airgap adustment in the field of a 25:1 ratio OT

Discussions and experiences with user adjustable gaps.
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dave slagle
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airgap adustment in the field of a 25:1 ratio OT

Post by dave slagle »

This is a little bit of pre-history on some experiences with playing with different gap sizes in a pair of output transformers.

Next, I took my amp back to my place and tried the amorphous trannies
as output transformers with no gap and with a gap of four post-it note
thickness. In the first case, no gap in the amorphous trannies, the
amorphous trannies were not as open and full as the Tamuras used as
output trannies. They were just a little worse than when I compared the
amorphous trannies with the mylar spacing to the Tamuras.
nothing really surprising there, thanks for giving it a try!
The last thing I tried was using four post-it notes as a spacer for
the amorphous trannies. This opened up a whole new world. The amorphous
trannies were definitely more open, airy, and at the same time more
detailed than the Tamuras. I was really surprised. I definitely like
your amorphous trannies better than the Tamuras in this case. I think
in the other cases, the iron was saturating and distorting the signal.
I'd like to mess around more with the size of the gap because I think
fine tuning the gap may give even better results than I got with the
four post-it note gap. Have any ideas?
I have a few ideas, but I am a bit surprised that the 4 postit note size gap made that large of an improvement.

Two things to mention at this beginning point: I chose postits as a form of universal gap size, it seems everyone has access to them and they all seem to measure .0035 inches.

The other point that needs to be taken into consideration is that this amp is a 75TL being loaded with a 10K reflected load and has a lowpass filter prior to it rolling things off below 100hz or so…

OK… onto a second reply.

Steve came over to my place yesterday and we played around with
the gap in your amorphous transformers. Before Steve came over I'd only
tried no-gap, a four post-it-note thickness, and a three post-it-note
thickness. I definitely like the four post-it-note thickness best.
There was a significant difference between three and four post-it-note
thicknesses. I was really surprised by this.
me too!… I have heard differences, but often wondered if I my mind was playing tricks on me.
When Steve came over to my place we first listened to the four
post-it-note thickness. Then we tried five, and six post-it-note
thicknesses. The six post-it-note thickness was much better than the
five post-it-note thickness.
for lack of a better way to say it, the addition of postit notes is not a linear progression, it follows more of a log taper. I have found that in order to get a nice family of inductance curves, you need to double the gap size to see an appreciable change in the inductance vs. current plots. The math predicts the doubling of the gap will cut the inductance in half and my plots roughly validate this. Seeing an appreciable change between 6 and 7 postit notes at the points you are operating at seems unlikely to me so I will have to think a bit about what you might be hearing.

Next, I took off the top two C-cores from
each transformer and we listened. With this setup it sounded bad, thin
and no juice.
good so at least we found the airgap can be too big. I was just wondering if an aircored trannie might work… thanks for giving it a try.
The last thing we tried was a seven post-it-note
thickness. This sounded definitely worse than the six post-it-note
thickness. A difference of one post-it-note thickness made a big
difference in the sound.
I do not doubt what you heard, but I must say I am really surprised since the difference in the inductance and linearity from 6-7 postit notes is really small. In order to find out what you really were hearing I think we have to look beyond simple inductance and linearity.

The two things that initially come to mind for me are the slope of your crossover and the point where your AC flux becomes greater than your DC Flux.

