What is the 3rd 6V6GT tube for?!

[mwaller]

Hello!
I'm in the process of recapping the amplifier from a Bell & Howell 179 film projector. My first goal is to get it running as it was originally designed.
This amplifier has THREE 6V6GT tubes. One pair is used to drive the output transformer. However, I can't figure out the function of the third.
Any ideas?

B&H 179 Schematics.PDF

[martin manning]

Voltage regulator, I believe.

[didit]

Voltage regulator, I believe.

My assumption as well. Quick schematic reading suggests a stable low current supply, though the trace through is somewhat muddling.

Best .. Ian

[sluckey]

I think it's a blocking oscillator used to drive the exciter lamp.

[martin manning]

I think it's a blocking oscillator used to drive the exciter lamp.

I see, yes, looks like those coils are coupled.

[mwaller]

Very interesting. Here's a closer look at the circuit.
Why would an oscillator be required to drive a lamp?!

[martin manning]

It’s for avoiding ripple voltage buzz in the audio. Evidently ultrasonic AC was easier to produce than really clean DC.

[mwaller]

Interesting! I would have never guessed.
Thanks for the input!

[didit]

I think it's a blocking oscillator used to drive the exciter lamp.

I see, yes, looks like those coils are coupled.

This makes much more sense.

..