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Thread: Attempt on a 7591 Se design

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    Attempt on a 7591 Se design

    Hi,

    here my first steps toward a 7591 SE amp:

    Click image for larger version. 

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    And here its .ac analysis:

    Click image for larger version. 

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    This one is based on an Epiphone Valve Junior and a Jan Wstens ATRA0211 output transformer with a primary impedance of 5.2 kOhm and a max current of 60 mA. Within the limits of the simulation it looks as if the ideal anode voltage of 315 V of a 7591 at 5.2 kOhm quite exactly corresponds to the 60 mA of the OT. The G2 current meets the expectation as well.

    I have yet to estimate the power and to check the dynamic behavior of the amp - a nice opportunity to learn how to do that with LTSpice. The preamp is still work in progress. It is based on the preamp i am using in the G2000 but with a different tone stack (AMZ control vs. Bone Ray control).

    Suggestions / comments welcome.

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    Supporting Member loudthud's Avatar
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    What is the power rating of the OT? 5.2K is on the high side and the 7591 can take a lot more Voltage.

    RCA suggests 3K OT with B+ 300V for 11Watts.

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    Quote Originally Posted by loudthud View Post
    RCA suggests 3K OT with B+ 300V for 11Watts.
    I know, and at Ra=3.3k. But i do not believe. Class A means 50% of the max anode dissipation which is 19 W. So the 11 W would mean driving the tube beyond its specs. If the specs are sufficiently conservative that might be still ok. (What about 7591 from recent production... would You dare?)

    The Epi Valve Junior is an amp with a single ended EL84, hence the 5.2 kOhms. But its power supply delivers more that 300 V (which would have been perfectly suitable for the 7 kOhms used in the 1st release of that amp, but Epiphone changed that later to 5.2 kOhms to to pressure of the users). My main motivation for this project is to utilize the energy which is wasted to reduce the anode voltage to reduce the driving voltage by 50 V and obtain more output instead. This will obviously require a larger valve than the EL84 such as the 6L6 or the 7591.

    Back to physics:

    The 5.2 kOhms are only optimal for an EL84 if the anode voltage is ~250 V - according to
    Z = Ua^2 / Pa
    where:
    Ua = Anode voltage.
    Pa = Maximum anode dissipation.

    Applying this to a 7591 yields Ra=4.7 kOhm - and Ua=315 V as the optimum for Ra=5.2 kOhm. It is obvious that i cannot expect anything near to the 11 W. I did a rough estimate of 6.5 W from the curves for Ua=300V/Ra=6.5W. Extrapolating this via P=U^2/R would yield 7.2 W. BTW: taking the 300V/11W as given and extrapolating down would yield larger values.

    Now to the power supply. There is not much known about it except that it is reasonably oversized for an EL84 in SE mode and can successfully drive a 6L6 and even an EL34. It is said to be have the following specs: Usec=260V~ (known), Isec=100 mA (speculative). I did an estimate of the effective internal resistance from the voltages without tubes and in operation of the amp and found an effective internal resistance of 509 Ohms.

    Th OT: i did already change the original OT to the ATRA0211 which can do with 60 mA. Measured primary resistance: 365 Ohms.

    My simulation uses the following values: Ri=509 Ohms, V=376 V which will appear after the rectifier if i use the amp in its 220V setting but with 230 V from the wall. Due to lack of actual data i use "reasonable estimates" for the OT except of the measured primary Ri.

    I am aware that i am exploring the limits of both transformers.

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    Supporting Member jazbo8's Avatar
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    Quote Originally Posted by bea View Post
    I know, and at Ra=3.3k. But i do not believe.
    Please note the 11W output in the datasheet has a distortion figure of 11%, i.e., it's running on the ragged edge. But 8-9W would be easy to manage with much less distortion. If the goal is to get the maximum out of the existing PT, then using a higher Zpri seems to be counter-intuitive with the 7591. If you must use the 5k OPT, then another output tube might be more suitable... some of the more experienced builders can probably make some good suggestions.

