Depth mod gurus

lespaul6

Well-known member
How does changing the resistor value in a fixed depth circuit affect the tone and feel. right now i'm using the typical dave friedman style fixed setup: .0047 cap and 220k resistor in series with the 47k negative feedback resistor ( to the 4 ohm jack). If I change the 220k resistor to something like 150k or 100k how will that alter the tone and feel? all else being the same.
 

FourT6and2

Well-known member
How does changing the resistor value in a fixed depth circuit affect the tone and feel. right now i'm using the typical dave friedman style fixed setup: .0047 cap and 220k resistor in series with the 47k negative feedback resistor ( to the 4 ohm jack). If I change the 220k resistor to something like 150k or 100k how will that alter the tone and feel? all else being the same.

Think of a typical depth setup with a dial on the front of the amp. That dial is a potentiometer = variable resistor. A resistor is simply like a pot set to a fixed position. So what would happen if you increase the value of that resistor, like turning up the dial on the pot?
 

lespaul6

Well-known member
So it would be like turning the dial down then... ok, I get it. As opposed to changing the value of the negative feedback resistor or is that a different way of doing the same thing?
 

FourT6and2

Well-known member
So it would be like turning the dial down then... ok, I get it. As opposed to changing the value of the negative feedback resistor or is that a different way of doing the same thing?

The NFB resistor will have a different effect. If what you want to do is "turn the depth down" a bit, then leave the NFB as it is.
 

SpiderWars

Well-known member
Increasing NFB (decreasing the NFB resistor or going to a higher speaker tap) will tend to tame the amp. Tighter lows and less aggressive highs/mids and just less raw all around. Decreasing it does the opposite. And you know know what a Depth control does. Hope that helps.
 

lespaul6

Well-known member
Increasing NFB (decreasing the NFB resistor or going to a higher speaker tap) will tend to tame the amp. Tighter lows and less aggressive highs/mids and just less raw all around. Decreasing it does the opposite. And you know know what a Depth control does. Hope that helps.
right now the high end is perfect but the string to string definition on the bottom strings is a little rubbery and lacking definition, hard to explain but it squishes out a little rather than staying defined for each note played.
 

glpg80

Well-known member
Experimenting with different types of coupling caps as well as different types of cathode bypass caps can resolve that.

That problem isn’t necessarily a NFB problem.

What you describe isn’t even a different value thing, but a cap brand thing.

WIMA, xicon, high voltage ceramic, sprague orange drops, etc. can all provide sharper more defined highs. All of the vintage flavors or caps that are reissues tend to have that relaxed high end smear character.

However the latter do one thing the former dont, and that’s add a complex balance of good harmonics to the gain. In my 74 I have a mix of both types. It all comes down to personal preference. I tend to hate amps with all one kind though. I don’t like wizards with all orange drops, and I don’t like old Marshall’s with all original spragues. You need both for an amp to sound good. Where and what value is where it gets challenging.
 

lespaul6

Well-known member
Experimenting with different types of coupling caps as well as different types of cathode bypass caps can resolve that.

That problem isn’t necessarily a NFB problem.

What you describe isn’t even a different value thing, but a cap brand thing.

WIMA, xicon, high voltage ceramic, sprague orange drops, etc. can all provide sharper more defined highs. All of the vintage flavors or caps that are reissues tend to have that relaxed high end smear character.

However the latter do one thing the former dont, and that’s add a complex balance of good harmonics to the gain. In my 74 I have a mix of both types. It all comes down to personal preference. I tend to hate amps with all one kind though. I don’t like wizards with all orange drops, and I don’t like old Marshall’s with all original spragues. You need both for an amp to sound good. Where and what value is where it gets challenging.
And thats a very good point(s)... I know that something is off just not entirely sure where the issue is. I thought that the whole approach of thinning out the preamp and getting the bass back in the power amp section was the culprit, maybe it is? I just know it could track faster on the lower registers while maintaining the sound of each picked note a little better. My AFD and 2555 track riffage on point. This JEL mod with the standard fixed depth is a little bloated on the bottom, causing the recovery time between each picked note to be slower, if that makes sense. The soldano slo is also like this. to me anyway
 

psychodave

Well-known member
Don’t forget you can also run an additional resistor in parallel with the depth cap like Cameron does. Different values with focus the sound in different areas/ways. Cameron uses 39-47k for the depth switch on the CCV.
 

panhead

Well-known member
And thats a very good point(s)... I know that something is off just not entirely sure where the issue is. I thought that the whole approach of thinning out the preamp and getting the bass back in the power amp section was the culprit, maybe it is? I just know it could track faster on the lower registers while maintaining the sound of each picked note a little better. My AFD and 2555 track riffage on point. This JEL mod with the standard fixed depth is a little bloated on the bottom, causing the recovery time between each picked note to be slower, if that makes sense. The soldano slo is also like this. to me anyway
Most likely cause both those marshalls have a 100k pot on the midrange. Jmp/SLO etc have 25k.
 
