astable multivibrator issues - 2N3904s work, but not TIP31As

I built an astable multivibrator with blinking lights (much simpler
than using a 556!), as a test for a circuit that will use a power
transistor (or MOSFET) to pulse a transformer primary for future
experiments.

http://mrdarrett.googlepages.com/blinkenlights002.pdf

Strangely, when I replaced the two 2N3904 transistors with TIP31As,
the circuit would not oscillate unless I briefly disconnected then re-
connected R2 or R3. (I bumped the voltage up from 3V to 6V during
these tests.)

As a work-around, I'm considering just putting the 2N3904s back in,
and connecting the positive end of C2 to the base of a TIP31A.
Inelegant, but I think it will work.

I'm trying to figure out why TIP31As won't work, but it also doesn't
help me any that the TIP31A data *** does not specify a minimum
V_BE_on.

http://www.st.com/stonline/products/literature/ds/12292/tip31a.pdf

Input?

Michael
.

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