Building a 1996 Super Lead Overdrive SLO 100 style amp into a 15 watt chassis

[Snfoilhat]

I finished reading Richard Kuehnel's https://www.ampbooks.com/mobile/books/slo/ and want to work on a practical application, so that'll be this thread (my book report to the class) and an amplifier.

First, the SLO book seems to me to take a very particular approach to the '96 SLO 100 -- almost purely theoretical. A lot of the questions the book set out to answer using the schematic, SPICE analysis, and a comparative method with a selection of other amps and amp components, could alternatively been answered by playing, measuring, and tearing down a SLO 100, which I don't believe the author did. In a way this is really cool, because it foregrounds the general theory by showing how it can be used to answer a particular question (e.g. what were the specs of the SLO 100 power transformer?) and arrives at an answer that you could independently verify by going out and finding one IRL and looking at it.

This theoretical method has one big consequence for people like me: it makes the book nothing like a builder's guide, neither to help make SLO 100 counterfeits nor boutique/hobby adaptations. A real amp you can plug a guitar into has a physical chassis, a layout, components that aren't just a simple value of resistance or capacitance or inductance like a symbol on a schematic. So all the work of putting together a metal box in a real environment full of electromagnetic waves, physical vibrations, and heat, all that's left for us DIY people to tackle. Go team

The book's flow is from speaker to input, and Kuehnel's gives his reasons for that. I'll do the same. The big picture design philosophy of the SLO 100 is separation between the power amp (according to the author, more accurately thought of as the effects return + EQ + phase inversion + power amp module) and the preamp + effects send module. The power side is like the 1950s ideal the 5F6-a Bassman strove to be but couldn't achieve w/ 1950s parts and other constraints -- super stiff, super clean. The real SLO 100's chassis, power supply, transformer selection, all that comes from here.

The amp I'm building will follow the same design philosophy (clean, stiff power amp) but with some parts constraints that are not historical but based on size, expense, power consumption, availability. I'm setting out that the physical chassis has to be such and such dimensions (Princeton Reverb reproduction), that the 'biggest' power tube allowed is the JJ 6V6S, that the output transformer will be rated for 15 watts. I'll do my best to apply the SLO 100 design thinking within those constraints, and hopefully end up with a very accurate preamp + effects send running into an effects return + EQ + power amp that, below a certain stage volume, acts as much like the SLO 100 as I can manage. (Plus offers opportunities drive the power amp to distortion at lower volumes, something Soldano later tried out in models like the SLO 30).

***

Source material

Annotated 1996 schematic from EL34 world:

Transformer data sheets (for the 15 watt build):

Chassis and drill template, control panel graphics:

Note: first change from the SLO 100 -- I don't think I'm going to include a bright switch on the Clean channel. Not a function I've used on any amp I've had, and not enough space on the control panel. Always off (bright capacitor omitted), always on, "Pull for Bright" on the Normal volume? Not sure yet, will have to listen and decide.

More chassis work:

 

[Snfoilhat]

I started with the 1996 schematic, and am taking one little bite at a time changing what needs to be changed to fit the new constraints.

First, finding the best way to take the SLO 100's optical switching system out and replacing it with a simple panel-mounted slide switch. Kuehnel's book has a chapter on the switching system that helped me to begin to understand what it's doing in the circuit -- very high or very low resistances, depending on the setting of the switch. Not exactly like a relay-based system. I replaced a relay system with a panel switch in my Fender Prosonic-based amp with a lot of success. So I set to adapting that. Here's my first guess at how it could work with a 3PDT slide switch and some standard resistors to take the place of the photoresistive optocouplers. Proposed edits shown in green.

 

[Ten Over]

I started with the 1996 schematic, and am taking one little bite at a time changing what needs to be changed to fit the new constraints.

First, finding the best way to take the SLO 100's optical switching system out and replacing it with a simple panel-mounted slide switch. Kuehnel's book has a chapter on the switching system that helped me to begin to understand what it's doing in the circuit -- very high or very low resistances, depending on the setting of the switch. Not exactly like a relay-based system. I replaced a relay system with a panel switch in my Fender Prosonic-based amp with a lot of success. So I set to adapting that. Here's my first guess at how it could work with a 3PDT slide switch and some standard resistors to take the place of the photoresistive optocouplers. Proposed edits shown in green.

View attachment 1120357

I can't read this.

 

[Snfoilhat]

Thanks for the feedback @Ten Over. Not sure if it's too small/low res in the original or if the tdpri software is doing something. I'll try this and then check on my phone to see if it's legible and attach a full res PDF if it's not.

