# 12 ohm resistor

## Recommended Posts

i cannot find a 12 ohm 2 Watt resistor unless i move a long way away.

i was wondering if i could use a 10 ohm or 15 ohm wich i have laying around here

the resistor is placed in the power supply of the mc tubes II pedal i'm building, european version, just after the first bridge rectifier, it's used to heat the valves.

also could i have an explanation of what changing the value makes, cos i remember studying V=R·I but i forgot how this is applied, mine was a fairly light digital electronics knowledge. i've tried simulating a circuit in electronics workbench just to avoid having to ask, but i must be useless

i have all the components except for that resistance, and i'm sooooo willing to wire it all up and see how it sounds

##### Share on other sites

Given the 10% or even 20% tollerance of most garden variety resistors, you could use either a 10 or 15 to replace that 12.

If you have an ohm meter you could dig through a box of them until you find one that's close to 12 if you need it to be very precise.

Alternatively, you could take a 10 ohm resistor and start to file out the center. This will gradually increase the resistance as you nibble away the carbon inside. We used to do this sort of thing in the lab to create precision resistors.

Hope that helps.

##### Share on other sites

Given that the tolerances on filament voltages are about+/- 10%, use the 10 ohm, and check to see that the filament voltage isn't above 14 volts - if it is, use the 15 ohm.

i´ll try that.

##### Share on other sites

• 2 weeks later...

i'm trying to solve a problem i got now with the power supply now i found my resistor and thought i'd share it here to see if you can help (i'm sure you will )

so, i've wired the power supply of the mc tubes, and i should be getting 15V when there's nothing more plugged in the low dc voltage output, the one to the filaments.

but i'm getting no less than 17. i've tried 10ohm 12 ohm and 15 ohm resistors without going under 17, and in the high dc output i should be getting 150 but i got 180, what's the problem, i just wired the power supply as the diagram, if i could get an explanation of what elements i got to change to get the desired voltages it would be great

this is the diagram i'm following here

##### Share on other sites

instead of the 1n4001 diodes for the bridge i used 1n4004, i think this is not a reasson for the higher voltage, but it's after the bridge that i read 17 volts dc although gettig 12.6 ac from the transformer.... please

anybody who knows

##### Share on other sites

some of the transformers have different load specs.. try hooking up and put a load on them if it drops down the all is well. are you running a regulator. its a good way to eat up some extra power. and keep it under control

##### Share on other sites

Ansil's right, you're never gonna know what the transformer is actually gonna put out until you load it. I've seen 12 volt transformers in wallwarts that would measure as high as 20 volts with no load connected. Those voltages don't seem out of line to me - the only way you're going to know is to put the tubes in the circuit and check the voltages under load (and be careful - even these realtively low voltages are potentially dangerous). If you have access to a variac, you could slowly bring the voltage up to line level, but even at 180 volts the B+ won't hurt the tubes, and the filament supply should drop considerably under a load, and I doubt it would fry the tubes immediately, even at 17 volts. If it was mine, I'd recheck the wiring about three more times, install the tubes and power it up - if nothing immediately exploded, I'd check the filament voltage, and if it was less than 14 volts, I'd start checking out the ound of my new box. Your threshold of anxiety probably differs from mine, but that's how I'd do it. If it would make you more comfortable, you could power it up with the tubes in place but with the B+ disconnected to check the filament voltage, but since a 12AX7 will take well over 300 volts, that seems like a waste of time to me. Good luck with whatever you decide to do. Here's hoping you get a great result the first time around.

##### Share on other sites

Your threshold of anxiety probably differs from mine, but that's how I'd do it.

oh yeah, you can't imagine how anxious i am about finishing it, but you know, everybody always saying to be patient, just didn't want to screw it up XD

i'm gonna try now!!!

##### Share on other sites

i get 17 volts on the filaments, but the valve doesn't even glow, when you mean a load, you mean a guitar plugged??

is this an expected phenomenon or i just killed the valve cos it doesn't glow

##### Share on other sites

UMMM QUESTION. which tube are you using the 12ax7.. or 12au7 sounds like a real silly thing to ask but how did you wire the heaters.

one of the schematics i saw for that had them wired wrong. they had like was it pin nine grounded or somethign like that.

anyway. when i first started i didnt' know you wired up the filaments 4&5 tied and the ninth one as the second filament.

##### Share on other sites

when i first started i didnt' know you wired up the filaments 4&5 tied and the ninth one as the second filament.

He doesn't, Ansil , he's using a 12 volt filament transformer, so he wires just 4 and 5 (and in this case, grounds pin 9).

cr_XD, I'm sorry, I should have looked more closely at the power supply schematic - as shown, you're gonna get 17 volts sure as shootin' !! Let me go read back through the article and see if I can find a justification for that - if not, I'll see if we can get in touch with Fred and find out why he's running the filaments so hot.

##### Share on other sites

ahh.. silly me.. what was i thinking.. i should have paid more attention.

