ok i was concerned about the lack of an air gap on the vibro champ output,
further research shows that every lamination has a natural air gap,
and the air gap ratio is this gap divided by the magnetic path length,
you can imagine that small lams are harder to squeeze the air out of, a little bend goes a long way, where as you can clamp a big core down prettiy tight, and small irregulaities will not make as much air gap as with the tiny cores, no wonder Beyer uses the pre welded cores with the spin on coils, the perm on small nickel cores can change radically depending on how tight they are, so you want the small lams at a constant tension for quality control,
Magnetic Path Length, MPL is the average path around the core,
gap ratio = L-a/MPL, where L-a is the air gap, and MPL is the magnetic path length,
use the same units for L-a and MPL, like centimeters for europe, or inches for those of us still suffering the Imperial system, ;D,
for small lams like the 625 EI as used in the champ output, the MPL is small so the ratio of the natural airgap of a 1 x 1 stacked 625 core to it;s MPL is high enuff to reduce dc flux without gapping.
so the larger the lam, the less natural airgap, as the MPL gets big quick..so the gap becomes more important as the xfmr gets bigger, if unbalanced dc is running thru the coil.
interesting note:
total design flux = ac flux + dc flux,
ac flux does not depend on the air gap,
dc flux does,
so when you insert an airgap, you are reducing dc flux but not ac flux.
if dc flux did not depend on the gap, then there would be no reason to use an air gap.
(except maybe to help stablalize (sp) permeability of the xfmr, but thats in chapter 6,)
formula for dc flux caused by current to feed the 6v6,
B-dc=0.4PiNI-dc/L-a+MPL/u-r,
example: 2000 turn primary coil with 40 ma dc current ,
B=dc=1.256*2000*0.04/gap, B-dc = 100/gap +MPL/u-r
see that for hi perm cores the MPL/u-r will drop out, eg, hi perm 80 , 625 EI,
mpl = 3.75, u-r = 100,000, mpl/ur= a small number, so we can simply write:
B-dc=100/gap, if the gap is 0.005 cm, you have 100/.005 = 20,000 gauss of dc flux that we have to lower, (bigger gap)
0.01 cm would drop us to 10,000 gaus dc flux, which allows room for some ac flux from the guitar signal,
now if you use regular steel grain, 625 EI, which has MPL = 3.75, you have to consider the MPL/u-r term in the formula, 3.75/1500=0.0025,
so for a steel core with low perm, the MPL/u-r term should be included.
100/-005 +.0025=100/.0075 = 13,000 gauss dc flux, so for the same gap, the lower perm materials will get their dc bias lowered more than the super 80 stuff,
another thing to grasp is that dc flux goes up with turns, ac flux goes down with turns, so with an SE transformer, you are fighting 2 things at once, raise the turns to lower b max, and you ampere-turns go up, which increases dc flux, so you get your turns for ac flux first, then compute the dc flux, then the gap you need to reduce dc flux, which then lowers inductance, so you raise turns, see how this process requires a few tries to get into the ball park of happiness?
now the effective perm, a little air gap reduces 80 ni to nothing quicker than it will drop steel perm, but we are still working on this so 420 break needed before my head explodes,