Clip: DIY JCM800 Atomica/Jose

[FourT6and2]

Finished this build recently. Ceriatone chassis/turret board, but changed most of the circuit into an "Atomica" style Jose mod. But maybe a little more aggressive/modern.
In place of the trimmer on the back, I have a 6-position rotary for different clipping options. Though I find myself using 2N5401s most of the time.

[Chasin' Note]

Do you have a schematic for this?

[olddawg]

A very clean build...

[FourT6and2]

Do you have a schematic for this?

Typical Jose. Start with a JCM800 type of circuit with dual gains and a pre-tone stack master volume.

Cathodes are 2K7/.68, 2K7/.68, 820R/.68
First stage plate is 320K total. In this case 100K and 220K in series with a 330pF bypass.
First coupler is 0.022uF, second stage is 0.0022uF
4700pF bright cap on Gain 2.
68K/68K voltage divider on Gain 1 (68K from wiper to ground and 68K input grid on V1b)
Typical Jose mod with whatever zeners or transistors you like
Depth mod with whatever value cap you like
Everything else stock JCM800

And just go from there if you want to tweak something.

[J M Fahey]

Typical Jose means nothing to us.

... whatever zeners or transistors you like
... whatever value cap you like

Would you care to share an actual schematic?
Thanks.

[FourT6and2]

[Chasin' Note]

Thanks bro

[J M Fahey]

Thanks
NOW we know what are you talking about, .

So basically cold cathode stage removal and added clipping diodes , so end result is way less second
harmonic and a lot more third ... a woodier sound comopared to original reedier one,which is the core of JCM800 sound.

Total gain not much changed because what´s increased by bypassing (and rebiasing) cold cathode and increasing V1 plate load is lost by heavily loading first stage with a 68k resistor.

Rest of circuit looks roughly standard.

[FourT6and2]

Thanks
NOW we know what are you talking about, .

So basically cold cathode stage removal and added clipping diodes , so end result is way less second
harmonic and a lot more third ... a woodier sound comopared to original reedier one,which is the core of JCM800 sound.

Total gain not much changed because what´s increased by bypassing (and rebiasing) cold cathode and increasing V1 plate load is lost by heavily loading first stage with a 68k resistor.

Rest of circuit looks roughly standard.

The entire circuit is pretty standard. That's why I said "typical Jose." This thing is 40 years old by now.

[strato56]

I don't understand how this clipper works. I mean the BE-junctions of the 2N5401s.
Anyone cares to clarify ?

Thanks.

[nickb]

I don't understand how this clipper works. I mean the BE-junctions of the 2N5401s.
Anyone cares to clarify ?

Thanks.

They behave like back to back zener diodes. It's a guess at to what the zener voltage might be as it's not specified in the datasheet other than >5V.

[strato56]

Thanks ! I thought 5V was the breakdown voltage.

[FourT6and2]

Yeah, just using transistors for their internal zener-like behavior. They do sound different than using zeners of the same/similar voltage. The MPSA06 sound like 12v zeners and the 2N5401 clip a little more. Maybe like 9v-10v. But the character/feel is a little different than a zener. I have a rotary switch in this amp with a bunch of clipping options on it from 20v zeners, 18v, 16v, 12v, 9v, and a few transistor options. Not in the schematic, but you can see it in the photo.

[strato56]

Thanks for clearing that up !