Diode shootout - Zeners and transistors on a Marshall JMP comparison video.

  • Thread starter Thread starter RedPlated
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I have the 220k/.0047 per the JEL spec. That’s a great fixed depth value for most amps. It essentially simulating a 1 Meg audio taper pot at around 1 o clock.

I run a 3 way switch on my Detonator with 3 different value capacitances across the adjustable depth pot. Can really alter the tone pretty dramatically.

Another easy spot to dial this mod in would be the bright cap across gain pot. Lots of flexibly here.
 
You can try using a .0022 for a more Marshall sound (or .003, etc. I think VHT uses a .0068uf cap for his amps) You can also use an additional resistor like Cameron does and it really has a tonal effect. I added a 39k resistor to the depth pot on my JCA20 on a switch. Flick it on and the sub-mids get focused and tighter with more cut.
There are so many things to tinker with… as I’m sure you know. Makes for lots of tinkering. 😂
 
It's endless!

I've tried most of what you've mentioned and/or put it on a switch. But there is always something new to learn and I love hearing new ideas.

Where/how did you add the 39k on the depth pot?
 
I liked the 20 most overall. I also liked the 16 for the saturation but it lost a smidge of what I liked in the 20. I bet an 18 would be a great marriage for those two dynamics. You mentioned a 20/18 which I would be really interested in hearing and would probably suit my tastes perfectly. Anyway, awesome video, thank you for sharing it.
 
harddriver said:
You could also try two IN5400 diodes (oriented assymetrically) I'd have to pull the chassis. I read Dave Friedman liked those as well. I'm pretty sure I used those for a plexi saturation switch in my Jose/2203 build just off of the standard JCM800 master and it really adds alot of sag to the feel/tone when switched in. I'm running 20V zeners off of the Jose Master pot on a switch on that amp, and I added the plexi sat switch off the standard JCM800 pot that I left in that 2203 build.

I just looked up the voltage clipping threshold on the IN5400 and it is 50V. The IN 5408 was 1000V so it was the IN5400 that I used in the plexi switch. the IN5408 would be used in the mains rectification circuit in for B+ and bias supply. You'd have a hard time clipping 1000V diodes. I think the 50V IN5400 soaks up quite a bit of current giving a really saggy effect.
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I pulled the chassis to verify what IN5400 I used for the plexi/sat switch. Well... it looks like I used two IN5408's which are 1000V diodes. I would imagine I used those for s specific reason when I came across the recommendation to use it supposedly Dave F was using the 5400's prior to the MPSA06's, that's what I remember anyway.

So in short it appears the IN5408 functions just fine in this application so I'm going to go out on a limb and theorize that the 1000V spec really don't clip but they are absorbing current giving a sag effect instead of clipping the sinewaves like a lower voltage 10-20V zenere does.
 
@harddriver
I think 1N5408 would clip at the typical diode 0.7v reverse breakdown voltage.

Regarding the asymmetrical clippers, you need to try it in both directions. One way will sound better than the other. My Jose which is very Molecular-like has 20v/18v on 70s mode and 4x MPSA06 (connected BE-EB-BE-EB) in 80s mode. And a Cameron-inspired Depth control with added resistor but I can't remember what it is.
 
SpiderWars said:
@harddriver
I think 1N5408 would clip at the typical diode 0.7v reverse breakdown voltage.

Regarding the asymmetrical clippers, you need to try it in both directions. One way will sound better than the other. My Jose which is very Molecular-like has 20v/18v on 70s mode and 4x MPSA06 (connected BE-EB-BE-EB) in 80s mode. And a Cameron-inspired Depth control with added resistor but I can't remember what it is.
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I was just going off the specs I saw at Mouser. 0.7V would clip easily then.

Absolute Maximum Ratings: (TA = +25°C unless otherwise specified, Note 1) Peak Repetitive Reverse Voltage, VRRM DC Reverse Voltage, VR

