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Unexpectedly good EL84 amp
Unexpectedly good EL84 amp
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Old 10th September 2018, 07:51 AM   #51
andyjevans is offline andyjevans  United Kingdom
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Originally Posted by Fenris View Post
The gain of the 6N1P is about 35. Putting a LED in place of the cathode resistor should ensure that you get all of that gain.
I question whether you need 3 stages, since the EL84 is a relatively high mu tube. My Leak Stereo 20 uses 6S4 triodes instead, in PPP. They have a mu of 16.5. I reworked the chassis slightly. Currently it has an ECC40 front end as a diff pair. Mu 32 and sounds nice, but different socket. I'll find an alternative, and currently thinking of a directly heated 3a5 for that.

As regards bias I've been getting very interesting results with SIC diodes, specifically C3D02060F. Something I got off Ale Moglia's 01A preamp in a recent shootout. This is my bias of choice right now. Each diode is giving me .92v in my 26 preamp. In that preamp I add a 120R resistor to the chain for a little feedback. Sound is very clear and clean.
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Old 10th September 2018, 02:42 PM   #52
mark02131 is offline mark02131  United States
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I'm thinking by lowering the 30K resistors to 15K I can get the current up enough to drive the 6L6 grids without the need of additional driver pair. If it were EL34 then for sure but with 6L6's I may be able to drive them without the need for that extra pair.
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Old 10th September 2018, 04:12 PM   #53
Fenris is offline Fenris  United States
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Quote:
Originally Posted by andyjevans View Post
I question whether you need 3 stages, since the EL84 is a relatively high mu tube. My Leak Stereo 20 uses 6S4 triodes instead, in PPP. They have a mu of 16.5. I reworked the chassis slightly. Currently it has an ECC40 front end as a diff pair. Mu 32 and sounds nice, but different socket. I'll find an alternative, and currently thinking of a directly heated 3a5 for that.
Andy, Mark was talking about driving 6L6 tubes rather than EL84. That's about twice the voltage drive and probably more current if going into AB. And once you go into AB, those 6L6 grids can draw some current. The one weakness of the concertina using a tube is that due to the Rp, the output signals have uneven current capabilities. A MOSFET has a far lower Rds which gives more balanced current capabilities.

I've run this circuit with a 6N1P, EL84s and a MOSFET in place of the second tube, but without the bypass cap in order to lower the gain. It works brilliantly. You could do two stages instead of 3, but it's the same number of tubes and the first diff pair would need a CCS to get good results - which may require another power rail.

One of the subtle brilliances of the original circuit is that the resistors in the first two stages are matched. Any current increase in the first stage is matched by a current decrease in the second stage, and vice-versa. The same goes for the balanced output stage. This results in a constant current circuit, which keeps the power supply from being modulated by changing currents.
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Old 10th September 2018, 04:46 PM   #54
Lingwendil is offline Lingwendil  United States
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This circuit and variations thereof are probably my most commonly built schematic. I'v done many builds based on the same basic idea, just with values changed to suit.

Other than my flea amp (a real giant killer!!!!) my current daily use amplifier is this circuit, with 6N2P input/phase inverter, 6CW5/EL86 outputs, with garter bias cathodes. All toroidal power and ouput iron. It's powerful, affordable, easy to drive, and sounds fantastic. LED biased 6N1P up front would be a great way to go if you don't want/need the extra gain for feedback, or if you have hot sources feeding it.
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Old 15th October 2018, 10:34 AM   #55
vuohi is offline vuohi  Finland
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I'm building this amp and I've got one channel assembled so I was doing my initial testing and measurements on it. My voltages are pretty close, but I'd like to tweak them a bit to get closer to the values in the schematic. Both halves of the 6N1P have a 30k resistor at the anode, but I'd get closer to nominal voltages by replacing one of those resistors with a slightly lower value resistor. My question is would this do more harm than good? I'm only about 10V off and I was wondering if there's a particular reason why the anode resistors are the same value.
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Old 15th October 2018, 12:18 PM   #56
Lingwendil is offline Lingwendil  United States
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10 volts is nothing in the grand scheme of things with tubes in general. I wouldn't bother changing anything. Built as shown, it's a very good circuit.
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Old 15th October 2018, 06:36 PM   #57
Francois G is online now Francois G  United States
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Originally Posted by Lingwendil View Post
.... my current daily use amplifier is this circuit, with 6N2P input/phase inverter, 6CW5/EL86 outputs, with garter bias cathodes. All toroidal power and ouput iron. It's powerful, affordable, easy to drive, and sounds fantastic....
I don’t want to highjack the thread, but I’m really interested in more details of your EL86 amp. Do you have a link, or please send me a PM. Thanks

Last edited by Francois G; 15th October 2018 at 06:40 PM.
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Old 15th October 2018, 06:49 PM   #58
Lingwendil is offline Lingwendil  United States
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Sure thing, it's got it's own thread right here-

6N2P/6CW5 design with toroidal outputs
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Old 15th October 2018, 07:15 PM   #59
Francois G is online now Francois G  United States
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Quote:
Originally Posted by Lingwendil View Post
.... my current daily use amplifier is this circuit, with 6N2P input/phase inverter, 6CW5/EL86 outputs, with garter bias cathodes. All toroidal power and ouput iron. It's powerful, affordable, easy to drive, and sounds fantastic....
I donít want to highkack the thread, but I really interested in more details of your EL86 amp. Do ypu have a link, or please send me a PM. Thanks
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Old 17th October 2018, 06:32 AM   #60
vuohi is offline vuohi  Finland
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Originally Posted by Lingwendil View Post
10 volts is nothing in the grand scheme of things with tubes in general. I wouldn't bother changing anything. Built as shown, it's a very good circuit.
Yeah, I don't think I'll bother. All the voltages are close enough.

But still the question remains, is there a reason why the anode resistors are both 30k? I have a faint recollection of someone mentioning something about this, but I can't remember where I saw it. I don't understand what difference it would make even if they were different values as long as the voltages are right.
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