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#1
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Heater voltage
What is the +/- % of deviation that can be applied to heaters on say 12ax7
tubes. I am rebuilding a small single ended 7695 amp and after I got the heater supply wired the voltage on the 12ax7's is in the 13 volt range. Any help would be greatly appreciated. |
#2
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5% I believe. My GE datasheet doesn't state it, but +/- 5 or 10% is the
rule of thumb. Tim -- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @ http://webpages.charter.net/dawill/tmoranwms "Rick Ralston" wrote in message news What is the +/- % of deviation that can be applied to heaters on say 12ax7 tubes. I am rebuilding a small single ended 7695 amp and after I got the heater supply wired the voltage on the 12ax7's is in the 13 volt range. Any help would be greatly appreciated. |
#3
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Rick Ralston wrote:
What is the +/- % of deviation that can be applied to heaters on say 12ax7 tubes. I am rebuilding a small single ended 7695 amp and after I got the heater supply wired the voltage on the 12ax7's is in the 13 volt range. Any help would be greatly appreciated. About 5%. Use a 5 or 6 ohm resistor in series with the heater (assuming 12.6V hookup) to drop the excess 1V. This also lets the tube heat up with less stress on its heater. |
#4
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As the other fella's said, 5%. But a 7812 commonly used for those heaters
will hold 'em nice at +-0.1% or better :-) -- Gregg "t3h g33k" http://geek.scorpiorising.ca *Ratings are for transistors, tubes have guidelines* |
#5
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How about a 7812 with a Si diode in the ground leg ?
Ronald . "Gregg" schreef in bericht news:hZ9Pd.44303$gA4.10186@edtnps89... As the other fella's said, 5%. But a 7812 commonly used for those heaters will hold 'em nice at +-0.1% or better :-) -- Gregg "t3h g33k" http://geek.scorpiorising.ca *Ratings are for transistors, tubes have guidelines* |
#6
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Behold, Ronald scribed on tube chassis:
How about a 7812 with a Si diode in the ground leg ? Ronald . That works fantabulous :-) But regulation goes to about 1% then. Also don't forget, which most do, to place the I/O bypass caps to the *common* pin, not ground and use, preferably, a 2.2 to 10uF tantalum across the Si diode ;-) 7805 with 2 or 3 (depends on load) 1N400x diodes work great for 6.3V heaters. -- Gregg "t3h g33k" http://geek.scorpiorising.ca *Ratings are for transistors, tubes have guidelines* |
#7
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Hi Gregg,
How about a 7812 with a Si diode in the ground leg ? That works fantabulous :-) But regulation goes to about 1% then. When talking about regulated DC for heaters, CC makes more sense to me than CV - it is even more gentle to the heaters :-) Tom -- Anyone who considers protocol unimportant has never dealt with a cat. - R. Heinlein |
#8
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Behold, Tom Schlangen scribed on tube chassis:
Hi Gregg, How about a 7812 with a Si diode in the ground leg ? That works fantabulous :-) But regulation goes to about 1% then. When talking about regulated DC for heaters, CC makes more sense to me than CV - it is even more gentle to the heaters :-) Tom Yes, I remember that from your GeeK ZonE posts. I'll have to try that! -- Gregg "t3h g33k" Adle-brained valveaholic |
#9
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"Gregg" wrote
When talking about regulated DC for heaters, CC makes more sense to me than CV - it is even more gentle to the heaters :-) Tom Yes, I remember that from your GeeK ZonE posts. I'll have to try that! Also more efficient cos regulator drop is lower proportion of voltage. And you can use thinner wire. But all the heaters end up at different potentials, and all valves in the chain must have same heater current requirements. This latter point is a fundamental problem, because requirements tend to be for increasing perveance valves from input to output. Higher perveance tends to mean higher heater current. Also heater current spec is not as reliable as voltage spec. Morgan Jones did some testing on this. He advocates series supply but warns that you need to be careful. Is the current rating of ECC88 really different from E88CC, for example? So I use series (I reg) for the four power valves, and parallel (V reg) for the little chappies. Both are easily made switchable to half power in standby if adjustable regs are used. cheers, Ian |
#10
