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PP and SE OTs, defining the "Gap"?

12/18/1997 8:06 AM
Whit	PP and SE OTs, defining the "Gap"? G'Mornin, While scouring the net I came across this schematic http://www.infomaniak.ch/~bonavolt/6550.htm#6550 Single-Ended (Don't go there with a modem, it's a huge page with 8 schematics, but if you go, just press the Stop button when this schematic comes into view) It appears to be a 6550 running class A SE, but the OT looks like a conventional PP type, with the primary CT hooked to the screen, like an ultralinear hookup. Is this what's actually going on? I've read reference to modifying PP OTs, adding a mysterious "gap" (mysterious to me at least) for SE use. I've been unable to find info on the Tango transformer used here. What I guess I'm getting at is this: Does it sound feasible to consider transforming the old 6L6 PP amp I have into a 6550 SE circuit? Or is my ignorance of this Tango OT steering me off on a silly tangent? And what about this "gap" thing? As always, any wisdom thrown at this question is humbly appreciated... Happy Holidays... ... Whit

12/18/1997 8:06 AM

Whit

PP and SE OTs, defining the "Gap"?

G'Mornin,

While scouring the net I came across this
schematic

http://www.infomaniak.ch/~bonavolt/6550.htm#6550 Single-Ended

(Don't go there with a modem, it's a huge
page with 8 schematics, but if you go,
just press the Stop button when this
schematic comes into view)

It appears to be a 6550 running class A
SE, but the OT looks like a conventional
PP type, with the primary CT hooked to
the screen, like an ultralinear hookup.

Is this what's actually going on? I've
read reference to modifying PP OTs, adding
a mysterious "gap" (mysterious to me at
least) for SE use. I've been unable to
find info on the Tango transformer used
here.

What I guess I'm getting at is this: Does
it sound feasible to consider transforming
the old 6L6 PP amp I have into a 6550 SE
circuit? Or is my ignorance of this Tango
OT steering me off on a silly tangent?

And what about this "gap" thing?

As always, any wisdom thrown at this
question is humbly appreciated...

Happy Holidays...

... Whit

12/18/1997 8:47 AM
J Epstein	Most transformers are wound on coil bobbins which are then inserted into a "core" made of laminated special iron or steel (or nickel, etc.) On most transformers the goal is to maximize the inductance, the most effective way to do this is with a core that is continuous, so the magnetic flux travels in continuous lines, entirely through the steel (or whatever.) These cores are built by placing an "E" shaped lamination through the coil, and then taking an I shaped lamination and laying it across the "arms" of the "E" : EI. Then another layer is inserted the opposite way (I backwards E) and the core is built of many laminations like this. In a single-ended ouput transformer, there is DC current flow through the transformer primary (this current is cancelled in a pushpull circuit) and the optimum configuration is NOT a continuous path through the iron : an air gap is needed. I'll need some heavier weight help here to explain this, meanwhile take it for granted. These are built on a core that has all its "E" pieces inserted through the core. then a spacer (there's not really air in an air gap, but something with less magnetic permeability than the iron which affords a consistent spacing and performs a similar magnetic function) is laid across the arms of the E's, then the "I" pieces are placed there too. A retainer is wrapped around the whole thing to hold it together because it can't just be bolted through like the alternate-leaved trannys. People have disassembled all the interleaved laminations of a pushpull tranny and rebuilt them as single ended trannys, perhaps we'll hear from some of them as I haven't. There's an series of articles about doing all this (and more!) in the VALVE newsletter, email Bottlehead@prodigy.com for more info on obtaining VALVE. -j

12/18/1997 8:47 AM

J Epstein

Most transformers are wound on coil bobbins which are then inserted into a "core" made of laminated special iron or steel (or nickel, etc.)

On most transformers the goal is to maximize the inductance, the most effective way to do this is with a core that is continuous, so the magnetic flux travels in continuous lines, entirely through the steel (or whatever.)

These cores are built by placing an "E" shaped lamination through the coil, and then taking an I shaped lamination and laying it across the "arms" of the "E" : EI. Then another layer is inserted the opposite way (I backwards E) and the core is built of many laminations like this.

In a single-ended ouput transformer, there is DC current flow through the transformer primary (this current is cancelled in a pushpull circuit) and the optimum configuration is NOT a continuous path through the iron : an air gap is needed. I'll need some heavier weight help here to explain this, meanwhile take it for granted.

