Hello all! This is my first post here.
I just got an early/mid 1969 Super Lead, at least I think, and I’d like to do the right things to make this one true to the original spirit of the amps. So, hoping I can ask some questions and learn.
The amp has had some things done right, and some things done wrong. When I got it, I was shocked to find the bolts holding the output transformer floating around inside the chassis. Someone tried to put nuts with the wrong thread on, they stripped and fell off. The transformer was pretty loose and clunking around. Yikes.
Obviously, those knobs gotta go.
I think this is an early 1969, but please correct me if I’m wrong. Serial number is SL/A 10433 and the front and rear panels are plexiglass not metal. Since it says SL/A not SL I believe this makes it 1969 or later. And since 1969 was the transition year from plexi to metal faceplates I think it’s early 1969. Am I on the right track? It was advertised as 1969.
It looks to me like all the coupling capacitors, etc. are original. But the filter caps have been replaced, as they should be. Although I was expecting to see 4 or more, not just two. If anyone has observations or suggestions please share.
1969 Super Lead
Moderators: pompeiisneaks, Colossal
1969 Super Lead
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Re: 1969 Super Lead
unoriginal transformers. Messy wiring of power amp and filtering
Re: 1969 Super Lead
Safety ground is not connected to chassis.
Do not connect to mains!
Do not connect to mains!
Re: 1969 Super Lead
This should be a fun and rewarding project if you take the time to evaluate every component for proper specs and other operational integrity. I’m focusing on the trannies first. I see a choke by Classic Tone… What are the others? What’s your experience with working on amps? There’s quite a few extremely knowledgeable members here that will be very capable to guide you through this adventure… But that’s why you’re here, right?
Greg
Re: 1969 Super Lead
Yes!! That's why I'm here!syscokid wrote: ↑Mon Nov 24, 2025 4:36 pm This should be a fun and rewarding project if you take the time to evaluate every component for proper specs and other operational integrity. I’m focusing on the trannies first. I see a choke by Classic Tone… What are the others? What’s your experience with working on amps? There’s quite a few extremely knowledgeable members here that will be very capable to guide you through this adventure… But that’s why you’re here, right?
The picture was from right after I received the amp. With the transformer clunking around first thing I did was secure it and reattach the ground, which was on one of the transformer bolts.
I actually have a degree in avionics repair many years ago, but I have never worked professionally in electronics. I recently restored a '68 Fender Bassman, but I had a full set of instructions on how to do that. So I have plenty of general electronics knowledge, but only a little specific to tube amps.
The choke and output trannies are Classic Tone. I'm pretty sure the power transformer is too, the label is rubbed off, but I found a web site with a picture Classic Tone for Marshall that looks exactly like this one. And the seller told me they were Magnetic Industries which is Classic Tone. So I think these are all high quality replacement, but please correct me if I'm wrong.
Re: 1969 Super Lead
Welcome! I'm excited for you -- that's a really cool amp. I don't have firsthand experience with Classic Tone transformers, but people I trust and who have tried them liked them. As long as they're the right kind of transformers for the amp, I'd keep them. Do make sure they're right for the amp, though, since the transformers are a critical part of what makes these Marshalls special. Almost all of the 1969 100w amps I'm aware of have had C1998 output transformers, which had an unusual winding pattern, and Classic Tone made a replacement that was wound appropriately. The power transformer is essential to the sound since these amps have a lot of power supply sag. I've attached a datasheet for one of the shades of 100w Marshall PT's that Classic Tone made and the colors don't match, but it's totally possible that some individual transformers from Classic Tone had brown leads substituted based on availability.
I took a few notes based on the gut shot you sent:
- The output transformer impedance selector switch has obviously been replaced. That particular switch looks like an Alpha rotary, most of which are not meant to route very high power signals. I suspect this is the datasheet for the switch your amp has, which is rated only for 37.5W of power (0.3A @125VAC): https://www.amplifiedparts.com/sites/de ... _sheet.pdf
It may not be an issue if Alpha just under-rates their switches, but 37.5W is less than what a 100w Marshall will produce while still clean. If that switch fails it will break the the secondary of the output transformer connection to the load and potentially cause catastrophic failure of the output transformer and/or your power tubes. I'd strongly advise replacing it with a more robust switch with enough of a power handling margin over the amp's maximum power output (~200w when cranked).
