John Mayer Two Rock build

[jj205]

Okay, so I'm considering building a JM series Two rock 50W head. I did find the d_jm_967 schematics that have been posted. Does anyone have a good BOM to go off of? I've been considering doing a PR but really like the tone from a two rock.

Any suggestions and resources would be greatly appreciative. Kinda new to the amp build world, but do have a decent understanding of the components.

[M Fowler]

I did a parts count off the schematic when I built mine used CTS and Alpha 3/8" pots, 6PS coupling caps, Silver Mica pf caps, RN70D plate caps, Xicon resistors and IC filter caps.

For 50w you can use seasoned Bassman transformers or use Magnetic Components Super Reverb transformer set but change the OT for 4/8/16 secondary version rather then SR 2 ohm secondary.

https://tubeamparchive.com/viewtopic.php?t=21917

Schematic https://tubeamparchive.com/download/file.php?id=30389

[beasleybodyshop]

Here is a rough draft of a layout i am putting together on a 100 watter. The tone boards will be similar. Hope this helps.

[jj205]

Thanks, that is definitely helpful. I'm looking forward to seeing how your build goes. I'm probably going to end up doing a PR this first time but next I would really like to the the JM TR.

[beasleybodyshop]

Truth be told, a PR is probably near the same in difficulty as the TR. Throwing in the verb and vibrato, plus dealing with lead dress in a normal PR sized chassis was time consuming - for me anyways. If you do decide to build one i recommend using the original layout. Good luck with your build!

[jj205]

Hmm, that definitely makes me reconsider. The PR and the TR are my two favorite amps, but it would be really fun to have a TR. I guess what makes me more nervous about it is coming up with my own BOM.

I've bought a hammond ao-29 and I plan to use the PT, OT, chassis and tubes for whatever I build, so I will definitely have plenty of room.

Price wise, would you say they are fairly equal?

[beasleybodyshop]

PR chassis is going to be cheaper if you decide to buy chassis outright. other than that, its probably going to be close in price.

PR is a very established amp to build, its easier to follow than the TR IMHO. I am still making up my directions as i go on my build, whereas with the PR I just followed everyone who came before me.

BOM is not hard to do - print the schematic, and highlight all the resistors, ledger out all the values, and move on to the caps, etc. All plate resistors are going to be Metal Film, Mark used RN70s which I like as well. All cathode resistors are Metal film as well.

I use 6PS caps, you can tally all the different ones you need off the schem and order from tubesandmore or the like.

For me, the most fun part besides building is planning out the builds, doing the research and digging through the thousands of posts here at TAG for info.

Good luck, and ask away. I will help as much as I can.

[jj205]

Yeah, that makes sense just going off of the schematic to get everything I need. Just don't want to miss anything, but that's on me. If I went with the TR I'm not sure if I would be able to use the PT and OT out of the hammond organ I bought. That will probably be the deciding factor, because I don't want to have to buy another pair or transformers.

What voltages and amps should I be seeing from the PT and OT? Mark, I know you suggested the Super Reverb PT. I guess I will try to find what all it puts out and compare that to what I read off of my transformers.

[beasleybodyshop]

Do you know how to check voltages/OT impedance out of circuit?

PT is easy, tape down all your secondary wires on the PT, making sure you dont have any touching and can check voltage with a meter. hook up the 120 primary side to a wall plug, and check AC voltage across the secondary windings (heater to heater, B+ to B+, etc)

OT is easy too, just requires some math:

http://www.youtube.com/watch?v=QQNVyYEzcJ8

Super reverb PT has a 600-700 volt B+ secondary - I would imagine that if your hammond is close to that then you could use it. OT impedance would be around 4.2K

[M Fowler]

If the hammond organ transformers were powering a pair of 6L6's then there is no problem using them in your build. I've used many organ donor transformers and they sound great.

[jj205]

It was powering a pair of 6V6GT's so I'm thinking maybe it doesn't have quite the juice for the 6L6?

[M Fowler]

I bet the PT does have enough mA they were very stout transformers and the filament supply was usually 3A using 5U4.

[jj205]

@beasleybodyshop, in the video you posted about measuring the impedance of the OT he used a variac (sp). Don't have one of those so is there a way I could simulate what he is doing by some other method? There are some percussion transformers on the ao-29 that I am looking to possibly use for the reverb transformer. I've been searching online and can't find any specs for them. I've even found the new(newer) part number for them but still no impedance info.

[Bombacaototal]

I did a parts count off the schematic when I built mine used CTS and Alpha 3/8" pots, 6PS coupling caps, Silver Mica pf caps, RN70D plate caps, Xicon resistors and IC filter caps.

For 50w you can use seasoned Bassman transformers or use Magnetic Components Super Reverb transformer set but change the OT for 4/8/16 secondary version rather then SR 2 ohm secondary.

https://tubeamparchive.com/viewtopic.php?t=21917

Schematic https://tubeamparchive.com/download/file.php?id=30389

Are the RN70D caps or resistors? Any further details on the parts used?

[M Fowler]

RN70D are the blue plate resistors.