##### Share on other sites

well, i think the easiest solution is making a new power supply, i wanted to go the easy way using the tonepad layout, and i've learned some interesting things on the way. tomorrow i´ll be getting two voltage regulator (i don't know if it's called like that) for 12V and 140V and reuse the parts i already have.

gonna get 17 volts sure as shootin' !!

i asked a friend who knows about electronics more than me and that's what he told me. he said "just use the voltage regulator" but i wanted to know why it didn't work if the article said so, i'm just that pig-headed

##### Share on other sites

OK, I sim'd it in Duncan's PSU, and it comes out at about 17.8 volts DC at the transformer, and about 13 volts between pins 4 and 5. Take the tube out and check it for continuity between pins 4, 5 and 9 - you should have continuity between any two. Then power up without putting the tube back in, and very carefully check the DC voltage between pins 4 and 5 at the socket. If it's between 12 and 14 volts, you should be good to go. Turn it off, install the tube, hook up to your amp, and start debugging the audio (j/k, it might well work fine right off the bat). If the voltage is way out of line, increase the size of that resistor (use the 15 0hm, or an 18 or 22 if necessary). I would check the voltage again after installing the tube (again, very carefully) to make sure it's not running below 11 volts.

##### Share on other sites

cr_XD i came across a bunch of little resistors like this when i was cleaning out my stock are you stateside.????

##### Share on other sites

cr_XD      i came across a bunch of little resistors like this when i was cleaning out my stock are you stateside.????

no, i'm in spain.

lovekraft: i get 18.7 volts without the tube in, i'll try with a higher value(currently 12 ohms). the tube seems to be ok, if what you mean with continuity is a little resistance.

i´ll keep this updated till i get to make it sound .

btw: as i followed the schematics i saw, i didn't realize that the guy who made them for 230volts used as t2 a 18v 500mA. that won't give 140v will it? anyway it makes it cleaner, and changing the transformer is a 3 € mod, the easiest to get to work in my case, just four solderings to change and ready to go XD.

off to study now , got an exam on saturday, file structures

##### Share on other sites

lovekraft: i get 18.7 volts without the tube in, i'll try with a higher value(currently 12 ohms). the tube seems to be ok, if what you mean with continuity is a little resistance.

Yes, so long as there's not an open circuit, the filament will show some resistance. So you're getting a higher voltage at the pins than you are after the diode bridge? I'd be tempted to dump the rectifier section altogether and run the filaments on 12.6 volts AC, but that's liable to cause noise and hum problems. Probably not a good idea, but i'll check into it - it sure would be easier.

i´ll keep this updated till i get to make it sound  .

That's the spirit! I refuse to be bested by inanimate objects! Also, please let me know if my language causes problems or confusion - your English is so good I tend to forget that it isn't your first language, and write the way I talk, which may not translate well.

btw: as i followed the schematics i saw, i didn't realize that the guy who made them for 230volts used as t2 a 18v 500mA. that won't give 140v will it? anyway it makes it cleaner, and changing the transformer is a 3 € mod, the easiest to get to work in my case, just four solderings to change and ready to go XD.
You can run the B+ up to about 300 volts without causing any problems, so the only issue is the headroom - the 18 volt secondary should give you about 146 volts across the 220 volt primary, and that 6 volts should have no real affect on the headroom. If you used a 12 volt, you'd be getting 220 volts, and that might add too much headroom.

off to study now , got an exam on saturday, file structures
Good luck! When you get time, run through the power supply and check the voltages at the transformer, the bridge, and the socket pins one more time - there's got to be something I'm missing. I suppose you could always throw in a regulator, but the grounded center tap on the filaments could make that problematic.. We'll figure it out.
##### Share on other sites

OK, I couldn't sleep, I've been obsessing over this, and I may have found a solution - try putting the 12 ohm resistor in series with the 15 ohm between the two caps, install the tube, power up and quickly but carefully check the voltage across the second cap. That should give you between 13 and 14 volts - if it does, you are good to go, unless you want to replace the two resistors with a 27 ohm 5 watt. If you still have more than 14 volts, check to make sure the only ground connection on that circuit is on pin 9 of the tube - if you've accidentally grounded the caps or the transformer secondary, the ground on pin 9 is shorting out half of your filament, cutting the load in half and not dropping as much voltage (not to mention overstressing the hot side of the filament while leaving the other side cold). If that still doesn't fix the problem, talk to your friend about installing a 12 volt regulator - just make sure it can handle 300 mA or so to keep it from overheating (a 12AX7 draws about 150 mA, but double it for safety) and causing more problems. Don't worry about regulating the high voltage supply, it should be fine. Good luck with that exam.

##### Share on other sites

• 2 weeks later...

12.4 - transformer secondary ac

17.8 - filament supply dc

this were the values i was getting just when i got back to meassure it today

i had gone as i promised to the shop right after the exam.... only had a 22 ohm resistor ... what kind of shop is than

well well well ... right now i'm getting 11.2 volts on the filaments with the valve in the socket

it's a pity i could be playing right now with my brand new mc tubes, see if i can get to a good shop on saturday and get some more values and tweak a little bit more till i get it to work

##### Share on other sites

OK, I couldn't sleep, I've been obsessing over this, and I may have found a solution...

##### Share on other sites

just one last thing before i go to party

I really cannot express how thankfull i am to the people round here. it's not only you do rule . it's that besides you are some of the nicest people i've met.

i just can't explain this in english, it's just a lack of vocabulary in theese kind of things sorry, but i do think you are great.

okokok... more tomorrow

##### Share on other sites

well well well ... right now i'm getting 11.2 volts on the filaments with the valve in the socket

xd, that's great! BTW, at 11.2 volts it should work fine if you can't stand waiting to play it! I would change out that resistor eventually, but 11.2 volts is still within the safe operating range for a short test drive. Excellent work!

##### Share on other sites

the valve doesn't light up, is that normal. sometimes i can see a little glow. i pluged it and didn't hear anything.

what is the way to run the filaments on ac, what is the difference?

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
• Home