I'm refurbishing a tube unit that uses a 12AX7 and 12BH7. In both tubes only one section is used. Is there are recommended approach for what to do with the unused sections in each tube. Is it cool to just leave them floating in thin air not connected to anything. Or should just the cathodes get tied to ground with grid and anode not connected?
what to do with unused tube sections
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shabtek
Well-known member
I would think it is best to ground unused terminals because that could have a shielding effect within the envelope...also it might be good to not have the filaments of the unused sections passing current: less PS demand, etc.
shabtek
Well-known member
I would think it is best to ground unused terminals because that would have a shielding effect within the envelope...also it might be good to not have the filaments of the unused sections passing current: less PS demand, etc.
I've never had a problem letting them float disconected... if you want to save power you can kill the power supplies to the half of the tube not in use. (save about 2 watts / section ) this would only easily work if you have 12v supplies. (you could try the 6v heaters in series, they have the same current according to data sheets, i'd meter it first)
if you let them float (in my experience) it's fine. I dont see why this would be a problem. If you feel like grounding stuff, just make sure the heater-cathode voltages arn't exceded. this is 180 in the 12ax7. If (i dont know how) you have the cathode in one stage at a very high potential, and have adjusted the heaters to be within the range, make sure the cathode-heater-cathode volage isn't incredibly high (i'd guess twice the cathode-heater voltage or 360 volts).
ok so if you want to ground the unsed cathode go for it, unless you have some kind of crazy voltage setup.
Whats this for anyway?
if you let them float (in my experience) it's fine. I dont see why this would be a problem. If you feel like grounding stuff, just make sure the heater-cathode voltages arn't exceded. this is 180 in the 12ax7. If (i dont know how) you have the cathode in one stage at a very high potential, and have adjusted the heaters to be within the range, make sure the cathode-heater-cathode volage isn't incredibly high (i'd guess twice the cathode-heater voltage or 360 volts).
ok so if you want to ground the unsed cathode go for it, unless you have some kind of crazy voltage setup.
Whats this for anyway?
not a bad idea, but would it be noticeable?
if you ground all of the extra metal in the tube, you'll have more capacitance to deal with. the data sheet for the 12ax7 says about .9pf between anodes so keep in mind that grounding this is adding a .9pf capacitor from your anode to ground. (.8pf in the 12bh7)
(thanks Duncan for the tdsl)
sismofyt
Well-known member
float them. that's what i recommend doing with the 12SN7 slow blow
funkydiplomat
Well-known member
I'm not sure I follow you here. But 12ax7's and 12bh7's have different filament currents.
At 6.3 volts...
12ax7 is 300mA
12bh7 is 600mA
funkydiplomat
Well-known member
http://www.groupdiy.com/index.php?topic=10070&highlight=paralell+parallel+triodes
thanks for the tips guys.
It seems floating them will be cool
This unit is a Fairchild conax and is tubed up for the stereo version, whereas what i have here is the mono. So i'd love to make it stereo and use those other tube halves, but i'd need input and output transformers and the magic conax module.
I've reverse engineered the magic module and so could build one of those.
Transformers needed are Stancor PWV 535 ( cat no 80-1 ) and UTC Y-6431. ....... any offers?
It seems floating them will be cool
This unit is a Fairchild conax and is tubed up for the stereo version, whereas what i have here is the mono. So i'd love to make it stereo and use those other tube halves, but i'd need input and output transformers and the magic conax module.
I've reverse engineered the magic module and so could build one of those.
Transformers needed are Stancor PWV 535 ( cat no 80-1 ) and UTC Y-6431. ....... any offers?
it depends on who's data sheet you read...
and its really not worth trying to get two tubes w/ the same current draw. If you had a number of these channels (each with 2 disconnected stages) and wanted to save wattage... but didn't want to do 6.3 volts...
i dont know. lol just an idea
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