- There is no V1 bypass cap on the bright channel (should have a 0.68uF capacitor across the cathode resistor). Interestingly, there appears (at least in my eyes' read of the picture), to be some residual red warranty pain on that turret, so it may not have had a bypass cap on that channel from the factory. Relatedly, it looks like the value of the same cathode resistor may be "wrong" there also. By 1969 lead-spec amps had 2k7 cathode resistors there, but some 1968-era amps had 820R on both cathodes like in your amp; I'm not confident this isn't a factory variant.
- in general, all but one of the Phillips "mustard" caps are gone. You'll hear different things from different people about whether or not this makes a difference in the sound assuming the capacitance of the capacitors in question is the same. What will uncontroversially make a significant difference in how the amp sounds is if the values of the capacitors are different, and I can't see the values outside of the correct 2n2 coupling cap off of the bright channel's first gain stage. Would recommend checking those to ensure that the values are correct and that the coupling capacitors have suitable voltage ratings since those will have hundreds of volts' potential across them.
- They appear to have increased the capacitance in the peaking network associated with the mixers by adding the bright red ceramic in parallel with the factory 560pF capacitor. Easy fix is to just remove the extra capacitor.
- Both channels' volume controls have bright caps. Marshall would most likely have built it with only the bright channel's volume having a bright cap (most typically 4.7nF for this era amp).
- 1969 lead amps would have had a 680nF capacitor bypassing the V2 gain stage's cathode. The original appears to have been removed with most of the other mustard caps, but adding a bypass cap back (mustard or otherwise) will make a significant difference in sound by making it much more aggressive.
- Most of the pictures of 1969-era amps I've seen have Philips mustard caps at the presence control. I can't read the value of the capacitor there, but it should be 100nF. My 1974 has a very similar appearing blue and white cap (can't remember the manufacturer) functioning as a rectifier snubber, but that cap measures 47nF. Someone more knowledgeable may be able to say if this is a factory variant.
- The dropping resistors linking the phase inverter node to the V2 node appear to have been replaced with parallel 22K carbon comp resistors. That is basically the right value since 11k would be within tolerance range for the original 10k, but carbon composition resistors are noisier and more prone to value drift than the carbon film resistors. Some Marshall amps from this period do use carbon comp B+ dropping resistors but yours appears to have had 2W Iskra carbon film resistors from the factory. In other words, replacing those with a carbon film resistor would be the "right" thing to do from a restoration perspective while also likely helping improve the amp's noise floor and reliability. Metal film would be even lower noise, but carbon comp is the worst possible choice from a noise perspective -- juts make sure that the resistor is rated at least for 2W. It also doesn't look like a particularly clean soldering job there anyway, so even if you keep them I'd recommend cleaning it up to ensure reliable connections.
- There also are the two metal film bias feed resistors tacked together so it may be worth considering replacing that if you're trying to restore the amp. I can't confidently read the values there with how the picture's colors render on my screen, but the resistance at that position is critical to having the appropriate range for output section bias voltage.
- I can't really see what's going on with the auxiliary board, but I can say that it doesn't strike me as very safe with bare leads hanging off the side of the board where the HT rectifier is mounted.
- To my eye it almost looks like they converted this to a full-wave (non-bridged) rectifier since the bias voltage is derived from one of the plain HT secondary leads contributing to the B+ supply rather than drawing from a dedicated secondary winding for the bias supply as would have been stock. This is very different than the original power supply topology and if the power transformer is indeed spec'd right for this application it will result in dramatically lower B+ voltages and a very pronounced difference in the amp's sound and performance.
I'd strongly caution against securing the safety ground to the transformer fasteners. Those are some of the most likely fasteners to work themselves loose in the amp, and the safety ground is one of the most important connections in the amp from a safety perspective. That should have its own dedicated fastener securing it to the chassis. That grounding point should have no other connections to it, and it should be fairly close to where the power cord inserts into the chassis so that there is considerably more length for its lead in the power cord than for either of the leads going to the power switch. That way if the power cord gets pulled out the hot leads get disconnected before the safety ground, preventing you from getting shocked by an amp that might seem fine since it powers on.
I took a few notes based on the gut shot you sent:
- The output transformer impedance selector switch has obviously been replaced. That particular switch looks like an Alpha rotary, most of which are not meant to route very high power signals. I suspect this is the datasheet for the switch your amp has, which is rated only for 37.5W of power (0.3A @125VAC): https://www.amplifiedparts.com/sites/de ... _sheet.pdf
It may not be an issue if Alpha just under-rates their switches, but 37.5W is less than what a 100w Marshall will produce while still clean. If that switch fails it will break the the secondary of the output transformer connection to the load and potentially cause catastrophic failure of the output transformer and/or your power tubes. I'd strongly advise replacing it with a more robust switch with enough of a power handling margin over the amp's maximum power output (~200w when cranked).
