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Old 17th July 2007, 04:07 PM   #1
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Default Stereo EL34 amplifier

Hi, this is my first post. I have lots of questions. I decided last christmas to design and build a tube amplifier for reasons I dont even remember. Let me get this started by stating that in this schematic I have decided to use 12AT7-WA. I also need someone to check this schematic who knows much more knowledge than I contain at this moment. I have talked to my professor last semester and said there is a load line method to determine some of the resistor values in this circuit, has anyone done this for either one of these tubes, keep in mind I would like to use the 12AT7-WA? Any criticism or input on what I am doing here would be, well, very helpful.
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Old 17th July 2007, 05:35 PM   #2
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Looks like reasonable approach for a guitar amplifier application, but this circuit is rarely used for stereo amplifiers, it will work particularly with the 12AT7WA but available loop gain will not be sufficient for good linearity closed loop, for this reason damping factor with the pentode connection is likely to be low as well. AC balance in this type of phase inverter can be a problem at high frequencies as well.

What is the intended application for this amplifier? Do you need this much power or could you settle for about 15W and use EL84/6BQ5 - results are likely to be better with this topology.

Any particular reason why you choose pentode connection over UL or triode?

Others will weigh in soon with more relevant comments and many things I have missed.
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Old 17th July 2007, 11:13 PM   #3
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I have intended to use it for a hi-fi; I realize that these tubes are generaly used for guitar. Also, I already have the tubes which makes it apeal to me more. Also I dont want anything too complex as this is my first to try to have two channels in one package. I will be driving a pair of JBL L-88 or 33 or I cant remember their model number right off hand but much like the L-100's. Also choice not for UL is I dont have that type of output transformer. Choice not for triode the power transformer I have currently is a 450-0-450 thordarson. I am trying not to spend to much on this, so using the existing major components is what I am after per the 12at7 choice and the el34 choice. So your comment on a different tube however a very good suggestion, and if i did not already have the el34 would probably take that road. I also agree with the feedback statement but I do not really know how to adjust that to get what it needs.
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Old 18th July 2007, 03:44 AM   #4
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Build it for a start. The EL34's will have a long life as they are conservatively run.
The performance of the power supply is important in this pentode configuration esp for anode/screen grids (g2) as the no sig to max sig current change is quite high. The feed to the screen grids should go through a 470R resistor to a well stabilised 300V. This 300V should come up with the anode voltage or after to avoid excessive current damaging the screen grids.
Despite kevinkr comments on open loop gains, I'm finding that some amps actually sound more dynamically pleasing when the front end (1st stage) gain is modest which requires higher signal input level and lower global nfb resistor. In otherwords when the 1st stage is a standard pentode stage with perhaps 40dB gain before global nfb feedback applied there is some performance advantage by strapping it as a triode which usually gives around 25dB. I'm finding that it is easier to get better control of 10Khz square wave tests i.e less ringing and overshoot created by output transformers.
This is tricky, and in your design there is alot of merit in adopting a simplest p-p and it should delight you. Remember if it howls on first test, reverse secondary on o/p transformer and reconnect nfb loop.

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Old 18th July 2007, 04:09 AM   #5
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I agree, go ahead and try it. However, you might get parasitic oscillations without grid stopper resistors on the EL34. I would use 4.7k stoppers on the control grids and 150 ohm on the screens, soldered as close as possible with very short leads to the respectgive pins. They need to be non-inductive resistors (definitely not wire wound), preferably carbon if you have them.
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Old 18th July 2007, 05:58 PM   #6
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In that case, I will get on that tonight and see if I can get it running. Last night I put the transformers and my inductor on a piece of MDF board as well as the sockets and terminal strips. I will let ya'll know what I find out tomorrow morning.
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Old 18th July 2007, 06:04 PM   #7
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The power supply section: perhaps worth a pic or scribble so we can see whats going on ?
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Old 18th July 2007, 10:30 PM   #8
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I am very sorry I did not post that in my orignal section. I thought at first that there would be more work to do on the signal amplification part of the circuit. In this picture you will see that above the inductor I have written in that I have two inductors in serries. The main reason I have done this is to get the voltage down to a reasonable voltage. I also mis spoke about the power transformer that I am using as it is a 400-0-400 as listed in 1949 by Thordarson. I also am going to assume that each set of EL34's will pull about 70ma at idle and the grids will pull about 10ma a piece. The 12at7's I am not sure about idle current draw but I would think about 10ma per triode. So to add that up 140ma for the EL34's plates, 20ma for the EL34's grids, and 20ma for the 12at7's for a grand total of 190ma or there abouts. I think that sould be the idle current draw. I am not sure that the voltages I actually have written in on this picture are entirely correct. So the DC voltage befor the inductors should be 425V, after the inductors roughly 368Vdc(193M DC resistance is about 50 ohms plush 250 ohms from the other transformer)throught the 1.6kohm next voltage should be about 304 and the plate voltage for the triodes should be through a 5kohm and the voltage on that should be 200Volts or so seems low but that is my circuit analysis for this section.
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Old 19th July 2007, 03:24 AM   #9
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Those 47uF caps in your power supply: if you're going to use them in series like that, you should connect a resistor (about 270k) across each one, to ensure that the voltage is shared equally.
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Old 19th July 2007, 05:32 AM   #10
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The 5U4 will give peak switch-on DC about 550V+ before power tubes warm up, so be careful. I'm suspicious of the screen grid voltage fed from a high value resistive source. This voltage will fall on signal transients and create distortion. The main thing is get amp working then look at improvements.

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