These are built on a core that has all its "E" pieces inserted through the core. then a spacer (there's not really air in an air gap, but something with less magnetic permeability than the iron which affords a consistent spacing and performs a similar magnetic function) is laid across the arms of the E's, then the "I" pieces are placed there too. A retainer is wrapped around the whole thing to hold it together because it can't just be bolted through like the alternate-leaved trannys.

People have disassembled all the interleaved laminations of a pushpull tranny and rebuilt them as single ended trannys, perhaps we'll hear from some of them as I haven't. There's an series of articles about doing all this (and more!) in the VALVE newsletter, email Bottlehead@prodigy.com for more info on obtaining VALVE.
-j

12/18/1997 9:00 AM
R.G.	Damn. Can't press the send button fast enough.

12/18/1997 12:51 PM
J Epstein	You face the handicap of thinking before you post : not me! -j

12/18/1997 1:34 PM
R.G..	Then how come we got the same answer???? Huh?? I think (ack! that again!) that I have a handicap.

12/18/1997 8:58 AM
anonymous	> And what about this "gap" thing? The "gap" in single ended transformers is just that - a space where there is no iron. The way this is done with E-I cores is to stack all the e's together and insert them into the bobbin in one chunk, then put a paper or fiber spacer across the end of the "E"'s, and then the I's are laid on in a chunk. The point of this is that there is now a gap where there is no iron bridging it that makes the magnetic field jump across. This linearizes the magnetic properties of the structure as a whole, as the properties of the gap are so different from iron that they dominate, and the gap cannot saturate like the iron can. The use of PP OPTs for SE is cautionary. The problem is that you must run the bias current through the primary as DC, and this offsets the core magnetically. A transformer which uses interleaved laminations (almost all PP OPT's) has a much higher primary inductance, but is easy to offset and make the iron saturate. Introducing a gap makes the primary inductance lower, but makes the transformer much more resistant to saturation, good for SE OPT's. Using an interleaved PP OPT in SE use necessarily limits the amount of DC bias that can be used without saturation and the corresponding distortion. It works if you keep the DC bias current low enough to not saturate the core, but that limits the amount of power you can use. So in general it probably works if you use a BIG PP OPT for a much smaller SE amp. As a tube hacker, I can understand the urge to just hook up whatever you have that might be a good output, but as a former transformer designer, there are going to be cases where it will not work or won't work well. >Is this what's actually going on? I've read >reference to modifying PP OTs, adding a >mysterious "gap" (mysterious to me at least) for >SE use. I don't know if that really is what is going on, but the process of adding a gap means carefully unstacking an interleaved PP transformer, restacking the E's and I's into non-interleaved chunks and reinserting the bobbin with a spacer, as I noted above. This gets you a transformer that is better, maybe good enough, that won't saturate as easily. It's probably not what you would originally have designed for the circuit, as the primary inductance is now lower, but it might work. And it REALLY appeals to the home-handyman tube hacker crafstman instinct. Now back to the first questions: >It appears to be a 6550 running class A SE, but >the OT looks like a conventional PP type, with >the primary CT hooked to the screen, like an >ultralinear hookup. The most common UL tap is at 42% of the primary, usably close to the 50% of a CT. Some people have just used CT's for SE ultralinear, with mixed results. and >Does it sound feasible to consider transforming >the old 6L6 PP amp I have into a 6550 SE circuit? It's cheaper than getting one originally designed for that use, but you do have to consider it as experimental - it may or may not return good results, and you may have irretrievably damaged a working 6L6 PP OPT, which may or may not be a tragedy, depending which transformer it was and whether you paid real dollars for it and how many.

12/18/1997 8:58 AM

anonymous

> And what about this "gap" thing?
The "gap" in single ended transformers is just that - a space where there is no iron. The way this is done with E-I cores is to stack all the e's together and insert them into the bobbin in one chunk, then put a paper or fiber spacer across the end of the "E"'s, and then the I's are laid on in a chunk. The point of this is that there is now a gap where there is no iron bridging it that makes the magnetic field jump across. This linearizes the magnetic properties of the structure as a whole, as the properties of the gap are so different from iron that they dominate, and the gap cannot saturate like the iron can.

The use of PP OPTs for SE is cautionary. The problem is that you must run the bias current through the primary as DC, and this offsets the core magnetically. A transformer which uses interleaved laminations (almost all PP OPT's) has a much higher primary inductance, but is easy to offset and make the iron saturate. Introducing a gap makes the primary inductance lower, but makes the transformer much more resistant to saturation, good for SE OPT's.