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    As i mentioned i have a fully working EL84 amp (actually two, but one still requires some work in the preamp anyway...), and i am exploring to which degree it can be tweaked. What i am showing here is currently just a feasibility study. Therefore i do have the power supply and i do already have installed the fat OT.
    (And i know of several successful conversions to 6L6 and EL34.)

    A note to the data sheet: if You do the math of designing an SE amp with the 7591 with the standard approach You will end up with a much larger Ra than the suggestion of the data sheet. Something You should be aware of that. The designs shown in data sheets should be taken as "just proposals" which might or might not fully exploit the possibilities of a tube. It might always be meaningful to try something different.

    BTW: i noticed need to reduce the input grid resistance; it is a bit large for fixed bias operation.

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    Supporting Member jazbo8's Avatar
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    Quote Originally Posted by bea View Post
    A note to the data sheet: if You do the math of designing an SE amp with the 7591 with the standard approach You will end up with a much larger Ra than the suggestion of the data sheet.
    Kindly elaborate.

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    i need to leave for work.

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    Quote Originally Posted by bea View Post
    i need to leave for work.
    No worries, whenever you get a chance.

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    Quote Originally Posted by jazbo8 View Post
    Kindly elaborate.
    It is about minimizing THD - it reaches a minimum for Ra=Ua/Ia for given values of Ua and Ia where Ia is controlled by biasing.

    The abovementioned formula Ra = Ua^2 / Pa also takes this argument but implies that the anode current is chosen to make Ua*Ia = Pa_max, the maximum anode dissipation.


    I am aware of another rule of thumb: Ra ~ 10...15% of the internal resistance of the valve which for the 7591 is given as 29 kOhms. That would explain the suggested load of 3 kOhms - but apparently on cost of increased THD even at smaller signal levels.

    It is also clear that in my simulation the max dissipation of the 7591 is very closely approached - it is 18.9 W and therefore really close to the limit of the tube.

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    Quote Originally Posted by bea View Post
    It is about minimizing THD - it reaches a minimum for Ra=Ua/Ia for given values of Ua and Ia where Ia is controlled by biasing.
    Hmm, not sure where that came from... in any case, in guitar amps, seldom is minimizing the distortion a concern, usually maximizing the output power is the top priority.

    It is also clear that in my simulation the max dissipation of the 7591 is very closely approached - it is 18.9 W and therefore really close to the limit of the tube.
    That may be true, but the distortion is still rather high (before NFB is applied) even when compared with the datasheet example. As you mentioned earlier, there are some successful 6L6 and EL34 conversions, is there a reason that you are not following those examples?

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    As it can be seen, the 7591 matches pretty nicely into the framework of the amp, so why should i not use it?

    The 7591 is closest to the EL84 with about 50% more possible anode dissipation.
    I'll also simulate the 6L6, but it's amplification is smaller, and it is quite obvious in advance that i will not be able to obtain more power from it (i actually did the analysis on paper). Same fort the EL34.

    A valve larger than the 7591 will give me not advantage except of the lower prices of 6L6 or EL34.

    Something else i would like to mention: heater current. The 7591 draws a tiny bit more current than the EL84, the 6L6 even more, and the EL34 needs 1.5 - twice as much as the EL84. And that IS a concern to me.

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    Quote Originally Posted by jazbo8 View Post
    Hmm, not sure where that came from... in any case, in guitar amps, seldom is minimizing the distortion a concern, usually maximizing the output power is the top priority.
    But the output power is determined by the capabilities of the power supply and the load resistance. The valve acts like a voltage controlled resistor, it must be sufficiently large to handle it. The 7591 is sufficiently large for the given hardware, and it will not help if a larger valve is used. I actually expect similar values in the simulation if i insert a larger valve and adopt the biasing accordingly.

