Plexi Build Questions?

RadRob · Post by **RadRob** » Mon Oct 14, 2013 5:54 pm

I've been hammering out a Mojo Plexi kit with Marstran transformers. I have gotten to the point where I am about to drop in the board. I do have a few questions about wiring the Choke, Output CT, and B+ fuse. The layout and schem from Mojo appears to be incomplete or I am dense.

Here are some photos. I am unsure of what questions to even ask, so let me know if you need more information.

Thanks,

Rob

http://i46.photobucket.com/albums/f116/ ... 230838.jpg
http://i46.photobucket.com/albums/f116/ ... 215204.jpg
http://i46.photobucket.com/albums/f116/ ... 215259.jpg
http://i46.photobucket.com/albums/f116/ ... 215243.jpg

RadRob · Post by **RadRob** » Wed Oct 16, 2013 4:26 pm

I discovered the more complete layouts for the 69 in the forum. I think I have it figured out. Here is an updated pic. Let me know if anything looks awry. I will be installing the 5.1ks, wiring the bias pot, and checking voltages today.

http://i46.photobucket.com/albums/f116/ ... 235959.jpg

stef · Post by **stef** » Wed Oct 16, 2013 10:22 pm

Did you reverse v1a and v1b cathodes?

RadRob · Post by **RadRob** » Mon Oct 28, 2013 4:46 pm

Maybe, I could have mixed up the pin out? Channel I is definitely the treble input. I double checked the layout/schematics and it shows 820r/250uf to v1a (pin8) and 2.7r/.68 to v1b (pin3).

I fired it up and found that I forgot to jump pins 3/8 on the PI, fixed that. tried it out again, v5 flashed and blew the fuse. All my voltages checked out so I swapped out for a different set of tubes and all is well. Great sounding amp, I am going to try some Sozo's in the near future after I give this amp a good break in.

I also went in after getting it going and installed the Lar/Mar PPIMV. I think that master sounds great as long as you run it at or over 12 o'clock.

Anybody have any recommendations for making the amp a little less bright? It's got a 33k slope resistor, no bright cap on the vol, and 47k 8ohm tap NFB.

Thanks,

Rob

stef · Post by **stef** » Mon Oct 28, 2013 10:41 pm

Tad (mojo dijon) caps are bright

neikeel · Post by **neikeel** » Tue Oct 29, 2013 5:41 am

stef wrote:Tad (mojo dijon) caps are bright

Original mustards are smoooooth!

stef · Post by **stef** » Tue Oct 29, 2013 2:37 pm

One more thing - the ppimv messes with the NFB (decreasing it) makes the sound brighter and looser - more NFB fuller (darker) tone
I like the original mustards but white M 150's and sozo's are also good choices

RadRob · Post by **RadRob** » Tue Nov 12, 2013 4:40 pm

I've been looking into redoing the amp with Sozo. On another note, if turning down the LarMar PPIMV decreases the NFB, could one find a triple gang pot for the PPIMV and have it add NFB as the MV is turned down or am I crazy?

Tonight after I get home I will record some clips of the amp as is.

RadRob · Post by **RadRob** » Thu Nov 21, 2013 8:39 pm

I made a new thread as I am heading in a EVH direction with the build. Clips can be found here http://forum.metroamp.com/viewtopic.php?f=16&t=42690.

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