Mark IIB to IIC+ Conversion

[RicherHertz]

I can put something together. Any sounds in particular you’d be interested in?

@Jay Omega just a demo of the clean and lead channels. I’d love to hear how much gain and saturation is on tap.

Typical settings I use on my Mark IIC+/++ HRG
Volume 1 = 7
Treble (pulled) = 6
Bass = 2.5
Middle = 2
Master 1 = 4.5
Lead Drive (pulled) = 6.5
Lead Master = 4

GEQ settings
80hz = above middle bar
240hz = barely below middle bar
750hz = just above lowest bar
2200hz = middle bar
6600hz = barely below middle bar

 

[Jay Omega]

@Jay Omega just a demo of the clean and lead channels. I’d love to hear how much gain and saturation is on tap.

Typical settings I use on my Mark IIC+/++ HRG
Volume 1 = 7
Treble (pulled) = 6
Bass = 2.5
Middle = 2
Master 1 = 4.5
Lead Drive (pulled) = 6.5
Lead Master = 4

GEQ settings
80hz = above middle bar
240hz = barely below middle bar
750hz = just above lowest bar
2200hz = middle bar
6600hz = barely below middle bar

When you say +/++, is it switchable with a mid pull pot? I’d love to do an amp with a switchable ++, but haven’t seen a schematic or good pictures. JRB32 sent me some pictures of a ++ amp that also had an R2 mod, so I wasn’t able to tell what was ++ and what was R2

 

[Matador]

Stunning work!

From what I have gleaned between schematics and looking at exactly one ++ mod in person, the mod is:

1) A 3.3M + 20pF cap (in series) are added parallel to the existing 3.3M + 20pF cap (that are in parallel) that bridges across the lead channel gain stages. The tonal change by itself was pretty subtle if memory serves.

2) A 15uF tant cap was added across the 1.5K cathode resistor in the third gain stage (the one that drives the reverb tank and the 'send' part of the effects loop). This has the largest impact on the sound - the amp gets significantly louder, and the level driving the EQ is significantly hotter.

3) A 330K resistor is added in parallel with the 'lead drive' pot, and the 680K series resistance is increase to 1M. This cuts the signal going into the lead gain stages and softens the distortion a bit.

In some situations I find #2 a bit too much, and it can really eat into the clean headroom of the preamp, however you can get a more aggressive lead tone with it engaged. In one of my preamp boards, I added a Vactrol to this cap so that it only kicks in when then lead mode is engaged, and that has been my favorite iteration of the ++ mod.

EDIT: in the Mark 3, the cap in #2 above was switchable in and out via a relay in that amps 'Rhythm 2' channel. In addition, the voltage divider leading to the 'send' of the amp also kicked in (R130), to reduce the gain jump when adding in the cathode bias cap. The R2 mod involved converting R130 into a pot so you could dial in the amount of gain reduction to your tastes.

 

[Jay Omega]

Stunning work!

From what I have gleaned between schematics and looking at exactly one ++ mod in person, the mod is:

1) A 3.3M + 20pF cap (in series) are added parallel to the existing 3.3M + 20pF cap (that are in parallel) that bridges across the lead channel gain stages. The tonal change by itself was pretty subtle if memory serves.

2) A 15uF tant cap was added across the 1.5K cathode resistor in the third gain stage (the one that drives the reverb tank and the 'send' part of the effects loop). This has the largest impact on the sound - the amp gets significantly louder, and the level driving the EQ is significantly hotter.

3) A 330K resistor is added in parallel with the 'lead drive' pot, and the 680K series resistance is increase to 1M. This cuts the signal going into the lead gain stages and softens the distortion a bit.

In some situations I find #2 a bit too much, and it can really eat into the clean headroom of the preamp, however you can get a more aggressive lead tone with it engaged. In one of my preamp boards, I added a Vactrol to this cap so that it only kicks in when then lead mode is engaged, and that has been my favorite iteration of the ++ mod.

EDIT: in the Mark 3, the cap in #2 above was switchable in and out via a relay in that amps 'Rhythm 2' channel. In addition, the voltage divider leading to the 'send' of the amp also kicked in (R130), to reduce the gain jump when adding in the cathode bias cap. The R2 mod involved converting R130 into a pot so you could dial in the amount of gain reduction to your tastes.

