S.I.R. 100W SuperLead Schematic pt. II

[Unique]

That is one view. It's not clear enough to say. There appears to be something "brownish" with solder at the bottom, on the right side of the pot and it appears to be connected as the normal 25k circuit. The 4.7k resistor is most often soldered to the buss wire, not the back of the pot. That is where the components are usually mounted in the 25k circuit, but is not a very clear image. Santiago drew it as a 25k pot with the cap connected as in the 5k circuit, or it's a simple mistake in his schematic. One thing for sure, removing the 4.7k and leaving the cap connected as in the 25k circuit, is the wrong thing to do. It will affect aspects of the PI operation. Without the 4.7k and with improper wiring, the PI cathode has no proper ground because the presence cap, as connected in the 25k circuit, blocks the DC from ground. I repaired an amp with this mistake once. Before the repair, the PI voltages were all screwed up and it sounded like poo. Try it all three ways if you want to. Just let us know your thoughts afterward.

I am definitely going to take a look at it again. I have been going through a lot of PI tubes, that's for sure. So this is beginning to make sense to me as why that is. But it throws nothing but more confusion into the fire with #34 and there being no resistor on the presence pot and it looks wired like mine as far as I can tell.

[Unique]

I've been trying to post some pics but all I keep getting is this:

"It was not possible to determine the dimensions of the image. Please verify that the URL you entered is correct."

So until I can figure this out pics will have to wait...

[CoffeeTones]

Coffee first let me say that your circuit knowledge is unmatched in this thread and I have learned a lot from you. So I can't really comment on the technical aspects as you do. All I can say is what I see, with the knowledge I have, and speak from my own experiences. I have had this resistor off for about 3 years and have never had any issues and everything works as it should. Although I do seem to go through a lot PI tubes? Could that be the reason?

It's possible but I have not put any thought into why and would need to recreate the scenario to measure voltages and such. I'd assume your PI voltages are pretty abnormal, depending on exactly how things are connected. Take some readings from the cathode and plates and lets compare them to the original voltages.

From my understanding, the resistor on the presence pot is only there to keep DC off the pot with a 25k pot as DC can be scratchy sounding. From my experience, removing this resistor did not make the amp sound that scratchy, more like a bit more 'raspy' sounding. I found removing this resistor is what gives that wah sound some depth and it's woolliness. Something I don't really hear from most #34 mods outside of #34.

The resistor is there to give the PI a ground reference as it forms a part of the total tail resistance and also the ratio divider for the NFB. It also references DC to ground, so the pot is not experiencing the load, thereby not creating noise when the pot is rotated. No doubt the sound changes because of the PI bias and NFB has changed

When I first noticed that the resistor was not on the presence pot in #34, I researched this as much as I could to learn more about it. From what I could see in the pics, and then compared it to how my presence circuit was wired before removing that resistor in my amp, I seen no difference other than the absentee resistor.

I see the same in the pics but the resistor being there or not is not clear to me in that video

From all I read, it seemed to be no big deal removing this resistor other than your allowing for more DC on the pot.

Again, the PI current and NFB changes - it has to because you changed the resistance. Ask yourself this... what happened to the DC that was referencing ground through the 4.7k tail resistor before you removed it? How did this resistance change affect the PI voltages and PI cathode bias current, because now you have changed the cathode resistor value? What has happened to the NFB ratio?

Answer: The DC can't get to ground through the presence pot because the presence cap wired as in the 25k circuit, won't let the DC pass through and it has to float around. The way to solve this issue is to replace a proper ground reference by re-installing a resistor or re-configuring the pot wiring to the 5k pot circuit connections, either with 5k pot or 25k pot. If you use the 25k pot, the NFB ratio, along with PI voltage and current is still altered by the 25k tail value, although now the PI tube may live longer and you still may have a sound you like.

The total of the 470r, 22k in #34 and final tail, whether it be 4.7k resistor, 5k pot & circuit or 25k pot & cap connected as in the 5k circuit, will all affect the PI and NFB as a fixed value, but will affect presence as an adjustable resistor. The ground reference is maintained. The 25k circuit without the 4.7k to ground will affect the NFB and presence as an adjustable resistor. The ground reference is compromised. In using a 5k pot and circuit, the 5k (close enough to 4.7k) tail value and NFB ratio is still maintained, because of the way the pot is grounded and the way the cap is connected.