    (And if i want even more power, i'll use different hardware - i have some other stuff lying around: an EL84 PP powerstage (again with the question if it is possible to achieve a bit more by using larger valves, but i need to collect more data before i can evaluate that), and a few Dynacord transformer sets which allow to drive either 2 EL34 at 750 V. The would be a pretty straightforward project: restoring the power stage and adding two of the preamp stages i just developed for the G-2000. No math at all...

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    Supporting Member jazbo8's Avatar
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    Perhaps I did not make my point well enough... yes, 7591 can indeed handle the output power (at least for the NOS, not sure about the current production), but its plate characteristic does not appear to be the best choice for the PT and OPT that you plan to use, unless of course, you just prefer how the 7591 sounds compared to the others.

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    From what I have gathered the current 7591 tubes do not hold a candle to the NOS design.

    Not even close.

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    Quote Originally Posted by Jazz P Bass View Post
    From what I have gathered the current 7591 tubes do not hold a candle to the NOS design.

    Not even close.
    Yep, generally best practice to design around the tubes that are being made now, not those of yesteryear, unless you have a stash of the good stuff. And at the risk of sounding unromantic, the 7591 was basically a refined 6L6: a slightly cooler filament, but higher transconductance and voltage ratings to squeeze more juice out of fewer tubes in an amplifier (economical!) Drive voltage isn't really hard to come by, and the high voltage ratings are of much more use in a Class AB push-pull design than a single-ender.

    Remember that with an SE amp, the maximum plate dissipation happens at idle. The 8-11 watts of output power that we're talking about goes to the load, actually reducing the heat generated at the plate.

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    Let me just mention that the setup would also work with the 6L6GC (i am aware of the different pinout...) - the range of the bias pot is sufficient. So if i build it would be relatively easy to migrate to a 6L6GC if the 7591 turned out not to be useful.

    Another remark: the 7591 is a real pentode. The 6L6GC is a tetrode, and its abovementioned high voltage cousin has been the 7027A.

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    Senior Member ThermionicScott's Avatar
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    You sure about that? The pictures of JJ 7591s that I can find look like they have beam-forming plates inside, not a suppressor grid.

    As far as the old ones went, perhaps they were like EL84s/6BQ5s in that they were made both ways. (I have some GEs at home with beam-forming plates.)

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    If you want a mathematical formula for working out load resistance for output stages, Merlin's page on SE output stage design has information on optimum load resistance for centre-biased class A operation This is applicable no matter what tube you're using because it calculates load resistance based on plate power and plate voltage.

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    Quote Originally Posted by ThermionicScott View Post
    You sure about that? The pictures of JJ 7591s that I can find look like they have beam-forming plates inside, not a suppressor grid.

    As far as the old ones went, perhaps they were like EL84s/6BQ5s in that they were made both ways. (I have some GEs at home with beam-forming plates.)
    This: http://www.mif.pg.gda.pl/homepages/f...127/7/7591.pdf calls it a beam power pentode, whatever that means. I always thought that they were very similar to a 6L6 in construction, but that is just a memory from long ago.

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    Quote Originally Posted by tubeswell View Post
    If you want a mathematical formula for working out load resistance for output stages, Merlin's page on SE output stage design has information on optimum load resistance for centre-biased class A operation This is applicable no matter what tube you're using because it calculates load resistance based on plate power and plate voltage.

    I would think the only way to do get the maximum power is to try load lines on the tube curves, and then derive the impedance from that. "Any tube" covers a lot of possibilities.

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    Here's my two cents worth:

    EDIT: OK It looked like the top cathode resistor was 820 ohm but now I see it's 0.1m and R20. It's just too blurry for my poor old eyes, I guess Those two resistors are just a hangover from simulation, I guess.

    The bias arrangement itself presents too low an impedance to the vol pot & driver. I would move the 150K to be in series to the bias pot wiper and connect the other end to the grid and C4.

    One you've fixed these things, the loop gain will shoot up and you might find you have stability issues that need to be addressed, assuming it is stable as shown.

    What is the thinking behind having 390K anode resistors?

    Having C2 and C6 in parallel to form a 3.3nF seems a little wasteful.

