12ax7 heater problem

[Ice-9]

I have finished building my psu and the postman delivered a new 12ax7 for me today so i started to build a pcb to hold the valve, (as in the picture). I have all the correct voltages on my psu and switch board, the 12v regulated DC for my heaters is working and i have used a 7812 capable of 1.5A output so should have enough current for the heaters. From the same 12v i have wired the led for effect on/off which is working great but when i connect the heaters pin 4 and 5 to the 12 volt the led goes out (as if there is a current overload) Nothing else is connected at this stage other than the heater, the the AC adapter is set at 18V and has a rating of 800ma.

If i dissconnect the 12v from the valve and wire a 9v battery direct to the heater it works fine with a little red glow (so the valve is fine)

Any ideas what could be wrong are appreciated.

[R.G.]

More data needed.

What voltages are on the regulator pins with/without the tube in place?

[Gus]

Is the circuit built like the picture?  If so you need a heatsink on the regulator it is most likely overheating and shutting down you will not feel much heat because it is safeguarding "itself" (1.5 amps with a heatsink).
  What is the other TO220 package device?  does it need a heatsink?

[Ice-9]

With the valve not in the socket i get 11.85v across the output of the regulator and 0v with the valve in place. If i dissconnect the psu from the valve pcb and solder a a pp3 connector to the same connections i get 8.7v across the heaters and a glow.

[Ice-9]

If it's shutting down it does it instantly (as soon as the voltage is connected to the heater terminal) the other T0220 is a 15v reg to power the the 4049 chip for the voltage multiplyer. (i only had a T0220 device in my parts bin but a T092 is all thats needed there and at the moment isn't being used) Also there is no heat at all in any of the components.

[markusw]

Maybe give it a try to set the AC adapter to 15V AC or even 12V AC.
With 18V AC the 7812 has quite some wattage to dissipate.
Alternatively, you could try to stick a 2-3W resistor in front of the 7812 to drop voltage a bit. 

[Ice-9]

I have just tried different voltages from the AC adapter and i get the same problem with each setting.What i have noticed is the othe parts of the psu are affected. I have 3 parts to the circuit.

1. is the 12v for the heaters
2. is +/-12 for opamps.
3. is 15v for the voltage multiplier

all give the correct voltage with the valve not in the socket.
Once the heater is connected all the + voltages are wrong the only correct voltage is the -12v .

I think i will need to draw up a schematic to post to shed more light on what i have here. 

[Gus]

Read my post again when it safeguards it often does feel hot.  The sensing is done at the die inside so it can be fast.

  Put a heatsink on the 7812 use thermal compound and an isolator or an all in one thermal pad and see what happens.  The 7812 is only good for 1.5 amps with a good heatsink.  You can calculate the watts dissipated in the 7812 knowing the voltage drop across it and current draw then you can figure out what size heatsink or you can put a heatsink on it and see what happens.  A 7812 works it best at >= 3VDC drop from in to out.

Go the national semiconductor site and look at the PDFs app notes and spec sheets for the 78xx regulators.

[Ice-9]

Gus I read your post again and i had read it correctly i just didn't think the shutdown would of been instant. I will try a heatsink on the reg. If the reg is shutting down i would think it shouldn't effect the other regulators in the circuit which go low as soon as the heater is connected as well

Here is a schematic, the voltage multiplier part can be ignored as i have removed the reg that supplies that so it is effectively isolated now, and not part of the circuit.

[R.G.]

Let's play ohm's law.

Quick, what voltage drops across a 390 ohm resistor when you pull 150ma through it?

V = 0.15* 390 = 58.5V

Mother Nature is whispering in your ear "You ain't gettin' 150ma through that resistor, guy. Not with those voltages."

[markusw]

The 390R will drop the voltage too much. Maybe try something like 30-60R.
Withou doing a calculation you might want to get a voltage drop across the resistor of something like 10VDC with the 18VAC supply.

Argh: too late 

[Ice-9]

Thanks to R.G. and markusw

Problem now fixed .

Anyway i'm now happy i can move onto the next part of the circuit

[markusw]

I believe the 30R should be fine. Depending on how stable your mains voltage is you might even go a bit higher (35R ?)

[Gus]

Edit I see a few posts in the time I was writing this with some of the same things

The schematic helps.  18VAC at 800ma should be OK.  

Something to try with a heatsink.  Jumper R1 390 ohms on the +12 side.  WHY?  150ma is used by the heater(12ax7 etc wired for 12.6 heater) Voltage = current x resistance 390ohms x .150amps = 58.5 volt drop across the 390 ohm at 150ma this can't happen with you circuit because you don't have enough voltage so the current is limited.

 If this works with a heatsink on the +12 reg measure the in and out of the 7812 you want just over 3VDC for good ripple rejection and regulation so then you can(if you want) calculate a resistor value that will allow regulation and limit the heat in the 7812 by dissipating some of it in the resistor.  1V drop at .150amp =6.7ohm, 2V drop at .15amp 13.3ohms etc.  You might not need a resistor at all if the heatsink is a good size.

It is a good idea to use balance bleed resistors across C1 and C2, 22ks might be fine calculate the wattage needed.

http://en.wikipedia.org/wiki/Ohm%27s_law

http://www.the12volt.com/ohm/ohmslaw.asp

[Ice-9]

thanks everyone, thats was a problem i would have stared at for ages before realising the size of those resistors . I need a trip to the parts shop before i can go any further.
thanks again.

[Renegadrian]

Mick, what circuit is that BTW?!

[Ice-9]

Adriano, Its a circuit i'm working on myself, not quite totally from scratch. The voltage multiplier is similar to the Paia SIAB but later i might use switched psu to get 250v to the plates. (i want to get it working at 60-90v first) Now i have the psu working the way i want it to, with thanks to this forum members.
Its now time to start on the audio part of the circuit, which will be original and i will be building / testing in blocks and posting schematics as i go. Its not going to be all valve but a mixture of valve and solid state components.

[Renegadrian]

Right, thx for the answer - have fun while you keep it going!!! 

[Paul Marossy]

Let's play ohm's law.

Quick, what voltage drops across a 390 ohm resistor when you pull 150ma through it?

V = 0.15* 390 = 58.5V

Mother Nature is whispering in your ear "You ain't gettin' 150ma through that resistor, guy. Not with those voltages."

OK, dumb question time. How can you have 58.5V when your supply voltage is only 18V?!

[Renegadrian]

He has a 4049 based voltage multipler!