- There is no V1 bypass cap on the bright channel (should have a 0.68uF capacitor across the cathode resistor). Interestingly, there appears (at least in my eyes' read of the picture), to be some residual red warranty pain on that turret, so it may not have had a bypass cap on that channel from the factory. Relatedly, it looks like the value of the same cathode resistor may be "wrong" there also. By 1969 lead-spec amps had 2k7 cathode resistors there, but some 1968-era amps had 820R on both cathodes like in your amp; I'm not confident this isn't a factory variant.
- in general, all but one of the Phillips "mustard" caps are gone. You'll hear different things from different people about whether or not this makes a difference in the sound assuming the capacitance of the capacitors in question is the same. What will uncontroversially make a significant difference in how the amp sounds is if the values of the capacitors are different, and I can't see the values outside of the correct 2n2 coupling cap off of the bright channel's first gain stage. Would recommend checking those to ensure that the values are correct and that the coupling capacitors have suitable voltage ratings since those will have hundreds of volts' potential across them.
- They appear to have increased the capacitance in the peaking network associated with the mixers by adding the bright red ceramic in parallel with the factory 560pF capacitor. Easy fix is to just remove the extra capacitor.
- Both channels' volume controls have bright caps. Marshall would most likely have built it with only the bright channel's volume having a bright cap (most typically 4.7nF for this era amp).
- 1969 lead amps would have had a 680nF capacitor bypassing the V2 gain stage's cathode. The original appears to have been removed with most of the other mustard caps, but adding a bypass cap back (mustard or otherwise) will make a significant difference in sound by making it much more aggressive.
- Most of the pictures of 1969-era amps I've seen have Philips mustard caps at the presence control. I can't read the value of the capacitor there, but it should be 100nF. My 1974 has a very similar appearing blue and white cap (can't remember the manufacturer) functioning as a rectifier snubber, but that cap measures 47nF. Someone more knowledgeable may be able to say if this is a factory variant.
- The dropping resistors linking the phase inverter node to the V2 node appear to have been replaced with parallel 22K carbon comp resistors. That is basically the right value since 11k would be within tolerance range for the original 10k, but carbon composition resistors are noisier and more prone to value drift than the carbon film resistors. Some Marshall amps from this period do use carbon comp B+ dropping resistors but yours appears to have had 2W Iskra carbon film resistors from the factory. In other words, replacing those with a carbon film resistor would be the "right" thing to do from a restoration perspective while also likely helping improve the amp's noise floor and reliability. Metal film would be even lower noise, but carbon comp is the worst possible choice from a noise perspective -- juts make sure that the resistor is rated at least for 2W. It also doesn't look like a particularly clean soldering job there anyway, so even if you keep them I'd recommend cleaning it up to ensure reliable connections.
- There also are the two metal film bias feed resistors tacked together so it may be worth considering replacing that if you're trying to restore the amp. I can't confidently read the values there with how the picture's colors render on my screen, but the resistance at that position is critical to having the appropriate range for output section bias voltage.
- I can't really see what's going on with the auxiliary board, but I can say that it doesn't strike me as very safe with bare leads hanging off the side of the board where the HT rectifier is mounted.
- To my eye it almost looks like they converted this to a full-wave (non-bridged) rectifier since the bias voltage is derived from one of the plain HT secondary leads contributing to the B+ supply rather than drawing from a dedicated secondary winding for the bias supply as would have been stock. This is very different than the original power supply topology and if the power transformer is indeed spec'd right for this application it will result in dramatically lower B+ voltages and a very pronounced difference in the amp's sound and performance.
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Re: 1969 Super Lead
Fun little project!
Those coupling caps are original to the amp. Every now and then these capacitors were used. I have seen some amps with half Mustards and half these. Usually on the Bass amps.
Pots and input jacks look good too. Easiest way to tell what’s been replaced/messed with is the red paint on the solder joints. If it isn’t there then it’s had work done.
Original transformers do come up every now and then. Depends how much you’re willing to pay.
Good luck with the restoration.
Aaron
Those coupling caps are original to the amp. Every now and then these capacitors were used. I have seen some amps with half Mustards and half these. Usually on the Bass amps.
