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Old 11th August 2004, 10:34 AM   #11
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Default Re: Apples to apples

Quote:
Originally posted by smoking-amp
Try conecting up the IRF510 with your original SET xfmer this way for a more even comparison (see attached diagram). Use the secondary, disconnect the primary and watch out for High Voltage on the primary. This connection provides some feedback on the Source connection to make it act like a triode.
I built an amp like that. Has been used daily for almost 2 years now. I am still unable to hear any shortcomings in the sound.
My first ever Class A amp.
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Old 11th August 2004, 02:55 PM   #12
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Interesting.... I will have to try that. Thanks for the idea guys!

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Old 11th August 2004, 03:45 PM   #13
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Quote:
Interesting.... I will have to try that. Thanks for the idea guys!
You've really got time to burn if you're gonna follow these ideas. Your initial observations seem spot-on but annoy the hell out of the deaf engineering community.
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Old 11th August 2004, 04:01 PM   #14
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Default Re: Apples to apples

Quote:
Originally posted by smoking-amp
Try conecting up the IRF510 with your original SET xfmer this way for a more even comparison (see attached diagram). Use the secondary, disconnect the primary and watch out for High Voltage on the primary. This connection provides some feedback on the Source connection to make it act like a triode.
Um, no.
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Old 11th August 2004, 08:25 PM   #15
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Default Um sorta

"Um no"

Well, obviously the mosfet by itself is much more like a pentode with high output impedance, giving a poor damping factor without Neg feedback. But lets look at how things work here.

For the triode, current emission depends on the electric field (to the 3/2 power) at the cathode, which for a triode is something like Vgk + Vpk/Mu. (The pentode has the plate voltage Vpk mostly shielded by the screen grid so as to have little effect.) Since the output AC signal on the plate is inverted, the internal feedback field from the plate acts as negative feedback, subtracting 1/Mu of the output Vpk from the Vgk field.

Similarly, for the mosfet, current emission depends on the electric field at the source due to the gate or Vgs (to some power between 2 and 1 depending on current level). The drain voltage is shielded by the channel pinchoff effect so has little effect like for the pentode.

In the "Apples to apples" circuit, the transformer is connected so as to subtract about 30% of the output voltage from the Vgs, acting as negative feedback to current emission in much the same way as the triode does internally. The effective "Mu" of the "Apples to apples" circuit will then be about 3 (depending on the tap ratio of the xfmr, 4 Ohms is about 0.7 turns ratio to 8 Ohm)
Output impedance from a cathode, or source in this case, is about 1/gm of the device. Since the speaker output is taken across the xfmr., the output impedance will be transformed to (1/0.3) squared times 1/gm, giving a damping factor of about 8 ohms times gm/(1/0.3)squared, or 0.8 gm. For the IRF510, gm is typically around 2.0, so damping factor will be around 1.6. So the circuit will have output impedance similar to a triode SET. I rest my case your honor. (putting flame retardant suit on and sneaking quietly out the back door before mayhem breaks loose)

Don
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Old 12th August 2004, 12:45 AM   #16
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Default a familiar circuit

By the way, the "Apples to apples", circuit when implimented on the primary side for tubes, goes by the name "partial cathode feedback" and was very popular with N. Crowhurst and Quad and McIntosh too, in a slightly modified form using separate cathode windings for P-P. When used in P-P form (unmodified) it would be called the "Circlotron" if using 50% cathode feedback.

Another descriptive name would be the "Mu changer circuit" since it can be used effectively to make pentodes behave as triodes (without having to connect the screen grid to the plate). The effective Mu is close to 1/(fraction of the total primary winding for the cathode's use). (not much new under the sun)

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