Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: leftu2 on April 29, 2026, 11:57:17 am

Title: Can't get zener clipping diodes to activate
Post by: leftu2 on April 29, 2026, 11:57:17 am
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
Title: Re: Can't get zener clipping diodes to activate
Post by: johnnyreece on April 29, 2026, 12:26:11 pm
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. 
Title: Re: Can't get zener clipping diodes to activate
Post by: sluckey on April 29, 2026, 12:38:19 pm
Quote from: johnnyreece on April 29, 2026, 12:26:11 pm
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 11.5Vpp. That's only 5.75Vpk,which is what the zeners will see.
Title: Re: Can't get zener clipping diodes to activate
Post by: pdf64 on April 30, 2026, 01:14:36 am
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.