL seems pretty close to the A class in data sheet. My output tranny is somewhere near 5.3K as the friend who made it told me today. Are these values ok for the output tubes?
What shall I do next?
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Old 14th December 2010, 10:39 PM   #9
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hmmmn - the opts are NOT optimal for the EL84. THey should be around 8k a-a.

THe EL84s do not have sufficient voltage across them (anode to cathode) to operate optimally. They are biased at just over -7 volts. They should be closer to -12v.

I don't think your power supply is going to get you there - you are going to need to achieve about 320V at the primary centre tap of the opt with cathode bias, or 305V if you go to fixed bias on the finals (no cathode resistors). In which case your power supply will JUST get there, maybe.

I don't think your voltage amp (6N6p) is capable of delivering the gain required to drive the cathodyne splitter to achieve full voltage swing on the EL84s. It needs to be able to put out at least 50Vpp, preferably more. In any case, at 127V on the anode at idle you are waaaay down in the non-linear area of its operating curves. Its the wrong tube for this front end duty.

I haven't looked at the set-up of the splitter because frankly, I'd ditch it and change topology to a simple 2 stage - ltp to finals.

I know this looks pretty negative but take it as a learning experience! All is not lost, and you can remodel the circuit to acheive close to what you are looking for.
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Old 14th December 2010, 11:03 PM   #10
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Quote:
Originally Posted by Trileru View Post
My output tranny is somewhere near 5.3K as the friend who made it told me today.
How many taps on the secondary winding of your output transformer, and what is the nominal impedance for each tap?

And, welcome to diyAudio!

John
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