6021 tube heaters problem

[mlabbee]

I've got a 6021 tube on my breadboard.

I built a regulated power supply for the heaters. I'm using a LM317T regulator (rated at 1.5 A). When unloaded, the regulator is putting out 6.3 volts.  The regulator is set up exactly as recommended in the data sheet and I've used this arrangement in other projects with no issues.

The data sheet for the 6021 shows leads 3 and 6 as the heater leads (I assume polarity doesn't matter, since nothing on the datasheets suggests polarity).  Heater voltage bogey is 6.3V and startup current draw is supposed to be 300mA.

I plug in leads 3 and 6 of the tube into the regulator output and the voltage across the leads drops to .1V, current is 20mA.  The tube does not heat and the regulator appears to be staying cool.

Any clues as to why the voltage from the regulator is dropping and why the heaters aren't working?

[aron]

Weird.

I found this on the web:

8DG: 1.p-a 2.g1-a 3.h 4.k-a 5.k-b 6.h 7.g1-b 8p-b

So yes, pin 3 and 6 are the heaters. Could the tube be bad?

[mlabbee]

It's very weird - I'm really scratching my head on this one. I've tried two diferent tubes (one RCA, one Raytheon)with identical results - I guess they both could be bad, but the the readings are perfectly consistent . . .

I was regulating a 9 volt regulated power source, so I figured that might do something funky (weird interaction between regulators, so I plugged a 12V wall art directly into the heater regulator circuit, tweaked it to get the right voltage (6.3V) - exact same result.

Am I confused about what LM317s do?  I thought they maintained the voltage at a preset level and raised current as needed.  But it seems lik it is regulating the current and letting voltage vary . . .

Does the rest of the tube need to be hooked up?  It looks to me like the heater circuit is independent.

[aron]

I _think_ only the heaters need to be connected, but I could be wrong. I never tried those small tubes. I always wanted to try them, but I never got them.

[zachary vex]

try powering them with something else.

[Eric H]

and startup current draw is supposed to be 300mA.

What is the VA rating of those wall-warts?

-Eric

[R.G.]

Eric is likely correct - the 317 cannot put out more current than its raw DC supply can maintain.

LM317s **are** current regulators, not voltage regulators, by the way. The stock hookup uses two resistors to make them regulate current instead. However, this is not your problem, unless you have made some wiring mistakes.

In a cold tube, the heaters are the ONLY pins which show continuity between them - the others are open circuits, so an ohmmeter is a realiable way to find heater pins.

Tube heaters are wire filaments. They have a resistance of less than 1/10 their hot value when they are cold. That's another way of saying that startup current is typically 10X the run current.  You may need up to 3A to start a 300ma tube- it's often less, but can be that high.

Finally, measure the voltage IN and OUT of the LM317 when it's pumping out its collapsed voltage. I think you'll find that the supply voltage is also collapsed, hence Eric's correct answer.

[mlabbee]

First - thanks for all the comments - I am making some progress here.

Here are the results of my latest attempts:

Using a 1 A, 12V wall wart (unloaded output = 15 V) into the regulator board: input voltage is 15V, output, regulated , is 6.3V.  Attach the heaters, input voltage remains at 15V, regulated output drops to .1V.

So, I tried pluggin the 6021 directly into the wall wart.  I know the voltage is too high, so I just turned it on briefly to see if it lit up.  Lights up fine.

So, the tube is fine, the wall wart can handle the startup needs.  Why is the regulator causing the problem?  The 317 circuit has a .1uF input cap, a 2,2uF output cap, a 220 ohm resistor between out and adj and a 5k adjustable pot going from adj to ground.

Also, in an effort to get the heater voltage to the right number straight from the wall wart, so I tried a voltage divider with the heater as the second leg of the divider.  I got a reading of 3.5 ohms across the heater, does that sound right?  I tied in a pot set to about 5 ohms and the tube lit up, but the voltage was around 11 - what am I missing?  

