Re: NFB loop, R value, multi-tap OT

On Sun, 24 May 2009 09:54:49 -0400, "Phil S." <psymonds@xxxxxxxxxxx>
wrote:


"jh" <jh@xxxxxx> wrote in message
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Phil S. schrieb:
"Stephen Cowell" <scowell@xxxxxxxxxxxxx> wrote in message
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"Phil S." <psymonds@xxxxxxxxxxx> wrote in message
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What is the NFB resistor relationship to the selected tap on the output
tranny?

I have concluded that my 'wreck xpress variant howls on the Presence
control because it is out of phase, in other words, postive feedback.
(No, haven't put the 'scope to work yet.)

The OT is bi-filar wound, two separate windings, one 8 ohm, and one 4
ohm and they are wound in opposite directions as far as I can tell.
I'm thinking the 8 ohm just runs the wrong way and I should move the
connection to the 4 ohm winding.

The layout: Presence pot 5KB > 100K resistor > 8 ohm jack.

In other words, should I double or half the 100K if I'm running it to
the 4 ohm jack, or does this even matter?

Thanks, and sorry for the amp related content.
Shouldn't matter, very much... NFB is voltage,
output transformer is current. Try it and see, anyway.
__
Steve
.


Sure makes it easy. Thanks.

I've decided that I'll give this amp one last round of attempts to
address this problem and the hiss.

Interestingly, I read the other day (hiss related) that the 6-7-8 section
of the 12AX7 is the "A" section and there is supposed to be some noise
reduction benefit to putting the A section in the circuit first. I am
pretty sure that I've got the "B" section first and I'm going to rewire
it, as I've got nothing to lose there. I know, Steve, you say it's the
tube and I couldn't agree more, but there must be something to do to
interact with the tube on this item.


Hi Phil,
i never heard of a superior "A" section in a 12AX7. I've got problems to
understand why one of two identical systems in a glass envelope should b
better than the other.


This was offered as a possible explanation:
"Standard practice once was to ground one side of the heater, so if you use
a standard layout,
the triode on the grounded side would be measurably quieter. It's the same
with a series supply,
the ground end of the string had the valves most prone to noise. Now
measurably quieter does not mean you'll notice, may be for a record pre.
where the noise floor can be a real issue."

Which suggests that re-wireing the tube will do nothing, since it's the
standard raised twisted pair terminating with a pair of 100z.. Nothing much
to lose, either.


Please try it and report back. BTW what type of resistors are those in
your circuit?

Yup, carbon film...changing to metal film is on my list, but I didn't
mention it before. I was hoping to leave the CF in there, as the TW crowd
seems to like them that way. LOL, form over function?


just my 2cts

Jochen


I've also got a bunch of other crap, and I do mean crap, from the TW
forum to try with grounding and plate load resistors. One yo-yo suggests
that he got hiss reduction when using a 56K+47K in series instead of the
single 100K. Sounds like voodoo to me, but cheap enough to try.

It will then go on the auction block, as it is finally time to thin the
herd. While unique in it's tone, it's not my cup o' tea and I'm guessing
this one will bring at least $200 on ebait, especially if I post a notice
at the ampgarage. I made a miniumal head cab (just the outer 4 walls)
from poplar, with dovetail joints and it looks nice. I think this will
give it a boost. Too bad I don't know someone who could give me some
decent sound clips from it. I'll consider whether my limited playing
ability is adequate for a sample that will help or hurt a sale.

I've learned, I'm not a high gain type of guy. I'll be happy to let it
go to a good home.

I heard that you use cf only in the signal chain and mf everywhere
else..and SC can lay out how to use the scope to find noise..
.

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