How about using two more diodes in series with the two you have there.
Or you could use a Zener diode of the proper voltage and wattage on the center tap to ground.
Schematic and parts list for DC heater supply?
Moderators: pompeiisneaks, Colossal
Re: Schematic and parts list for DC heater supply?
Tom
Don't let that smoke out!
Don't let that smoke out!
Re: Schematic and parts list for DC heater supply?
Yeah, I might look into those options once I get a feeling for what the actual voltage is under load...Structo wrote:How about using two more diodes in series with the two you have there.
Or you could use a Zener diode of the proper voltage and wattage on the center tap to ground.
Tempus edax rerum
Re: Schematic and parts list for DC heater supply?
Connect it between + and ground, so it acts as a load, just as your heaters would.dehughes wrote:Right on. I think I might have a 10 ohm resistor lying about. So, I'd connect that from the + to ground in order to simulate a load? Or just connect it to the + and not ground the other end, and then measure?
It'll dissipate up to 10W depending on how the voltage drops, so make sure it's up to the job.
Re: Schematic and parts list for DC heater supply?
Sweet. Thanks. I'll try to tinker more tonight.paulster wrote:Connect it between + and ground, so it acts as a load, just as your heaters would.dehughes wrote:Right on. I think I might have a 10 ohm resistor lying about. So, I'd connect that from the + to ground in order to simulate a load? Or just connect it to the + and not ground the other end, and then measure?
It'll dissipate up to 10W depending on how the voltage drops, so make sure it's up to the job.
You guys are great...
Tempus edax rerum
Re: Schematic and parts list for DC heater supply?
So, I think I'm getting it:
If I have 9.56v as my supply DC voltage right now (without tubes installed), a 3.6k 5w resistor would, with a .9A draw (one 12AX7 and one 6SN7), drop about 3.24v, right? This, combined with the drop caused by installing the tubes, should put me under the 6.3v mark...probably just under 6v once the tubes have warmed up, right?
If I ordered some 3 ohm and 3.6 ohm 5w resistors, I'd probably be set, yes?
If I have 9.56v as my supply DC voltage right now (without tubes installed), a 3.6k 5w resistor would, with a .9A draw (one 12AX7 and one 6SN7), drop about 3.24v, right? This, combined with the drop caused by installing the tubes, should put me under the 6.3v mark...probably just under 6v once the tubes have warmed up, right?
If I ordered some 3 ohm and 3.6 ohm 5w resistors, I'd probably be set, yes?
Tempus edax rerum
Re: Schematic and parts list for DC heater supply?
Your calculations are correct, but that assumes your voltage is going to remain that high once you get a load on there. You may find that you're down in the mid 7's once there is any sort of load, in which case the couple of additional diodes would be enough.
Why not knock up a test rig with 3 or 4 1N4007s in series and put them between your positive terminal and the tube sockets and put the tubes in for a moment.
That'll knock a couple of volts off so even if the heaters end up too high, they won't be that much over that's going to do any harm for a moment. Then, if they're under, you can remove diodes to suit.
You'll know what you're actually up against at this point.
Why not knock up a test rig with 3 or 4 1N4007s in series and put them between your positive terminal and the tube sockets and put the tubes in for a moment.
That'll knock a couple of volts off so even if the heaters end up too high, they won't be that much over that's going to do any harm for a moment. Then, if they're under, you can remove diodes to suit.
You'll know what you're actually up against at this point.
Re: Schematic and parts list for DC heater supply?
Such wise advise...paulster wrote:Your calculations are correct, but that assumes your voltage is going to remain that high once you get a load on there. You may find that you're down in the mid 7's once there is any sort of load, in which case the couple of additional diodes would be enough.
Why not knock up a test rig with 3 or 4 1N4007s in series and put them between your positive terminal and the tube sockets and put the tubes in for a moment.
That'll knock a couple of volts off so even if the heaters end up too high, they won't be that much over that's going to do any harm for a moment. Then, if they're under, you can remove diodes to suit.
You'll know what you're actually up against at this point.
I'll do just that come Monday, and report back.
THANK YOU!
Tempus edax rerum
Re: Schematic and parts list for DC heater supply?
A tube filament is like a light bulb - the hotter it gets, the more resistance it exhibits. While it's not cool (no pun intended!) to run tubes at higher than normal heater voltage for their entire life, I'm almost certain your tubes could stand a brief test at that voltage.
If you're feeling brave, you could cross your fingers, fire it up, and see how much the voltage drops under load. The worst possible outcome is that you ruin a tube - hopefully, you've got some start-up tubes kicking around that don't have any sentimental value to you.
Just a thought...
W
If you're feeling brave, you could cross your fingers, fire it up, and see how much the voltage drops under load. The worst possible outcome is that you ruin a tube - hopefully, you've got some start-up tubes kicking around that don't have any sentimental value to you.
Just a thought...
W
Re: Schematic and parts list for DC heater supply?
Thanks Wayne. Makes sense. I have a bunch of other 12AX7s, but only a few octals....two of which are actually kinda nice. I'll save the dice rolling for when I have more $$$ available. 
I'll do the diode thing first and see where I land...
I'll do the diode thing first and see where I land...Tempus edax rerum
Re: Schematic and parts list for DC heater supply?
Certainly understandable. I actually had second thoughts after posting that. Most of my glass collection falls somewhere around "used to be nice" or "not nice at all, but man were they cheap" so if there's any doubt, I stick one of those in and go for it! I don't mind sacrificing the occasional component if it's in the name of further education. 
W
W
Re: Schematic and parts list for DC heater supply?
