VHO Bulbs don't fire, what could be wrong?

Hi folks-

Some of you may remember me from about two weeks ago; I had problems
with my MH bulbs popping the GFCI outlet on turn-on. That's all sorted
out. It turns out that replacing the GFCI outlet solved the problem.

Now I've got a different problem, that I noticed when I was working on
the MH issue. Sometime recently, the 2 110W VHO bulbs stopped firing.
I just get a dull orange glow near the ends of the tubes.

Figuring these bulbs were just old, I got new ones from Hamilton
Technology (supplier of the lighting hood), and swapped them out. The
new ones don't light at all, though I get a brief flash when turning off
the power. Tried combinations of old & new bulbs; no difference. The
old ones glow orange, the new ones give nothing.

I checked the connections; all is OK.

The ballast is a standard magnetic variety (not an electronic one), and
I believe it is (supposedly) instant start since there's no obvious
starting capacitor in the circuit.

Sounds to me like the ballast must be bad (it's six years old), but if
anyone can suggest an alternative cause for what I'm seeing, I'd like to
investigate that before plunking down $ for a new ballast.

Marc