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Old 25th December 2008, 08:23 PM   #1
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Default 6UA4-GTA Question

Hi,

I am using this tv toob for slow start-up after SS rectification. I see on the RCA data sheet that the max. DC Plate Current is 210 ma but the Sylvania data sheet lists it at 190 ma.

Just curious, has anyone ever attempted to measure these values? Can any sort of tester out there do this?

My thinking by using this tube in my PS I am also building in a current limiter which could prevent accidental over-biasing... not that I would rely on it entirely, but just for additional safety it is nice.

In any case the sound is very natural - still the solid bass response I experience with SS rectification - and the start-up is nice and slow....
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Old 25th December 2008, 11:09 PM   #2
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The GE data sheet show max. continuous DC as 190 mA., design center. Design center ratings are EXTREMELEY conservative and the 210 mA. rating is the 10% more that is associated with a design maximum rating. The 210 mA. number is quite safe to rely on. AAMOF, an occasional pulse to a greater current is OK.
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Old 25th December 2008, 11:32 PM   #3
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Default 6AU4GTA (I presume)

The maximum average current rating of a tube rectifier can't really be measured per se, since you can often go well above it without immediate damage. It is really a rating for maximum long-term reliability. What is more important here is the peak current rating. If you are using large filter caps right after the rectifiers, you may easily exceed the peak ratings, especially during initial turn-on. The symptom is arcing inside the tube. The series tube rectifier works best with modest capacitance (100uF or less) or choke input filters.

- John Atwood
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Old 26th December 2008, 01:58 PM   #4
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Thanks John and Eli.

I am using this sort of set-up:

SS bridge -> 6AU4-GTA -> C1 -> Fuse -> L -> C2

The rca spec sheet lists peak plate current at max. 1300 ma and the caps are actually 200uf. The choke is 8h. I have considered using a smaller C1 value though.

I have observed no arcing in the 6AU4-GTA and it has been over a month. Just to be sure though, I built in a T500ma fuse (it hasn't blown either). The amp is grid biased the amp to (about) 185 ma.
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Old 26th December 2008, 05:54 PM   #5
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Default 6AU4-GTA

Reducing your C1 capacitance will make life easier for the 6AU4GTA and not affect the amp much (except for slightly lower B+ voltage). C2 is your reservoir capacitor that supplies current surges to the amplifier.

I have been using 19AU4GT rectifiers in my main amps (push-pull 300Bs) for several years without any problems. I am using a choke-input filter, though.

- John
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Old 27th December 2008, 12:33 AM   #6
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Mr. Atwood's remarks are, of course, correct. However, there is an alternative to holding the value of the 1st filter cap. down, that will protect the damper diode. Use the arrangement that follows.

SS bridge -> C1 -> 6AU4-GTA -> L -> C2

A large cap. can now be used safely as C1, which suppresses the ripple fundamental and keeps the rail voltage up. A downside of such a large 1st cap. is a tiny conduction angle and concommitant trafo heating. TANSTAAFL. IMO, the use of CL150 inrush current limiting thermistor between the bridge and C1 is a good idea, when that 1st cap. is huge. Clanging power trafos are not amusing. Also, inserting a "hash" filter consisting of a RF choke and mica or NPO ceramic cap. makes sense, to keep high order ripple overtones from leaking through the interwinding capacitance of the main filter inductance. Per Fourier's theorem, smaller conduction angles imply more high order ripple overtone energy.
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Old 11th September 2010, 01:23 AM   #7
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Default Brownish stain and white powder ?

I bought 2 6AU4 the other day, one of them comes with 2 circular brownish stains on the inner wall of glass, around the getter splashes. And some white powder in the tube, looks like cracked insulation material.

Click the image to open in full size.

Click the image to open in full size.

And this very tube starts up much quicker than the other - both lit-up and the raise of HV. Almost twice as quick. Is this OK? Would there be any foreseeable bad? Shorter life span or... ? Should I return it?

I see no other unusual things, though, and it seems working fine in my amp.
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Old 12th September 2010, 01:00 AM   #8
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Default Heavily-used 6AU4

This tube (the one in the photos) has seen a hard life. The brown circles seem to be the edge of where the original getter flash was. Note that there is no getter flash left at all on the left. The white particles are probably bits of the heater-cathode insulation that have broken off either due to mechanical shock or lots of on-off cycles. It may still work, but the lifetime will likely be short.

If you bought this as a used but tested good tube, you might be able to get a return, based on the very poor condition of the getter. This is actually about the worst looking damper tube I've seen (and I've seen a lot!), so you should be able to get a used one in better shape.

- John
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Old 12th September 2010, 02:03 AM   #9
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Wow!

Thanks a lot for the information. I'll return it.

And interestingly, I have a 6S45 with abnormal getter flash, too.
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Old 13th September 2010, 02:29 AM   #10
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Default Strange getters

The tube on the right doesn't look quite right, although it may be perfectly OK. Normally the getter flash should be continuous, without a "shadow". On some tubes, especially some American miniature tubes with circular getters, the getter flash has a hole in the middle, but this is OK, since the getter is close to the glass.

When a tube has been run hard, two things can happen to the getter: gasses are liberated from the metal and glass parts and the getter reacts chemically to "clean-up" the vacuum. This results in a lighter-colored getter. In the worst case when air gets into the tube, the getter will turn completely white. The other thing that can happen is that if the glass bulb gets too hot, the getter will start to evaporate and deposit getter materials (typically barium and strontium) throughout the tube. This isn't good, since electrical leakage can result. The result of evaporation is that the getter flash starts to disappear, especially around the edge where the flash is the thinnest.

Sometimes you will see black areas in the getter flash. This is caused by gasses reacting with the getter as it is actually flashed. This is just the getter doing its job. As the flashing continues, the vacuum is cleaned-up and the flash reverts to the metallic-looking color on the inside of the tube. So don't get worried about black zones in the flash. For some reason, certain brands seem to have this more than others. British Mullard tubes and some Russian tubes show this a lot, but they are perfectly fine.

- John
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