2nd gen ODS with Erwin chassis.

[Dr d]

I have come back to the fold after a long break, and after getting the vintage Marshall bug. First time around, I built every Dumble variant in one chassis and ended up with nothing! The mistake I made was not to live with any of the variants and really get to know/understand what it did. So this time around, I am going to try to live with a 2nd gen ODS and see if I can fine tune it as best I can.
The chassis, as stated in the title, was purchased from Erwin….and a fine chassis it is! I used new iron from Hammond and tried to use as many NOS parts as possible. Everything I have learned from this forum was from guys like Tony, Ayan, Erwin, Martin to name but a few….and I thank them all.
Things that I learned in the build…..
Coupling cap orientation is completely opposite to later builds.
FET bias with a low Vdd sounds fantastic. Lovely fat tone.
Use a good balanced PI valve. I used a balanced GE 12AT7. This is where the harmonic content lies.
The middle and bass controls did nothing so I removed the caps and joined the outer lugs with a 0.0022 cap, and reduced value to 500K. Now the controls work!
It is evident to me that HAD must have selected high gain valves for the OD. With the valves I had to hand, the 1m2 resistor in the funky OD entrance is just too little gain. In the end I added a 680k plus a 500k trim in series and settled at about 820k. I also reduced the 180k to ground on the exit of the OD to 150k to help balance the OD/clean balance.
The accent switch is a bit “all or nothing” so I ended up putting a 500R trim in series with the 1uf cap to adjust to taste. Switching the accent in and out now works well.
Voltages in the preamp are CRUCIAL! Get V1 plates down to between 170-180k. I swapped V1 with V2 and this reduced the V1 plate voltages and slightly raised the V2 plates. The amp began to sing. Incredible transformation.
Finally ( I think!) I reduced the volume bright cap to 100pf.

Valves used were EH V1&2, GE balanced 12AT7 and TAD STR 6l6.
Ok here it is….

[Dr d]

Another…

[Dr d]

And another…

[bepone]

nice.. yesterday i was checking materials for this amp and schematic.. never built this version..are we sure about V2a entrance resistor connections? looking the blury pictures im not sure about this part of schematic... is there some clear pic of originals showing those parts?

[erwin_ve]

Hi Dr D,

Great looking build with lots of detail like the original 2nd gen out there and I enjoyed reading your tweaking process. I did the same thing with the accent switch, putting a trimmer before the presence cap.
The "funky" OD entrance may have the #40 values for more signal/bite and you're very close to that.

Bepone: yes the OD entrance is correct. I can post some pics, but check the files section, there are some good pics for several 2nd gen / #40.

Erwin

[Dr d]

Hi Bepone, Thank you. The "Funky OD entrance" to V2a is definately confirmed, and as shown on Tonys layout in the files section. The only errors that I have noticed on Tonys layout are of the lack of ground connection on the FET board and no ground connection for the relay switch.

[Dr d]

Hi Erwin! Thank you for your kind words. The amp took a few minor tweaks, but considering it is not burned in yet, it is sounding fantastic. As always with these amps, the devil is in the detail. Lovely amp!

[dbharris]

What an awesome build! Even all of the correct Mepcos, very impressive.

-Dan

[martin manning]

Looks great!

The middle and bass controls did nothing so I removed the caps and joined the outer lugs with a 0.0022 cap. Now the controls work!

As shown in the layout, the 2nd Gen stack is more Fendery, but the mid scoop is shifted down, too far down IMO. By changing to a 2n2 across the Bass pot (top to bottom) and deleting the 2n2 (top to wiper), you have essentially changed that bit to Skyline, and moved the action of the Bass pot and the mid scoop up in frequency. Note that the #040 schematic shows a 1n cap on the Bass pot from wiper to bottom (matching the Skyline in Rock mode when top to wiper is shorted), where the 2nd Gen layout shows a 20n. The other cap (top to wiper) is shown as unknown on the #040 schematic, and deleted in the Skyline.

IMO, the evolution of ODS through the various generations was a series of improvements in HAD's mind. That includes not just the tone stack, but also features like the PI trimmer and the OD drive trimmer, all of which you are anticipating here.

[ijedouglas]

Great looking build!

Thanks for the info on the tweaks.

[Dr d]

IMO, the evolution of ODS through the various generations was a series of improvements in HAD's mind. That includes not just the tone stack, but also features like the PI trimmer and the OD drive trimmer, all of which you are anticipating here.

I couldn't agree more!
Thank you for your encouraging words. I have learned a lot from your posts over the years.

[Dr d]

Looks great!

The middle and bass controls did nothing so I removed the caps and joined the outer lugs with a 0.0022 cap. Now the controls work!

As shown in the layout, the 2nd Gen stack is more Fendery, but the mid scoop is shifted down, too far down IMO. By changing to a 2n2 across the Bass pot (top to bottom) and deleting the 2n2 (top to wiper), you have essentially changed that bit to Skyline, and moved the action of the Bass pot and the mid scoop up in frequency. Note that the #040 schematic shows a 1n cap on the Bass pot from wiper to bottom (matching the Skyline in Rock mode when top to wiper is shorted), where the 2nd Gen layout shows a 20n. The other cap (top to wiper) is shown as unknown on the #040 schematic, and deleted in the Skyline.

This is really interesting. I hadn't noticed the discrepancy between the #40 schematics and Tonys layout. I also hadn't noticed that on the schematic it indicates a 470K on the OD entrance vs 1m2 in the layout. I always assumed that 470K was a Kittyhawk value. I suppose the confusion here lies in the fact that quite a few of the 2nd gen amps were exported to Germany. Still think 1m2 offers too little gain IMHO. Thanks again for your input.
BTW, still re learning how to quote etc on this forum. Its been a while!

[Dr d]

And here are some voltages with a dropping string of 2K2, 27K and 2K2......
With a B+ of 441V on the 6L6 plates
V1 168.8V + 175.2V
V2 184.8V + 185.5V
V3 298.5V + 291.5V (exactly 7V difference).

FET VDD 10.50V with 8k2 voltage divider. 5.25V across drain resistor with 6K source resistor.

[Raoul Duke]

Interesting and informative! Thanks Doc!

[Dr d]

Really enjoying this amp. In the meantime I have made a headshell and combo cab (just need to make a front baffle to complete). Details of the circuit to date to follow, including a master (hope Erwin doesn’t mind me drilling his lovely chassis ).