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    Quote Originally Posted by nickb View Post
    Here's my two cents worth:

    EDIT: OK It looked like the top cathode resistor was 820 ohm but now I see it's 0.1m and R20. It's just too blurry for my poor old eyes, I guess Those two resistors are just a hangover from simulation, I guess.
    Yes, i wanted to be able to switch between cathode bias and external bias with relatively few clicks.

    The bias arrangement itself presents too low an impedance to the vol pot & driver. I would move the 150K to be in series to the bias pot wiper and connect the other end to the grid and C4.
    I had some problem with blocking distortion and increased the grid stopper. But anyway, the preamp is work in progress. I'll probably use the setup from my G-2000.

    One you've fixed these things, the loop gain will shoot up and you might find you have stability issues that need to be addressed, assuming it is stable as shown.
    I am not sure. There is a (really) slight increase toward 10 kHz which might be suspicious.

    What is the thinking behind having 390K anode resistors?
    Maximising gain. In the current state, V1 has 390 k. When i'll change the preamp (what i will definitely do), i'll use 220k.


    Having C2 and C6 in parallel to form a 3.3nF seems a little wasteful.
    In reality, C6 is switchable.

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    Quote Originally Posted by bea View Post
    Yes, i wanted to be able to switch between cathode bias and external bias with relatively few clicks.



    I had some problem with blocking distortion and increased the grid stopper. But anyway, the preamp is work in progress. I'll probably use the setup from my G-2000.



    I am not sure. There is a (really) slight increase toward 10 kHz which might be suspicious.


    Maximising gain. In the current state, V1 has 390 k. When i'll change the preamp (what i will definitely do), i'll use 220k.



    In reality, C6 is switchable.

    A slight increase in gain? C4 is loaded by at most 5.5k (bias network) and the source impedance is 50K-250k so the attenuation is about 10 at best.

    Also 390k/3.3k won't give you max gain. For example 100k/1k will give you about 1.5dB more gain as the Gm increases with anode current.

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    Last edited by nickb; 03-28-2016 at 09:58 PM.
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    Quote Originally Posted by nickb View Post
    A slight increase in gain? C4 is loaded by at most 5.5k (bias network) and the source impedance is 50K-250k so the attenuation is about 10 at best.
    I am currently playing a bit with the model. Fixed the bias circuit (the bias was was initially larger - 100k). Now the grid stopper is 15k and a resistor of 220 k providing grid leak through the bias circuit has been added. Now the frequency response looks as expected.

    I also played a bit with the anode resistors and the values of the pots. I'll stick with the 390k anode resistors. This will give slightly more gain at V1 which i need if i want to overdrive V2, but change the gain pot to 500k. At V2 i am a bit unsure wether i want more gain or not. Overdriving the power stage is not necessarily intended, and with the 7591 so close to its max dissipation heavy overdrive of the power stage should probably be avoided.

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    Quote Originally Posted by Mike Sulzer View Post
    I would think the only way to do get the maximum power is to try load lines on the tube curves, and then derive the impedance from that. "Any tube" covers a lot of possibilities.
    Well yes - the tube needs to be biased correctly to ensure the operating point corresponds with the plate voltage and the target dissipation at the screen voltage you are working with. Nevertheless I did state 'centre-bias' 'Class A operation' as assumptions.

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    I played with some 7591 when I did an Ampeg Echo Twin restoration not so long ago.
    Did an A B on the AVO MK3 Tube tester betwen some old RCA and new JJ. The new JJ had half the gm of the old RCA. That is, they were not much like a "real" 7591. They did bias at the same voltage thou'.
    I made myself a note to try ElectroHarmonix istead of the JJ next time I needed 7591. May not be any better but I don't recommend the JJ.
    Cheers,
    Ian

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    Quote Originally Posted by tubeswell View Post
    If you want a mathematical formula for working out load resistance for output stages, Merlin's page on SE output stage design has information on optimum load resistance for centre-biased class A operation This is applicable no matter what tube you're using because it calculates load resistance based on plate power and plate voltage.
    The formula is the one i used in my estimate. It appears to me not as power maximum but as being derived from the distortion minimum condition. As the internal resistance of the tube is large compared to the load resistor, the available power is determined by the anode voltage, the current determined by the load impedance and the minimum usable voltage due to the "knee". To first order You will see the same output power from any tube provided it can handle the current and the voltages.