Pots and input jacks look good too. Easiest way to tell what’s been replaced/messed with is the red paint on the solder joints. If it isn’t there then it’s had work done.
Original transformers do come up every now and then. Depends how much you’re willing to pay.
Good luck with the restoration.
Aaron
Re: 1969 Super Lead
@cdemike Wow, thanks for all the very useful info!! I've read everything you said once, but I'll have to read again carefully and understand the specifics.
I was actually planning to solder the ground to the chassis. I just stuck in on the post to be able to turn the amp on and test it. It was sold to me as being in "good working condition." Which it is absolutely not.
As Aaron says, I'm pretty sure the blue and yellow caps are original. I've seen plenty of other pictures of these in original Marshalls. I'm not sure why they used these sometimes when clearly Marshall used the mustard caps more often.
As I started digging in, I have found some astonishingly horrible things done to this poor amp. The anode (pin 3) of the last EL 34 was completely disconnected.
The resistors connecting the control grid, pin 5, were disconnected. The power and output transformers are actually for a 50W amp. And the impedance switch, with all those capacitors stuck on it, was also not connected to anything. The choke, at least, is the correct transformer. So at this point I'm just carefully disassembling and documenting how it all goes together.
Any recommendations on the best transformers? If there's a good new option that does justice to the original, I don't feel the need to have actual originals on this amp.
@Aaron, didn't know about the red paint on the solder joints, that's good to know. I will make an effort not to re-solder those unless I have to.
I've been trying to find a correct schematic. I found a 1967 schematic that has 1500 ohm resistors connecting the control grids, which mine has (the one that was disconnected on one end). But mine has a split cathode on V1 which is not on this schematic. I found a 1970 diagram with the split cathode, but 5.6k resistors hooked up a little differently on the screen grid. By any chance does anyone know of a schematic in between these two?
I was actually planning to solder the ground to the chassis. I just stuck in on the post to be able to turn the amp on and test it. It was sold to me as being in "good working condition." Which it is absolutely not.
As Aaron says, I'm pretty sure the blue and yellow caps are original. I've seen plenty of other pictures of these in original Marshalls. I'm not sure why they used these sometimes when clearly Marshall used the mustard caps more often.
As I started digging in, I have found some astonishingly horrible things done to this poor amp. The anode (pin 3) of the last EL 34 was completely disconnected.
The resistors connecting the control grid, pin 5, were disconnected. The power and output transformers are actually for a 50W amp. And the impedance switch, with all those capacitors stuck on it, was also not connected to anything. The choke, at least, is the correct transformer. So at this point I'm just carefully disassembling and documenting how it all goes together.Any recommendations on the best transformers? If there's a good new option that does justice to the original, I don't feel the need to have actual originals on this amp.
@Aaron, didn't know about the red paint on the solder joints, that's good to know. I will make an effort not to re-solder those unless I have to.
I've been trying to find a correct schematic. I found a 1967 schematic that has 1500 ohm resistors connecting the control grids, which mine has (the one that was disconnected on one end). But mine has a split cathode on V1 which is not on this schematic. I found a 1970 diagram with the split cathode, but 5.6k resistors hooked up a little differently on the screen grid. By any chance does anyone know of a schematic in between these two?
Re: 1969 Super Lead
Here are the two schematics I've found, for what it's worth. I think mine is in between these two. It would be great to find a the correct schematic if it exists.
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Re: 1969 Super Lead
Those blue/yellow Dubilier caps were OEM around 1968.in general, all but one of the Phillips "mustard" caps are gone.
I have an amp from the same era. It essentially corresponds the "1967" schematic above.
Re: 1969 Super Lead
I stand corrected about the mustards! Thanks for helping me learn about those Dubilier caps, Aaron and Helmholtz!
AFAIK, foscoe944, the Unicord schematic from 1970 should nominally be the same circuit as an early 1969-spec amp with an SL/A serial number, though individual amps did have variations between them. Some amps came with two pairs of 10nF ceramic capacitors placed in parallel in lieu of the two 22nF tone stack caps, for example. Some amps from this period have two 820R resistors at V1 like yours does, and based on the warranty paint on the 820R cathode resistor for the bright channel, I suspect that may be original to your amp. That would also mean that your amp would have come with no cathode bypass cap there. There are also variations between amps in terms of filtering values, which make big differences in these amps.