So, here's what I've found...
I strung three 1N4007 diodes together in series after the DC cap(s), and with the tubes in and warmed up, I'm getting about 8.5vDC before the diodes (this being down from 9.56vDC...thus indicating the load of the tubes), and about 6.07vDC on the tube filament pins. SO, this is great! That's just where I want to be.
Regarding hum, it was a bit hummy still, so I put in ANOTHER 10000uf cap, and boom...all I hear is the sound of the PT humming along, just like when the 6v battery was installed. Perfect! All it took was 30000uf and DC rectification....
So, I assume then I'll need to find a resistor that would drop the same amount of voltage as the diodes. If memory serves, the diodes drop .7v each, so I'd need a resistor in the range of 2.1 ohms, yes? Probably about 5w?
Lemme know if I'm doing the math correctly...I'll place the order from Mouser tomorrow. I'm thinking nice silicone coated wirewound power resistors....
I strung three 1N4007 diodes together in series after the DC cap(s), and with the tubes in and warmed up, I'm getting about 8.5vDC before the diodes (this being down from 9.56vDC...thus indicating the load of the tubes), and about 6.07vDC on the tube filament pins. SO, this is great! That's just where I want to be.
Regarding hum, it was a bit hummy still, so I put in ANOTHER 10000uf cap, and boom...all I hear is the sound of the PT humming along, just like when the 6v battery was installed. Perfect! All it took was 30000uf and DC rectification....
So, I assume then I'll need to find a resistor that would drop the same amount of voltage as the diodes. If memory serves, the diodes drop .7v each, so I'd need a resistor in the range of 2.1 ohms, yes? Probably about 5w?
Lemme know if I'm doing the math correctly...I'll place the order from Mouser tomorrow. I'm thinking nice silicone coated wirewound power resistors....
Tempus edax rerum
Re: Schematic and parts list for DC heater supply?
You'll need to drop about 2.2 volts (8.5-6.3).
At .9 A draw, that's 1.98 ohms. 2 ohms would be plenty close, as would 2.2.
Power dissipated would be 1.62 watts, so a 5 watter would be a good safe bet.
W
At .9 A draw, that's 1.98 ohms. 2 ohms would be plenty close, as would 2.2.
Power dissipated would be 1.62 watts, so a 5 watter would be a good safe bet.
W
Re: Schematic and parts list for DC heater supply?
Killer. I'll order the parts tonight.
Any recommendations on a regulator (just in case)? I know people have recommended some previously in the thread, but figured others might have opinions. What about the LD1085 type? Or is there a better model?
Any recommendations on a regulator (just in case)? I know people have recommended some previously in the thread, but figured others might have opinions. What about the LD1085 type? Or is there a better model?
Tempus edax rerum
Re: Schematic and parts list for DC heater supply?
Great news.
As regards the regulator, what you need to look at is the dropout voltage.
You've got 8.5Vdc available and you want to get 6.3V, so you've currently got 2.2V headroom. You want to allow for some line voltage fluctuation so if you allow for -10% then your 8.5Vdc would drop to 7.65V and your headroom would be reduced to 1.35V.
You'll also still have some ripple in that 8.5V, although you've got it down to a very low level by using 30,000uF of filtering (coincidentally the same as I use on my regulator boards). If we say there's 0.15V ripple (which it won't have, but it's good for the calculations) then you're left with 1.2V headroom.
So long as you have a voltage regulator which has less than a 1.2V dropout voltage at 1A drawn then it will give you a fully regulated 6.3Vdc down as low as 108Vac mains.
Get a 3A rated regulator, and use the chassis wall as a heatsink ideally. Make sure you read the datasheet to see whether the tab of the package is connected electrically to the ground pin or to any of the other pins (sometimes it's +V), in which case you might need an insulating washer.
Then go and enjoy that sucker!
As regards the regulator, what you need to look at is the dropout voltage.
You've got 8.5Vdc available and you want to get 6.3V, so you've currently got 2.2V headroom. You want to allow for some line voltage fluctuation so if you allow for -10% then your 8.5Vdc would drop to 7.65V and your headroom would be reduced to 1.35V.
You'll also still have some ripple in that 8.5V, although you've got it down to a very low level by using 30,000uF of filtering (coincidentally the same as I use on my regulator boards). If we say there's 0.15V ripple (which it won't have, but it's good for the calculations) then you're left with 1.2V headroom.
So long as you have a voltage regulator which has less than a 1.2V dropout voltage at 1A drawn then it will give you a fully regulated 6.3Vdc down as low as 108Vac mains.
Get a 3A rated regulator, and use the chassis wall as a heatsink ideally. Make sure you read the datasheet to see whether the tab of the package is connected electrically to the ground pin or to any of the other pins (sometimes it's +V), in which case you might need an insulating washer.
Then go and enjoy that sucker!
Re: Schematic and parts list for DC heater supply?
Tsk, tsk, ampgaragers. Either no one caught my mistake, or you're all just being polite! (if the latter is the case, I thank you)Wayne wrote:You'll need to drop about 2.2 volts (8.5-6.3).
At .9 A draw, that's 1.98 ohms. 2 ohms would be plenty close, as would 2.2.
Power dissipated would be 1.62 watts, so a 5 watter would be a good safe bet.
W
The correct resistor is 2.4 ohms (meaning that 2.2 would still be close enough).
When I was younger I had trouble obeying some laws - "You can't drive that fast, son" and "Hey - don't smoke that - it's illegal" come to mind. I never had trouble until now adhering to Ohm's law!
W