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Old 21st April 2003, 01:44 AM   #11
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Default RE:RE:ON PAPER.

Hi,

It's been a while since I had so much fun on a Sunday night here...
Must be me...

Quote:
But negative regulators are almost always noisier than positive ones (I can't remember the details, but it's to do with the fact that it's easier to make NPN transistors than PNP, so the differential pair at the input of the op-amp is implemented with NPN, and their common-mode performance suffers).
Correct, and that's why you need to balance both halves for two reasons: imbalance in the heater and imbalance from the regs.
Once done it equals a battery supply in performance.
So, we need to correct two basic sources of imbalance here:
The heater and the regs.

Is all of this worth the effort?

Well, I use this technique in a MC headamp since 1985 and it's been earning its salt there big time...
It's only when I noticed so many people having hum problems with SET amps and even simple preamps that I thought of this little beauty again.

Cheers,
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Old 21st April 2003, 03:00 PM   #12
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Frank,

The active regged split rail heater setup you mentioned for DHTs was also written up by John Camille in one of the early Sound Practices (SP5?) with apparently very good results in a 211 SET. I'm contemplating it for the big momma 813/GM70(soon) PP that's currently singing in my system. I need to source a different powertrans to try it, and I'll probably use LT108x regs as I have some around here somewhere. Currently I'm using a passive LCLC (branch to each channel) LCLC setup, and whilst the noise and hum are fairly low, it's massive with those huge chokes (~3kg ea), so the active version has the potential to be a lot smaller.

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Old 21st April 2003, 04:08 PM   #13
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OK, EC8010 says, and I have read this from others, that DC on the filament of a DH tube (from triodes to pentodes, etc) causes uneven wear on the filament.

My contention is this:

From this picture, would not the schematic on the left show that the voltage of importance, the B voltage, is evenly distributed across the filament, inasmuch as it is in the center of both ends via the resistors? Can 2.5 or 5 volts actually impede current from a 300-500 voltage potential that much to cause the kind of current crowding you and others refer to? Whereas the circuit on the right is obviously going to cause one sided current flow. However, there doesn't seem to be a problem with any of my battery tube radios, let alone the heaters of the AC-DC radio tubes.

Gabe
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Old 21st April 2003, 07:10 PM   #14
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Default Tes, it can.

Remember that we are not talking about the voltage between the anode and cathode, but the voltage between the control grid and cathode. DC filaments mean that one end of the heater/cathode is at a significantly different voltage, so it emits a different number of electrons to the other end.

You can easily demonstrate this to yourself by looking at 300B or 845 curves. Hold anode voltage current constant, but chnage grid voltage by half heater voltage either side of your operating point, and you will see the problem.
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Old 21st April 2003, 07:19 PM   #15
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Default HEATERS.

Brett,

Quote:
The active regged split rail heater setup you mentioned for DHTs was also written up by John Camille in one of the early Sound Practices (SP5?) with apparently very good results in a 211 SET.
Interesting and flattering at the same time.
Little did I know that someone else had tried to tackle this already.

Gabe,

Quote:
OK, EC8010 says, and I have read this from others, that DC on the filament of a DH tube (from triodes to pentodes, etc) causes uneven wear on the filament.
This particular phenomenon is called "notching".
It has been discussed here already on the forum a few months ago.
The term was brought back from memory by co-member D'Haen.

When fed by DC as EC8010 explains you stand a good chance that the cathode isn't heated evenly creating cold spots AKA "islands".
This in turn has an impact on the performance of the tube.

Cheers,
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Old 21st April 2003, 07:23 PM   #16
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Default Even heating.

Hello Frank,

I'm not suggesting that the cathode is heated unevenly (although I would welcome discussion on "notching"), but I am saying that the DC voltage from one part of the cathode to the grid is different to another. AC heating averages these differences at a 50 or 60Hz rate, ensuring even wear.