Thanks for the compliment!

Interesting. When you say schematics, are you referring to the one JRB32 put together?

I would expect #1 to have negligible impact on the high gain mode and mostly increase the rhythm mode gain in upper frequencies. You'd especially have increased rhythm mode distortion in conjunction with #2 as there'd be effectively no more local negative feedback at that stage with a 15 uF bypass.

I understand how #2 would increase distortion at higher frequencies, when I traced part of my JP-2C I found a 680 nF in this position that was always active. If you look closely at my board, you can see that I added places for a resistor and capacitor to make this switchable if I wanted to add a mid pull pot for some extra juice. The Mark IV uses a 2.2 uF cap in this position for the mid-gain / harmonics switch I believe.

In one of the pictures I thought I saw a 150k instead of a 68k on the V4B grid, which would develop more signal for that last big clipping stage.

There are so many ways to tweak this circuit, it's really fun to mess around with. And fun to think that these guys at Boogie figured this all out when I was still in diapers.

 

[Jay Omega]

It's alive!!

It was pretty late by the time I finished wiring everything and finding / fixing a few goofs, so I haven't had much time to fully test and wasn't able to crank it up.

One thing I wasn't expecting, my IIB lead drive pot switch is pull to open rather than pull to close like the others. So basically it became a "PUSH LEAD" or "PULL RHYTHM" LOL. This doesn't bother me, but thought it would be good to let folks know in case they convert their own and want to change the pot.

Also, when changing from lead to rhythm I get a volume swell. This is probably vactrol hysteresis

I'd like to start looking at sound clips, does anybody have an IR they recommend? I have a Torpedo Captor and an Axe FX II, but might be able to figure out how to apply a software IR in a DAW.

 

[Henz]

It's alive!!

It was pretty late by the time I finished wiring everything and finding / fixing a few goofs, so I haven't had much time to fully test and wasn't able to crank it up.

One thing I wasn't expecting, my IIB lead drive pot switch is pull to open rather than pull to close like the others. So basically it became a "PUSH LEAD" or "PULL RHYTHM" LOL. This doesn't bother me, but thought it would be good to let folks know in case they convert their own and want to change the pot.

Also, when changing from lead to rhythm I get a volume swell. This is probably vactrol hysteresis

I'd like to start looking at sound clips, does anybody have an IR they recommend? I have a Torpedo Captor and an Axe FX II, but might be able to figure out how to apply a software IR in a DAW.

Does the above mods apply to the IIB ? There are similarities to the C+ schematic by JRB32. I tried the V2 B mod on the C+ schematic. I tried a 2.2uf tantulum with the 1.5kR . I really didnt notice much in the way of tonal differences just a slight increase in white noise .

 

[Jay Omega]

I finally had a chance to crank the amp, and it is certainly a glorious beast! Pretty thrilling to be next to the cab at drummer volume.

I started working towards sound clips for y'all over the past few days, and I'm starting to get annoyed. It has reminded me how much I dislike my playing, audio engineering and recording.. But, I've been eyeing this Tonex software since the pedal leak and I had a lightbulb moment. What if I just capture / profile the dang thing with different settings using a reactive load, then you all can try it out? It looks like they have a free version of the software if you only want to use it as a player.

Thoughts??

 

[Jay Omega]

Tonex pedal finally shipped so I should have it next week and hopefully some models for you all to try shortly after that.

Also, ordered ten boards from a different MFG today, they said around four week lead time. The new ones will be green with some minor adjustments, and 3 oz copper per square inch compared to 1 in the prototype run. PM if interested!

 

[PS Neely]

View attachment 696

Awesome !! I want one looks great !!

 

[dominique78]

Bonjour de France
est-il possible d'en avoir un svp ?
et quel est le prix d'un article svp ?

dom

 

[Jay Omega]

I've been messing with this Tonex for a few days and it wasn't as easy as I'd hoped. My old focusrite liquid saffire interface fed back like crazy when I hooked it up, and a presonus studio 26 had some funky background noise going on and the captures were way too bassy. I've ordered IK's Axe I/O interface, hopefully this will give me acceptable results. Should get here tomorrow, pretty nice to be located so close to Sweetwater.

