a 1:1 nickel IT for a 10

Using tertiary windings to offset DC.
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dave slagle
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a 1:1 nickel IT for a 10

Post by dave slagle »

So Jeffrey is one of the people that is going to play with the current cancel 1:1’s His initial intention is to use it to IT couple a 10 as a driver to an output tube. We decided that 350V and 30ma would be the general area where the 10 would operate. This should work well and a goal of 80 hy’s of inductance was arbitrarily set.

I came to the realization that if I use 3 windings doubling the turns for each section, they can be configured to offset a wide range of currents. Operating under the assumption that the 10 will be drawing 1.25A the various combinations of cancel winding configurations will net the following offsets in plate current (in ma)

4.8
9.6
14.4
19.2
24.0
28.8
33.7


This will take a little simple math on your part to interpret these numbers and determine where you are operating on the inductance curve below. What you need to do is subtract the actual current running through the tube from the offset current above to find the net current on the core from the gap selected. For example if the tube were running 30ma and the cancel winding were set at 24ma the net current on the core would be +6 ma and the inductance would be read from that point. Obviously if the two currents are equal, there will be no net DC on the core (and this can be easily confirmed with an unclamped core) Finally if the entire cancel winding were used, the net DC on the core would be –3.7ma and the core would be operating with a negative DC flux on the core. This will essentially place the even order harmonics that arise from a bias on a core on the opposite phase of the signal. What these various configurations sound like will hopefully be reported on in the future.

The choice of the intial gap is an interesting one. At first guess the .0005 gap (butt gap of the lams) looks wonderful with its 100+hy’s of inductance. The problem arises when you look at how narrow the peak is and realize this narrowness suggests two things. The choke is fairly nonlinear, and the margin for error of the current cancellation is also pretty narrow. If I would graph the L vs. I for a 1X1 alternatly stacked version, I would expect an even sharper peak that would near 1000hy’s but even a fraction of a ma of net offset would delier a fairly large reduction. (feel free to try it) The series of plots show how the airgap trades total inductance for linearity and a wide range of permissible offset. My gut feel for a starting point would actually be the .002 gap (pink) even though it has slightly less than the desired 80hy. It will offer a wide window of acceptable operation without running into trouble. Running the 10 at 30ma will allow the offset currents to be easily varied from –3.7 to +10.4 with little change in inductance. When placed at say 6ma small variations in plate current and filament current will not cause major issues.

I am going to send them so the gap can easily be adjusted so there will be a whole range of listening possibilities.
Attachments
general ballpark info for a few gaps.
general ballpark info for a few gaps.
10 IT cancel.gif (11.42 KiB) Viewed 29091 times
DowdyLama
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very cool!

Post by DowdyLama »

Please give us a report, Jeffrey
JeffreyJ

Funny man Dowdy...

Post by JeffreyJ »

Hello Jim, Dave...

this looks killer...

I have been wanting to give IT another shot for a while now... I looked around for a good bifilar 1:1 for a type 10 and couldn't find one.. didn't exist... so I emailed Slagle... well, as we were already saying DHT driver, which implied a nice DC supply, he started spinning these ideas... which I ate up.. and have committed to... can't wait...

I think we will learn a lot... I especially wonder what offsetting currents to either side the same amount will sound like..

so.. I am thinking (and things never end up as we think) that those Cu plate GM70's would be a good output for this... maybe direct couple the input stage to the 10 which will be IT to the gm70... since we aren't stacking supplies to direct couple the driver and output, then I may be able to get away with one HV supply.. wouldn't that be nice.... and the stacked input/driver would just about line up with the B+ for the GM70...

Peace,
Me
JeffreyJ
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couldn't resist...

Post by JeffreyJ »

Dave spoke:

"If I would graph the L vs. I for a 1X1 alternatly stacked version, I would expect an even sharper peak that would near 1000hy’s but even a fraction of a ma of net offset would delier a fairly large reduction. (feel free to try it)"


this seems the perfect IT for a low level stage when the plate impedances rise and the currents are low... and most importantly, no grid current of the next stage to mess up the balance...

Dowdy, your phono? my 6299's?