 

[Ten Over]

Looks like you'll be taking the bias voltage from the HT since I don't see a bias tap on your PT. Attached is one possible way:

 

[dan40]

Good luck with your build. I built a 50 watt SLO clone over the winter and I'm loving it. The high gain channel is perfect and the crunch channel is pretty good also. The clean channel was much better than I was expecting also. I look forward to seeing how a lower power version will sound when you are done.

 

[jsnwhite619]

Nice faceplates!

 

[Snfoilhat]

Thanks, everyone! I really appreciate it.



Layout stuff

Here's a 1992 SLO 100 that was photographed years ago and posted by someone on rig talk. Not awesome image quality but enough to check out some of how the original layout was done. I flipped images horizontally so that the input is on the right and power transformer is on the left, like in my chassis.

Looking at all that space in the SLO 100 is making me envious. So much space that even the filter choke could be inside!

It's not a very big circuit, but I think the Princeton Reverb chassis is going to be stuffed. I don't plan on adopting Soldano build style, like all those long parallel leads connecting tube pins, and my chassis isn't set up to place the small tubes in a row right up beside the controls, but I still think there's value in seeing how the original was done. Grid stop resistors are right on their tube pins. I'm guessing the fatter white leads are shielded.

I've been waiting on parts like the filter choke and a second eyelet board, so this week has been just filling in stuff where the placement was dictated by the chassis.

Power is a DPDT oriented left-right rather than up-down and in the original ground polarity switch spot. That's not as intuitive but I thought it was the path of least resistance w/ respect to fitting it around the fuse holder and speaker impedance switch (which takes the original power switch hole).

Just on pencil and paper, I've been playing around with layout for the circuit board. A single 10 inch board (shown) is not going to cut it.

This draft is especially not compact, since I'm trying to leave the top row just for ground connections, and the bottom row for the high voltage supply. A little bit emulating the PCBs I've been looking at lately, the Prosonic and the SLO. Working from right to left I ran out of board before I even got to components associated with the phase inverter (V5).

So, a tighter layout would be good. A longer board. A full-width board that flies over the capacitor can terminals rather than a long plus a short power supply board. Gotta take some time thinking about it. Gotta locate the choke, too. It's just the little 4H@50mA guy from the deluxe reverb. I haven't done any of the kind of heavy work of figuring out just how much filtering the SLO is doing at it's current draw and then trying to scale everything down to the much smaller draw here. Neither do I want to try to copy every component and value from the original (since with the much smaller draw, we wouldn't expect it to work the same anyway). I'm just aiming at a general target like "reasonably stiff for a 6V6 amp", not a meticulously rigged replica ship in a bottle.

 

[Snfoilhat]

Not quite

Maybe

Nudge the gain down a little, please?

Struggle

Wait if the switch is up then ... ...

Trying to figure how long each lead coming off the switches should be, shielded or not, and anything else, since it's all gotta be soldered as a package before sticking them into the chassis. No way for me anyway -- too small! Sure don't want to do it twice

 

[jsnwhite619]

Why don't you start off practicing by fitting a real 454 into Matchbox car?

I don't doubt the ability - I'm just glad it's not me!! Damn that's a tight job! Best of luck on this one.

 

[2L man]

If long signal wires concern you installing signal relays does not make that board too much more complicated. Then adding a footswitch later is easy if it is needed. Relay control is DC and it tolerate amp circuits very well and what DC does to amp circuits is very little.

Are you going to us DC for filament? SLO seem to have straight filament wiring so I assume it is DC?

 

[sds1]

SLO seem to have straight filament wiring so I assume it is DC?

It's AC.

Pretty sure they've started putting DC on the first stage or 2 on newer production models but this is not reflected in the clone schematics.

Relay control is DC and it tolerate amp circuits very well and what DC does to amp circuits is very little.

Actually in middle of this analysis right now. The ripple on these DC lines will cause noise problems, I've done it on past 2 designs now. I think PCB layout makes crazy things possible because I've never heard complaints about relay DC power supply noise otherwise. Just 5mV of ripple running too close to sensitive signal and enough gain results in a noisy amplifier.

 

[Snfoilhat]

If long signal wires concern you installing signal relays does not make that board too much more complicated. Then adding a footswitch later is easy if it is needed. Relay control is DC and it tolerate amp circuits very well and what DC does to amp circuits is very little.

Are you going to us DC for filament? SLO seem to have straight filament wiring so I assume it is DC?

One thing I'm really hoping to accomplish is simplifying the switching; we'll see if it will work and then come up with a better plan like you describe if necessary.

For heaters, I managed to fit a couple more resistors and a capacitor to add DC elevation of of the regular AC w/ virtual center-tap

Moved the OT over to make room for the little filter choke.

 

[dan40]

Are you going to us DC for filament? SLO seem to have straight filament wiring so I assume it is DC?

