mic pre circlotron ?

[mseddon]

Hi, I have been thinking about winding a small Nickel OT for a mic pre project - basic design has been cinemag input and various output trannies with many iterations both bal and SE, fet and tube and hybrid in the middle - but never been really satisfied with it. Have done it parafeed , but want series feed for low B+ (and simplicity)

It occured to me one doesnt really need a separate cancellation winding - cna do the same thing feeding the cancellation current back up the same winding as the plate supply. I guess this is a bit like a circlotron, but with a ccs instead of the mirror image amp. Presumably it behaves exactly as a discreet cancellation winding?

Reading here suggests that it might well be easier to build the mirror amp than the ccs. I have gone battery for quietness - but its double teh batteries. What would be a good ccs?

Question also is will I permantly magnetise the Nickel lams in the process of setting this up and swithching on?

Same old care pkg lams given good service in so many projects

Any advice?

best rgds, martin

[dave slagle]

hey martin,

I need a picture (crude schematic) to see what it is you are thinking and i highly doubt you will permanently magnetize the nickel lams. (i think the whole concept is an urban legend)

dave

[mseddon]

Hi Dave,

Pls find sketches attached showing tertiary winding morphing into circlotron.

I guess the other way of describing it is - DC goes round the long loop, AC goes round the short loop.

This is only for a few milliamp - maybe a simple fet ccs would work? and adjust tube bias to match it.

Best rgds, Martin

[dave slagle]

i'll have to think about that... here is the image.

[Dave Cigna]

Martin, some things you probably don't need to be told:

You don't get any real voltage gain with a circlotron. But, it does have low output impedance, so you might get away with using a step up transformer. Maybe you don't need gain, I don't know ...

You need two floating supplies. By symmetry, neither one is ground referenced, so if you have other stages you need more power supplies for them. The ground reference is usually provided by a center tap in the primary.

You can get around the ps issue (if it is an issue) by making the primary a pair of bifilar windings with a CT in each. Then you can use a single supply and use the same supply for other stages if you want.

[mseddon]

Hi Dave C,

Neat - once you unravel it it's pp - with an equal cathode load. Move the taps down and its normal 1:1 pp.

Focussing on the ccs with shared winding (for my application) it is conceptually similar to the circlotron in that AC and DC have separate paths. But the benefit is one ps at tube voltage level grounded conventionally, and a low voltage ccs supply floating.

Ultimately I guess theres appealing simplicity but little difference to a separate ccs winding. And I dont have wind a new bobbin.

My current mic pre has a battery B+ of 48v to a 6DJ8, with 6v heater supply. I could probably use the 6v heater supply in the ccs circuit.

I will wire it up and give it a go witha fet ccs. Bit of a worry about oscillation I read about here.

Rgds,

Martin

[dave slagle]

hey martin,

if you were referring to the oscillations from the CCS in series with a transformer primary I realized i was somewhat in error with my test setup since i was not loading the other windings. This meant that the CCS saw a pure inductance which is what caused the issues. In reality the reflected loads would dominate and make the CCS see a much more "resistive primary" in series.

I redid my tests with the appropriate loads (8 ohm resistor across the speaker taps) and it seemed rock stable with the CCS feeding the offset winding.

dave