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Rick Ralston wrote: What is the +/- % of deviation that can be applied to heaters on say 12ax7 tubes. I am rebuilding a small single ended 7695 amp and after I got the heater supply wired the voltage on the 12ax7's is in the 13 volt range. Any help would be greatly appreciated. RDH4 says +/- 10%. That means about 5.7v to 7.1 v for 6.3 v heaters or 11.4 v to 14.2v for 12.6v heaters. Getting heater voltages closer to proper working voltage prolongs the tube life. But you should be able to make sure its close to 12.6v. Using some experimentally sized series R between the transformer winding and heaters can trim the heater voltage closer to where it ought to be. The 12AX7 would be better off with rectified heater voltages, and if there is a 13v winding and you have a bridge, it will give you about 17v dc at a first filter cap of say 4,700 uF, and then you will need to have a resistor to drop down to 12.6v to a second cap of another 4,700 uF. Caps should be rated at 25v, and C1 for an amp of ripple. If you can make a 12.6v shunt regulator using a power transistor and a 12v zener diode, then the heater voltage will never exceed 12.6v even when no tubes are in the circuit. This is very basic electronics, and you should learn how-to-do. Patrick Turner. |
#11
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"Patrick Turner" wrote
RDH4 says +/- 10%. ... Brimar says mains-derived heater voltage should be within 5% at nominal mains voltage, as long as mains itself remains within 10% Does that mean 15% is the limit? cheers, Ian |
#12
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Ian Iveson wrote: "Patrick Turner" wrote RDH4 says +/- 10%. ... Brimar says mains-derived heater voltage should be within 5% at nominal mains voltage, as long as mains itself remains within 10% Does that mean 15% is the limit? I can't quote the exact reference from RDH4, but its in there alright. And there is no mention of +/-15% afaik. So whatever that book says is about right. I try to make the heater voltage for all gear stay within +/- 2% of the official proper voltage. Mains might vary +/- 5%, but I have never seen more than between 235v and 253v for 240v nominal, so I guess heater voltage would vary say +/- 7%, but over time its not enough to significantly shorten expected tube life. One can place 12.6 volts onto a 12AX7, and gee it looks bright, but not for very long, and you might stuff the emission of the cathode.... The tube will also work with 8 volts on the heater, but it'll be noisy, and Gm will be low... Patrick Turner. cheers, Ian |
#13
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"Gregg" wrote in message news:hZ9Pd.44303$gA4.10186@edtnps89... As the other fella's said, 5%. But a 7812 commonly used for those heaters will hold 'em nice at +-0.1% or better :-) I hate simple solutions... BTW, there's a 7806, really. Still, though, ain't it a bit more sportin' to use a 317, if you're already usin' them darned things grumpy old man voice? That way, you can *twiddle* things... Though you prob'ly just tap into your 'puter for the 12V, knowin' you (I like the PS's from old macs (macs are good for something - go figure!) - nice & small & have a switch right on the box... Replace the 7805 with 7806, and you're off and runnin'. And spin the wires through one of 'dem ceramic doughnuts. Oh, did I mention that I get much better S/n from my "projects" when i unplug the wireless & the router? -dim -- Gregg "t3h g33k" http://geek.scorpiorising.ca *Ratings are for transistors, tubes have guidelines* |
#14
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Behold, shiva scribed on tube chassis:
"Gregg" wrote in message news:hZ9Pd.44303$gA4.10186@edtnps89... As the other fella's said, 5%. But a 7812 commonly used for those heaters will hold 'em nice at +-0.1% or better :-) I hate simple solutions... Ya nutz! ))))))) -- Gregg "t3h g33k" http://geek.scorpiorising.ca *Ratings are for transistors, tubes have guidelines* |
#15
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"Patrick Turner" wrote in message ... : : : Ian Iveson wrote: : : "Patrick Turner" wrote : : RDH4 says +/- 10%. ... : : Brimar says mains-derived heater voltage should be within 5% at : nominal mains voltage, as long as mains itself remains within 10% : : Does that mean 15% is the limit? : : I can't quote the exact reference from RDH4, but its in there alright. : : And there is no mention of +/-15% afaik. : : So whatever that book says is about right. : : I try to make the heater voltage for all gear stay within +/- 2% of the : official proper voltage. : : Mains might vary +/- 5%, but I have never seen more than : between 235v and 253v for 240v nominal, so I guess heater voltage would : vary say +/- 7%, but over time its not enough to significantly shorten : expected tube life. : : One can place 12.6 volts onto a 12AX7, and gee it looks bright, but not : for very long, : and you might stuff the emission of the cathode.... : : The tube will also work with 8 volts on the heater, but it'll be noisy, : and Gm will be low... : : Patrick Turner. : Constant power is a good thing to strive for - the cathode coating material doesn't 'like' thermal fluctuations. A 12AX7, run below 11 V will have altered characteristics: the electron emission is lowered, therefore the 'virtuel cathode' due to the space charge will be physically closer to the cathode: Gm will get lower (curves will tilt to the right), depending on the bias this may actually increase linearity. Even lower, say below 10 V or so, the cathode will no longer be uniformly heated, resulting in increased noise and the risk of cathode sputtering - this *does* decrease lifespan! Rudy |
