16 mfd cap on the board

Slammintone · Post by **Slammintone** » Tue Oct 04, 2005 12:26 am

George, I've noticed that your kit amp for the JTM 45 contains a large blue 16uf cap which is wired on the board, as was done on the original Marshall JTM 45s I've checked out. But I've also noticed that the same cap is missing from the RI PTP board and it is missing from the board from my Mojo JTM 45 combo amp as well. Do I need this cap or is that capacitance handled elswhere in the circuit, such as from one of my 2 32X32uf filter caps? Is my amp technically still a JTM 45 without that board mounted cap? Thanks!

Slammintone · Post by **Slammintone** » Tue Oct 04, 2005 12:30 am

PS. I have the Valve Art KT66s you sent and they look fantastic compared to the Groove tubes KT66HPs. The really look like GEC clones. I should have them installed and up and running as soon as my NOS Mullard relabled Valvo GZ34 arrives. I'll post the results here afterwards.

Flames1950 · Post by **Flames1950** » Tue Oct 04, 2005 7:01 am

George used the 16uF board cap to be authentic. But the can mounted on the side of his JTM45 kit is a dual 16/16 with one side left unused (the only 16uF-rated can available for use.) The RI and the MoJo use the other half of the cap can instead of the board-mounted 16uF cap. The Marshall RI has this can mounted above the circuit board instead of one the side of the chassis; I don't know where the MoJo cap is mounted exactly.
The JTM45 circuit requires four filtering caps total, so two dual section cap cans can cover everything; George just likes to be accurate as possible......

Guest · Post by **Guest** » Tue Oct 04, 2005 8:06 am

Thanks Flames, my Mojo amp has both of it's dual 32uf filter caps mounted on the inside near the OT. Now I'm wondering if I should figure out which of them is handling the capacitance normally done by the 16uf cap on the board, and have my amp guy replace it with a dual 16uf cap and rewire. Would that make much of a difference soundwise? As it is the sound is pretty hellaciously good. If I ever do another JTM45 or Plexi amp I wouldn't hesitate to go with a Metro kit. Although the Mojo guys sell good stuff the tech support (2 years ago anyhow) was pretty much non existant and at the time Metro wasn't doing JTM45 kits. Otherwise I can see that the job of building it from a Metro kit would have been so much smoother and faster.

Flames1950 · Post by **Flames1950** » Tue Oct 04, 2005 4:23 pm

I think it would be worth a shot trying the dual 16uF can in place of the dual 32uF. 16uF is the original value for the preamp and phase inverter B+ filters, and you very well may hear a difference in tone, and a bit more compression.

Slammintone · Post by **Slammintone** » Tue Oct 04, 2005 10:22 pm

Yeah, I think I'll go ahead and order another dual 32uF and the dual 16uF caps, both F&Ts this time, and replace the black JJ dual 32s caps the amp kit came with. What the heck, might as well go as correct as possible now that I got better tubes to go in. THanks again.