I found this to be a key part of #34's sound. Whether it's right circuit wise or not as you say, I'm not sure, this is how #34 appears to be set up as far as I can tell. I'm not saying what you've stated is incorrect, you are probably right. I will have to take another look at the schematic Santiago made. But if I remember, it gets cut off right there and does not show the 4k7 resistor.

Yes, even if the 25k pot is connected as Santiago left it in the schematic, the NFB and PI experience the changes I have discussed (47k total tail resistance of the 22k resistor and 25k pot in series, instead of the original 10k and 4.7k which totals 14.7k and then the 25k pot instead of the 4.7k resistor as the grounded portion of the NFB). The only difference is that with the pot & cap connected as in the schematic, you now have a proper ground reference for the PI. The 25k pot wired as the 5k circuit will always apply it's total value of 25k as part of the PI the tail and NFB divider portions, no matter the pots rotation. The 5k circuit does the same, which is similar in resistance value to the 4.7k, so you always have either 4.7k (with resistor) or 5K (with pot). The rotation of the pot should only affect the AC signal being sent to ground and not affect the DC potential. Marshall did it this way to retain the PI and NFB operation but get rid of the scratchy pot. As you apparently have it wired, you are affecting the NFB and AC. DC is affected because the ground reference has changed

Let me add... what a lot of folks do not realize, is the fact AC and DC travels on the same conductor / path unless the DC is blocked by a capacitor. Once this is realized, a lot of things about tube amps become more clear.

As far as the pics goes, I stated them being "clear enough" to see what is what. I do have the same pics of #34's circuit from every angle as anyone else who has went through that video frame by frame. I even spent a lot of time trying to enhance them, brightening them up and sharpening them some to clear them up a little. But they are clear enough to see what is what.

Now here is another mystery that really stumps me. And that is, there is no 150k resistor on the back of the PA pot in #34, neither is there a bright cap. That little brown mark that everyone says is a resistor is actually a burn mark on the green wire, where Frank or someone hit it with the iron. Zoom in on the photo's, enhance the color and brightness, and you will see it clearly. I tried my amp without these two components and it didn't sound too good. I believe without a doubt that they are part of the mod. So why are they missing?

I'll have a look. I just wish they had filmed it with a decent camera.

There is three possibilities for this I can think of. One, when they shot the video for the AFD100 vlog, they had taken the components off for whatever reason and did not get them back on in time for the video shoot. I also thought about this for the presence resistor. But it doesn't make much sense that they would be removing components unless they were trying to measure them, or something. Even then, that doesn't really seem like a good reason.

Two, it's possible that Slash had a 150k pot installed for the PA and did not need the 150k resistor anymore. But from what I can tell, the pot looks to be the original. I think a newer pot would show a slight difference in color and age when looking at the pics and would stand out somewhat. I could be wrong, but that is my thinking.

And three, which to me seems most likely, I think that maybe at some point Slash had them removed so he could have more options with setting the PA. In the early days when recording he normally set his PA between 1 and 2 (as per Adam Day). As of late, when you look at the studio track sheets for his first solo album he is now setting the PA between 5 and 7, and you see this also in pics of #34 in studio settings on the web. If you do that with the 150k on the back of the PA pot, it doesn't sound too good, and definitely not like what you hear with Slash's sound at anytime. But the tubes your using definitely can have a huge effect on this. Since he doesn't use any kind of attenuator or PPIMV with #34 he really has no need for the bright cap either. I believe these components were there in the early days, but have since been removed. Frank still continued to implement these in his #34 mods when he did them. But he also did a few things different today than how he did it in #34 from the pics I've seen of Franks more recent #34 mods.

A 150K pot would be much different than the other config, but would go in that direction as far as brightness and attack, even without a bright cap. Either way, I'd prefer to have these parts on switches because I like to play more than GNR music.

Now here's where things seem to get cloudy. Santiago has stated that Slash told him nothing in the amp has ever been changed since he's got it. So that leaves this a mystery to me. Because without the 150k on the back of the PA pot, the amp loses a lot of it's drive, especially with the lower PA settings, and just sounded kind of flat to me. But this would be the case since that is what that resistor does, lowers the pot's value to achieve more gain at lower settings, correct?

