Dual Rectifier Cold Clipping Mods

I know I previously stated I would discontinue writing about mods, but this one has been in the back of my head for a long time.  A lot of the grind produced by the cold clipping stage is due to the presence of third harmonics (H3).  By forcing the idle point to be so close to grid current limiting, the amount of information loss from the distortion is going to produce it.

Yesterday, I was playing around with the Trioda load line program and found an interesting effect.  With the parameters set about where the resistances would be for V2b, the cold clipping stage, I was adjusting the cathode value.  As the cathode resistance moves from 39k to 10k, the amount of H3 falls to an amount which would become inaudible.

The Test

The initial experiment was to find where the H3 reaches zero, or very close to zero.  The value where that could be found was about 9.5k, where the amount of H3 was measured in thousandths of a percent.  Moving it up to to 10k increased the H3 to roughly half of 1 percent.

I’m not sure whether Marshall intended to have their JCM 800 cold clipping at that point, or if it was done by ear, but it’s a really great spot; the golden zone.  If the power is scaled up or down, the effect of a 10k cathode resistance on the ratio of harmonics remains fairly stable, which I found to be amazing.

Now, of course, the total amount of harmonic distortion (Ht) decreases as the cathode resistance decreases.  From 39k to 10k, the second harmonic drops from almost 49% to roughly 35% when the grid is completely cutoff.

Application

For today’s guitarist, the distortion produced by 10k at the cathode may not be enough, particularly in a Dual Rectifier.  It occurred to me, rather than completely eliminate the H3, what if it is brought down enough to be overpowered by H4 or to be just short of equal size?

There is a “sweet spot” for this, as well.  With a cathode resistance of 15k, the H4 is always higher than H3.  However, H2 is reduced to a maximum of 40%, which is still significantly nasty distortion for people who want to play Metal, but it is a big loss.

An alternative for those who want to really push the distortion and still reduce the H3 grind, is to use a cathode resistance of 22k.  At lower input levels the H3 is dominant to H4.  However, when the maximum signal is applied, the H3 drops and H4 increases.  This could provide a good balance between regular Recto tones and reduced harshness.

Of course, this is all speculative and based on simulations.  A person will need to try it out and experiment to find the sound they desire.  One additional consideration is that reducing the cathode resistance increases gain, which will create more pressure going into V3 and will drive that stage harder.

Data For Maximum Signal Conditions

• 39k: H2 49%, H3 7.5%, H4 5.5%, Ht 49.5%
• 22k: H2 44%, H3 3.9%, H4 5.8%, Ht 44.6%
• 15k: H2 40%, H3 1.7%, H4 5.3 %, Ht 40.5%
• 10k: H2 35%, H3 0.5%, H4 4.3%, Ht 35.5%

(Update) Rather than changing the cathode resistor, the anode resistor can be changed.  This would affect the cutoff frequency for the filter.  For instance, using 150k anode and 39k cathode, the frequency cutoff is 1.06 kHz, the gain is increased from 2.2 V/V to 3.3 V/V, and the harmonics would be:

• H2 46.2%, H3 5.5%, H4 5.7%, Ht 46.9%.

A person can choose to also change both the anode and cathode resistors to meet their requirements for distortion and frequency cutoff.  Using 82k at the anode and 22k at the cathode puts the frequency cutoff at 1.94 kHz, sets the gain at 3.3 V/V, and produces the following harmonics:

• H2 45.2%, H3  4.6%, H4 6%, Ht 46%

Using 68k on the anode and 22k at the cathode, the frequency cutoff becomes 2.3 kHz, gain is 2.6 V/V, and harmonics are:

• H2 46.3%, H3 5.5%, H4 6%, Ht 47%

I hope this has been helpful.  Good luck.