There's a few schematics on the web, but most of them assume you know how to power the heater filaments. Could someone explain how to power the heater filaments of say a couple of 12ax7's, I know there's a way to do it with 6Va.c. or 12Vd.c., but that's about all I know.
How to power heater filaments
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If you're heating directly with AC, with 6.3V in parallel for a 12AX7, one side of the AC goes to pins 4 and 5, and the other goes to 9. 12V is used when wired in series. But you mentioned 12VDC--for this the AC would have to be rectified and filtered, but still the same deal 4+5, and 9, but usually 4+5 get the positive and 9 to ground.
For most applications wiring in parallel with 6.3VAC (or even down to 5.8VAC is usually fine). Meaning pins 4+5 are connected to the other 4+5 pins, which is powered by one side of the AC, and likewise the pin 9's are all wired together powered by the other side of the AC.
Is that confusing?
For most applications wiring in parallel with 6.3VAC (or even down to 5.8VAC is usually fine). Meaning pins 4+5 are connected to the other 4+5 pins, which is powered by one side of the AC, and likewise the pin 9's are all wired together powered by the other side of the AC.
Is that confusing?
You can also do it with 6VDC or 12VAC. In a 12AX7, you got 2 triodes with one 6.3V heater each. The heaters are in series with a tap in the middle that goes to pin 9. In other words:
PIN4----(heater1)----PIN9----(heater2)----PIN5
So you need 12V across pins 4 and 5, with pin 9 floating. Or ground pin 9 and put 6V on pins 4 and 5.
Peace,
Al.
PIN4----(heater1)----PIN9----(heater2)----PIN5
So you need 12V across pins 4 and 5, with pin 9 floating. Or ground pin 9 and put 6V on pins 4 and 5.
Peace,
Al.
CJ
Well-known member
Or you could series the two tubes and run 24 volts.
In pin four out pin 5, then into the other pin four etc.
Saves a little wiring.
In pin four out pin 5, then into the other pin four etc.
Saves a little wiring.
Bjorn Zetterlund
Well-known member
Not sure about the 12V AC thing...
Essentially the 'pin in the middle' is at 0 volts, which means that for a dual valve you have two heaters; each has one side grounded and the other side connected to 6V AC...that will certainly make your cathodes hum!
Even with circuits that use AC heaters, it is quite common to find some means of 'balancing' the heater, like a 100 Ohm or so pot with each end connected to either end of the heater, and the centre tap connected to ground. This allows for some adjustment to minimize hum.
Bjorn
Essentially the 'pin in the middle' is at 0 volts, which means that for a dual valve you have two heaters; each has one side grounded and the other side connected to 6V AC...that will certainly make your cathodes hum!
Even with circuits that use AC heaters, it is quite common to find some means of 'balancing' the heater, like a 100 Ohm or so pot with each end connected to either end of the heater, and the centre tap connected to ground. This allows for some adjustment to minimize hum.
Bjorn
[quote author="Bjorn Zetterlund"]Not sure about the 12V AC thing...[/quote]
It can and has been done - that's where the "12" in 12AX7 comes from. :wink:
[quote author="Bjorn Zetterlund"]Even with circuits that use AC heaters, it is quite common to find some means of 'balancing' the heater[/quote]
Just looking quickly through my schem collection, I found no circuits that did this. I've seen it done, but I'm just not sure that it's "quite common" :?...
It is - generally speaking - better to use DC for filaments, but whether and how much AC heaters cause hum problems depends on the circuit, the tube used and your layout.
Peace,
Al.
It can and has been done - that's where the "12" in 12AX7 comes from. :wink:
[quote author="Bjorn Zetterlund"]Even with circuits that use AC heaters, it is quite common to find some means of 'balancing' the heater[/quote]
Just looking quickly through my schem collection, I found no circuits that did this. I've seen it done, but I'm just not sure that it's "quite common" :?...
It is - generally speaking - better to use DC for filaments, but whether and how much AC heaters cause hum problems depends on the circuit, the tube used and your layout.
Peace,
Al.
> how to power the heater filaments.
They are just like light bulbs. Feed the rated voltage and they do their job.
You can plug-in a desk lamp, right? You could probably wire-up a lamp from a plug, socket, and some wire. Or wire a car tail-light bulb to either a 12VDC battery or a 12VAC transformer. Same deal.
Big difference is that most heaters can work red-hot instead of white-hot like a lamp. So they last forever, pretty near.
It is simpler than you think.
Complications:
There are 7, 8, or 9 pins down there, be sure you get the right ones.
AC heat causes hum. A thin direct-heat filament cathode will get hot/cool twice per cycle, humming-up the signal. Separate heater-cathode tubes can be heated with AC and give very low hum, because the separate cathode holds the heat over a 50/60Hz wave. Hum in indirect heat tubes is mostly about those AC wires near your grid and plate signals. Keeping the AC balanced and well away from signal pins can give excellent results.