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    Quote Originally Posted by bea View Post
    It appears to me not as power maximum but as being derived from the distortion minimum condition.
    No, that is just not the case, the quiescent variables Ep0, Ia0, and Pmax have little to do with the harmonic distortion per se. To calculate the distortion, as others have already mentioned, you need to do load line and/or transient analysis.

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    Well, here the latest simulation. The IMO logical next step - large signal behavior.

    This time i'll show the result for the 6L6, which i did in parallel to the experiments with the 7591. This time i did not repeat the small signal analysis but ran a sine of 1000 Hz through the circuit, the amplitude large enough to reach saturation. I adjusted the bias for max undistorted and symmetric output.

    As You see, the max clean amplitude at the output is quite exactly +-10 V, i.e. 7 V eff which means 6.1 W at 8 Ohms. For the 7591 i found 5.5 W. In order to find this, i needed to slightly reduce the cathode current of the output tube to 60 mA.


    EDIT: at least to a large degree that's the preamp.
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    Last edited by bea; 03-30-2016 at 08:35 AM.

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    Quote Originally Posted by bea View Post
    Well, here the latest simulation. The IMO logical next step - large signal behavior.

    This time i'll show the result for the 6L6, which i did in parallel to the experiments with the 7591. This time i did not repeat the small signal analysis but ran a sine of 1000 Hz through the circuit, the amplitude large enough to reach saturation. I adjusted the bias for max undistorted and symmetric output.

    As You see, the max clean amplitude at the output is quite exactly +-10 V, i.e. 7 V eff which means 6.1 W at 8 Ohms. For the 7591 i found 5.5 W. In order to find this, i needed to slightly reduce the cathode current of the output tube to 60 mA.


    EDIT: at least to a large degree that's the preamp.

    Let me throw a couple of simulation tools/ ideas your way.

    First, until you get to some level of refinement, keep it simple. this means lose as many non-essential components as possible and simulate one section at a time. The schematic below will give you the idea.

    Second, which is handy for seeing the linearity visually, is rather than to step the driving voltage source, is to use an amplitude modulator fed by a linear ramp. You can then see at a glance where the linearity starts to fail off.

    Schematic:
    Ramp Input Sch with Power & THD 7591 SE Power ramp out sch.pdf

    Waveform:
    Waveform Ramp Input 7591 SE Power - ramp out.pdf



    Lastly, you can estimate the power out and THD when stepping the input using '.four' and '.meas' commands. Note the TRANS statement parameters are crucial to get meaningful results.

    Schematic:
    Step Input Sch 7591 SE Power step.pdf

    Results (in err/log file):
    Step Input THD and Power out log 7591 SE Power.log.txt


    One more thing, to report the power dissipated in anything when doing a trans, put your cursor over the component and hold the ALT key. On the newly added waveform, hover over the name, hold CTRL and click. The power will be in the pop-up dialog.

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  31. #31
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    Thanks, i'll go through this.

    BTW: my results seem pretty close to yours.

    Using a 6L6GC, i ended up with something around 7.1W clean and some 8-8.5 V clipped. This appears to me as a really nice result - compared with the data sheet of the 6L6GC i should not expect more.

    So if i would do the mod in practice i would probably use the 6L6 instead of the 7591: cheaper and significantly more output power. In actual practice tweaking the preamp is more important because of some weaknesses of the present circuitry.

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    Quote Originally Posted by nickb View Post
    First, until you get to some level of refinement, keep it simple. this means lose as many non-essential components as possible and simulate one section at a time. The schematic below will give you the idea.
    I actually did this in the static analysis. At some point I wanted to see the complexity of the power supply and the effect of the NFB loop as well.