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RedPlated

Well-known member
Increase the slope resistor value an you'll get faster tracking of low notes. I like 39k in a Marshall. 33k can be bloated, depending on the rest of the circuit. 33k works great in some scenarios. It just depends on the overall setup. I even use 36k sometimes.

Also, what is the value of your bias splitter resistors? I prefer 220k over 150k or 82K.

Lastly, check your preamp voltage at the phase inverter. Marshalls can be notoriously low from the factory, like sub-300vdc. Adjust the dropping resistor value to get it to around 330-340vdc at least. This will improve string separation as well.
 
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SpiderWars

Well-known member
And thats a very good point(s)... I know that something is off just not entirely sure where the issue is. I thought that the whole approach of thinning out the preamp and getting the bass back in the power amp section was the culprit, maybe it is? I just know it could track faster on the lower registers while maintaining the sound of each picked note a little better. My AFD and 2555 track riffage on point. This JEL mod with the standard fixed depth is a little bloated on the bottom, causing the recovery time between each picked note to be slower, if that makes sense. The soldano slo is also like this. to me anyway
To me, that recovery time sounds like its a preamp thing more than a power amp thing. But Depth circuits can make it seem bloated. Are you sure it is the Depth circuit that is causing this? That 220k to ground in parallel with the gain pot should keep the low end intact when in hi gain mode. This circuit should be really bright.
 

Cap217

Active member
Increase the slope resistor value an you'll get faster tracking of low notes. I like 39k in a Marshall. 33k can be bloated, depending on the rest of the circuit. 33k works great in some scenarios. It just depends on the overall setup. I even use 36k sometimes.

Also, what is the value of your bias splitter resistors? I prefer 220k over 150k or 82K.

Lastly, check your preamp voltage at the phase inverter. Marshalls can be notoriously low from the factory, like sub-300vdc. Adjust the dropping resistor value to get it to around 330-340vdc at least. This will improve string separation as well.

Are you checking the PI voltage on pin 1 or 6? And if you adjusted these voltages, would you change the stock 100k/82k resistors?
 

glpg80

Well-known member
Are you checking the PI voltage on pin 1 or 6? And if you adjusted these voltages, would you change the stock 100k/82k resistors?
Careful, those 100k/82k resistors are chosen to balance the phase inverter triode offsets that occur by design of a normal 12AX7 tube. You want those to be exactly 100k and 82k. If they have drifted, replace them with 100k and 82k resistors. To increase the voltage to the PI itself you’d adjust the dropping resistors up stream.
 

Cap217

Active member
Careful, those 100k/82k resistors are chosen to balance the phase inverter triode offsets that occur by design of a normal 12AX7 tube. You want those to be exactly 100k and 82k. If they have drifted, replace them with 100k and 82k resistors. To increase the voltage to the PI itself you’d adjust the dropping resistors up stream.

That makes more sense. But wouldnt changing the dropping resistor values effect V1 and V2 as well?
 

glpg80

Well-known member
That makes more sense. But wouldnt changing the dropping resistor values effect V1 and V2 as well?
The Pi triodes are part of a design called a long tailed pair. Increasing the voltage to both sides equally and maintaining the set ratio is far more important than adjusting those values separately. You’re right though, it does walk the linearity curve of a 12AX7 however that’s what Jeremy was making an intentional point about :)
 

FourT6and2

Well-known member
Increase the slope resistor value an you'll get faster tracking of low notes. I like 39k in a Marshall. 33k can be bloated, depending on the rest of the circuit. 33k works great in some scenarios. It just depends on the overall setup. I even use 36k sometimes.

Also, what is the value of your bias splitter resistors? I prefer 220k over 150k or 82K.

Lastly, check your preamp voltage at the phase inverter. Marshalls can be notoriously low from the factory, like sub-300vdc. Adjust the dropping resistor value to get it to around 330-340vdc at least. This will improve string separation as well.

Be careful. You might want to explain what else happens when changing the slope resistor value in case someone reading this doesn't know. It's all personal preference, but I've found the tried and true 33k value works the best for me. Anything higher, while it can sound good at first, over time it just gets old and I found myself reverting back to 33K. I think there are other things in the circuit you can do instead. But if one must... I guess 39K is a good starting point.

For bias resistors, you're talking about the grid leak/PI output resistors?

For preamp voltages, this is very important. But you gotta make sure your PT can supply the right voltages in the first place. If the main dropper is already 10K and your preamp voltage are too low, then you're kind of in trouble lol. I like to just make sure my V1a B+ is at least 160v in an amp like this.
 

FourT6and2

Well-known member
Are you checking the PI voltage on pin 1 or 6? And if you adjusted these voltages, would you change the stock 100k/82k resistors?

He's talking about B+, measured at the PI node. I wouldn't touch the stock 100k/82k. You want to adjust the main B+ dropper. I wouldn't go lower than 10K. And it's a balancing act to make sure all the preamp voltages are where you want them. For me, I don't want to go lower than 160v on V1a. And I tune the amp's droppers at every node as well as V1a plate resistor to get at least 160v there. Because then I know all the other voltages are high enough. There are a few other reasons as well that I won't go into.
 
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