You need the "right" voltage. Most general-purpose tubes use 6.3V, a relic of using lead storage batteries as heater supply. We call these "6V", and in RMA type-numbers these usually start with "6" (6SN7, 6L6, etc). The nominal voltage is really 6.3V, but 10% or even 20% variation makes very little difference. And in some situations, under-volting the heater works better for small-signal use.
But design for nearly any voltage is possible, and lots of tubes were made for 12V, 25V, 35V, 50V, 60V, and even some 117V heaters (for direct operation from a US wall outlet). You often find the "same" tube in several heater voltages: 6SN7 and 12SN7 are the same except for heater needs. (But 35L6 has nothing to do with 6L6!)
Some tubes like 12AX7 are dual-voltage. They have two heaters, brought out on three pins. Typically: parallel both heaters and feed 6V, or series the heaters and feed 12V. This comes naturally when you have a twin-triode (two heaters) on a 9-pin base, and it avoids having to stock two versions of the "same" tube for different uses. Be sure you wire right. Putting 12V on a 6V series connection will light-up white-hot and die in hours. Putting 6V on a 12V series connection won't get hot enuff to boil any electrons, won't work.
Oh: Welcome to the Lab!
They are just like light bulbs. Feed the rated voltage and they do their job.
You can plug-in a desk lamp, right? You could probably wire-up a lamp from a plug, socket, and some wire. Or wire a car tail-light bulb to either a 12VDC battery or a 12VAC transformer. Same deal.
Big difference is that most heaters can work red-hot instead of white-hot like a lamp. So they last forever, pretty near.
It is simpler than you think.
Complications:
There are 7, 8, or 9 pins down there, be sure you get the right ones.
AC heat causes hum. A thin direct-heat filament cathode will get hot/cool twice per cycle, humming-up the signal. Separate heater-cathode tubes can be heated with AC and give very low hum, because the separate cathode holds the heat over a 50/60Hz wave. Hum in indirect heat tubes is mostly about those AC wires near your grid and plate signals. Keeping the AC balanced and well away from signal pins can give excellent results.
You need the "right" voltage. Most general-purpose tubes use 6.3V, a relic of using lead storage batteries as heater supply. We call these "6V", and in RMA type-numbers these usually start with "6" (6SN7, 6L6, etc). The nominal voltage is really 6.3V, but 10% or even 20% variation makes very little difference. And in some situations, under-volting the heater works better for small-signal use.
But design for nearly any voltage is possible, and lots of tubes were made for 12V, 25V, 35V, 50V, 60V, and even some 117V heaters (for direct operation from a US wall outlet). You often find the "same" tube in several heater voltages: 6SN7 and 12SN7 are the same except for heater needs. (But 35L6 has nothing to do with 6L6!)
Some tubes like 12AX7 are dual-voltage. They have two heaters, brought out on three pins. Typically: parallel both heaters and feed 6V, or series the heaters and feed 12V. This comes naturally when you have a twin-triode (two heaters) on a 9-pin base, and it avoids having to stock two versions of the "same" tube for different uses. Be sure you wire right. Putting 12V on a 6V series connection will light-up white-hot and die in hours. Putting 6V on a 12V series connection won't get hot enuff to boil any electrons, won't work.
Oh: Welcome to the Lab!
Cainester
Well-known member
This thread should go in the META...
I searched for a long time with no usable results.
Caine
I searched for a long time with no usable results.
Caine
CJ
Well-known member
Filaments are connected directly to the cathode. Heaters, as you find in most low level audio tubes, are not. From RDH4:
Here is a Phillips 7025/12AX7A. The heaters go down the middle of that small tube The 7025 is supposed to be a low noise version of the 12AX7A. This is accomplished with a special heater:
Here is the heater pulled from the tube. It is supposed to be twisted like the double helix heater shown in the first pic. Only a few twists here. How much hum reduction do you think this is going to give you! Those lying @#$%^&*&*(*^^&'s!
Here is what happens when you run a 12 volt heater at 20 volts outside it's vacuum. It lasts about ten seconds:
Just for the heck of it, here is a grid.
That white thingy is the cathode:
Heaters don't always put out the same amount of heat from tube to tube. The companies tried to get them so that 5.5 volts on a 6.3 volt tube would not alter the characteristics much. So there is a lot of variation there.
Here is a Phillips 7025/12AX7A. The heaters go down the middle of that small tube The 7025 is supposed to be a low noise version of the 12AX7A. This is accomplished with a special heater:
Here is the heater pulled from the tube. It is supposed to be twisted like the double helix heater shown in the first pic. Only a few twists here. How much hum reduction do you think this is going to give you! Those lying @#$%^&*&*(*^^&'s!
Here is what happens when you run a 12 volt heater at 20 volts outside it's vacuum. It lasts about ten seconds:
Just for the heck of it, here is a grid.
That white thingy is the cathode:
Heaters don't always put out the same amount of heat from tube to tube. The companies tried to get them so that 5.5 volts on a 6.3 volt tube would not alter the characteristics much. So there is a lot of variation there.
