Glow Tube (0B2) Help

[dsavitsk]

I am having a glow tube issue, and I am totally stumped what is going on. I have a stereo line stage as attached. When I turn it on, one side's glow tube will glow, and the other won't. When I move the tubes from side to side, sometimes the side that strikes changes, and sometimes it doesn't. In fact, while certain tubes strike more often than others, it is somewhat random which side will light up. Moreover, I have a pile of these tubes, all are good, and some of them seem faster than others.

The 6T4 runs with about 70V on the plate when biased with a red LED. The 0B2 is supposed to strike at 133V (or maybe 115 -- different sources say different things). At any rate, if the mosfets are left out, both sides strike with no issue (essentially with just a 30mA CCS running through them). So, it is definitely the addition of the mosfets that is causing the issue.

I tried increasing R8 to 22K in case there was current being drawn, but it does not seem to be the case as the 6T4 is still running at the right spot. Also, the mosfet's source resistors are dropping the right amount of voltage. So, there must be current running through the 0B2's (~20mA each) but only one lights up or gets warm. Though, even the non-glowing one seems to be regulating.

I think I am at the limit of my abilities here to diagnose this. Can anyone offer any suggestions?

[SY]

What's the starting current spec? I'll bet you're not supplying it. Without the MOSFET at startup, the tubes can draw the full 30mA.

[dsavitsk]

I think what might be happening is that the voltage is initially high and the first one strikes. This causes the voltage to sag. At this point, the other mosfet is biased to B+ since the 6T4 is not yet conducting. As such, it draws too much current which keeps the second 0B2 from striking.

If this is right, then either increasing the source resistor or cap coupling and biasing with a voltage divider, might work. The former will reduce the current it can supply, the second adds another cap, so I'm not really in favor of either. Maybe a higher B+ is an option. But, I am going to try these other options first to see if one works.

[astouffer]

Hmmm a similar thing was happening to me with an 0A2. Putting a .047uf cap across the tube made it strike every time. Not sure exactly what caused it or why the cap helps.

[dsavitsk]

Was that the first cap you tried? I've tried both 0.1u and 0.022u, and neither seems to help. Increasing the source resistor to 15K didn't help.

I think a higher B+ would help, and the power transformer I have hums something awful, but I am feeling too impatient to wait for a new transformer to to ship.

[kevinkr]

I see a number of problems mainly relating to insufficient starting voltage.. Raising the B+ slightly will probably help..

[Wavebourn]

Hmmm... What if to use a LED lighter?

[dsavitsk]

Hmmmm. I used a 144VAC transformer rather than the 120 I was using which increased B+ to 170V or so. I also tried cap coupling the mosfets and biasing them to 1/2 B+, and using 15K source resistors to cut down on the current. Still, one glow tube lights, the other doesn't.

[dsavitsk]

Another clue? The mosfet on the side with the lighted tube has ~5mA through it, which means the glow tube has 25mA. On the non-lighted side, the mosfet has 3mA with 27mA for the non-lighted tube. B+ measures at 163V. The lighted tube gets warm, the other doesn't. It is like the CCS isn't actually putting out current, or it is going somewhere else. Which doesn't make sense as the one glowing switches sides.

[Hearinspace]

Have you tried lighting up the 6T4 heaters with a separate supply before applying B+?