While checking a 6EJ7 for my IM-21, I noticed a whiff of smoke coming from my trusty TT-1.

Per a suggestion of someone in the Vintage Test Equipment group, I turned to my trusty TU-75 and finally used it’s series bulb feature.  An incandescent bulb in series acts as a current limiter, which both protects the Device Under Test, and gives a visual indication of when the DUT is trying to draw more current than you’d like. Sure enough, when the tube was connected using the ‘normal / disconnect’ switch, the lamp glows, and I see the voltage drop appreciably.

Side note: I finally got around to measuring and labeling the lamps in my TU-75.  According to the wattmeter, the largest lamp is 500W!  I seem to remember an even larger one that unfortunately got broken due to shoddy packing when I bought this 4 – 5 years ago.

What led me to the problem was this line adjust rheostat starting to smoulder.

See me. Spin me. Smell me.

It was suggested that this is not the source of the issue, rather an indication of something upstream drawing too much current.  Hopefully not an underlying issue with the transformer. With 40 taps across a half dozen windings, It’s basically unobtanium.

I didn’t recall ever having an issue with this tester in the past, and sure, things age, but what’s different about this test than countless one’s I’ve done before.  Well, for one thing, I don’t ever recall testing a tube with a 600mA before.  I decided to try testing something with a fixed voltage filament instead.  A 12AT6 with it’s 6.3V filament did not yield the same excess current draw, and tested fine under this setup.

This Page has a great document on refurbishing the TT-1.  It also has the last published tube data addendum.

Here’s another in-depth document on the TT-1.  It references Kent’s document as well, and pages 12 – 15 describe the theory behind the ballast capacitor. The reactance is inversely proportional to the capacitance, so as the capacitance goes up, it’s equivalent resistance goes down.   In my case, they’re all measuring a bit high:

The sum is what’s important for this range – looking at the switch, it appears that the capacitors are paralleled sequentially through the last three filament settings (the current settings).  Is a 47% overage enough to account for this error?  That would mean at 600mA, it’s theoretically allowing ~900mA: Not great, but I’m surprised it’s enough to cause this problem.  There’s nothing else in this circuit – does this mean I should suspect a short somewhere in this tap?  It’s the top-most tap of the filament winding, #29. 

I’m going to try a lower current filament and see if I end up with the same issue, as well as some larger, higher current tubes.

Testing a 6L6 with it’s 6.3v filament at 900mA gets me a little bit of a dip in voltage down to 105V when testing with the dim bulb, but nothing like I was getting with the 600mA setting. With the bulb bypassed, it seems to work fine, and nothing is getting warm or stinky.
Testing a 6HZ6 with it’s 450mA gets me a dip down to 80V.  With the bulb bypassed, I get a good test, but started to get just a hint of fresh roasted components, so I killed it before I started to see smoke.
Went back to test a 6AW8 which calls for a 600mA, this time with a current meter on the common line on the ballast capacitor.  With the bulb, the line voltage dipped down to 70V & the filament is only able to draw ~400mA.  With the bulb bypassed, the filament drew ~700mA. I didn’t leave it on this setting long enough to summon the stank.
With the same tube & settings, switching to the 6.3V tap doesn’t cause the same brown-out.

My working theory is that there’s a short somewhere in the part of the winding that feeds the current filament settings, but that does not affect the other taps.  Not being able to use the current settings actually isn’t too big of a problem, as the tubes that call for it also have a published filament voltage.