I did notice looking back over the pictures that it looks like they may have bypassed the 10k dropping resistor separating the V1 and V2 B+ nodes. JTM45's also shared that node, but it would not have been stock for an amp from late 1968 or early 1969.
AFAIK, foscoe944, the Unicord schematic from 1970 should nominally be the same circuit as an early 1969-spec amp with an SL/A serial number, though individual amps did have variations between them. Some amps came with two pairs of 10nF ceramic capacitors placed in parallel in lieu of the two 22nF tone stack caps, for example. Some amps from this period have two 820R resistors at V1 like yours does, and based on the warranty paint on the 820R cathode resistor for the bright channel, I suspect that may be original to your amp. That would also mean that your amp would have come with no cathode bypass cap there. There are also variations between amps in terms of filtering values, which make big differences in these amps.
I did notice looking back over the pictures that it looks like they may have bypassed the 10k dropping resistor separating the V1 and V2 B+ nodes. JTM45's also shared that node, but it would not have been stock for an amp from late 1968 or early 1969.
Re: 1969 Super Lead
Thanks cdemike, I also noticed the V1 820R cathode capacitor is not on mine. And both channels have an 820R resistor there, not an 820 and a 2.7k like in the diagram. The other capacitor has been replaced with a 220uf instead of the 250uf. I wonder if I should find a better capacitor and replace it again? Not sure if the capacitor type would matter much since it’s just sending signal to ground. According to a Johan Segaborn video, the channel without the cap should have more bass.
Those next set of, I guess 50uf and 50uf caps on the B+ though, I believe were originally on the power board? But I’m not sure. I don’t seem to have those and I’m still trying to figure out what the heck was done to mine and what it should look like originally.
And re your earlier post:
The 10k dropping resistor between V1 and V2 B+ nodes wasn’t bypassed. But the solder joints are new because there are new wires running the the 32+32 can capacitor right behind the board. My can cap is a new JJ. I hope this is the way it was originally wired?
I also see the bright red capacitor you mentioned earlier. That one will get yanked. And the Volume II bright cap looks like the one that was added. That will also get yanked. The presence cap is 0.1 uf, so yes, 100nf and it’s original.
The dropping resistors linking the phase inverter node to V2 node… Something is not right here. Those new 22k resistors that are in parallel are then in series with the 10k with nothing connected between them. So this is roughly 21k all together. I’m still trying to figure this one out…
The two metal film bias feed resistors are actually 3 resistors in series, 360 ohms total. I also haven’t gotten to figuring that out exactly but yeah, I’ll certain put the right value in there.
Also, yes, on the rectifier, although I don’t know myself what it is I can see that it is wrong. Again, I’m hoping I can see what some other original boards look like. I haven’t tested the square diode chip, but I’m guessing it’s the original. Do you know?
Those next set of, I guess 50uf and 50uf caps on the B+ though, I believe were originally on the power board? But I’m not sure. I don’t seem to have those and I’m still trying to figure out what the heck was done to mine and what it should look like originally.
And re your earlier post:
The 10k dropping resistor between V1 and V2 B+ nodes wasn’t bypassed. But the solder joints are new because there are new wires running the the 32+32 can capacitor right behind the board. My can cap is a new JJ. I hope this is the way it was originally wired?
I also see the bright red capacitor you mentioned earlier. That one will get yanked. And the Volume II bright cap looks like the one that was added. That will also get yanked. The presence cap is 0.1 uf, so yes, 100nf and it’s original.
The dropping resistors linking the phase inverter node to V2 node… Something is not right here. Those new 22k resistors that are in parallel are then in series with the 10k with nothing connected between them. So this is roughly 21k all together. I’m still trying to figure this one out…
The two metal film bias feed resistors are actually 3 resistors in series, 360 ohms total. I also haven’t gotten to figuring that out exactly but yeah, I’ll certain put the right value in there.
Also, yes, on the rectifier, although I don’t know myself what it is I can see that it is wrong. Again, I’m hoping I can see what some other original boards look like. I haven’t tested the square diode chip, but I’m guessing it’s the original. Do you know?
Re: 1969 Super Lead
And thanks to everyone for all the help, I hope everybody here in the USA is having a fantastic Thanksgiving!!!
Re: 1969 Super Lead
You may find this helpful:
https://roef.folk.ntnu.no/JTM100.html
Your amp looks like it lines up with version "7B."
A truly useful site, btw, Roe!
https://roef.folk.ntnu.no/JTM100.html
Your amp looks like it lines up with version "7B."
A truly useful site, btw, Roe!