Can't get zener clipping diodes to activate

[leftu2]

I'm trying to implement clipping diodes between the cathode of the last preamp stage and the tone slope resistor, but the diodes won't engage.
I've checked everything I can think of, continuity, voltages, tracing signal with my scope all the way to the slope resistor, everything check out good.
I'm showing 2 pics, 1st one is from the schematic I'm following with the measured voltages.
The 2nd pic is the way I have the components laid out on my small board.

I'm wondering if there's not enough voltage to engage the diodes? I'm not sure how much it takes when using 2 diodes set up this way to get clipping. I've already got both voltage dropping resistors at 10k

[johnnyreece]

I'm no expert, but if your voltage isn't high enough to hit the zener limit, there won't be any clipping.  It would seem you would need much lower voltage zeners. 

[sluckey]

I'm no expert, but if your voltage isn't high enough to hit the zener limit, there won't be any clipping.  It would seem you would need much lower voltage zeners.

I agree. You only have 11.5V_pp. That's only 5.75V_pk,which is what the zeners will see.

[pdf64]

Why not feed it with a higher signal level, eg sufficient to get the DCCF stage clipping?
That should provide the diodes something to work with.