Quote:
Originally Posted by Phil Allison[_3_]
"John L Stewart"
'Phil Allison
Sounds like by either intent or serendipity the 300B and 6SN7 2'nd
harmonics are canceling.-
** Bingo !!
Seems an eccentric Russian gent was responsible for the circuit design.
There was an article on 2H cancellation in SE amps published about 10
years ago, I think in Glass Audio. Used the curvature of the driver to
cancel curvature in the OP Tride. One would think this possible at only
one level on the output.
** More testing with the FFT feature on a Rigol scope:
3H is dominant from about 9 watts through to 21 watts.
From 1watt to 4 watts, 2H dominates, then 4H, then 3H.
If load R is changed to 16 ohms, 2H dominates all the way.
2H cancellation is occurring over a fairly wide range, but it is not
complete.
I can swap the 6SN7 with almost no change.
Neat eh ?
..... Phil
|
I’ve found four articles covering Second Harmonic Correction in SE Amplifiers. All are now in a pdf of about 2M. If anyone would like a copy please email me direct as follows-
johnnhelen4 at gmail dot com
In Glass Audio 3/96 we find a fix authored by Graeme J Cohen. It uses a pair of side by side SE amplifiers, driven in antiphase. The combined output is added to drive the speakers. At first look it appears the even order harmonics are simply cancelled at the secondaries of the OPTs rather than in the primary of a single PP OPT. Redrawing the cct it looks a lot like a PP amp but needs two expensive SE OPTs. And it doesn’t get the cancellation of the zeroth order harmonic which is also even order, the DC component.
In Glass Audio 4/96 the cct is a 5842 driving into a 300B, authored by Reid Welch. It is a fact of life that in an RC coupled amplifier the AC load line is critical & is a cause of distortion. Interstage transformer coupling can avoid that problem at the expense of cost & BW limiting. An improvement possible with IT coupling is by reversing the leads driving the 300B grid. Curvature in the 5842 may subtract from the curvature in the 300B. Reid comments that the 5842 driver does not have the best transfer characteristic. But his amplifier measured a lower THD after that fix.
In Sound Practices Fall 1994 John Atwood of One-Electron discusses the SE 3x 2A3 amplifier developed for testing the OPTs to be built by Electra-Print Audio. It is RC coupled thru 6J5 input followed by the paralleled sections of a 6BL7GTA for the driver.
The 6BL7 family is much like a 6SN7 on steroids, same mu with twice the G.
In the same issue of SP is an article by Jack Elliano of Electra-Print. The amp in this case is again a 300B OP. The driver is a rather complicated mu-follower using paralleled sections of a 6AQ8 as the gain stage & a pentode 12HG7 as the constant current source. In my opinion the reason for the improvement in the THD figures is due solely to the mu-follower stage & nothing to do with curvature correction of the OP stage at all. For a triode (And pentodes too) 2H declines as the load R is increased. In this cct the 12GH7 looks like a very good current source at AC, hence a high Z plate load for the 6AQ8. And the mu-follower OP to the following grid is low impedance so easily drives the 300B.
In all these I find a lot of work sorting for the best sounding tubes & parts. Many of the tests are subjective. I could not find any tests for individual harmonics or intermodulation distortion.
My opinion, curvature correction is smoke & mirrors. Might work for some for a while & might not.
Phil has shown us that the harmonic content of the amp he has for repair varies quite a bit as the power level is changed. I would not depend on curvature correction as a cure for amp distortion. The THD may go down but what about the higher order harmonics & IM? It has been shown by many tests that these are the objectionable components in the reproduced sound.
Cheers to all, John