A somewhat steep crossover would make the point where you don’t have enough inductance show up in a more pronounced manner

Dc Flux (from the DC plate current) is stronger than any banding method to keep the gap tight and secure. As you approach the point where the DC flux is equal to the AC flux you will hear an un-banded core start to chatter. Try pulling a core with just DC apart, then play a large signal 20hz sine and watch things chatter (and listen though a speaker to hear artifacts of this chatter) As you increase the number of postit notes, you decrease the DC flux, so you may have found the point where the average AC flux overtook the DC flux. With sinewaves this is easy, but with music I would expect it to depend on a bunch of factors like music type and listening level.
Next we went over to Steve's and had a nice steak dinner. We
also listened to some music. Steve lent me a digital thickness gauge he
had to measure the thickness of the post-it notes. Each post-it note
was around 0.0035 inches thick, so the six post-it note thickness we
liked best was about 0.020 inches. I looked at the plots you sent me.
With a gap of 0.020 inches the inductance was a flat 30hy across a wide
range of bias currents.
here is the rub… since each postit note enters the magnetic path twice, you need to double the value so you were actually off the graphs with a .040 gap!! Do note my comment about the doubling of the gap to make an appreciable difference from the measurement POV. *IF* i had to guess, i would expect your 6 postit note (.021) physical gap (.042 actual gap) to give about 18 hy's of inductance and be flat out past 100ma... this is part of the beauty... with all of the variables known, we have a much better chance of pinpointing the important ones.
Anyway, it was very interesting to hear the difference in sound
quality created by varying the gap. As you mentioned earlier, it would
probably be better for long-term reliability to use 0.020 inches of
Mylar instead of the paper.
Yup, but postits work well as a sort of universal gap since not everyone has access to a micrometer. Since this whole thing is a work in progress, hopefully as others join in with the experiments we will see a pattern emerge.

Thanks!

dave
Attachments
this is the estimated inductance vs. current for the primary of the OT in question.  Please note that the gap sizes are actual and represent 2X the actual spacer thickness since the gap enters the magnetic path twice in this design.  The "gold Standard" o
this is the estimated inductance vs. current for the primary of the OT in question. Please note that the gap sizes are actual and represent 2X the actual spacer thickness since the gap enters the magnetic path twice in this design. The "gold Standard" o
est. LvsI.jpg (15.83 KiB) Viewed 8045 times
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Re: airgap adustment in the field of a 25:1 ratio OT

Post by Guest »

Hi Dave,

I do not doubt what you heard, but I must say I am really surprised since the difference in the inductance and linearity from 6-7 postit notes is really small. In order to find out what you really were hearing I think we have to look beyond simple inductance and linearity.

The two things that initially come to mind for me are the slope of your crossover and the point where your AC flux becomes greater than your DC Flux.

A somewhat steep crossover would make the point where you don’t have enough inductance show up in a more pronounced manner

Dc Flux (from the DC plate current) is stronger than any banding method to keep the gap tight and secure. As you approach the point where the DC flux is equal to the AC flux you will hear an un-banded core start to chatter. Try pulling a core with just DC apart, then play a large signal 20hz sine and watch things chatter (and listen though a speaker to hear artifacts of this chatter) As you increase the number of postit notes, you decrease the DC flux, so you may have found the point where the average AC flux overtook the DC flux. With sinewaves this is easy, but with music I would expect it to depend on a bunch of factors like music type and listening level.
I thought I would describe the filter section of my amp a little more because it might explain what we are hearing. I use this amp to drive Oris horns from ~230Hz and above. I use another amp, 572 based, to drive Klipschhorns. Originally, I sent the output of the preamp to a Marchand active cross-over. The Marchand split the signal at 230hz. Grant Gassman suggested it might be better to eliminate the path through the Marchand for 230Hz and above by building a high-pass filter between the driver and output stages of my 75tL. I tried it and it was much better.
Here's the filter I finally used; a cap from the plate of the driver tube, rs241, to a 4:1 autoformer Dave build for Steve. I used the 4:1 autoformer because I wanted to drop the voltage at the grid of the 75TL. This created a second order high-pass filter or a constant K filter according to the Radio Designer's Handbook. The Handbook explains at the cross-over point, 230hz, the slope is much steeper than the 12db/octave you would usually get with a second order filter. At lower frequencies the slope is the usual 12db/octave. Because the slope is so steep at the cross-over point, I wonder if it made the sound more sensitive when adjusting the gap of the transformers?

Johnny
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