Of course, these arguments only apply to directly heated valves.
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Old 21st April 2003, 07:39 PM   #17
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Questions:

fdegrove, I remember the comment about notching. What is not clear is how that occurs. Is it a physical phenomenon that occurs due to the grid, inasmuch as the grid suppresses flow of current in certain areas of the filament? If that is so... then it would stand to reason that this would occur with AC as well as DC. The grid doesn't change location with AC does it?

Otherwise I would suspect more even heat distribution with DC versus AC, due to harmonics on the part of AC. Assuming clean DC.

EC8010, Is one end of the filament further away from the grid than the other? Or are you saying that one end of the grid is at 1/2 the DC voltage plus the bias voltage further away from the grid than the other, +/-? In other words, one end of the filament of a 300b is at 52.5 volts, while the other is at 47.5 volts, given a 50 volts bias? (I will have to check that out for myself!)

This is interesting and explains why in my battery tube radios when connected one way the tube doesn't conduct, since the filament voltage brings it into cut off.

But, given the age of my 201a tubes, and the fact that so many others do it, I do not see a problem with using DC on a 300b in the long run. I think it is a phenomenon as real but not as significant as is cathode stripping. It may only affect the tubes after 7-10 years of use! So... what of it?

My humble opinion,
Gabe
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Old 21st April 2003, 07:39 PM   #18
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Default RE:Even heating.

Hi,

Quote:
I'm not suggesting that the cathode is heated unevenly (although I would welcome discussion on "notching"), but I am saying that the DC voltage from one part of the cathode to the grid is different to another.
I wasn't trying to put words into your mouth...I take full responsibility for this claim I made.

A local search on the word "notching' will yield about 4 or 5 threads on it.

The reason I brought this up is that it's not commonly know amongst designers, but important nonetheless.

Too bad that file I intended to send you seems too large but it was discussed in there as well...in Dutch.

Another place holding useful info is the 1957 Sylvania Tube Manual and Tubecad.

Cheers,
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Old 21st April 2003, 07:48 PM   #19
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Gabevee,

yes, I'm saying that one end of your 300B filament is at 47.5V, and the other at 52.5V, causing unequal emission. It's probably gilding the lily, but isn't that what we are about? And 300B's are expensive, so it's our duty to ensure longevity...

Frank,

time for me to learn some more...
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Old 21st April 2003, 08:59 PM   #20
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Default NOTCHING.

Hi Gabe,

Quote:
fdegrove, I remember the comment about notching. What is not clear is how that occurs. Is it a physical phenomenon that occurs due to the grid, inasmuch as the grid suppresses flow of current in certain areas of the filament? If that is so... then it would stand to reason that this would occur with AC as well as DC. The grid doesn't change location with AC does it?
From what I understand of it: somehow particles present in the heater material clogg together at one end either causing a heater to cathode short or, when it gets too heavy it actually breaks it.
This is a phenomenon that is well known in the lightbulb industry and between you and me I wouldn't loose sleep over it for audio use unless you want to run the same tubes for fifty years.
One way to prevent it is using AC for the heaters which, due to its pulsating action tends to clean this up.
Another solution, but no cure all, is to switch polarity of the DC heater occasionally.

What's more important is to heat the cathode evenly so electron emission is not concentrated on a particular end of the cathode.

BTW, the controlgrid doesn't really come into play here.

Quote:
Otherwise I would suspect more even heat distribution with DC versus AC, due to harmonics on the part of AC. Assuming clean DC.
On the contrary, the alternating nature of the AC pulses heat the cathode more evenly, with DC applied you create an imbalance which is why I suggested the split -/+ DC arrangement using a virtual CT.

Other nasty problems can develop when using standby arrangements without really understanding how tubes react to them, IMO more tubes get poisoned by these arrangements than saved by them.

Cathode stripping, cathode poisoning, sleepy cathodes are all terms that come to mind.

Cheers,
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