Friedman Dirty Shirley build

[daveyajd]

There are no layouts or schems for this anywhere so I figured I would try my hand at reverse engineering something close from any pics or info I can find. I need to add a Marshally -thing to my arsenal. I thought I would start a new thread for this project as I get things going. I have pretty much completed a layout for my board but its still not quite clear how the filtering will all connect. Attached are a couple of pics from the web, Ceriatone's JTM 45 layout, and my version of the board.


A couple of notes:
The resistors around the PI differ on my layout because I have found the 470R to be the more likely value to pins 8-3.
The 470k/500pf pair going to the gain will likely not be on the board but I left it for reference.


A couple of questions:
-Should my filtering connect like Ceriatones (32uf to one side of 10k, 16uf to the other)? Can I just eliminate that half power switch?


-It seems bias is pulled from pin 6 of the rect tube(I dont use those diodes). Can I use a Bias tap from the Xformer and just lose the dropping resistor? Is one better than the other?


-Except for an old Fender I have all of my builds have been cathode bias. What would a change like that look like?


Please let me know if you see anything I have screwed up so far! 


Aaron

[daveyajd]

More pics

[vampwizzard]

Id setup the power filtering in the same way as the JTM 45. You can remove the half power switch by removing the switch and all the connections to the tubes per that ceriatone layout.

Using a bias tap off the transformer is different than deriving it from one of the pre-rectified AC legs. compare https://el34world.com/charts/Biascircuits.htm to what youve got on the JTM45.

[daveyajd]

Thank you vamp. I will just derive my bias from the rect.
I am checking out options for transformers and it seems to be common for a JTM type build to use a 350-0-350 PT. What kind of mA should I try for?  I’ve tried using a current calculator but I’m not sure if I should plan on using an 8k OT with the 5881s.


What size should I make the choke?


Aaron

[SILVERGUN]

>What size should I make the choke?
I would use this one or equivalent:
http://www.classictone.net/40-18032.html

If you haven't seen it yet, this thread might have something for you:
https://ampgarage.com/forum/viewtopic.php?f=9&t=30395

[daveyajd]

Thank you for that. I will look for a choke in that range. I’m still trying to figure out my PT and OT sizes. It seems most JTM type amps use around 350-0-350 but how close do I need to be to that and how does that affect the amp’ sound? For example using a 325-0-325?


I’m using this calculator:
https://thesubjectmatter.com/calcptcurrent.html


So assuming a 345-0-345, GZ34, (2) 5881, (3) preamp tubes. And a OT with 8k primary it says I should have a PT with about 130mA. Does that seem right?


Aaron

[shooter]

Does that seem right?

it's early but looks good enough, I always error on the higher current rating side, voltage can be "manipulated"

[SILVERGUN]

For your application (and future potential tweaking) I would be all over this PT :
http://www.classictone.net/40-18095.pdf
http://www.classictone.net/40-18095.html

As far as the OT goes, I thought I read somewhere in the TAG thread that someone said he used a standard Marshall 50W OT.
"Dave has confirmed that the DS has a 50w plexi style OT - with a 3k6 primary."

Like this:
http://www.classictone.net/40-18025.html

[daveyajd]

As far as the OT goes, I thought I read somewhere in the TAG thread that someone said he used a standard Marshall 50W OT.
"Dave has confirmed that the DS has a 50w plexi style OT - with a 3k6 primary."

Yes I have read that also. I’ve also seen mention that JTM used 8k. I was thinking of going the 8k route and then I could have the option to reflect a 4K load by running 8 ohm speaker off 16 ohm tap.


I really like that PT. Multiple voltage option is really nice!


Aaron

[SILVERGUN]

>Yes I have read that also. I’ve also seen mention that JTM used 8k. I was thinking of going the 8k route and then I could have the option to reflect a 4K load by running 8 ohm speaker off 16 ohm tap.

Then I think you just talked yourself into this one (or equiv.):
http://www.classictone.net/40-18039.pdf
http://www.classictone.net/40-18039.html

[daveyajd]

Then I think you just talked yourself into this one (or equiv.):

   I always do that!


Question though, should I get something with higher current rating if I will use that trick?


Aaron

[shooter]

should I get something with higher current rating

If you "reflect back" the proper impedance current stays the same, if you DON'T reflect back the proper impedance it won't matter 

[daveyajd]

Ok so I am going to have MPS build me my transformers again on this amp.


Going to get an upright 345-0-345 with 150mA, a 5H 120mA choke, and a 40 Watt(maybe 50) OT at 7.5K with multiple taps. Should be quite a ride! I will update as things progress.


Aaron

[purpletele]

This project looks real intriguing.  Looking forward to seeing it develop.

[daveyajd]

Alright, well this project is moving forward again. It has been an interesting few months but I’m back on track to build this. Got my transformers,chassis, and a small order of parts.


(side note: placed an order with Doug on a Saturday, packaged delivered Monday in the mail. Unbelievably top notch service!. Thank you.)


Finishing the designs for the faceplates and drilling out the chassis. Looking forward to getting this completed. I’ll post a couple pics soon.


Aaron