Why a follower in a parafeed triodeamp? - diyAudio
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Old 9th June 2012, 03:55 PM   #1
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Default Why a follower in a parafeed triodeamp?

In an attemp to sort out if there is anything good about followers from the booooring Ju-thread I modelled three versions of a typical SE triode-amp.

Any ideas what came out ouf this wrt uneven order harmonics and THD? It anyway indicates to follow only one route......

I actually also modelled three MJK-style parafeeds with Schaded 6L6 with unexpected results, but that will be saved for later.
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Old 9th June 2012, 06:18 PM   #2
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With the first circuit you will see the distortion of the output valve, loaded by the OPT-transformed load. The third circuit will largely give you the distortion of the 'follower'. The second circuit will be somewhere between. The actual distortion levels depend on active device types and bias points as much as architecture, so difficult to say more than that.
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Old 9th June 2012, 07:56 PM   #3
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DF, can tell you the second will not even be close to "somewhere in between".

Don, Michael, kenpeter and a few others have explained this briefly before. Triode-antitriode behaviour seems to be no gimmick.

Donīt mind the possible refinements possible of the circuit. This is just a runofthemill design made for comparision use.

I already know the predictions. But to some it will be surprising to know their indicated behaviour.

One thing for sure though, even if the follower looks right to some it is the one to be avoided due to the highest uneven harmonics of them all.

Last edited by revintage; 9th June 2012 at 08:02 PM.
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Old 9th June 2012, 10:21 PM   #4
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I reckon fig 1. sounds best, but that's just my hunch because..
.. I once wired up an SRPP so the output was taken from the lower anode - it sounded better than the 'fix' to change it to the upper cathode...
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Old 9th June 2012, 10:31 PM   #5
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1st circuit a CCS loaded triode. Probably the worst distortion.

The 2nd (anti-triode version) should ideally have a little more resistance in the bottom 82 Ohm resistor to provide for the Mosfet drive signal component.
This then centers the dynamic range too. I think this should have the 2nd lowest distortion if set up optimally, since it 2X's the load Z on the triode.

The 3rd circuit I presume is supposed to be a Mu follower? The Mu-F normally has a high resistance up top the bottom tube plate.
As actually set up, the high gm Mosfet probably drives the load with next to no distortion and still lets the triode below it run with nearly no load, for minimal distortion of them all.

SRPP model?
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Last edited by smoking-amp; 9th June 2012 at 10:39 PM.
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Old 10th June 2012, 12:20 AM   #6
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The 3rd circuit has the greatest assymmetry of current drive, so likely has the higher odd harmonics. Putting the usual high value resistor above its plate for a typical Mu follower would likely reduce its power output capability way below the other circuits.

On second thought, the 3rd circuit is still putting a 164 Ohm load onto the tube, since it still has to develop the drive signal for the Mosfet. So I'm demoting its standing to 2nd or 3rd worst distortion.
Anti-triode takes the lead.
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Last edited by smoking-amp; 10th June 2012 at 12:44 AM.
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Old 10th June 2012, 01:03 AM   #7
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Ah, but the 164 Ohms is bootstrapped. Looks like the tube sees (164 gm Rl) for its load. So maybe about 10 Rl load (10 x load), which is the lightest load of the three, so it (3rd circuit) stays as the lowest distortion. Anti-triode as 2nd (with 2x load) and CCS+load (1x load) as worst dist.

If you put a triode or pentode up top though, the (164 gm Rl) factor will drop to more like (1 x load). Its the high Mosfet gm that is making the big deal there.

OK, I give up, whats the answer?
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Last edited by smoking-amp; 10th June 2012 at 01:14 AM.
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Old 10th June 2012, 09:57 AM   #8
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Hey Don,

You have already found out. Used the "gyrator-style" to have it work on top of pentodes too. But these are the predictions/indications:

THD:
1 G
2 F 1,2x higher than G
3 AT, 1,3x higher than G

Uneven:
1 AT
2 G 3rd H 2,5x higher than AT
3 F 3rd H 7x higher than AT

The higher harmonics where all a lot better for the AT.

In other words: Stay away from the follower!

Going for the MJK-style 6L6/depl.fet-hybrid, both the F and AT where odd-order killers compared to G. AT just marginally better though.
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Old 10th June 2012, 12:03 PM   #9
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FORGET!!! about my post above. Found some gross errors. Will redo!
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Old 10th June 2012, 12:17 PM   #10
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Corrected the errors and indications are AT will have the highest odd order harmonics. About the hybrid sims that where correct, AT seems still marginally better.

Last edited by revintage; 10th June 2012 at 12:32 PM.
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