Another thing this made me realize is that my desktop is way out of date. With the default model training, I think it took roughly four hours and the CPU was pegged at 100% during that time. I ordered a refurbished video card off Ebay, hoping that will help a bit. I think it took 15 minutes or so on my newer laptop for a default model, but the advanced model would not run, and on the laptop the memory was pretty much max'd out. Not sure if I got into paging which may be why the neural network wasn't built after 8 hours.

The fun never stops!

 

[Jay Omega]

Pretty cool news today. As some of you have discovered, one of the challenges with the conversion is sourcing the vactrols as they are obsolete. I've been scouring for a replacement for some time and finally found a device that looks like it will work on paper, but I can't find anyone selling them. One downside is that the device is non-RoHS, just like the originals, due to the CdS cell.

I reached out to the MFG, they have ordered samples that should be here in 4-6 weeks. I plan to test their performance with the stock IIC+ switching circuit. Since minimum quantities are cost prohibitive for my small volume, I've pinged some distributors about possibly carrying this line. One large US-based distributor has expressed interest in stocking these for me if they test out. They also said they'd be willing to setup a kit on their website for all the parts that they stock associated with the preamp board. So this could be handy for you all that are interested in purchasing, or have purchased boards.

If everything goes well it will be a few months at least, but thought I'd share.

 

[Jay Omega]

Alrighty, let's see if this works. I made an advanced Tonex tone model of the 60W amp setup for a lead sound as a test. I made a raw capture while connected to a speaker cab, so you would have to add a Tonex cab or your own IR to the tone model. The audio preview for the tone model was done with the the stock Tonex Greenback cab, I figured it was better than nothing.

60W Lead Sound No Cab Advanced

Also, I've attached a Tonex preset which hopefully has a random IR included.

Not sure what it will take for you guys to get it up and running, LMK what you encounter!

 

[Jay Omega]

Also, these beauties arrived today, I have ten on hand. Send me a PM if you’re interested in one!

 

[Henz]

Alrighty, let's see if this works. I made an advanced Tonex tone model of the 60W amp setup for a lead sound as a test. I made a raw capture while connected to a speaker cab, so you would have to add a Tonex cab or your own IR to the tone model. The audio preview for the tone model was done with the the stock Tonex Greenback cab, I figured it was better than nothing.

60W Lead Sound No Cab Advanced

Also, I've attached a Tonex preset which hopefully has a random IR included.

Not sure what it will take for you guys to get it up and running, LMK what you encounter!

Thats one facet that I never experienced. The digital realm. That is a plug in ? I've read where ppl love em or not so much. Ty for sharing!

 

[Matador]

Pretty cool news today. As some of you have discovered, one of the challenges with the conversion is sourcing the vactrols as they are obsolete. I've been scouring for a replacement for some time and finally found a device that looks like it will work on paper, but I can't find anyone selling them. One downside is that the device is non-RoHS, just like the originals, due to the CdS cell.

I reached out to the MFG, they have ordered samples that should be here in 4-6 weeks. I plan to test their performance with the stock IIC+ switching circuit. Since minimum quantities are cost prohibitive for my small volume, I've pinged some distributors about possibly carrying this line. One large US-based distributor has expressed interest in stocking these for me if they test out. They also said they'd be willing to setup a kit on their website for all the parts that they stock associated with the preamp board. So this could be handy for you all that are interested in purchasing, or have purchased boards.

If everything goes well it will be a few months at least, but thought I'd share.

I have used the NSL-32 from Advanced Photonix (previously Luna Optoelectronics) successfully in the circuit with stock values. The SR2 variant has a maximum on resistance of 40 ohms, however all of the samples I tested were below 10 ohms (@15mA forward current). The maximum off resistance is listed as 5M minimum, but most tested well above that. On and off times are also pretty fast at 8 and 80ms respectively.

They also sell a 'SR2S' model which are matched to within 5%, which means they make good replacements for those looking to use as digital pots (like what you see in the Triaxis).

See: https://media.digikey.com/pdf/Data Sheets/Photonic Detetectors Inc PDFs/NSL-32SR2.pdf

All of the typical online vendors like DigiKey, Mouser, and Farnell carry them.

 

[quad48uF]

Are the audible differences between the original Vactec and the Advanced Photnix very apparent? Could you comment on what you noticed?

