i seem to develop very audible hum running my amp using 6A3's with AC fils. i know this isnt due to the 6SL7 because it runs on DC fils. i did read somewhere that the point of low voltage heaters was to reduce hum, so i switched the 6A3's to run on the 2.5V winding and see what happens. the output level dropped (duh), and the hum seemed to go away. so should i get some 2A3's (expensive) or try and DC the 6A3 filament supply? could it also be that i have bad hum pots (alpha wire wound 100ohm)? although after the adjustment of the pots to eliminate as much hum as possible there is a somewhat different hum present (what im trying to get rid of) i though it was power supply problems at first because the hum is somewhat different from the type i get when i turn the hum pot to one extreme. any suggestions?
i dont think 2A3 has thorium filament.. thoriated tungsten is more common in higher power tubes..
you can usually tell a thoriated tungsten tube by the fact it is very bright, it needs to be to get sufficient electron dissipation.. (not as bright as lightbulb usually, but much brighter the the dull orange most tubes have..)
i think 2A3 uses just oxide coating (oxide of what? i dont remember.. not thorium though)
you can usually tell a thoriated tungsten tube by the fact it is very bright, it needs to be to get sufficient electron dissipation.. (not as bright as lightbulb usually, but much brighter the the dull orange most tubes have..)
i think 2A3 uses just oxide coating (oxide of what? i dont remember.. not thorium though)
Don't suffer with AC filaments... I got bored of the hum on my 6B4 SET.
Get a 50 cent diode bridge into a C-R-C filter (I use 1000uF - 2 ohm (5W) - 6800uF) 10V or 25V caps are cheap.
Silent, smooth and 6.25V. If you don't like it then you have spent about $5! no great loss...
If you want to be flash use shottkey or UF4007 diodes for clean switching.
Hum is not hi-fi.
Get a 50 cent diode bridge into a C-R-C filter (I use 1000uF - 2 ohm (5W) - 6800uF) 10V or 25V caps are cheap.
Silent, smooth and 6.25V. If you don't like it then you have spent about $5! no great loss...
If you want to be flash use shottkey or UF4007 diodes for clean switching.
Hum is not hi-fi.
ShiFtY said: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 ? 316a
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?
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.
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.
Re: thorium
How the heck do you figure!?
http://www.mif.pg.gda.pl/homepages/frank/sheets/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.
Tim
Positron said:
How the heck do you figure!?
http://www.mif.pg.gda.pl/homepages/frank/sheets/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.
Tim
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.
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.
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.
JDeV,
good for you. Proper heater wiring rules OK. Anything else is laziness.
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