CRS3 Build - Rootz

[Marshmallow]

Good day all,

I've been a long time lurker here. About to build rootz amp based on the CRS3 schematic rootz provided. I promised rootz that I will post the amp here when the build started.

[Marshmallow]

Chassis has been ordered. Since the guy is on holiday last week it will most likely arrive 2 weeks later.

[rootz]

Some pictures to clarify.

I'd not use the hand written layout. Connecting the reverb driver/send pot requires some extra circuitry to work properly. The way it is drawn will work, but loads the high end of the preamp (2n2/250k pot combo) and also the drive levels are high for a 12AX7 reverb drivers which will mean overdriving it very early on the reverb send dial. I did do an amp with a post reverb master only, but altered the reverb driver, recovery ànd mixer stages for it to work properly and cleanly. It is a variation on the CRS mk3.

- The master pot is right after the OD relay. There is a second master on the CRS mk3 on the back of the amp. U64 in my schematic. The reverb send control is also on the back, as is the second contour control on the Two Rock amps. I did not copy that feature, but you could.
- The bumps at the red arrow indicate the location of the effects loop. Per standard Dumble fare right after the Master Volume/U64.
- The 1 Ohm resistor (r52) should be omitted. It is not in the original amp, there is no space for it. I added it to make it possible to adjust the mix of wet and dry signal at the mixer. I did only use one 150k resistor, R54.
- In some amps C20 is there, in some others not. Some have the 220k/470p combo inside the local feedback loop (like a Dumbleator), some have it outside (like a Wonderland/JM). I opted for outside the loop in the end, which makes a minor change in the layout mandatory. That is quite simple to do, with the same amount of parts.
- I do not fully understand what you mean by #60 reverb. The tank? That can be any tank with 8/10 Ohm input and 2250 Ohm output impedance. I use 3 spring long delay types, but 2 springs or medium or short delay should work equally well.
- The bias is standard Dumble/Fender type. Simply copy it from the the #102 or #124 layout and you're good. Same for HV supply (diodes), FET and relay supply.

There is a lot room to play in the structure of the mixers, the reverb stages and off course the rest of the amp. You, e.g., could play with different feedback loops on the reverb driver and recovery, or use a 12AT7 there. You can alter the voltage divider in front of the dry mixer stage. You can connect the local feedback network around U63 (the dry mixer) to either the junction of R70 and R69 (like in the schematic, or connect R68 directly to the grid of U63. You could alter the value of C35 and U11 to e.g. 270pF and A1meg to get a different loading on the high end (as in: a bit less). So many possibilities!

some other fun parts of this design:
- You can (but not should) replace U64 by a fixed resistive divider;
- You can use either single or dual presence or contour controls;
- You can implement a fixed reverb send control. AN did in one of the attached pictures, although that picture does not show the mandatory grid leak resistor on the reverb driver.
- You can use this design with virtually every possible ODS schematic you have ever seen. I've done #124, #102, #94 and they all work.

[rootz]

Here's what I did to move the master volume after the reverb. Note the amount of differences and a mixer stage that is more like the one in ODRS #60.

[Marshmallow]

Good day all,

I've been a long time lurker here. About to build rootz amp based on the CRS3 schematic rootz provided. I have also ordered the chassis.

[Marshmallow]

Some pictures to clarify.

I'd not use the hand written layout. Connecting the reverb driver/send pot requires some extra circuitry to work properly. The way it is drawn will work, but loads the high end of the preamp (2n2/250k pot combo) and also the drive levels are high for a 12AX7 reverb drivers which will mean overdriving it very early on the reverb send dial. I did do an amp with a post reverb master only, but altered the reverb driver, recovery ànd mixer stages for it to work properly and cleanly. It is a variation on the CRS mk3.