Using an interleaved PP OPT in SE use necessarily limits the amount of DC bias that can be used without saturation and the corresponding distortion. It works if you keep the DC bias current low enough to not saturate the core, but that limits the amount of power you can use. So in general it probably works if you use a BIG PP OPT for a much smaller SE amp.

As a tube hacker, I can understand the urge to just hook up whatever you have that might be a good output, but as a former transformer designer, there are going to be cases where it will not work or won't work well.

>Is this what's actually going on? I've read >reference to modifying PP OTs, adding a >mysterious "gap" (mysterious to me at least) for >SE use.
I don't know if that really is what is going on, but the process of adding a gap means carefully
unstacking an interleaved PP transformer, restacking the E's and I's into non-interleaved chunks and reinserting the bobbin with a spacer, as I noted above. This gets you a transformer that is better, maybe good enough, that won't saturate as easily. It's probably not what you would originally have designed for the circuit, as the primary inductance is now lower, but it might work. And it REALLY appeals to the home-handyman tube hacker crafstman instinct.

Now back to the first questions:
>It appears to be a 6550 running class A SE, but >the OT looks like a conventional PP type, with >the primary CT hooked to the screen, like an >ultralinear hookup.
The most common UL tap is at 42% of the primary, usably close to the 50% of a CT. Some people have just used CT's for SE ultralinear, with mixed results.

and
>Does it sound feasible to consider transforming
>the old 6L6 PP amp I have into a 6550 SE circuit?
It's cheaper than getting one originally designed for that use, but you do have to consider it as experimental - it may or may not return good results, and you may have irretrievably damaged a working 6L6 PP OPT, which may or may not be a tragedy, depending which transformer it was and whether you paid real dollars for it and how many.

12/18/1997 8:59 AM
R.G.	> And what about this "gap" thing? The "gap" in single ended transformers is just that - a space where there is no iron. The way this is done with E-I cores is to stack all the e's together and insert them into the bobbin in one chunk, then put a paper or fiber spacer across the end of the "E"'s, and then the I's are laid on in a chunk. The point of this is that there is now a gap where there is no iron bridging it that makes the magnetic field jump across. This linearizes the magnetic properties of the structure as a whole, as the properties of the gap are so different from iron that they dominate, and the gap cannot saturate like the iron can. The use of PP OPTs for SE is cautionary. The problem is that you must run the bias current through the primary as DC, and this offsets the core magnetically. A transformer which uses interleaved laminations (almost all PP OPT's) has a much higher primary inductance, but is easy to offset and make the iron saturate. Introducing a gap makes the primary inductance lower, but makes the transformer much more resistant to saturation, good for SE OPT's. Using an interleaved PP OPT in SE use necessarily limits the amount of DC bias that can be used without saturation and the corresponding distortion. It works if you keep the DC bias current low enough to not saturate the core, but that limits the amount of power you can use. So in general it probably works if you use a BIG PP OPT for a much smaller SE amp. As a tube hacker, I can understand the urge to just hook up whatever you have that might be a good output, but as a former transformer designer, there are going to be cases where it will not work or won't work well. >Is this what's actually going on? I've read >reference to modifying PP OTs, adding a >mysterious "gap" (mysterious to me at least) for >SE use. I don't know if that really is what is going on, but the process of adding a gap means carefully unstacking an interleaved PP transformer, restacking the E's and I's into non-interleaved chunks and reinserting the bobbin with a spacer, as I noted above. This gets you a transformer that is better, maybe good enough, that won't saturate as easily. It's probably not what you would originally have designed for the circuit, as the primary inductance is now lower, but it might work. And it REALLY appeals to the home-handyman tube hacker crafstman instinct. Now back to the first questions: >It appears to be a 6550 running class A SE, but >the OT looks like a conventional PP type, with >the primary CT hooked to the screen, like an >ultralinear hookup. The most common UL tap is at 42% of the primary, usably close to the 50% of a CT. Some people have just used CT's for SE ultralinear, with mixed results. and >Does it sound feasible to consider transforming >the old 6L6 PP amp I have into a 6550 SE circuit? It's cheaper than getting one originally designed for that use, but you do have to consider it as experimental - it may or may not return good results, and you may have irretrievably damaged a working 6L6 PP OPT, which may or may not be a tragedy, depending which transformer it was and whether you paid real dollars for it and how many.

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