For heaters, I managed to fit a couple more resistors and a capacitor to add DC elevation of of the regular AC w/ virtual center-tap

When I built my 50 watt SLO clone, I decided to keep it simple and stick with AC voltage for the filaments. I did use the elevation circuit to put about 60v on the filament and the amp turned out extremely quiet.

 

[JohnnyCrash]

I love these types of builds.

One of my recent builds was putting a 50 watt 1987/2204 hybrid Marshall circuit into a tiny Hammond box and an almost (it’s a little larger than a) lunchbox sized birch ply head cab.

I’m planning on a Mesa MkI in a Champ chassis soon.

 

[Snfoilhat]

Seeing the parts laid out helped me realize there was room for a more local filter cap for V2, drawn in green. I'm making informed guesses for power supply values. With so many fewer volts to play with (the real SLO starts w/ around 500V B+, this amp may be low 300s), it's a tradeoff between voltage drop and filtering and I don't know which to prioritize. Adding the cap to the circuit board will let me parallel 20||20 µF in the cap can at B+2 (screens).

?

Changes include DC elevation of the heaters, clearer drawing of the switches, and best guesses at the high voltage supply. I think all the component values show what I actually have

 

[rschiller]

I started with the 1996 schematic, and am taking one little bite at a time changing what needs to be changed to fit the new constraints.

First, finding the best way to take the SLO 100's optical switching system out and replacing it with a simple panel-mounted slide switch. Kuehnel's book has a chapter on the switching system that helped me to begin to understand what it's doing in the circuit -- very high or very low resistances, depending on the setting of the switch. Not exactly like a relay-based system. I replaced a relay system with a panel switch in my Fender Prosonic-based amp with a lot of success. So I set to adapting that. Here's my first guess at how it could work with a 3PDT slide switch and some standard resistors to take the place of the photoresistive optocouplers. Proposed edits shown in green.

View attachment 1120357

The opto-isolator Vactec 5C1 work well which is why they've become a standard swtiching device. With current applied the resistance is ~ 500ohm which is virtually no resistance in the circuit. No current and the resistance is ~50M. Done correctly they are also a virtually silent switiching means and reliable. I've used them successfully in a number of multi-channel builds. You have to be sure to clamp voltage w/ a 6.2v zener diode.

But I'd be curious how relays work in your final build. To be fair my dual channel builds always included separate tone stacks for the two channels and Soldano uses a single tone stack. That may effect your choise of switching circuit. Looks like you are carefully thinking this through.

 

[jumpbluesdude]

Be aware that as you change layout, change the space the amp occupies, change the orientation of the components, change component values, change the wiring scheme, change lead dress, and eliminate parts of the circuit, you most likely alter the sound of the amp you seek to emulate and in the end have something very different. There is a reason that the schematically identical 1962 Marshall amp differs from the Bassman Jim Marshall sought to copy. It may be an improvement, but if your goal is to get that sound in a smaller package, you may end up disappointed. I’m not a fan of circuit board amps, but when I built my SLO, I built it on a circuit board for that reason. Just an FYI.

 

[rschiller]

Be aware that as you change layout, change the space the amp occupies, change the orientation of the components, change component values, change the wiring scheme, change lead dress, and eliminate parts of the circuit, you most likely alter the sound of the amp you seek to emulate and in the end have something very different. There is a reason that the schematically identical 1962 Marshall amp differs from the Bassman Jim Marshall sought to copy. It may be an improvement, but if your goal is to get that sound in a smaller package, you may end up disappointed. I’m not a fan of circuit board amps, but when I built my SLO, I built it on a circuit board for that reason. Just an FYI.

Layout is another reason to consider staying with the Vactec opto-isolators as often they switch in/out master vol pots and the Vactec's hang nicely on the back of the pots taking up no circuit board room.

 

[Snfoilhat]

I'm cautiously optimistic this will work because it's what I i did to replace the channel switching relays in my Prosonic-inspired amp (panel only, not footswitchable). But I can see that it just might not go well in this amp, in which case I'll get to give those switching components a try, thanks!

Each spot in the circuit (VR1 - VR4) where there was originally an optoisolator, i replaced it with a 10MΩ resistor so that all the usual electrical connections of the original are preserved. That should work reasonably like to how each optoisolator in its high resistance state works. Across each 10M resistor are the terminals of a simple 3PDT switch (VR3 and VR4 which switch between the Normal and OD master volumes could be combined into two resistors but only one pole).

Then i just picked terminals for the two switch positions OD/Normal according to the schematic: OD mode VR 2 and 3 on, Normal mode VR 1 and 4 on. On in this case is a short across the 10M resistor rather than the optoisolators' low resistance state. So 10MΩ instead of 50MΩ, and 0Ω instead of 500Ω, but I expect that that won't really mean much w/ respect to the impedances for the signal. We'll see.

I built the crunch/clean DPDT switch today too