Hello.
It is the same Bell-Howell 34 vacuum tube oscilloscope from another previous thread... Works good -or I think so-

What is the function of C29 by the bottom, is it shorting AC signal to GND?
V5B pin 6 puts good AC signal at both sides of C28, like 14Vpp ;
V6A pin 2 shows good signal; V6B pin 7 does not.
Am feeding the horizontal input only and was expecting a larger amplification. Found not much explanation on its manual. What deflection should be expected ?
V6A plate is +280V
V6B plate is +260V

Light please ?

 

I agree with your thought that V6b has no AC signal.
I think A6a amplifies the signal while V6b sends a DC "offset" to the plates.
The deflection is the difference between the two plates. One has the ramp that deflects left to right. The other plate only has a DC on it to canter the trace.

 

V6A and V6B form a differential pair for perfect phase inversion for the Horizontal Sweep. One of the inputs can be "zero" as in this case. The effect is from the changing Cathode-Grid voltage brought about by the other half of the pair.
The same with the Vertical Sweep setup V2A-V2B pair.
Here's an explanation for a Transistor version, but the principle is same for vacuum Tubes too.