1N5400 ..................................................................... 50V
1N5401 .................................................................... 100V
1N5402 .................................................................... 200V
1N5403 .................................................................... 300V
1N5404 .................................................................... 400V
1N5405 .................................................................... 500V
1N5406 .................................................................... 600V
1N5407 .................................................................... 800V
1N5408 ................................................................... 1000V
Maximum RMS Voltage
1N5400 ..................................................................... 35V
1N5401 ..................................................................... 70V
1N5402 .................................................................... 140V
1N5403 .................................................................... 210V
1N5404 .................................................................... 280V
1N5405 .................................................................... 350V
1N5406 .................................................................... 420V
1N5407 .................................................................... 560V
1N5408 .................................................................... 700V
Average Rectified Current (.375” Lead Length, TA = +75°C), IO ............................ 3A
Peak Forward Surge Current, IF(surge)
(Superimposed on a Rated Load, 8.3ms Single Half−Sine Wave) 200A ..................
Total Device Dissipation, PD ....................................................... 6.25W
Derate Above +25°C 50mW/ ..................................................... °C
Operating Junction Temperature Range, TJ .................................. −55° to +150°C
Storage Temperature Range, Tstg .......................................... −55° to +150°C
Thermal Resistance, Junction−to−Ambient, RthJA .................................. +20°C/W
 
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Yeah, they're not zeners just regular diodes so they should clip a ton.
 
They work great for the application of providing sag to the input signal. I think I read about Dave using them on Metro on some early builds.
 
RedPlated said:
It's endless!

I've tried most of what you've mentioned and/or put it on a switch. But there is always something new to learn and I love hearing new ideas.

Where/how did you add the 39k on the depth pot?
Click to expand...
So I have two .001uf caps in series across the pot. Then have .0033 on a switch (old Erie .0022 and Mallory .001up). The switch up engages the 39k resistor to focus some mids. (Keep in mind the pic is of the bottom so “up” would be with the amp in its operating position). I wired it like this since I didn’t want to ruin that old red Erie cap. That cap has a major effect on the amps sound. The second I put it in my jaw hit the ground.
The bottom pic is from a Cameron amp. The input is coming from the board to the back of the switch then to the lugs. Pull the pot and it engages the 39k resistor and .002uf caps (two .001uf in parallel).

1625599115218.jpeg

1625599529154.jpeg
 
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Ahhhhh, now I see. Thanks for pics/ info.

So it’s 39k in series with one leg of the depth cap, reducing the overall effect of the cap over the resistance. Makes total sense now that I see it.

I’ve been contemplating experimenting with bright caps and resistance in series more lately. I’ve done it before and it’s a great way to tune the circuit. Sometimes a bright cap can be too much and a little resistance will lessen the effect.
 
RedPlated said:
Ahhhhh, now I see. Thanks for pics/ info.

So it’s 39k in series with one leg of the depth cap, reducing the overall effect of the cap over the resistance. Makes total sense now that I see it.

I’ve been contemplating experimenting with bright caps and resistance in series more lately. I’ve done it before and it’s a great way to tune the circuit. Sometimes a bright cap can be too much and a little resistance will lessen the effect.
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Yeah, it’s hard to explain, but that picture is worth a thousand words. 😂. Changing the value of the resistor changes the way mids are heard. Of course you could add another pot there, but damn, when is enough enough 😂. On my CCV I prefer the depth switch to the left… which engages that 39 (I’ve seen 45k used as well) resistor, so modding my amp, I was replicating the CCV’s sound. I had to put it on a switch since the louder you play, the sound changes and I tend to want less high end.
 
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I think you should go a step further and add a pot for each cap value on the depth control! Haha…
 
blackie13 said:
I would put the 20v zener and the MPSA06 in a switch...

Actually I have already done this...!!!!
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I’d go even further and do a 3 way switch but add a 10v Zener in that mix. Or what about a 6 position rotors knob!? ;)
 
I picked up a bunch of these Wima 470pf 400v caps. Put one on the JMP today. Works great. I made a video with that cap in and out. Might be hard to hear in video, but the amp does get a bit harsh without it. But you do give up a little bite. You can especially hear the ice pick on the upper single strings… I left the cap in place.

I have 16v Zener in it at the moment.

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I see you have a electrolytic bypass cap on the V2A 820 what is it 10uf/25uf?

I can hear the dampening the 470pf cap imparts when you remove the 470pf cap on the V2B 100K cathode resistor.

You keep adding more and more to this amp soon it will be a Detonator! :D:shocked:
 
Haha… this is one is just the Jake E Lee spec. Part of the mod is a 22uf cap on the 820 ohm. I prefer Sprague Atoms so this is a 25uf. I think above 22uf you’re not going to pass any additional noteworthy frequencies.

The JEL spec is:

.0022 uf first stage coupler

10k cold clipper changed to 2.7k / .68uf

220k resistor parallel on gain pot

22uf or bigger on v2a 820 ohm

470pf on 100k v2b cathode

.0047/ 220k depth

I also added a shielded coax input and ditched the hot shield.
 
So the 220K resistor on the 1 Meg gain pot is shunting it down to what value pot?
 
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