Jose clipping diodes - less effect when Master set high?

[mxrshiver]

working on a customized 50W JMP plexi that's somewhere between a #1986 Bass and #1987 Lead. i've added a 250KA Master after the cathode follower, with a small resistor and coupling cap between the CF and the Master, and i've also added 2x anti-parallel clipping diodes on a switch between the top of the Master and ground. Master is followed by tonestack.

it's sounding and working great, but to my surprise, the clipping diodes only have an effect with the Master set low! when i turn the Master up past around 2 o'clock, the diodes no longer have any audible effect when switched in. but they work just fine and sound great at lower settings...

planning on scoping it today, but i was wondering if anyone knows what's up with that offhand? i was expecting the diodes to make just as much, if not more of an impact, at high Master settings, considering how much they limit the signal... perhaps the clipping later in the circuit (PI and power amp) swamps the diode clipping?  to be specific, i have anti-parallel 20V's on one switch, and asymmetrical 20V/13V on another switch. both have the intended effect at low Master settings.

[passaloutre]

Do you have a schematic? It seems unusual to have the "Master" volume *before* the tonestack.

[stratomaster]

The order of the components matter, so we'll need to see a schematic. There are many variations on the Jose theme, and they will behave differently.

[mxrshiver]

thanks for your time folks. here's the relevant section.

i've heard Jose would sometimes do post-tonestack Master, sometimes pre tonestack, and sometimes make it switchable between the two. we went with pre tonestack because there's already a series Mojotone FX loop after the tonestack with Send and Return controls, but he wanted a front panel Master as well, and max possible control over how hard the signal's hitting each stage. (plus a little extra loading for the cathode follower AC output suits the gain staging we're going for) i didn't draw or mention the FX loop because the diodes behave the same whether it is switched into the circuit or not.

[passaloutre]

Who is Jose?

[stratomaster]

With that circuit, the clipping is happening before the attenuation, so instead of the volume control feeding the clipping diodes and there being a corresponding increase in the clipping of the signal (until it goes fully square) the signal is clipped before the volume control. In this case the signal is square from the get-go, and the volume control is just determining how much of that squared signal you're feeding to the tonestack and rest of the amp.

Paradoxically, the signal becomes less square as you overdrive later stages with a high amplitude square wave.  This may be the effect you're hearing. This is also why a Fuzz Face into a clean amp doesn't sound as pleasant as one into an overdriven amp.

[stratomaster]

Who is Jose?

Jose Arredondo, amp tech known for Marshall amplifier mods in the hair metal days. Famous/infamous for adding master volume controls and clipping diodes (usually zeners). Was able to deliver hard edged, saturated tones at more reasonable volumes to those that needed it.

The hot rod Marshall guys look to his ideas for inspiration.

[mxrshiver]

Paradoxically, the signal becomes less square as you overdrive later stages with a high amplitude square wave.  This may be the effect you're hearing. This is also why a Fuzz Face into a clean amp doesn't sound as pleasant as one into an overdriven amp.

...whoa 

your first paragraph is exactly how i was expecting this circuit to work. but *this* one is not an effect i was familiar with! can you elaborate and/or link to further reading? thanks so much!!

the part i don't get is this. when i have the Master set low to moderate, and the preamp volume and tone controls dimed, the diode clipping makes a BIG difference in both tone and volume, in exactly the expected ways. so i know that the preamp/cathode follower is able to deliver at least 40Vp-p to get the zeners to clip the wave. and i know that the signal is actually much larger than that, because of the huge volume difference when the zeners are switched in or out of circuit. (still haven't scoped it or fed a clean sine wave yet, currently just using guitar as i'm tuning some values by ear)

let's say for sake of example that the cathode follower is able to produce 80Vp-p output, it's probably more than that but it'll serve this purpose. so when the diodes are switched into the circuit, the max voltage hitting the top of the Master pot should be 40V, and when the diodes are switched out, it should be 80V.

so i would assume that no matter where the Master is set, the diodes being switched in and out of circuit would cause a big drop in volume. with the Master set halfway up, it would output 20V with diodes and 40V without... and wide open, 40V or 80V.

but what i'm experiencing is that this effect only happens when the Master is set quite low... and there's absolutely no difference by the time it's set moderately high. what am i missing? it's just weird that the diodes wouldn't at least clamp and limit the voltage and have some major effect on the tone and/or volume, even if it is pushing it more towards a sine wave again!