    Lastly, you can estimate the power out and THD when stepping the input using '.four' and '.meas' commands. Note the TRANS statement parameters are crucial to get meaningful results.
    I followed especially that suggestion and got results like the following (using a model of the JJ 6L6GC which AFAIK has been derived from -tracer-data):
    Code:
    .step in=0.9 (Remark: step 4)
    N-Period=1
    Fourier components of V(out)
    DC component:0.0836239
    
    Harmonic	Frequency	 Fourier 	Normalized	 Phase  	Normalized
     Number 	  [Hz]   	Component	 Component	[degree]	Phase [deg]
        1   	 1.000e+3	 1.099e+1	 1.000e+0	 -161.96	    0.00
        2   	 2.000e+3	 5.030e-1	 4.577e-2	    6.81	  168.76
        3   	 3.000e+3	 1.060e+0	 9.644e-2	 -118.59	   43.36
        4   	 4.000e+3	 1.861e-1	 1.693e-2	  135.32	  297.28
        5   	 5.000e+3	 1.267e-1	 1.153e-2	  -28.53	  133.43
        6   	 6.000e+3	 9.270e-2	 8.435e-3	 -132.46	   29.50
        7   	 7.000e+3	 4.893e-2	 4.452e-3	  101.51	  263.46
        8   	 8.000e+3	 3.807e-2	 3.464e-3	  -35.18	  126.78
        9   	 9.000e+3	 2.748e-2	 2.500e-3	 -157.58	    4.38
    Total Harmonic Distortion: 10.920017%(10.923670%)
    
    ...
    
    Measurement: power_out
      step	vrms**2/8
         1	4.09692
         2	5.0647
         3	6.19534
         4	7.6529
         5	9.67627
    So with a 6L6 i can obtain about 7.5 W at 10%THD.
    After these results i concentrated on the EL84 version. I both versions i had to struggle a bit with stability issues, especially with the EL84. I am not sure to which degree these are artefacts by the models and to which degree they are real.

    Code:
    ...
    .step in=0.55 (Remark: Step 4)
    N-Period=1
    Fourier components of V(out)
    DC component:0.466336
    
    Harmonic	Frequency	 Fourier 	Normalized	 Phase  	Normalized
     Number 	  [Hz]   	Component	 Component	[degree]	Phase [deg]
        1   	 1.000e+3	 9.276e+0	 1.000e+0	 -178.70	    0.00
        2   	 2.000e+3	 2.531e-1	 2.729e-2	   21.10	  199.80
        3   	 3.000e+3	 7.622e-1	 8.216e-2	 -165.69	   13.01
        4   	 4.000e+3	 1.307e-1	 1.409e-2	  105.87	  284.57
        5   	 5.000e+3	 1.182e-1	 1.275e-2	  -17.80	  160.90
        6   	 6.000e+3	 6.183e-2	 6.666e-3	  175.17	  353.87
        7   	 7.000e+3	 5.235e-2	 5.644e-3	   62.64	  241.34
        8   	 8.000e+3	 3.023e-2	 3.259e-3	  -64.73	  113.97
        9   	 9.000e+3	 2.397e-2	 2.584e-3	  153.30	  332.00
    Total Harmonic Distortion: 8.916135%(8.923619%)
    
    ...
    
    
    Measurement: power_out
      step	vrms**2/8
         1	2.86495
         2	3.71463
         3	4.65839
         4	5.58837
         5	6.25489
         6	6.67498
         7	6.96952
    5.6 W at 9% THD (Step 4) ... that's pretty close to the predictions of the data sheet for an anode voltage of 250 V (Valvo, Telefunken). Even the values for k2 and k3 do match reasonably well.

    Overdrive:
    when the EL84 is overdriven, the output power can reach 8 W. To my understanding that means a heavy thermal overload of the anode - and indeed, my amp does not like to be overdriven for a long time.
    With the 6L6GC the fully clipped output corresponds to about 12W which should be on the safe side

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  33. #33
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    Sounds like you are getting on well.

