First test of a Jose in my 1987 JCM800 4 holer

  • Thread starter Thread starter scottosan
  • Start date Start date
Since modding this amp, I felt that the volume was a bit lower and a little lower gain than other Jose mods that I’ve done, but I don’t have the reverse audio taper pots in hand yet and I attributed the gain to out of spec zeners

Well, today I got parts to put in the punch/depth control. 2 things really stuck out. Very scratchy depth and when depth all do the way up, the amp would actually mute. I opened it back up to inspect the presence and NFB circuit. Whomever gutted and hand wired the board used a 25k presence without the 4k7 shunt resistor, which meant I was getting g way too much NFB resulting in less gain and output. Added the 4k7 shunt and with the Cameron pinch circuit is simply menacing. I will probably try some different zeners again now that I found the issue.
 
scottosan said:
Since modding this amp, I felt that the volume was a bit lower and a little lower gain than other Jose mods that I’ve done, but I don’t have the reverse audio taper pots in hand yet and I attributed the gain to out of spec zeners

Well, today I got parts to put in the punch/depth control. 2 things really stuck out. Very scratchy depth and when depth all do the way up, the amp would actually mute. I opened it back up to inspect the presence and NFB circuit. Whomever gutted and hand wired the board used a 25k presence without the 4k7 shunt resistor, which meant I was getting g way too much NFB resulting in less gain and output. Added the 4k7 shunt and with the Cameron pinch circuit is simply menacing. I will probably try some different zeners again now that I found the issue.
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That also means the tail of the PI wasn't properly set either lol?

Which punch setup are you running? I've tried a few variations. Most recently was 47K NFB/8ohm, 1M depth, 470pF across pot, and a switch with another 47K and 4700pF in series. But I kinda just like a normal depth control with 2200pF cap more.
 
FourT6and2 said:
That also means the tail of the PI wasn't properly set either lol?

Which punch setup are you running? I've tried a few variations. Most recently was 47K NFB/8ohm, 1M depth, 470pF across pot, and a switch with another 47K and 4700pF in series. But I kinda just like a normal depth control with 2200pF cap more.
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Similar to stated 470pf across the lugs with 2200pf and 39k in series. 8 ohm tap and 47k NFB. The PI was good, just the 4k7 omission. I don’t like the taper of the shunted 25k so it will get swapped to a 5k
 
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scottosan said:
Similar to stated 470pf across the lugs with 2200pf and 39k in series. 8 ohm tap and 47k NFB. The PI was good, just the 4k7 omission. I don’t like the taper of the shunted 25k so it will get swapped to a 5k
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Well without that 4K7 resistor, isn't the tail of the PI the pot? Which would have been 25K (variable) instead of the correct 4K7. In the version with the 5K pot, the pot itself intentionally acts as the tail of the LTP. No big deal since you're changing it anyway. Just something interesting to be aware of.
 
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FourT6and2 said:
Well without that 4K7 resistor, isn't the tail of the PI the pot? Which would have been 25K (variable) instead of the correct 4K7. In the version with the 5K pot, the pot itself intentionally acts as the tail of the LTP. No big deal since you're changing it anyway. Just something interesting to be aware of.
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I don’t think so. The long tail is the 10k resistor. The 4k7 was added in the design later since before it was just a 5k pot that scratched when turning because of DC on the pot. Using a 4.7k resistor in parallel with the 25k accomplishes both shunting the DC to ground since is a much lower value to ground than the 25k pot and it also change the variable resistance of the 25k pot down to around 4k.

This removes the scratchiness from the pot, but most of the audible changes happen within a much narrow sweep of the pot.

The scratchiness is what made me check the presence pot resistance because I remember not seeing a shunt and it increased with the depth, which it wouldn’t have.
 
scottosan said:
I don’t think so. The long tail is the 10k resistor. The 4k7 was added in the design later since before it was just a 5k pot that scratched when turning because of DC on the pot. Using a 4.7k resistor in parallel with the 25k accomplishes both shunting the DC to ground since is a much lower value to ground than the 25k pot and it also change the variable resistance of the 25k pot down to around 4k.

This removes the scratchiness from the pot, but most of the audible changes happen within a much narrow sweep of the pot.

The scratchiness is what made me check the presence pot resistance because I remember not seeing a shunt and it increased with the depth, which it wouldn’t have.
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Weird. I always read, and was told by people much smarter than me, that the 5K pot acts as part of the LTP. I know the 4K7 shunts DC and is part of the reason why it was added later with the 25K pot (to remove the scratchy sound). But that is not it's only function... or so I thought. The presence control is between the PI 10K tail resistor and ground. So isn't it part of the LTP?


For example, from SDM:

"The 5K pot or the 4.7K resistor are both there connected directly to ground. They both serve as a tap in the phase inverter's tail resistor (which combined is 15K: the 10K plus the 5K pot or 4.7K resistor). Either the 5K pot or 4.7K resistor will have the DC bias from the tube flowing through them.

With the 5K pot used as your presence control, the pot's wiper is connected to the .1u presence cap, and turning this control causes the static sound due to DC running through the pot directly.

Marshall was apparently bugged by this, so they dumped the 5K pot and put in the 4.7K resistor instead. This keeps the phase inverter biased the same as before and provides the same ~5K tap in the tail resistor."


All I was saying is that without that 4K7 resistor, your PI tail resistance is no longer 15K. It's 35K or whatever value of pot you used. It doesn't really matter because you're changing it anyway. But I just thought it was an interesting thing to mention.
 
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FourT6and2 said:
Weird. I always read, and was told by people much smarter than me, that the 5K pot acts as part of the LTP. I know the 4K7 shunts DC and is part of the reason why it was added later with the 25K pot (to remove the scratchy sound). But that is not it's only function... or so I thought. The presence control is between the PI 10K tail resistor and ground. So isn't it part of the LTP?


For example, from SDM:

"The 5K pot or the 4.7K resistor are both there connected directly to ground. They both serve as a tap in the phase inverter's tail resistor (which combined is 15K: the 10K plus the 5K pot or 4.7K resistor). Either the 5K pot or 4.7K resistor will have the DC bias from the tube flowing through them.

With the 5K pot used as your presence control, the pot's wiper is connected to the .1u presence cap, and turning this control causes the static sound due to DC running through the pot directly.

Marshall was apparently bugged by this, so they dumped the 5K pot and put in the 4.7K resistor instead. This keeps the phase inverter biased the same as before and provides the same ~5K tap in the tail resistor."


All I was saying is that without that 4K7 resistor, your PI tail resistance is no longer 15K. It's 35K or whatever value of pot you used. It doesn't really matter because you're changing it anyway. But I just thought it was an interesting thing to mention.
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I don’t disagree with any of that. I was more so implying that the 4k7 more often referenced as the shunt resistor while the long tail resistor is the 10k in a Marshall. I will take a scratchy 5k with no shunt 10 times out of 10 for the even taper.
 
scottosan said:
I don’t disagree with any of that. I was more so implying that the 4k7 more often referenced as the shunt resistor while the long tail resistor is the 10k in a Marshall. I will take a scratchy 5k with no shunt 10 times out of 10 for the even taper.
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I totally agree. I prefer the 5K pot and no shunt resistor as well.
 
Matt300ZXT said:
Eh, if your neighbors ever complain, just play some Marty Friedman licks. They LOVE him over there lol
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I just assume Marty Friedman is like Jackie Chan over here or David Hasselhoff in Germany.
 
I like the cocked wah sound too but without the additional "metal" voicing this now has.. everything is going to sound the same
 
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