Peace,
Me
dave slagle
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Re: Funny man Dowdy...

Post by dave slagle »

JeffreyJ wrote: so.. I am thinking (and things never end up as we think)
i like your thinking... but remember, staked supplies suggest a large DC voltage difference between primary and secondary. This may not make the bifilar transformer happy. Do note that given enough resources it isn;t an issue, but for now can we keep the difference between primary and sec as low as possible. I would hate to have a good idea shelved because the wire was too expensive... and i would also hate to buy really expensive wire for an idea that sucks. So lets keep it at a fe hundred volts and see if the need for special wire exists. If the whole concept shows promise, i have no problem pushing things to find the failure rate, but it seems pointless to but the cart before the horse.

dave
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Re: couldn't resist...

Post by dave slagle »

JeffreyJ wrote: this seems the perfect IT for a low level stage when the plate impedances rise and the currents are low... and most importantly, no grid current of the next stage to mess up the balance...
i was referring more to its use in a plug and play system for the masses. the 2A3 It for jim is just that... any 2A3 you plug in will be fine, no need to worry about tube to tube drift or filament and plate current variations.

my suggestion of the gap for your 10 had the same thought in mind. Not having to worry if your 10 draws 1.2 or 1.3A of filament current is a big comfort to me. in the early stages with DHT's they just fit.

i will agree as current goes down the window of variation of the tube goes down, so maybe we just need to zoom in? (just like zooming in on the 12AX7 curves gives you parallel lines from .1-1ma on the 0-1ma scale)

dave
buzz
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Post by buzz »

Jeffery directed me over here from the Yahoo Boatanchors Group, and this thread caught my eye as these are two fantastic tubes.

A few years later... have these IT become readily available? What was the final thought of a 10 driving GM70?

Dave do you have time to wind a pair for me? ;)
dave slagle
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Post by dave slagle »

Hey.

For the most part everything I do is custom so these aren't a problem.

The should work nicely for the 10, since you need DC filaments anyways, Just set up a CCS for 1.25A and tell me the plate current you want to run and I'll size the offset winding appropriately.

dave
JeffreyJ
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Post by JeffreyJ »

hey fellas..

we were talking about a bifilar 1:1 for a 46 driving a GM70... not sure which I would like better in this application... the GM70 has pretty good tone... so then I guess I would just choose the one I think just makes more music.. and, to me, that is the 46...

but I know more than a few people that would vote otherwise... heck, half of salt lake city is still in love with 10's...

fwiw, I bet I have purchased maybe a dozen custom nickel bifilars from dave since this thread started.. all have a good home.. none have been displaced...

and the really cool thing about these is that you can regap them for different inductances and currents... and still get that couple hundred kilohertz bandwidth...

Peace,
Me
dave slagle
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Post by dave slagle »

I'll second my preference for the 46. Those things are still my favorite driver. One other nice thing about the 46 is the Rp is lower and you can get away without using the offset winding.


dave
buzz
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Post by buzz »

Well I'm not gonna argue :) I know the 10 sounds great as a final but we are talking drivers here.

So my thinking is this... 46 'triode' > Daves IT > GM70 -40v fixed bias @150ma, grounded cathode. B+ is 630V@200ma for one channel.

If I can get enough swing off the 46 for grid current that would be nice as that is one reason for using an IT with low DC resistance on the GM70 grid.

One nice thing about using an 801A (or even the 30mu 841?) for the driver instead of the 46 is we wouldn't have to worry about 630v on the driver plate. :x
buzz
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Post by buzz »

I have theoretical schematic using the 801A. Let me know if the bias arrangement is acceptable for use with the Slagle Current Cancellation technique. I lowered voltage upon the primary IT windings as well. If I were to use the 46 I would need an additional filament supply and bias arrangement. Gain would also be lower.

Image
dave slagle
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Post by dave slagle »

Hey I need to look back at some old spice sims. I looked at the use of filament bias as the offset current a while back and you get some feedback. I think the value is pretty small since you have a roughly 40:1 ratio on the offset winding so the 4000 ohm Rp will appear as 2.5 ohms in series with the 30 ohm bias resistor.

