Understanding the dim bulb limiter?

[Phil_S]

I was inspired by another builder to cobble together the Priceless tube tester/matcher from some spare parts. It's been a fun project. I'm at the point where I plugged it in using the dim bulb limiter. I'm using the 109-109 no CT PT with a doubler that I asked about a few weeks ago. The HT winding meters at 2.9Ω.

Ordinarily on power up, I see the bulb flash bright and then grow dim. When that happens, I'm content there is no obvious short, I put in the tubes, and move to the next steps.

On this one, the bulb appears to be fairly bright. I'm using a 40W bulb. I'm thinking it dims just a little after the initial surge, but it is not dim. I'm having difficulty understanding it there really is a problem and I'm wondering if I botched the doubler. The other possibility of a problem is the bias supply, which is a 2-wire 37V winding where one wire is grounded.

Using the limiter, I see 3.25VAC on the HT secondary and about 12VDC at the B+ take off. I expect if I left the power on for a few more minutes 12VDC would sink closer to the expected 9V range. This suggests to me the doubler is OK.

I didn't meter what's up on the bias supply mostly because I wasn't comfortable with the bright bulb and wanted to turnoff the power before something bad happened. Additionally, given the expectation that I'd only see 3% of 37VAC = ~1VDC, I simply guessed that any reading might be too low to be meaningful. I suppose it is possible I botched the wiring of the bias supply, but I think not. I'm wondering now as I write if there is a problem with grounding one side of the 37V winding. When I did that it seemed to make sense.

What would you do given what I describe? I'd be very grateful for your wise counsel.

[xtian]

Is there any load that should be converting current into heat? I mean, do you have tubes installed in the tester? Is there a bleeder resistor? An indicator lamp? If not, and it's just a power supply with no significant load, it should not be drawing much current at idle, and the bulb should not be glowing much.

[JMFahey]

Using the limiter, I see 3.25VAC on the HT secondary

If you measure 3.25VAC at a nominal 109VAC winding then something is very very wrong.

Start by disconnecting all secondaries leaving them "in the air" and remeasure.

I suspect you made a big mistake with your doubler, or somehow shorted one scondary.

Triplecheck your circuits and reconnect secondaries one by one to reduce the search area.

[Jana]

My bulb starts to dim after two beers. After that, I start blowing fuses.

[Phil_S]

As always, Jana gives top notch advice. I'm going to toss the thing and go drink beer to dim the bulb. It is the path of least resistance.

So, more seriously:
No tubes installed.
220K bleed resistors across the caps in the doubler.

In free air: HT 109-109, Bias 37, Filament 6.9; this determined before installing the PT. DCR on secondary windings, in order 2.9Ω, 2.9Ω, 0.1Ω.

I really didn't want to, but I guess I have to lift the PT secondary connections and see when the bulb goes dim.

Curious about the comment on output voltage on the HT secondary. It is always quite low with the bulb in place. I'm not sure how low it should be.

Sit tight. I'll post what I find in a few hours. Thanks for the input. It's helpful.

Edits:
Jana, you know that approach has been used successfully by the medical profession for centuries: treat the symptoms not the underlying problem

I botched the doubler wiring. All is well now. The dim bulb barely glows. The doubler, with a pair of 100uf caps is at 285VDC. Not bad...was hoping for 300VDC. I imagine it will sink towards 260-ish with a pair of tubes, which is all this thing is meant to hold at one time.

Also, great news on the bias supply. Since I socketed this for octal and noval, it calls for 12.5V +/- for EL84's and 6V6's, and 37-ish for 6L6's. I used a switch to select 4.7K or 47K on one side of the voltage divider, and a 25K pot + 1K fixed on the other side. Ranges are 7.5 to 39 and 31-46. When I modeled this, it did not predict quite the range I get with the 4.7K. We'll see how this behaves with tubes in sockets but it goes without saying that I'm very pleased with how the bias supply worked out.

Filaments are at 7.1 today with no tubes. I expect the upper 6's with tubes and I think that's good, too. Line voltage today is a shocking 123.3VAC.

I'll see about some gut shots tomorrow.

Thanks everyone!