    You might want to check the models you have for inter-electrode capacitances. The thing that will have the biggest impact on stability is the output transformer. At a minimum I would add DC resistance, primary self-capacitance and inter-winding capacitance. You'll just have to guess at values (unless you have access to a network analyzer).

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  34. #34
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    Quote Originally Posted by nickb View Post
    Sounds like you are getting on well.

    You might want to check the models you have for inter-electrode capacitances. The thing that will have the biggest impact on stability is the output transformer. At a minimum I would add DC resistance, primary self-capacitance and inter-winding capacitance. You'll just have to guess at values (unless you have access to a network analyzer).
    I looked at Daten_A and found nothing even there. Well i could estimate from the Ls and the resonance frequencies. But is that really worth the effort?
    When i modded the amps i had a lot less experience - and i did neither own an oscilloscope and a signal generator and could not look what i thought would happen (in terms of oscillation). The simulations give me strong hints what to look for and how to fix it - but reality ids different anyway.

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    Preliminary final model, 6L6 version

    Maybe i'd show important results as replies to my first posting....

    So here it goes - Schematics and output form intermediate level into saturation. As there is no output voltage of the bias voltage i'll have to derive it from the main HT. If i decide to do the mod i will work on only one of my two amplifiers. Simple reason:

    Amp A has one volume control and one tone control, and it has the switch for the NFB on the front.
    Amp B has an additional "master" volume and the NFB switch on the back.

    So Amp B looks like the ideal candidate for a circuit like the following, and Amp A will keep the EL84 unless i find something better. Maybe the russian military variant of the EL84 with a Pa_max of 14W instead of 12 (my results for the JJ-7591 are not better than what would be expected from that tube, and the effort is smaller.)

    Because of the small value of the grid leak resistor the 6L6 requires with static biasing a master volume is a bit difficult. Therefore i omitted it in the following plans. You might notice the unusually large anode resistors of the ECC83. This is well and safely within the limits, the headroom remains sufficient to fully overdrive the 6L6GC, and the valve has less distortion. Disadvantage is the larger output impedance - but in connection with the small load impedance this is rather an advantage than a disadvantage - the effective (dynamic) load will be roughly as usual.

    Click image for larger version. 

Name:	VJ_B_with_6L6.png 
Views:	351 
Size:	24.6 KB 
ID:	38450

    Click image for larger version. 

Name:	VJ_B_with_6L6.out.png 
Views:	219 
Size:	56.0 KB 
ID:	38451

    And the log:

    Code:
    .step in=0.85
    N-Period=1
    Fourier components of V(out)
    DC component:0.157466
    
    Harmonic	Frequency	 Fourier 	Normalized	 Phase  	Normalized
     Number 	  [Hz]   	Component	 Component	[degree]	Phase [deg]
        1   	 1.000e+3	 1.002e+1	 1.000e+0	 -161.36	    0.00
        2   	 2.000e+3	 2.870e-1	 2.864e-2	   48.65	  210.01
        3   	 3.000e+3	 5.369e-1	 5.357e-2	 -101.84	   59.52
        4   	 4.000e+3	 1.652e-1	 1.648e-2	 -176.60	  -15.23
        5   	 5.000e+3	 8.401e-2	 8.383e-3	   74.13	  235.49
        6   	 6.000e+3	 5.443e-2	 5.431e-3	  -47.40	  113.96
        7   	 7.000e+3	 4.279e-2	 4.270e-3	 -157.69	    3.68
        8   	 8.000e+3	 3.093e-2	 3.086e-3	   89.65	  251.01
        9   	 9.000e+3	 2.360e-2	 2.354e-3	  -26.24	  135.12
    Total Harmonic Distortion: 6.398771%(6.408589%)
    
    .step in=0.9
    N-Period=1
    Fourier components of V(out)
    DC component:0.264288
    