The resistor makes an easier path to ground for the AC signal, therefore reducing gain and bass. All else being unchanged. The load seen by the following stage is also different. If you want to really know how this affects the sound and gain, put a 500k pot wired as a variable resistor in place of the 150k resistor and play guitar while rotating that pot in small increments. You could also leave it in place as a gain trim pot to get more versatility out of the amp.

All in all, my mod is exactly like what you see in the pictures with the presence cap removed, except I have the 150k on the back of the PA pot, and a bright cap (that came stock and I left it there), and it sounds spot on. It's only my playing that makes the biggest difference.

All I can say to that is, the only amp I have heard with the presence wired as you say yours is, sounded awful and IIRC, lacked power until the presence and tail was connected properly.

[blacklabel]

I'm loving your conversion

a little quote

Another thing people have missed on the schematic is that the resistor on the hot-shield is at the input and not the socket. This is also clearly shown in the pics of #34's. You can clearly see the resistor at the input. With my amp, having the resistor at the input does make a difference in the sound compared to having it at the socket.

So how it should be soldered this hot-shield to make it unsafe like in #34 ?
I understand that it has to go from the valve pin to the input with the resistance soldered here.
in my case it's welded to the valve pin

I've been trying to post some pics but all I keep getting is this:

"It was not possible to determine the dimensions of the image. Please verify that the URL you entered is correct."

So until I can figure this out pics will have to wait...

I'm using my mediafire account to upload images

[CoffeeTones]

a little quote

Another thing people have missed on the schematic is that the resistor on the hot-shield is at the input and not the socket. This is also clearly shown in the pics of #34's. You can clearly see the resistor at the input. With my amp, having the resistor at the input does make a difference in the sound compared to having it at the socket.

So how it should be soldered this hot-shield to make it unsafe like in #34 ?
I understand that it has to go from the valve pin to the input with the resistance soldered here.
in my case it's welded to the valve pin

What needs to be said here concerning the UNSAFE method, is that only ONE END of the shield portion / outer most wire lead, of the shielded wire unit is connected to the plate voltage at the socket. All other parts of the shield portion or outer most lead are heavily insulated with heat shrink tubing, thereby being isolated from ground and all other components. If not, it is potentially dangerous. The shielded wire assembly is normally fastened in place to prevent movement. The resistor is placed inline, in series with the signal wire or inner most portion of the shielded wire unit, either at the jack or socket / grid pin. The shield wire is not grounded at all, it has high voltage, DC power continuously. I am not telling you to do this, just explaining how it has been done by those willing to take the risk. I don't recommend it and will not be held responsible for those who choose to do this, as the risk of electrical shock has been made abundantly clear. (This method is UNSAFE)

If you want to put the resistor at the jack, with the normally, grounded (safe) shield wire you have now, the shield ground can be left where it is and the resistor or a new resistor, can be relocated from the socket to the jack. Then the inner most, signal wire can be re-attached to the socket pin after making sure the heat shrink insulation is isolating the assembly from other components.

[blacklabel]

You were very precise and very kind as always.
Thank you

[Unique]

It's possible but I have not put any thought into why and would need to recreate the scenario to measure voltages and such. I'd assume your PI voltages are pretty abnormal, depending on exactly how things are connected. Take some readings from the cathode and plates and lets compare them to the original voltages.

The resistor is there to give the PI a ground reference as it forms a part of the total tail resistance and also the ratio divider for the NFB. It also references DC to ground, so the pot is not experiencing the load, thereby not creating noise when the pot is rotated. No doubt the sound changes because of the PI bias and NFB has changed

I see the same in the pics but the resistor being there or not is not clear to me in that video

Again, the PI current and NFB changes - it has to because you changed the resistance. Ask yourself this... what happened to the DC that was referencing ground through the 4.7k tail resistor before you removed it? How did this resistance change affect the PI voltages and PI cathode bias current, because now you have changed the cathode resistor value? What has happened to the NFB ratio?

