12ax7 heater voltage problem

[bent]

Hello!

you can see my other topic for reference:
http://www.diystompboxes.com/smfforum/index.php?topic=89461.msg759722#msg759722

i have build a lm350 circuit:
Adjustable Regulator with Improved
Ripple Rejection, page 7  http://www.national.com/ds/LM/LM150.pdf

I have a nice 12.6vdc, and when i connect the heater (pin 4 and pin 5) of the 12ax7 , pin 9 unconnected :
voltage drop to 7.2 vdc at the output of the lm350 circuit ! 
I try 2 other tube (in case that the heater was malfunction)... they all do the same...

The rest of the tube is unconnected, i just try to solve the heater problem...

HELP !
Bent

[thedefog]

This may help you : http://www.enotes.com/topic/12AX7

[bent]

This may help you : http://www.enotes.com/topic/12AX7

Thank's  , but no really helping me....

i was more wandering why the voltage is dropping (12.6vdc to 7.2vdc ) on the out of the lm350 when i put a load on it...

and how to correct this....

bent

[R.G.]

Your regulator cannot supply more current than the raw unregulated DC supply before it.

When the output drops under load, what is the DC voltage on the INPUT to the regulator?

[bent]

Here's the set up:

- 110 vac in
- from 110vac,  goes to 229d24 transfo to get 24vac
- from 24vac --- go to 3 place:
    - another 229d24 in reverse + bridge and cap/resis to get 307vdc
    - phantom power circuit http://www.extremecircuits.net/2010/05/48v-phantom-microphone-power-supply.html to get 48vdc
    - another diode-bridge to supply 3 individual LM350 circuit to get:
                                                - 12vdc for IC board ( not connected for this moment)
                                                - 12.6vdc for heater
                                                - 24vdc for vu meter ( not connected for this moment)
          All ground connected together
The problem i see (or i think), is that the phantom power circuit is affecting the power of the other circuit.... cause i have 50vdc at the IN of the LM350...

I build 2 LM350 circuit to seperated the heater from the IC board to avoid HUM !

A BIG Thank's R.G. to drop by    , i'm totally lost....

Bent

[bent]

I need:
-  +/- 300 volt for 4 tube circuit, total +/- 30mA
-  12.6vdc for heating 4 tube, and 4 meter light, total +/- 850mA
-  12vdc for 4 IC board , total of +/- 500mA
-  24vdc for 2 vu meter board, +/- 5mA
-  48vdc for 4 mic phantom power

i got :
- 110vac in
- 229d24 (hammond) X 2
- TL783 ( few of them)
-  LM350 (few of them)

Solution anybody?

Bent

[gtudoran]

Had the same problem never solved - i'm using AC for heaters. DC 12v 1.5A - LM5806 - 12AX7 (6n2p) heaters => voltage drop to 7.9 / 8.2 (heaters in both cases are drawing between 350-400mA so there is another problem that i don't see).

Best regards,
Gabriel Tudoran
Analog Sound

[bent]

Thank's Gtudoran,

i just think in my shower (best place to think  ), i will remove the 48vdc circuit and take the 24vac in a diode bridge to supply the rest, if this work, i will order a 110v to 48v transfo, and make my phantom power from the transfo directly with diode bridge and rectifier like lm350 or TL783....

bent

[bent]

YES!
so far so good....
i disconnect the phantom circuit... plug 24vac throught a diode bridge with a 1000uF cap on the exit....
now all my 3 lm350 circuit have the good power....12vdc,  12.6vdc,  24vdc
when i connect a heater on the 12.6vdc...it stay at 12.6vdc....heater work !
when i connect vu meter light on it, stay at 12.6vdc but after a minute overheat and lm350 switch off...
i didn't put compound between the lm350 and the heatsink....might be the problem !

[bent]

can sustain 1 heater.... at 2 heater the lm350 switch off by the heat...

[defaced]

If you are not using a heat sink, I would consider adding one to improve heat dissipation.  This solution assumes the circuit is working correctly, which may or may not be the case. 

Or, omit the DC heaters and use elevated AC heaters.  Four resistors and a cap and you're there. 

[bent]

i use a heat sink, but didn't put compound between the heat sink and the lm350...

can you tell me more about the 4 resistor and cap.... the Ac i have is 24vac....

[defaced]

Under "DC Elevation".  http://www.freewebs.com/valvewizard/heater.html

R1 = 1M
R2 = 100K
C1 = 22 to 47u.  100v.

All can be user adjustable, I just know these values give you about 30v elevation with 300v B+ and good decoupling.  If that's not enough to eliminate heater/cathode coupling hum, increase R2 until you're in the 40-60v range. 

[bent]

thank's defaced....

very usefull page....

i just found a nice little transfo 115v to 12vac.... but its giving 14.3vac....how to drop it to 12.6vac...?

[R.G.]

i use a heat sink, but didn't put compound between the heat sink and the lm350...

The LM3xx series will always shut down before dying from overheating.

Heat production is the product of the current through the regulator times the voltage difference across it - that is, Vin minus Vout of the regulator. Measure this difference, then multiply by the current through the load.

If it's overheating and you did not use compound, then... use heat sink compound. Heat sinks can be almost useless unless they are properly attached to the heat producing device.

i just found a nice little transfo 115v to 12vac.... but its giving 14.3vac....how to drop it to 12.6vac...?

For what use?

[bent]

12vac transfo no good..... its 2.4vac....not enought current....

i use compound and.....one heater is good....but when i connect the second one it took about 1 minute to overheat...

[bent]

i use the 229d24.....

the second parallel is 12vac.... so when i pass it in a diode bridge i got 12.3vdc.... do i need to put a 1000uF cap at the dc side of the bridge ?
cause when i do it rise at 19.6vdc, so i would need a regulator, and im trying to avoid since i got problem with heat... but the 12.3vdc without the cap would be nice for me......
except that everything will be connect on this .....would it cause humm?


just connect 2 heater and voltage drop from 12.3vdc to 10.9vdc.... 

[bent]

well well well !

i took a chance and connect the 12vac on a bridge with 1000uF cap, then past it into the lm350 regulator....and the circuit can supply the 4 heater ! without overheat ! ( hot but not switch off)....but i need 24vdc so...

i think i will try to replug has it (24vac) then use a lm350 to drop to about 24vdc then use another lm350 to drop to about 16vdc, then lm350 to drop to 12.6vdc

[bent]

OK !
So far so good !
24vac -> diode bridge + 1000uF  ( 30vdc) -> lm350 to drop to 24vdc ( for vu meter) -> lm350 to drop to 16vdc -> lm350 to drop to 12.6vdc

all 4 heater are working for 5 minutes and still running...no shut down....

heat sink are hot ! but dont look overheat ! i will had a small fan for that !

i think ill will build another lm350 to drop 16vdc to 12vdc for the IC board....
and still have the 48vdc phantom power to find a way to get it...

[G. Hoffman]

well well well !

i took a chance and connect the 12vac on a bridge with 1000uF cap, then past it into the lm350 regulator....and the circuit can supply the 4 heater ! without overheat ! ( hot but not switch off)....but i need 24vdc so...

i think i will try to replug has it (24vac) then use a lm350 to drop to about 24vdc then use another lm350 to drop to about 16vdc, then lm350 to drop to 12.6vdc

If you already must have 24V_dc, then think about wiring your heaters in series.  You'll use less current, and reduce your parts count.  I'm not sure it is the way to go, and it does raise some concerns (if one of your heaters dies, you have got a problem!), but it would solve some of your problems.  You can decide for yourself if the trade off is worth it.



Gabriel