    Harmonic	Frequency	 Fourier 	Normalized	 Phase  	Normalized
     Number 	  [Hz]   	Component	 Component	[degree]	Phase [deg]
        1   	 1.000e+3	 1.098e+1	 1.000e+0	 -161.47	    0.00
        2   	 2.000e+3	 3.623e-1	 3.299e-2	   18.50	  179.97
        3   	 3.000e+3	 8.716e-1	 7.938e-2	 -108.56	   52.91
        4   	 4.000e+3	 2.035e-1	 1.853e-2	  164.12	  325.59
        5   	 5.000e+3	 1.213e-1	 1.104e-2	   33.53	  195.00
        6   	 6.000e+3	 9.677e-2	 8.813e-3	  -89.59	   71.87
        7   	 7.000e+3	 7.245e-2	 6.598e-3	  158.27	  319.74
        8   	 8.000e+3	 5.153e-2	 4.693e-3	   34.78	  196.25
        9   	 9.000e+3	 4.332e-2	 3.945e-3	  -86.17	   75.30
    Total Harmonic Distortion: 8.951782%(8.978948%)
    
    .step in=0.95
    N-Period=1
    Fourier components of V(out)
    DC component:0.327321
    
    Harmonic	Frequency	 Fourier 	Normalized	 Phase  	Normalized
     Number 	  [Hz]   	Component	 Component	[degree]	Phase [deg]
        1   	 1.000e+3	 1.173e+1	 1.000e+0	 -161.18	    0.00
        2   	 2.000e+3	 5.127e-1	 4.370e-2	   -3.86	  157.31
        3   	 3.000e+3	 1.301e+0	 1.109e-1	 -114.23	   46.95
        4   	 4.000e+3	 1.984e-1	 1.691e-2	  136.24	  297.41
        5   	 5.000e+3	 1.857e-1	 1.583e-2	   -4.12	  157.05
        6   	 6.000e+3	 1.378e-1	 1.175e-2	 -120.85	   40.33
        7   	 7.000e+3	 9.415e-2	 8.025e-3	  118.47	  279.65
        8   	 8.000e+3	 7.524e-2	 6.413e-3	  -17.86	  143.31
        9   	 9.000e+3	 6.267e-2	 5.341e-3	 -139.23	   21.95
    Total Harmonic Distortion: 12.256270%(12.306296%)
    
    .step in=1
    N-Period=1
    Fourier components of V(out)
    DC component:0.297474
    
    Harmonic	Frequency	 Fourier 	Normalized	 Phase  	Normalized
     Number 	  [Hz]   	Component	 Component	[degree]	Phase [deg]
        1   	 1.000e+3	 1.220e+1	 1.000e+0	 -160.12	    0.00
        2   	 2.000e+3	 7.063e-1	 5.792e-2	  -11.61	  148.51
        3   	 3.000e+3	 1.820e+0	 1.492e-1	 -117.08	   43.04
        4   	 4.000e+3	 1.895e-1	 1.554e-2	   72.07	  232.19
        5   	 5.000e+3	 2.616e-1	 2.145e-2	  -29.51	  130.61
        6   	 6.000e+3	 1.683e-1	 1.380e-2	 -149.09	   11.03
        7   	 7.000e+3	 1.089e-1	 8.929e-3	   76.29	  236.41
        8   	 8.000e+3	 9.909e-2	 8.125e-3	  -57.29	  102.83
        9   	 9.000e+3	 6.333e-2	 5.193e-3	 -178.03	  -17.91
    Total Harmonic Distortion: 16.338583%(16.386446%)
    
    
    
    Measurement: ig2
      step	RMS(ix(u5:screen))	FROM	TO
     
         5	0.00184526	0	0.01
         6	0.0024257	0	0.01
         7	0.00289037	0	0.01
         8	0.00317757	0	0.01
    
    Measurement: power_out
      step	vrms**2/8
    
         5	6.22658
         6	7.57143
         7	8.81957
         8	9.76553

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    Last edited by bea; 03-31-2016 at 10:46 PM.

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