Answer: The DC can't get to ground through the presence pot because the presence cap wired as in the 25k circuit, won't let the DC pass through and it has to float around. The way to solve this issue is to replace a proper ground reference by re-installing a resistor or re-configuring the pot wiring to the 5k pot circuit connections, either with 5k pot or 25k pot. If you use the 25k pot, the NFB ratio, along with PI voltage and current is still altered by the 25k tail value, although now the PI tube may live longer and you still may have a sound you like.

The total of the 470r, 22k in #34 and final tail, whether it be 4.7k resistor, 5k pot & circuit or 25k pot & cap connected as in the 5k circuit, will all affect the PI and NFB as a fixed value, but will affect presence as an adjustable resistor. The ground reference is maintained. The 25k circuit without the 4.7k to ground will affect the NFB and presence as an adjustable resistor. The ground reference is compromised. In using a 5k pot and circuit, the 5k (close enough to 4.7k) tail value and NFB ratio is still maintained, because of the way the pot is grounded and the way the cap is connected.

Yes, even if the 25k pot is connected as Santiago left it in the schematic, the NFB and PI experience the changes I have discussed (47k total tail resistance of the 22k resistor and 25k pot in series, instead of the original 10k and 4.7k which totals 14.7k and then the 25k pot instead of the 4.7k resistor as the grounded portion of the NFB). The only difference is that with the pot & cap connected as in the schematic, you now have a proper ground reference for the PI. The 25k pot wired as the 5k circuit will always apply it's total value of 25k as part of the PI the tail and NFB divider portions, no matter the pots rotation. The 5k circuit does the same, which is similar in resistance value to the 4.7k, so you always have either 4.7k (with resistor) or 5K (with pot). The rotation of the pot should only affect the AC signal being sent to ground and not affect the DC potential. Marshall did it this way to retain the PI and NFB operation but get rid of the scratchy pot. As you apparently have it wired, you are affecting the NFB and AC. DC is affected because the ground reference has changed

Let me add... what a lot of folks do not realize, is the fact AC and DC travels on the same conductor / path unless the DC is blocked by a capacitor. Once this is realized, a lot of things about tube amps become more clear.

I'll have a look. I just wish they had filmed it with a decent camera.

A 150K pot would be much different than the other config, but would go in that direction as far as brightness and attack, even without a bright cap. Either way, I'd prefer to have these parts on switches because I like to play more than GNR music.

The resistor makes an easier path to ground for the AC signal, therefore reducing gain and bass. All else being unchanged. The load seen by the following stage is also different. If you want to really know how this affects the sound and gain, put a 500k pot wired as a variable resistor in place of the 150k resistor and play guitar while rotating that pot in small increments. You could also leave it in place as a gain trim pot to get more versatility out of the amp.

All I can say to that is, the only amp I have heard with the presence wired as you say yours is, sounded awful and IIRC, lacked power until the presence and tail was connected properly.

Sorry for the late response, I've been tied up for that last few days. What your saying here makes a lot of sense, and when I researched this back when, never once did I read about the resistor completing the NFB circuit or I probably would have looked at this differently. So I'm going to re-examine this and before I put my resistor back on. I might try this and see how it sounds:

"Answer: The DC can't get to ground through the presence pot because the presence cap wired as in the 25k circuit, won't let the DC pass through and it has to float around. The way to solve this issue is to replace a proper ground reference by re-installing a resistor or re-configuring the pot wiring to the 5k pot circuit connections, either with 5k pot or 25k pot. If you use the 25k pot, the NFB ratio, along with PI voltage and current is still altered by the 25k tail value, although now the PI tube may live longer and you still may have a sound you like."

My amp, the way it is now, sounds spot on and just fabulous. With the PPIMV low, or up high, everything thing sounds as it should. Along with that, because of the PPIMV, my presence pot works, but it's not that effective when the PPIMV is set low, but it does work and you can hear changes made. But they are more subtle. Either way, the presence pot itself is not scratchy at all when I turn it, nor do I hear any hum coming from the circuit. In fact, I hit the strings on my guitar sometimes just to make sure my amp is on making sure I took my amp out of standby. It's dead quiet when idling.

Thank you for pointing this out to me Coffee. It is greatly appreciated!

