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Old 14th April 2009, 10:35 PM   #21
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"Wrong! Donīt forget there is no external load on the Aikido-triode as it is feeding a CF."

Whoaa, wait a minute.
Assuming the upper triode Rp matches the Rp of the of the lower triode in the Aikido, it just cuts Mu in half by paralleled Rp's. (well, some corrections needed for the Rk's lowering Mu a bit) I worked out a theoretical 3/2 power math model for the Aikido once and it comes out quite clean.

For the shunt feedback thru a diode case: (configuration: output pentode plate connects to the diode's plate besides it's output load. The diode's cathode connects to the triode driver's plate. Grid drive signal for the output pentode is taken from the triode driver plate, thru a cap etc. as usual):

With a diode load of say 10X Rp of the triode below (for the same current), and an (O.L.) gain of say 20X in the output stage, the diode load will then look like 1/2 Rp of the triode effectively (due to the 20X anti phase of the output signal, or 20X anti-bootstrapping). (there will be increased current thru the triode and diode due to this extra loading factor, so this lowers their Rp's even further yet. but they will remain in a constant ratio still, since the same current goes thru both triode and diode)

Higher feedback would use a lower Rp diode, becoming even less than 1/2 triode Rp effectively. The triode in each case just gets a lower Mu effectively from the non-linear loading, but still retains its linearity. (assuming the non-linearities of the diode and triode track, which should be reasonable for plate/cathode Rp's)

So the driver loading could be brought all the way down to 2K effective if needed, but would still give a linear gain, just less. The key here is the load/feedback diode is matching the non-linearity of the driver's plate, maintaining a fixed attenuation ratio. Whereas the usual fdbk resistor is just "too" linear.

Don
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Old 15th April 2009, 05:47 AM   #22
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Don,

I looked at the Aikido as a simple mu-follower with a very small bootstrap resistor of 250 ohms, albeit with the output taken below instead above the bootstrap-resistor. I did a quick calc and came to the conclusion that the lower triode should look into something like 15-20k.

When trying to verify I was then a little surprised when simming and finding out that the Aikido/mu-f./srpp/whatever 12AT7 simmed exatly like a 12AT7 with a anode resistor of 27k and an unbypassed Rk of 250ohm. Both gain, freq. and distortion alike. So the bootstrap-effect seems to be higher. Can do the same sims with a few other triodes to verify.

So I still believe there is a big difference between 27k and 1,75k ohms load and that "circuit 1" is a very bad way of adding feedback. Will still only work OK with pentodes as it was initially intended for them.....
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Old 15th April 2009, 04:32 PM   #23
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Rev, could you zip up your .ASC files relevant to this thread?
I'd like to crank them through the mill for learning purposes.
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Old 15th April 2009, 05:19 PM   #24
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Kenpeter,
Hope this works.
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Old 15th April 2009, 05:30 PM   #25
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And another.
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Old 15th April 2009, 05:33 PM   #26
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And a third. Just change txt to *.asc.
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Old 15th April 2009, 06:21 PM   #27
Svein_B is offline Svein_B  Norway
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Since the topic title refers to O.H.Schade, i played with LTspice to make a SE version of the circuit in his paper "BEAM POWER TUBES" from 1938. (the paper can be downloaded here: http://www.one-electron.com/Misc_Docs.html)

The interstage transformer coupling removes the problems we are discussing of (over)loading the driver tube and/or reduced gain.

Yes I know we may select a better driver tube than the 12AT7, and I also suspect that IT coupling present some new challenges, as well as the added cost, but it sure sims nice

SveinB
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Old 15th April 2009, 08:37 PM   #28
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Heja Svein,

Seriously, you can not mean it sims nice like it is . First of all you have to bypass the 12AT7 cathode resistor. The IT also has to be something like 200H to work OK. As it is -3dB is at 300Hz.


But IRL it would maybe not be worth the extra money . Will sim it against my other circuits.

EDIT: Simmed the two side by side and the IT is a tad better with respect to THD. Sensitivity is doubled. But due to the IT lowend response is worse.
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Old 16th April 2009, 05:36 PM   #29
vega65 is offline vega65  Norway
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O.H.Shades paper on beampower tubes is with respect to some issues hard to follow. On Pete Millets site there is an article http://www.pmillet.com/Books/bbc_feedback.pdf
where the construction of ia/ua for a Shaded tube is easier to understand.

I am working on a project with 13E1, and as an alternative to a std triode connection, I am about to have a closer look at plate to grid feedback.

I have spent some hours making tetrode ia/ua curves for 13E1 as the ones in the datasheet does not cover area of interest.(measurements not finnished)

Sorry for interrupting the thread, but as the heading refers to Shade....
What I am searching for is an article going through some of the calculations to set up a driver and a Shaded beampower tube output stage.
Also looking for a more detailed explanation of the use of interstage trafo as shown in Svein's drawing.
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Old 16th April 2009, 09:52 PM   #30
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Yet another way of getting P-G feedback...

http://www.pmillett.com/uc300b.htm
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