I also learned that my pots are not rated for the current.  The pot made a pretty orange flame  :-)

Again - thanks for all your comments - you guys are definitely helping me figure this out!

[R.G.]

Pinout.

[mlabbee]

Aha!  Always double check your circuits!  I missed the tie between the adjust pin and the R1/R2 junction.  D'oh!!!!  :oops:

Soldered it up right, but still no help on the regulated supply. (Although the input voltage now drops to zero as well).  Aaargh!

Now it is:
Vin to C1 (.01 uF) to 317 Vin
C1 to ground
317 Vout to R1 to C2 (2.2uF)
C2 to ground
R1 (220 Ohm) to 317Adj/R2 (variable pot - 5k)
R2 to ground

The application notes show a 1.2V to 20V regulator with minimum program current where R1 is 1k2 and R2 is a 20k pot. I'm not entirely clear on how the 317 works, but would this make a difference?

Why would a wall wart that can power the tubes just fine on its own not be  able to supply enough power when run through a regulator?

Should I just use a 7806 chip?

[puretube]

yes;

(or a 7805 plus 1 or 2 1N4001`s towards ground)

[R.G.]

(Although the input voltage now drops to zero as well).

Hmmm... Mother Nature is trying to tell you something. Maybe several things.

If the supply voltage drops to 0 when the load is applied, then your supply cannot support the (presumably) 0.3A of load current it's being asked for. There is no regulator will help in that situation. The raw DC supply can't handle it.

There are two possibilities here.
(a) the raw DC supply is defective
or
(b) there is too much impedance between the raw DC supply and the regulator/load to allow any current through, perhaps through mis- or broken wiring.

A different regulator will not help, unless you coincidentally correct the raw power supply flaw in putting it in.

For LM317s, I prefer R1 about 240 ohms. For 6.3Vdc, that makes R2 be 1K (OK, 969 ohms...). That should give you about the right voltage IF the DC supply can handle it, and IF you have decoupling caps near the IC to keep it from oscillating, per the National app note.

You say that your heaters cold measure 3.5ohms, right? OK, let's let Mr. Ohm tell us some things. The start up current is 6.3V/3.5 ohms = 1.8A. So we know right away that the LM317 is going to go into current limit as long as the heaters are cold. That's OK, as long as the heaters heat up and their resistance comes up.

Their resistance should come up to 6.3V/0.3A = 21 ohms, a factor of seven to one. That's roughly what I'd expect.

You didn't say whether your wall wart is a simple one (just transformer, diodes, and filter) or a regulated one. If it's regulated, it **might** have an internal current limiter that refuses to play when it's overloaded - as it is clearly being. A dumb transformer/diode/filter supply will just sit there and cook under overload until the overload clears or until smoke starts.

But your regulator is not your (only) problem. Ohm says so.

[mlabbee]

Well, I'm flummoxed.  There is obviously a serious problem with how I'm hooking up the 317, but I can't for the life of me figure out what it is.

I tried a 7806 and the tube lit up like a charm using the same wall wart. I  followed the recommended set-up from the data sheet for the 7806 and everything came up on the first try. :?:

It is a simple wall wart rated for 1 amp, so I guess it is just overloading for a second or two while the heaters get up to full resistance.

I can move forward on the project using the 7806, but I will continue to ponder why the 317 would just not work for me . . .  I'll keep fiddling with the circuit to see if I can figure out what I was doing wrong.

Thanks again for the input!

[puretube]

I recommended the 78xx because there`s less chance of hooking something up wrongly (except pinout, or oscillation problems in a few cases), or having conduction issues (cold joints).

R.G.: mlabbee pointed out in a previous post, that the raw wallwart
in fact did provide the power to light up the tube.

[R.G.]

R.G.: mlabbee pointed out in a previous post, that the raw wallwart
in fact did provide the power to light up the tube.

So he did. Sorry. I gotta read more thoroughly.

That leaves pinout and hookup. I've never had an LM317 NOT work once hooked up correctly.