[mxrshiver]

i=56W6S8SD5kF0qL0G

this was my main reference for first learning about this circuit block, i LOVE Headfirst Amps videos, he's got a lot of well informed and considered ideas that he explains with a lot of patience, his amps always sound FIRE and he's GREAT at showing them off 🔥

[pdf64]

I find such videos too tedious, sorry.
The power amp is the final cliing stage. Once the input level is large enough to make it clip, increasing the input level any further can't increase the output peak voltage.
And if the tone controls are set for a mid scoop, overdriving the output will make the tone more mid rangey.
It's the same scenario as daisy chaining 2 or more overdrive pedals.

[mxrshiver]

The power amp is the final cliing stage. Once the input level is large enough to make it clip, increasing the input level any further can't increase the output peak voltage.

the issue i'm having is not that i'm increasing the input voltage more and it's not increasing the output voltage.

on the contraty, i quite like the 250KA value chosen for the Master pot resistance as it continues to increase volume throughout its whole range, whereas a 1MA pot may have become a bit less useful for volume towards the top end of the range, as the phase inverter/power amp are already well overdriven.

the issue i'm having is that introducing the diodes ceases to reduce the volume, only when the Master is set high, and i don't know why.

to my mind, the diodes reduce the signal level when switched into the circuit (as long as the signal is large enough for them to clip it), and since that reduction happens before the signal goes through the Master pot, it's a reduction that happens before and separately from the reduction the Master pot introduces. and since it reduces the amount of signal hitting the Master pot, it should produce a pronounced audible effect no matter where the Master pot is set.

to use your analogy, let's imagine an overdrive pedal using clipping diodes in front of an amp's input. let's say for simplicity that the pedal can output 1V as a clean boost, but introducing the diodes clips the signal down to 0.5V. i would expect this pedal to produce a significant volume drop when the diodes are switched in, or at the least a very significant impact on tone, no matter how the amp's controls are set. if having the amp set loud completely swamped the effects of an overdrive pedal, no one would use them... right?? obviously there's something i'm still missing here, heh

[AlNewman]

Maybe try a 500k pot for the master volume.

[astronomicum]

With the master down, the load on the follower is 250k. With it up, the tone stack is in parallel with the master so the load drops. I would expect that the AC signal will drop also. Inject a signal and measure the cathode AC with your meter at both settings and see what is happening.

[Merlin]

The power amp is the final cliing stage. Once the input level is large enough to make it clip, increasing the input level any further can't increase the output peak voltage.

the issue i'm having is not that i'm increasing the input voltage more and it's not increasing the output voltage.

When the input voltage to the power tubes drives them to clipping, it will override or erase the clipping that your diodes added earlier. Turn the MV up further and the PI will clip too, erasing the effect of the diodes even more. Sounds like your circuit is working as expected.

[mxrshiver]

When the input voltage to the power tubes drives them to clipping, it will override or erase the clipping that your diodes added earlier. Turn the MV up further and the PI will clip too, erasing the effect of the diodes even more. Sounds like your circuit is working as expected.

ahhhh i knew i was missing something! i scoped it, and the diodes do produce the expected signal level drop at the top of the Master, regardless of how its set - but you're right, the power tubes and PI are just clipping deeper into the signal than the diodes did, with the Master turned up far enough. so simple in retrospect! this was very instructive as to the effects of different gain staging, and what stage clips first... thanks very much Merlin, and everyone else.