- The master pot is right after the OD relay. There is a second master on the CRS mk3 on the back of the amp. U64 in my schematic. The reverb send control is also on the back, as is the second contour control on the Two Rock amps. I did not copy that feature, but you could.
- The bumps at the red arrow indicate the location of the effects loop. Per standard Dumble fare right after the Master Volume/U64.
- The 1 Ohm resistor (r52) should be omitted. It is not in the original amp, there is no space for it. I added it to make it possible to adjust the mix of wet and dry signal at the mixer. I did only use one 150k resistor, R54.
- In some amps C20 is there, in some others not. Some have the 220k/470p combo inside the local feedback loop (like a Dumbleator), some have it outside (like a Wonderland/JM). I opted for outside the loop in the end, which makes a minor change in the layout mandatory. That is quite simple to do, with the same amount of parts.
- I do not fully understand what you mean by #60 reverb. The tank? That can be any tank with 8/10 Ohm input and 2250 Ohm output impedance. I use 3 spring long delay types, but 2 springs or medium or short delay should work equally well.
- The bias is standard Dumble/Fender type. Simply copy it from the the #102 or #124 layout and you're good. Same for HV supply (diodes), FET and relay supply.

There is a lot room to play in the structure of the mixers, the reverb stages and off course the rest of the amp. You, e.g., could play with different feedback loops on the reverb driver and recovery, or use a 12AT7 there. You can alter the voltage divider in front of the dry mixer stage. You can connect the local feedback network around U63 (the dry mixer) to either the junction of R70 and R69 (like in the schematic, or connect R68 directly to the grid of U63. You could alter the value of C35 and U11 to e.g. 270pF and A1meg to get a different loading on the high end (as in: a bit less). So many possibilities!

some other fun parts of this design:
- You can (but not should) replace U64 by a fixed resistive divider;
- You can use either single or dual presence or contour controls;
- You can implement a fixed reverb send control. AN did in one of the attached pictures, although that picture does not show the mandatory grid leak resistor on the reverb driver.
- You can use this design with virtually every possible ODS schematic you have ever seen. I've done #124, #102, #94 and they all work.

Hi Rootz,

Thanks a lot for this and thanks for giving different options. Chassis will arrive within 2 weeks then the production will begin. I will stick with your first CRS reverb with the modifications above as I like the sound coming from that amp. I'll post images all 4 amps after this amp.

[Marshmallow]

Had time this weekend since it's the long holiday in Japan.. Will put it in the cabinet in a few days. Probably my current fav. OD is raw and the cleans of the JM amp. . Didn't follow rootz layout but a layout were all familiar with. Sorry rootz!

Had massive help from AmpGuy (Guy77) throughout this build.

[norburybrook]

beautiful build there

Turrets and Tag board


M

[Marshmallow]

beautiful build there

Turrets and Tag board


M

Thanks! I'm a turret guy haha. Its usually PCB for the relay. Just ran out of parts lol. Should I post the other amps here too?

[Guy77]

Had time this weekend since it's the long holiday in Japan.. Will put it in the cabinet in a few days. Probably my current fav. OD is raw and the cleans of the JM amp. . Didn't follow rootz layout but a layout were all familiar with. Sorry rootz!

Had massive help from AmpGuy (Guy77) throughout this build.

It came out beautiful Tom , super clean! The turret boards look really solid. I like how you sprayed the boards black.

Congrats!
Guy

[rootz]

Not any need to apologise for a different build tech!! I love it and as a Marshall and Vox guy for years, I had to adopt a bit to the eyelet build style. Different, but both very nice approaches.

You need to help me a bit: is this a high plate classic? Or low plate?

[Marshmallow]

Not any need to apologise for a different build tech!! I love it and as a Marshall and Vox guy for years, I had to adopt a bit to the eyelet build style. Different, but both very nice approaches.

You need to help me a bit: is this a high plate classic? Or low plate?

I believe It's the low plate classic #124 for the OD. Will upload more photos later. Probably make a page for all the builds. Need to sort out the photo link! they keep disappearing.