 

[Jay Omega]

I have used the NSL-32 from Advanced Photonix (previously Luna Optoelectronics) successfully in the circuit with stock values. The SR2 variant has a maximum on resistance of 40 ohms, however all of the samples I tested were below 10 ohms (@15mA forward current). The maximum off resistance is listed as 5M minimum, but most tested well above that. On and off times are also pretty fast at 8 and 80ms respectively.

They also sell a 'SR2S' model which are matched to within 5%, which means they make good replacements for those looking to use as digital pots (like what you see in the Triaxis).

See: https://media.digikey.com/pdf/Data Sheets/Photonic Detetectors Inc PDFs/NSL-32SR2.pdf

All of the typical online vendors like DigiKey, Mouser, and Farnell carry them.

I found them to be good replacements for the VTL5C4, but I haven't used them in place of the VTL5C1. In the 60W build I used SR2 alongside Vactec VTL5C1, worked well as the forward voltage was close enough for the lead master just ended up with higher on resistance than if I'd used a Vactec 5C4. This is negligible though as I leave the pot and LDR combined resistance to be 10k a la the fixed resistor in the JP-2C. Since EQ LED drive is separate, the SR2 had a lower on resistance than a Vactec. Maybe the graphic common resistance difference is splitting hairs, it's clear from Boogie's follow-up designs that they don't think it's critical. Would be a good test, put a 1k log pot in place of the LDR and see if it's audible.

 

[Matador]

Can take these on a case-by-case basis:

1) LDR1 - shorts across a 10M, and even hundreds of ohms is small compared to the follow-on resistance of the treble pot which is 250k.
2) LDR2 and LDR3 engage the lead channel - LDR2 is in series with 680K, and LDR3 terminates into a 680K grid resistor, so even 1K isn't going to change much.
3) LDR4 is in series with the 250K lead master pot, so again, the resistance of the LDR is small in comparison.

LDR5 might come into play, however the default EQ 'disable' resistance is 68K, so anything lower than 6.8K (10%) is likely going to work just fine. Note that the resistance adds to the series RLC value, and doesn't change the resonant frequency of any of the bands.

I'd be curious to see if the pot has any audible effect for LDR5. The stock MK2C+ resistance values put a pretty hefty current (around 20mA if memory serves) through the LDR's when engaged, so even the VTL5C1 should be less than 500 ohms, and for the NSL-32, should be less than 10 ohms.

 

[Jay Omega]

Can take these on a case-by-case basis:

1) LDR1 - shorts across a 10M, and even hundreds of ohms is small compared to the follow-on resistance of the treble pot which is 250k.
2) LDR2 and LDR3 engage the lead channel - LDR2 is in series with 680K, and LDR3 terminates into a 680K grid resistor, so even 1K isn't going to change much.
3) LDR4 is in series with the 250K lead master pot, so again, the resistance of the LDR is small in comparison.

LDR5 might come into play, however the default EQ 'disable' resistance is 68K, so anything lower than 6.8K (10%) is likely going to work just fine. Note that the resistance adds to the series RLC value, and doesn't change the resonant frequency of any of the bands.

I'd be curious to see if the pot has any audible effect for LDR5. The stock MK2C+ resistance values put a pretty hefty current (around 20mA if memory serves) through the LDR's when engaged, so even the VTL5C1 should be less than 500 ohms, and for the NSL-32, should be less than 10 ohms.

I agree for the most part.

Measured LDR5 on-resistance from an actual IIC+ was around 150 ohms which is concerning to me, particularly in the lower faders. Agree it doesn't impact resonant frequency, but does affect the Q. For the lower three bands you have the inductor resistance (surprisingly low), the external Q-setting resistor of 470 ohms, and the LDR on resistance that determine Q - assuming you make the approximation that there is no bleed from band to band. 100 ohms difference (~50 actual for the SR2 vs ~150 actual for the 5C4) is more significant here compared to the other four LDRs. 6.8k is low enough to effectively remove the 68k from the circuit but way too high for EQ performance as your Q would drop dramatically. I suspect there would be noticeable difference between 0 ohms and 1k ohms but I could be wrong. Best way to find out is to try it, I'll probably do this test later this year and report back

Whether the amp sounds great or not with a 5C4, 5C1, or SR2 is a different matter, but if you want to stay as close to the original as possible you should be shooting for around 150 ohms on resistance in that spot.