#16
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"Ruud Broens" wrote
...Even lower, say below 10 V or so, the cathode will no longer be uniformly heated, resulting in increased noise and the risk of cathode sputtering - this *does* decrease lifespan! Aha, right, cathode sputtering, er, what's that? Searched and got a zillion hits. Seems sputtering involves disintegration of the cathode coating by ion bombardment (what we call stripping) and subsequent deposition elsewhere. You got me interested because I thought of it as a deposition process and hadn't considered the stripping part. So where does the stripped material get deposited? I thought it ended up as crumbs and powder that just fall to the bottom of the envelope. My chemistry is hopeless. If part of the cathode is not protected by space charge, but the valve is operating, then I suppose that part could get bombarded? But where does the continuing supply of ions come from? Also, the most common cathode material replenishes its surface by leaching, er, I thought, dimly. I had in mind some kind of dynamic equilibrium. Cathode poisoning has the opposite cause: leaving heaters on with no HT. Is that sputtering, with the heater acting as cathode? Presumably poisoning stops the leaching. cheers, Ian |
#17
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"Ian Iveson" wrote in message . uk... : "Ruud Broens" wrote : : ...Even lower, say below 10 V or : so, the cathode will no longer be uniformly heated, resulting in : increased : noise and the risk of cathode sputtering - this *does* decrease : lifespan! : : Aha, right, cathode sputtering, er, what's that? : : Searched and got a zillion hits. Seems sputtering involves : disintegration of the cathode coating by ion bombardment (what we : call stripping) and subsequent deposition elsewhere. You got me : interested because I thought of it as a deposition process and : hadn't considered the stripping part. : : So where does the stripped material get deposited? I thought it : ended up as crumbs and powder that just fall to the bottom of the : envelope. My chemistry is hopeless. : : If part of the cathode is not protected by space charge, but the : valve is operating, then I suppose that part could get bombarded? : But where does the continuing supply of ions come from? : : Also, the most common cathode material replenishes its surface by : leaching, er, I thought, dimly. I had in mind some kind of dynamic : equilibrium. : : Cathode poisoning has the opposite cause: leaving heaters on with no : HT. Is that sputtering, with the heater acting as cathode? : Presumably poisoning stops the leaching. : : cheers, Ian : Cathode failure mechanisms: if heated above 850-900 C, Ba will evaporate from the monatomic layer the 'work function', the escape energy required for electrons, increases; the emission is lowered. at even higher temperatures, the current density will enable sputtering, that is macroscopic particles from the cathode get 'ripped loose'. ....so, better not go much if at all above rated filament power ;-) If the cathode temperature is too low, the brittle oxide layer becomes locally overheated, chips 'fall off' - hot spots. all tubes contain some air molecules. when the accelerating force for the electrons is large enough, anode voltage above 20 Volts or so, collisions with air molecules result in ionization - these ions love to meet the cathode , hot date ;-) there, they poison the cathode so, running indirectly heated tubes at high voltages shortens the usuable cathode's lifespan. cheers, Rudy |
#18
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Rick -
Heater voltage can affect noise, lifetime, and characteristics; here's a link (translated) that shows changes in characteristic curves with heater voltage changes: http://honyakuinfoseek.infoseek.co.j...onyakuinfoseek Here's the origianl japanese page:http://www2.famille.ne.jp/~teddy/datalib/heater.htm Rick Ralston wrote: What is the +/- % of deviation that can be applied to heaters on say 12ax7 tubes. I am rebuilding a small single ended 7695 amp and after I got the heater supply wired the voltage on the 12ax7's is in the 13 volt range. Any help would be greatly appreciated. |
#19
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Rick -
Heater voltage can affect noise, lifetime, and characteristics; here's a link (translated) that shows changes in characteristic curves with heater voltage changes: http://honyakuinfoseek.infoseek.co.j...onyakuinfoseek Here's the original japanese page: http://www2.famille.ne.jp/~teddy/datalib/heater.htm Rick Ralston wrote: What is the +/- % of deviation that can be applied to heaters on say 12ax7 tubes. I am rebuilding a small single ended 7695 amp and after I got the heater supply wired the voltage on the 12ax7's is in the 13 volt range. Any help would be greatly appreciated. |
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