The thing I cannot remember is if this is positive or negative feedback. If it is positive I think the whole thing becomes unstable.

i'll look and post back later.

Oh yeah, the 350V makes me much happier!

dave
buzz
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Post by buzz »

I rearranged the schematic so that there would be less reflected signal to the driver cathode. To make the cancellation winding DC resistance less critical, the value of the drivers' cathode resistor will be adjustable to attain the value of 10R minus the DCR of your cancellation winding, whatever that ends up being.

I thought that the lower primary voltage may make you happy, but note that I increased current to compensate. It is now ~30ma. :twisted:
DowdyLama
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Post by DowdyLama »

Guys:

I don't know off the top of my head exactly what a Hammond 715 is, but I'm guessing that you're exceeding the 83's voltage ratings.
Is a mu of 8 (or 5.6 with the 46) adequate? Buzz, don't tell me you're falling for Jeffrey's "we don't need no stinking gain" mantra :wink:

Jim
buzz
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Post by buzz »

Thanks Jim,

Yes I am worried about the gain as well. I bet Dave is planning on winding iron for my linestage that I will need.

Ha, now everybody can't be using those 826 tubes as drivers :wink: Although I have a couple in my stash, won't you be offended if I build an 826 / GM70 and drive up the cost of 826 tubes ? :P

The peak inverse voltage is right at max for the 83 choke input. Because of the low impedance Triad chokes used, rectification behavior is pseudo cap input. The Hammond '715' is 510-0-510v 400ma. If I put just 0.01uF before the first choke the peak drops to less than 900V.
DowdyLama
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Post by DowdyLama »

Buzz:

Aside from asking that you not produce it commercially, you may feel free to use the 826. It's an excellent driver, but be forewarned that its filament draws a LOT of current!

The 3C24 sounds virtually identical to the 826, and will save you about 20% on filament current. Of course, the mu is a bit less [24 vs 31].
Please note that I offer the 3C24 as my 'standard' front-end tube for customer's GM70 amps...back when I had some customers, I mean :D
The 826 is offered almost entirely for the purpose of avoiding plate caps.

841 is another viable option...I don't think it has *quite* the magic of
the 826/3C24, but it's pretty darn close and a heck of a lot easier to
use!

Jim
JeffreyJ
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Post by JeffreyJ »

hey guys....

I just feel obligated to say that although I like those tubes, a 1:1 and any A2 is pretty much out of the question...

maybe three stage... 826 > 46 > gm70....

but, as Jim says, with my horns, a 46 > GM70 with a 71A linestage is plenty of gain.. and, yes, I realize that is unusual.. :^O

I think we all want that mythical mu 18, Rp 4k DHT that will take 500Vp... and, for some reason, I am not sure the EML20 is always the answer... though it sometimes is...

have any of you guys ever used a metal plate 211 as a driver?

Me
DowdyLama
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Post by DowdyLama »

In my estimation, A2 is out of the question here.
Heck, if you want A2, I'd say you're better off to start from scratch with a tube designed for A2.
You know, the 826 would make a dandy A2 output device with no dangerous plate caps to worry about.

Re: metal-plate 211s: yep, I've thought about it, but those things are pretty pricey...metal-plate 203As are a more affordable (and therefore more likely) possibility in my book. But school keeps me from doing much more than just idly pondering such things!

Jim
JeffreyJ
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Post by JeffreyJ »

hey Jimmy...

the project goals developed when you were likely buried in the school books..

the sound of A1... so biased in A1...
voltage *well* below 1kV...
need a little power, so good behavior into A2...
sort of like randy's 'il monstro...

and I think you and I can both agree that straight A2 sounds quite a bit different than an A1 biased GM70... and I also bet we both agree on which one our ears like... :^)

and that is why I was trying to stay with valves that we could at least IT couple.. especially considering the sound of Dave's nickel bifilar 1:1's...

Me
DowdyLama
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Post by DowdyLama »

I should've known that I was missing part of the story...maybe one day I'll have a life again :cry:

In any case, two-stage A2 is pretty difficult to pull off in my estimation...and while I'm being a cynic: won't a bifilar have problems with grid current?

Jim
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