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Darius Loftin White explained
Darius Loftin White explained
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Old 18th May 2008, 02:47 PM   #11
Sheldon is offline Sheldon  United States
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Default Re: #8 300B hum cancellation...

Quote:
Originally posted by oldeurope
frequency hum caused by the low thermal
inertia.
Practically, I don't know if it matters, but this author discounts thermal inertia as the main cause of hum from AC powered DHT filaments: http://www.geocities.com/dmitrynizh/...c.htm#abstract

Sheldon
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Old 18th May 2008, 04:43 PM   #12
kenpeter is offline kenpeter  United States
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My misunderstanding was that the output cathode follows the
interstage Mu. And that CHB coupled both hum and feedback.

C2 completely bypasses the output cathode back to the top rail.
There is no follower copy of interstage Mu in the presence of the
strong C2 bypass cap, only power line hum as intended...

You give up pig wrestling too easy. I was not wresting against
your efforts. Only with my own inability to describe what was
confusing to me. I never doubted you were right for a second.
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Old 18th May 2008, 04:59 PM   #13
oldeurope is offline oldeurope  Germany
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Default #11

Hi Sheldon,
this is interesting.
But read the last sentences of the conclusion.
Filaments don't have comparable mass. The thinner a filament
is, the lower is the thermal inertia. One inch of a 2A3 cathode
has more mass than one inch of a 6A3 cathode.
There are other points i disagree ...
Since this hum can be cancelled, it is not that important for me
where it comes from. Maybe the truth is between both theories.


Kind regards,
Darius

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Old 18th May 2008, 05:26 PM   #14
oldeurope is offline oldeurope  Germany
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Default follower copy of interstage Mu ???

Quote:
Originally #12 posted by kenpeter

There is no follower copy of interstage Mu in the presence of the
strong C2 bypass cap, only power line hum as intended...

Hi,
what is a follower copy of interstage Mu?



Kind regards,
Darius
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Old 18th May 2008, 07:30 PM   #15
Sheldon is offline Sheldon  United States
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Location: San Diego
Default Re: #11

Quote:
Originally posted by oldeurope
Since this hum can be cancelled, it is not that important for me
where it comes from. Maybe the truth is between both theories
Yes, I agree. In the context of this thread it's not important. I have seen other discussions using higher frequency supplies, on the assumption that filament mass was the dominant issue. If harmonic distortion is a significant contributor, then all that would do is shift the hum frequency higher, and in the case of square waves, generate lots of high order harmonics. If the frequency is high enough, maybe it doesn't matter there either. Or maybe it does, I don't know.

Sheldon
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Old 18th May 2008, 08:44 PM   #16
kenpeter is offline kenpeter  United States
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Default Re: follower copy of interstage Mu ???

Quote:
Originally posted by oldeurope


Hi,
what is a follower copy of interstage Mu?



Kind regards,
Darius
A figment of my imagination.

Don't worry about it.
I think I got it now.
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Old 20th May 2008, 04:01 AM   #17
Sheldon is offline Sheldon  United States
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Darius,

You got me interested in this Loftin-White stuff, so I looked a little on the web. I find lots of schematics called Loftin-White, but none that have the output current loop as shown in the original design, or in your version. But you knew that I guess. Everyone seems to have focussed on direct coupling, but ignored the really cool part of the design - at least to my poorly trained eyes. Surprisingly, even John Broskie of TubeCad has got it wrong.

If I get to this down the road on my 801 amp, I may use a mosfet follower in place of the second triode (no room to add another, the FET's are smaller).

Sheldon
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Old 21st May 2008, 06:52 AM   #18
oldeurope is offline oldeurope  Germany
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Default #17

Hi Sheldon,
thank you very much.
Your reply makes me happy and gives me hope ...

Kind regards,
Darius
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Old 1st June 2008, 03:16 AM   #19
Sheldon is offline Sheldon  United States
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One thread I came across when searching the term Loftin-White, is this one: http://www.vintage-radio.net/forum/s...ad.php?t=20849

Now, it's clear that the one poster did not understand the difference between feedback and what is going on in this circuit. I must confess that I was confused too. Pretty interesting how that happens. I think I'm beginning to see the difference now. Feedback implies that the input signal is compared with the output and the output corrected, based on the difference. I see now that the signal is not compared to the output.

In the Loftin-White, we are looking only keeping the power supply constant. The error signal that is compensated is only this one and has nothing to do with the input signal, except as it cancels ripple in the supply, whether caused by output demands or by residual supply ripple. You are right, it's pretty darn clever.

Here's an amp I intend to convert at some point:
Attached Images
File Type: gif 801ampfinal.gif (6.7 KB, 1484 views)
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Old 1st June 2008, 03:17 AM   #20
Sheldon is offline Sheldon  United States
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Default Loftin White version 1

To this:
Attached Images
File Type: gif 801loftin white fet bootstrap.gif (5.0 KB, 1451 views)
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