Altec 353A oddity

[Captn Dave]

The attached drawing is from an Altec 6L6GC puch pull amp. Notice the .0015uf 1600V capacitor between the OT primary leads; between pin 3 of each power tube? I can't recall seeing that in another design. What is the reason for or benefit from that cap?

Well that is a pretty small drawing. Here is a LINK to the full scale drawing.

[HollowState]

Originally Posted by Captn Dave

What is the reason for or benefit from that cap?

This capacitor reminds me of the old buffer capacitor that was used across the secondary of the old car radio vibrator HV supplies. It's purpose was to reduce sparking in the vibrator points and to reduce noise from same. Extreme potiental voltage spikes were buffered out which reduced wear and tear on the rectifier (0Z4) and transformer insulation. I suspect something similar is being done here. (Excluding the vibrator of course)

[smoking-amp]

The cap could just be a crude brute force killing off of the top HF end to preserve global neg. feedback stability. Ie, dominant pole roll-off before the OT's nasty double pole roll-off.