[Unique]

So how it should be soldered this hot-shield to make it unsafe like in #34 ?
I understand that it has to go from the valve pin to the input with the resistance soldered here.
in my case it's welded to the valve pin

I'm using my mediafire account to upload images

Okay, so that's how your getting pics to post here. Pics are important for a thread like this and it used to be so much easier to post some. But I understand why it changed.

As far as the hot-shield goes, Coffee explained it about as good as one can explain it. You just have to make sure you use a lot of heat shrink and make sure there are no stray strands sticking out anywhere. Then check it with a DMM for continuity making sure the ground and hot aren't in contact anywhere. You also have to make sure your using a cable that is rated high enough voltage-wise. I use a cable that is rated at 3000v, the Belden 8411. It's an older coax that was used in the '60-'80's as a mic and instrument cable, and they don't make it anymore. But you can find it on eBay. I've experimented with a lot of different cables, and to me, this one sounded the most correct and was one of the highest rated cables. No matter the length, it always retained about the same brightness, and tonal balance.

In fact, I tried one cable where I actually did have a little voltage come back on me. When I started playing my guitar, checking it out, I noticed my fingers started aching and slightly cramping, the hairs on my arm were standing up. This cable had a very thin foil shield under the braided shield and I didn't pay enough attention to that and it was never grounded. I think that was the reason voltage came back on me, I'm not 100% sure. Everything else was connected as it should have been. Maybe it was the resistor at the input that knocked the voltage down some, I don't know. It was like the second or third cable I tried, either way, since then I always double checked and triple checked everything even more. Including, making sure I always did a continuity test with all of my connections with my DMM.

In truth, I think the safe shield sounds almost as good as the hot-shield, and better than the AFD100's capacitor set up to mimic a hot-shield. When I tried the safe shield I had it like yours, with the resistor at the socket. To me, it sounded better with my amp than the hot-shield did with the resistor at the socket. And If your going for GN'R tones, with the 150k on the PA pot, your not going to turn your PA much up over 2 anyway.

I'm in no way telling anyone to do a hot-shield, that is a choice for each individual to make on their own, at their own risks. My amp originally came with a hot-shield from the factory. Marshall did this in the late 70's and early 80's with some amps, but it was set up a little different than how it is in the #34 mod, as there was no resistor connected to it. A capacitor across the pins on the socket to mimic a hot-shield has every bit as a chance to fail as a cable does. Only the safe shield removes any risks.

[Peter London]

Hi all!
This in my first post here. My amp (2203x) has the #39 mod , done by a tech here UK. My amp sounds fantastic, but seems like it is missing that nasally character.My question is that this method on the following picture was "voted" as safe , to get that effect? http://i443.photobucket.com/albums/qq15 ... 222222.jpg

Thanks
Pete

[CoffeeTones]

It is no different than placing two caps in series, on the socket pins, from plate to grid. You just wouldn't have the length of wire or terminal strip.

[Peter London]

Thank you for your help!
So if I put for example two 22pf 1kV ceramic caps in series between pin 6 and 7, then I should be alright safety wise?

[CoffeeTones]

Your call. There really needs to be a blocking cap in series with the the signal wire from the jack, so DC should never get to the guitar if the cap or caps on the socket happen to short circuit. Input jack -----> .22uF cap ------> 1M from .22uF cap's output to ground ----> .22uF / 1M junction to signal wire -----> grid stopper ------> input grid

[Peter London]

Thanks for Your reply!
I think I will leave that for the tech...
I see that You've been here for a while , so would You mind to listen to my quick noodling? I would love to hear Your (and others) thoughts!
https://youtu.be/eaJ4jBpGFAg
Cheers
Pete

[CoffeeTones]

I sounds decent, but I'd change all the metal oxide and metal film resistors to carbon film and I'd change all of the ceramic caps, except maybe the bright cap and snubber you have across the plate of the cathode follower, to mica. I might also increase the value of the 470pF cap bypassing the 470k, before the volume pot, to 1nF or more. The amp has a harshness that can be caused by those parts, IMO. That is unless you love it the way it is.

[Peter London]

You mean the ones are between the jack input and the preamp volume? I might try that, see what happens. Do I have to adjust the resistor value as well if I change the cap to 1nF or I can leave that alone?