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Old 5th January 2013, 11:29 PM   #51
JMFahey is offline JMFahey  Argentina
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Quote:
120VAC= about 160VDC
True enough.
And where are the 120VAC coming from ?
Besides, I'm trying to have the OP answer me, not somebody else.
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Old 6th January 2013, 01:38 AM   #52
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Quote:
Originally Posted by JMFahey View Post
True enough.
And where are the 120VAC coming from ?
Besides, I'm trying to have the OP answer me, not somebody else.
Thanks
That was my question too. You're not making DC off the line. At minimum you'll need a 1:1 isolation transformer...
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Old 6th January 2013, 01:41 AM   #53
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Originally Posted by counter culture View Post
Better with a 12AU7 I think, they work better at low voltages. Something like this?

It'll fit in 4 sq in... showing ground traces... still needs a bit of work. I think the tone controls are the wrong way round (anticlockwise for boost)

Here's the tube stage driven with 10mV

And with 1V... you can improve the symmetry a bit with the trimpots

50 watts into 8 ohms. It'd probably be better not to throw the full 35V at it.
That's a completely different concept. I count 16sq inches, 4 inch square
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Old 6th January 2013, 11:39 AM   #54
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Quote:
Originally Posted by JMFahey View Post
True enough.
And where are the 120VAC coming from ?
Besides, I'm trying to have the OP answer me, not somebody else.
Thanks
Quote:
Originally Posted by doozerdave View Post
That was my question too. You're not making DC off the line. At minimum you'll need a 1:1 isolation transformer...
Ah, I see. I assumed a trafo was implicit. But maybe not.
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Old 6th January 2013, 02:35 PM   #55
JMFahey is offline JMFahey  Argentina
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Hi friends, I *think* I have solved the mystery.
Like most others, I clicked m145 's link provided in post #43 , and landed in DigiKey's page attached here:
there we all clearly read its description:
Description TRANSFRM 120CT FILMNT 12.6V 26VA
where we can "decode" the following:
120 and 12.6V = clearly primary and secondary voltages, huh?
FILMNT = the intended use, huh?
CT= well, that one is easy for old hounddogs like us, it means it has a centertap somewhere, huh?
Being a filament transformer, that's a very common spec.
So the standard deduction would be that it has a center tapped 12.6V filament secondary ... which would be fine to feed "6.3V only" tubes, huh?
Well, everything above is wrong

Very intrigued and somewhat pissed off at the mistery, got into Hammond's own site.
And found that said 262F12 is properly described as:
*115VAC primary
*dual secondary; 120V and 12.6V AC
* NO centertap anywhere
Oh well

So, in a nutshell, M145 can power his preamp with it.

EDIT: just in case it was not clear, I was quite worried that he might try to rectify line voltage to power his preamp.
In the last 6 months I saw it twice in Forums; in both cases OPs said "ah !! but I'll be *careful* "
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Last edited by JMFahey; 6th January 2013 at 02:41 PM.
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Old 7th January 2013, 05:30 AM   #56
m145mcc is offline m145mcc  United States
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Pardon my absents, I have been reading. So I think I have come up with something that should work.. maybe. Tube = 12ax7 HT = 150vdc Rl = 330k and if I remember right I biased at -1.5, I fear I may destroy a good tube with this one, and the clipping may not be symmetrical. Thanks to those who have taken an interest in my thread. Especially the crazy SOB who came up with a real design, which I will be referring too after I read some more. Oh... anybody know of good cad software that supports tubes, the the behavior of the triodes in multisim are very in-accurate.
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Last edited by m145mcc; 7th January 2013 at 05:45 AM.
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Old 7th January 2013, 12:31 PM   #57
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I've been using TinyCAD. It's free, and has some tubes in the library by default. Just be sure to double check the pinout of any parts in free libraries.
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Old 8th January 2013, 11:08 PM   #58
m145mcc is offline m145mcc  United States
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Please disregard my last design (If you can call it that). what I'm posting now should actually work, If somebody could give it a quick look over that would really be great. I have been reading Merlin Blencowe's book on preamp designs I think its helped.
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Old 8th January 2013, 11:55 PM   #59
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That looks a lot more sensible but.... you want some grid stoppers (R2 & R7 in mine). What's the point of 1M after C1 when it's shunted by (in parallel with) 115k? I don't see much point in throwing away the gain at R4/R3, and R10/R11, better to put a pot in there, although the 12AX7 does have a lot of gain. R12 should be 10* R11 minimum anyway. R11||R12 = 2857 ohms, neglecting pot setting and loading. C4 is too big, cuts off @ ~160Hz (3pF cuts off @ 160kHz).

It's not too bad, but it won't work as it stands, it's full of beginner's errors.

Changed mine around a bit...

Click the image to open in full size.

Crazy SOB huh?
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Old 9th January 2013, 12:00 AM   #60
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CC, am curious about your biasing. What voltages show on the cathodes and grids??
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