Success story -- 2204/1987 50W new build

[PharmRock]

Finally got around to firing this up today.  This all started when, while browsing Reverb, I came across a Hoffman Plexi-100 kit that Doug put together back in the old days:
https://el34world.com/Forum/index.php?topic=27043.msg296657#msg296657
I made an offer on it and about a week later it was in my mailbox.

With a blank chassis and a few spare parts, I set about trying to figure out what I might do with this...I knew 100 watts was too much for a hobbyist like me, so I started scouring the forums and happened upon Sluckey's Dual 50, which looked like a pretty neat combination of a 1987 and 2204.  Some further research uncovered "Neil's Special", which was posted on the Metro forums.  This basically is a mod to a 1959 circuit that allows for switchable 2204, including the Lo Input.

After a quick crash course in DIYLC, I ended up going with this as my V1 input stage, which is based off Neil's Special: https://el34world.com/Forum/index.php?topic=27077.msg297144#msg297144

Along the way, I got some great feedback on this idea from forum members, and ended up with the following:
Elevated Heaters https://el34world.com/Forum/index.php?topic=27216.msg298901#msg298901
Lar-Mar PPIMV https://el34world.com/Forum/index.php?topic=27335.msg300301#msg300301
and a pretty neat grounding scheme inspired by Sluckey.

All of this required some modifications to the board, but using the Plexi-100 board and converting it to a 50W layout freed up some turrets where I could easily install some of the other features. 

Everything is working exactly as it should, with the only issue being some squealing when I am in the high input 2204 cascade mode.  I think if I use some shielded cable runs for the cascade switch (akin to the Hoffman Hot Switch), I can hopefully get that tamed down. 

Anyways, the Plexi tones are incredible, and the JCM800 tones are good but I haven't been able to really crank it yet while in this mode due to the squealing (FWIW, my workbench is in the basement with fluorescent lights directly overhead, with an HVAC unit 10 feet away). 

My voltages just about matched Mark Huss's JCM800 schematic, all within about 10%.  Funny story...while taking all of my readings and doing the biasing, my wall voltage would swing from 110-122VAC.  Had to wait a while until it stabilized.  It was really throwing me off.

I'll try to post some pictures of the completed build soon.  Like I said, I still have to catch a few gremlins to get this squealing under control. Otherwise, its great.   I plan on installing a Metro FX loop board once I get everything figured out. 

Probably would do some things differently if I did it again....I might make a "lessons learned" thread. 
 
Thanks to all the forum pros who helped me out along the way.  I really appreciate it.
 

[thetragichero]

i like your process: get the amp itself working properly then add the additional bells and whistles like effects loop
and i like the attitude of "just working through gremlins" as, in my opinion, amp/pedal building is about 30% assembly and 70% figuring out why the durn thing won't work right

[PharmRock]

i like your process: get the amp itself working properly then add the additional bells and whistles like effects loop
and i like the attitude of "just working through gremlins" as, in my opinion, amp/pedal building is about 30% assembly and 70% figuring out why the durn thing won't work right

Yeah it always seems that's the way it works out.  Usually at this stage is when bang my head against the wall for a week and its a simple error that ends up taking 2 minutes to fix.  I'll try the RG316 for the switch-to-board runs and see if that helps and go from there.

[PharmRock]

So here is a little bit of the build history in case anyone is interested in seeing this from start to finish.  Along the way I'll share what I wish I had done differently or perhaps some things to think about so maybe someone else can learn from the process.

 This started with having a blank 17.5 x 8 x 2.5" aluminum chassis I picked up several years from somewhere.  Btw...having the 8" depth for the chassis really restricts you as to which empty head cabs you can buy, as most (not all) are pre-fabricated for 6.5" depth.  I had a head cab that already accommodated an 8" depth from a previous build so no problems there. 

A Plexi-style circuit was definitely where this project was going, and I found a pristine untouched Plexi-100 Hoffman board on Reverb and snagged it.  Now to figure out what to do from here.

As I mentioned in the first post, I ended up settling on 50W version of Neil's Special, which basically modifies a 1959 by changing one of the input jacks to a dedicated volume control for the JCM cascaded gain, and changes the normal channel volume to a pre-PI MV. 

Using a free version of CorelDraw, I mocked up a faceplate.  I bought some clear acrylic from Lowe's and printed off the faceplate, stitched it together, and could "test fit" the pots and switches before drilling holes in the chassis.  This worked out really well and only cost about $10 for the acrylic board, which was big enough to allow me to do several iterations.  Attached is one of the many versions I tried out.  When I printed off, I would replace the "knobs" with just a 1/8" circle to drill a pilot hole.  I used the Maven Pro font from one of those free font websites.  Its pretty close to the original Marshall font.

[PharmRock]

After settling on a faceplate design, I sent off the CDR file to Sandy at Precision Design.  http://www.precisiondesignin.com/ (Highy Recommend!).  BNP Lasers used to be my go-to place for panels but they retired a few years ago.  I didn't even know Sandy's business existed but found her info on one of the forums.  About an hour after sending her the file, she emailed me back and asked me to give her a call to discuss the project.  This was all after hours, and she was very nice and professional.  I went with the traditional reverse-etched gold plexi panels.  I received the panels less than a week after ordering. 

I then used these as the template for drilling the front and rear of the chassis.  Used a step bit and used the 1/8" pilot holes which were pre-drilled on the panels. For the IEC socket, I traced the outline on the chassis and drilled a 5/8" hole centered within the outline, and used files from that point and got it fairly decent looking.

After all that, it was time to back-fill the etched panels...I picked up some black acrylic paint from the craft store. They turned out looking pretty good.

