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Old 13th June 2003, 09:47 AM   #11
316a is offline 316a  England
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Quote:
Originally posted by ShiFtY
If you want to be flash use shottkey or UF4007 diodes for clean switching.

Hum is not hi-fi.
If you want to ride the ragged edge of operation use a UF4007 . 6B4G as far as I know have a 1A filament . I would have thought a more substantial schottkey bridge would have done the trick far better than UF4007 But is the problem with the filaments ?

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Old 13th June 2003, 04:49 PM   #12
mig-ru is offline mig-ru  Russian Federation
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6A3 and 6B4G are same tubes, only difference is the base, 4pin, octal, respectively. i have been playing around with trying to make an adequate DC PSU for the two 6A3's but i keep getting something in the 5V range. this was just using a simple cap filter, i've tried different values to no avail. should i perhaps use two diffferent PSU's for each tube so there is more resistance in each circuit?
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Old 13th June 2003, 05:33 PM   #13
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Default thorium

The filament is a Thoriated tungsten filament with Thorium, in the form of thoria oxide, is added. The thorium is brought to the surface during manufacture.
Being simplistic, during the life of the tube, thorium "evaporates" or "boils" off the filament.

No problem running the tube for 30 seconds at 2.5 volts. Not nearly long enough to reduce the cathode's life.

Floating the 6sl7 filaments will cause a hum problem. If possible, I would DC the filament at some DC voltage where the heater/cathode voltage isn't exceeded.

Hope this helps.
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Old 13th June 2003, 05:44 PM   #14
JDeV is offline JDeV  South Africa
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Default Re: hum problems with my amp, still

Quote:
Originally posted by mig-ru
i seem to develop very audible hum running my amp using 6A3's with AC ...
I had similar problem with AC filaments. Hum disapeared completely when I simply twisted the filament supply wires around each other (all the way) and tugged them into the corners of the box. Try it, if you haven't yet.
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Old 13th June 2003, 05:51 PM   #15
dhaen is offline dhaen  Europe
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JDeV,

You are just too neat and tidy I bet, no hair is out of place either....

cheers,
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Old 13th June 2003, 06:39 PM   #16
mig-ru is offline mig-ru  Russian Federation
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Default Re: thorium

Quote:
Originally posted by Positron
Floating the 6sl7 filaments will cause a hum problem. If possible, I would DC the filament at some DC voltage where the heater/cathode voltage isn't exceeded.

Hope this helps.

for some reason people keep missing that the 6SL7 filaments ARE DC'ed.

JDeV, i do have the wires out of the way and twisted. i also have tried moving wires inside with no success. i guess ill try a beefier PS filter.
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Old 13th June 2003, 09:46 PM   #17
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Default Re: thorium

Quote:
Originally posted by Positron
The filament is a Thoriated tungsten filament with Thorium, in the form of thoria oxide, is added. The thorium is brought to the surface during manufacture.
How the heck do you figure!?

http://www.mif.pg.gda.pl/homepages/f.../049/2/2A3.pdf

You'll note that it says "Filament, Coated". And if you've ever seen one, it glows red/orange (like e.g. a 5U4) and not yellow-white like a lightbulb.

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Old 14th June 2003, 12:59 AM   #18
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Default Answer to both questions

First question, that the filaments are DC.

If the filament supply is "floating", there will be an AC component, from the filament circuit to ground, because of leakage in the filament transformer. This will be a considerable voltage, although a high impedance source, so it won't take much to minimize it.

2nd Question, on thoriated tungsten filaments. There is more than one way to manuifacture this filament, my mistake. Been too long since I read this material. I was refering to transmitting tubes, so my mistake. For transmitting tubes, "Care and Feeding of Power Grid Tubes", Varian, who used to own Eimac:

"Thorium is added to the tungsten in the process of making tungsten wire. Typically, about 1.5% of thorium in the form of thoria (thorium oxide, ThO2) is added. By proper processing during vacuum pumping of the tube envelope, the metallic thorium is brought to the surface of the filament wire, and emission increases approx 1000 times. The thoriated tungsten filament is also carburized. The small amount of tungsten carbide formed in the carburizing process reduces the evaporation rate of the thorium and thus increases the life of the filament."

This filament, although Thoriated Tungsten, operates at a higher temp and works on a slightly different principle.

Sorry about mixup.
Have a nice weekend, guys.
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Old 14th June 2003, 01:16 AM   #19
EC8010 is offline EC8010  United Kingdom
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I've just looked up 6A3 on Frank's site. Up came an (old) French Sylvania data sheet with virtually no information. In other words, 6A3 is an early valve. Eimac may well have come up with interesting variants on traditional techonology, but they don't apply here, we're looking at an early oxide-coated cathode.

JDeV,

good for you. Proper heater wiring rules OK. Anything else is laziness.
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Old 14th June 2003, 03:47 AM   #20
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Default See what you mean

Yes, I got mixed up, been so long.
Kinda neat.


Have a good weekend.
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