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Old 22nd January 2009, 04:07 AM   #31
puginfo is offline puginfo  Malaysia
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Discussion of non-isolated power supplies is prohibited on this forum and will result in posts being removed or the thread being closed.

> oh, hang on! im not promoting or suggesting it here. just want to know why is it dangerous, thats all.



In most of the world one side of the mains or the center of the mains is connected to an earth ground. That makes one or both sides of the mains HOT (120 volts +) with respect to ground. Omit the isolation transformer and you DIRECTLY CONNECT this hot circuit to the ground circuit of your amp.

> now thats so much clearer now to me. yes, it makes all the sense NOT to go that way. cheers pal.
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Old 5th April 2009, 05:29 PM   #32
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Quote:
Originally posted by Jeb-D.


Futterman spinoffs are a lot less prone to biasing drift causing DC offset on the output when you leave the center tap of the power transformer floating.
Jeb-D are u saying the CT should float respect to ground? Can u further explain your point here if u have a moment or two. I am currently rebuilding a futterman h-3 which was converted before i got it to use 6LF6's and I am interested in what u are talking about here.

Thx!!!!
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Old 5th April 2009, 06:20 PM   #33
pjanda1 is offline pjanda1  United States
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I've been reading a tiny bit about OTL's lately. It seems like a difficult and complicated way to make a cheap guitar amp. Why not buy an old tube organ or PA amp? Some looking should net you something that will donate all your iron, chassis, sockets, pots and most of a useable circuit for less than $50. If you look less hard, you can find plenty for under $100. Besides that, if you are playing big enough halls to need that kind of power, you need something highly portable, safe and darn reliable. One gig should buy new Edcor iron. Dragging around a pile of isolation transformers and tubes that works sporadically will not win you more gigs.

If you don't want to go the PA/organ amp route, this guy probably has an OPT that will work for $5-$20. www.oldradioparts.com

pj
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Old 5th April 2009, 07:06 PM   #34
Jeb-D. is offline Jeb-D.  United States
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Quote:
Jeb-D are u saying the CT should float respect to ground? Can u further explain your point here if u have a moment or two. I am currently rebuilding a futterman h-3 which was converted before i got it to use 6LF6's and I am interested in what u are talking about here.
Yes, assuming bi-polar power supply.

The transformers CT should be left disconnected for direct coupled operation. If the transformers CT is connected, a current imbalance between the tubes will cause DC offset on the output.

If the transformers CT is NOT connected, an imbalance between the tubes current will not cause DC on the output, but simply cause your +/- rails to become mis-matched(asymmetric ex. +150V & -130V).

I'm not too familiar with the H-3 design off hand. Not sure if it is a direct coupled or cap coupled design.
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Old 5th April 2009, 07:21 PM   #35
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Quote:
Originally posted by Jeb-D.


Yes, assuming bi-polar power supply.

I'm not too familiar with the H-3 design off hand. Not sure if it is a direct coupled or cap coupled design.

The H-3 is cap coupled and does not have assymetrical bipolar supplies. Top is higher voltage then the bottom. Thx
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Old 6th April 2009, 11:50 PM   #36
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Quote:
Originally posted by Jeb-D.


Yes, assuming bi-polar power supply.

The transformers CT should be left disconnected for direct coupled operation. If the transformers CT is connected, a current imbalance between the tubes will cause DC offset on the output.

If the transformers CT is NOT connected, an imbalance between the tubes current will not cause DC on the output, but simply cause your +/- rails to become mis-matched(asymmetric ex. +150V & -130V).

I'm not too familiar with the H-3 design off hand. Not sure if it is a direct coupled or cap coupled design.
Hi,

Absolutely correct.
Ideally you don't want a DC offset at the output and you'd still want relatively matched supply rails as well.
A well executed powertransformer is therefore important.

Cheers,
__________________
Frank
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Old 7th April 2009, 12:10 AM   #37
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Quote:
Originally posted by fdegrove


Hi,

Absolutely correct.
Ideally you don't want a DC offset at the output and you'd still want relatively matched supply rails as well.
A well executed powertransformer is therefore important.

Cheers,

Thx I was wondering if you might be interested in looking at the following thread I have going and my last post on this page.

Futterman H-3

in general I have concluded that the measurements are not to out of line since i wrote that post but i am still a bit concerned why In the end the adjustment pots for the Upper Screens as well as the Lower Screens do nothing when I turn them. They reach a voltage at power up as stated in that post and hold there. if i play with grid voltage i can get it to change in general. Does anyone here have any thoughts. it just seem odd that there would be adjustment posts for something that does not change anyway.

thx
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Old 10th April 2009, 02:58 PM   #38
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Quote:
Originally posted by fdegrove


Hi,

Absolutely correct.
Ideally you don't want a DC offset at the output and you'd still want relatively matched supply rails as well.
A well executed powertransformer is therefore important.

Cheers,

Frank what do you think about this mod for the Futt or the NYAL OTL 3?

The output cap network is not shown so the pic is clear obviously that would go back in at actual build time. The lower grids have their own power supply off of the PT so I would split that at the resistor before the control pot in that supply and then just double every going out from there as you would do say in any push pull amp which had a single supply for the output tubes.

On the upper grids it is shown clear what I believe will work.

The reason for this is that it is so difficult to get matched output tubes these days for the 6FW5's, 6BH5's & the 6LF6's that this should allow for separate adjustments for each unless I am missing something.

Any comments?

Thanks!!
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Old 10th April 2009, 09:26 PM   #39
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If you want a reliable OTL for guitar amp use, consider that it has to deal with overdrive on a continuous basis. The 'totem pole' OTLs might object to that treatment with oscillation that can fry a driver and/or power tube.

That's why, at least for this application, the Circlotron output is preferred. You can run it with zero feedback, so that amp can be quite stable, and if you have several drivers in series, you won't need an output coupling cap, DC Offset control, or even a bias control if you aren't worried about class A operation.

The trick is really the load. with 4 6AS7s and 4 8-ohm drivers in series, or 2 16-ohm drivers in series, you will be easily able to make 50 watts. The plate voltage will have to be around 170V or slightly more, and the the bias about 30ma per section at idle. Under such conditions if a tube arcs the speakers will be perfectly safe.

Some years ago I did a buy of power transformers that will suit nicely for the B+ and filament circuit. Its a torroid and runs slightly warm after hours of operation. If anyone is interested contact me- they are $80.00 each 'till gone. I think I have about 10 or 11 left. You will still need a driver power supply, but only B+.

There are a variety of ways to do the driver. Obviously you want symmetrical drive and you can do that a number of ways quite easily. The thing that has intrigued me recently is this: the amp sounds very good, has the cleanest clean tone in town. I think it could be a bit more aggressive in overdrive- right now it sounds sort of like a Fender with bandwidth, only smoother. I was thinking that an interstage coupling transformer would be fun- driven by a single 6SN7 section. You might be able to build the power amp section with the 4 power tubes and one 6SN7!

The nice thing is that if you do this right, the resulting combo is light, and can keep up with a Fender Twin easily, so long as you are careful about the drivers.
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Old 10th April 2009, 09:28 PM   #40
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I forgot one thing about the transformer, for those who are interested. It is rated for 50/60 Hz, and has a dual primary.
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