Lessons learned...Even with all of the acrylic panel mock-ups, I still had a couple places where the jack or switch nuts covered a little bit of the lettering.  In the future I will also mock up the actual nut and/or washer for the jacks and switches so I can be sure there will be a sufficient gap. 

[PharmRock]

The next step was to do the chassis top.  I downloaded some images I found of 50W chassis from sites like ValveStorm and Mojotone, imported these into CorelDraw, and ended up with a fairly accurate layout for tube sockets and cap cans.  I also downloaded the specs from ClassicTone for the transformer and choke mounting bolt spacing.  Finally, I snapped a picture of the turret board and with all the measurements I was able make a drill pattern for the chassis. 

The noval sockets were done using a chassis punch, and I had to use a step bit for the larger holes for the octals and cap cans.  Lesson learned: aluminum will get "goopy" if it gets too hot, and this results in a hole that has rough edges.  If drilling out an aluminum chassis, I would recommend going a few "steps" on the step bit at a time and letting the metal cool down before proceeding.

After all the holes were drilled, I test-fitted the board and components.  Then I removed everything and wet-sanded the chassis going to I think 600 grit.   That did a pretty good job of getting most of the scratches removed.

Once that was done, I installed all of the "infrastructure" hardware. Overall it turned out pretty good. 

Another Lesson Learned:  While the "off the shelf" chassis measurements and spacings for a 50W Marshall was a good starting point, they weren't the overall best for my build.  I could've placed the pre-amp sockets in a place more suitable for the board I had.  I also wish I went ahead and drilled and mounted another noval socket, just in case I wanted it for later modifications.

[PharmRock]

After I got all of the hardware installed, I temporarily installed some terminal strips, hooked up the PT secondaries and took some initial voltage measurements to make sure everything checked out.  I also took resistance measurements of the OT and recorded those for future reference.

Next step was to wire up the heaters, and I went with the black/orange color scheme that came with the ClassicTone OT.  I left the pin-9 heaters unsoldered for now as I wanted to get the rest of the noval socket pins wired up first.  I took the pin-9 heater wires directly over the socket vs. the circuitous path around the socket.  Figured there was less chance for hum generation doing it that way. 

I started mounting the fuse holders, pilot lamp, progressive power switch, etc.  Overall that was uneventful.  Located the power ground and mains ground bolts. 

Lessons learned:
1. the original plan was to use the "snap in" fuse holders.  Turns out the chassis + panel thickness was just too thick for the metal clips to "catch" onto and keep the fuse holder in position.  I ordered some new fuse holders that had the threaded housing with a nut.  These are much more secure as well as look better in my opinion.
2. The chassis + panel thickness was also a little too much for the snap-in Neon lamp.  Since I already had the hole drilled, I was pretty much stuck with using that style.  It is in place, but could easily be pulled out from the front panel. There's nothing for the lamp housing clips to hold onto.
3. Thickness issues came into play again with using toothed washers for the pots and switches.  There just wasn't enough of a gap for me to install these and also be able to get the nut threaded onto the shaft from the panel side.

[PharmRock]

Now its time to modify the board.  Since I am building a 50W amp and this was a 100W board, I didn't need the 2 extra screen resistors or the 1-ohm bias measurement resistors.  I also contemplated an elevated heater circuit as discussed here [url]https://el34world.com/Forum/index.php?topic=27216.msg299759#msg299759[/url with excellent help from the forum members (side note: this ended up giving ~49VDC output at the cap).  I also needed to squeeze in two extra diodes to run in series with the originals.  This all ended up working out remarkably well.  All I had to do is install a few extra turrets on the board.  I used some from TubeDepot which could be mounted by drilling a 1/8" hole in the board and securing the threaded turret shaft with a tiny nut on the underside of the board.  If I had to do it over, I would've installed regular old turrets using the tool Doug makes for this.  It would've looked as well as functioned better. 

The elevated heater voltage comes off one of the PT secondaries, and the bias supply off the other side. 

Additionally, I converted the 20K PI node resistor to 2 x 10K in series because I will end up tapping off the junction of the 10Ks for the FX loop board later.  Easier to do this part now.

Other modifications included tying the unused bright channel 470K mixing resistor to ground; replacing the original V1a cathode bypass cap and resistor with a single 10K bypass resistor (JCM 800 cold clipper), repositioning the NFB resistor to where the screen resistors used to be (shortened the runs to/from presence pot and better placement for connecting the NFB resistor to the impedance selector), and setting up board for Lar-Mar PPIMV. 

[PharmRock]

Here's the final installment of the build...

Overall the control wiring was pretty straight-forward.  The trickiest part by far was the switch that cascaded V1b into V1a (Plexi/JCM switch).  Initially I had a couple of wires running to the wrong lugs but caught the error during the review of the schematic. 

I also installed a copper ground buss for V1 and V2.  The presence pot is grounded to one of the old lugs that was for the 1-ohm resistors for V4.  There is a dedicated power amp ground bolt near the PT. 

As I mentioned, the amp sounds great except for the squealing, which I think is manageable.  If that ends up being too hard to get under control, I may just go the full Plexi route. 

On the front panel, the Lo Input only works when the amp is switched to JCM.  This is a great-sounding clean channel.  When plugged into the hi input, you can switch between Plexi or JCM.  If JCM is selected in Hi Input, it is the cascaded JCM800 preamp.  The amp has a Pre-PI MV (like a JCM) as well as a Post-PI MV.  If I had to do it again, I would move the post-PI MV to the rear panel where one of the speaker jacks currently is.  That would free up front panel space and also make the cable runs shorter.  Only one output jack is necessary...I would only use a single 2x12 cab anyways.  And another cab could always be daisy chained. 

Thanks again for all the help along the way.  Not finished with it yet so there may be more questions popping up.  Hope this documentation will serve a future builder in some small way.