JTM45 Schematic and Layout

[PharmRock]

Hello everyone,
First a disclaimer...this is the first time I've tried my hand at doing a full schematic and layout, so there may be some areas I didn't follow the normal conventions.  I used DIYLC and imported Doug's DIYLC symbols/parts, and was able to put together what I hope is a fairly accurate representation of what this will look like.

I attached both the PDFs and DIY files of the schematic and layout.  The board and chassis layout is an amalgamation of what I've seen from Metro Amps, Hoffman's Plexi 50, and Sluckey's Plexi-6V6.  Somewhat unique to my board is the inclusion of an elevated heater circuit, which is located near the bias circuit.  Additionally, I implemented Merlin's "the least bad way" version of the standby switch, which places a resistor across the switch (I used a 100K) as well as the reservoir cap prior to the switch. 

Other deviations from the typical JTM45 schematic/layout:

• All grid stoppers are located at the tube sockets. 
• The normal and bright channel mixing resistors are at the volume pots tied to a chassis-mounted insulated turret, which then goes to V2 via shielded cable (borrowed this from Sluckey).
• The bright volume bright cap is shown in the schematic but not on the layout.  I may end up leaving this off. 
• The board is currently set up as a shared cathode on V1, but there is space to split the cathode. 
• There is space on the board immediately after the PI dropping resistor to add a parallel dropper to an optional Metro FX Loop board, which UI may install later. 
• The schematic shows only the 16-ohm tap, but it will be wired for 4,8,16 as shown in the layout.  I couldn't find the correct symbols for a 3-position switch to make it work for the schematic.
• The OT and choke access holes are actually under the board, but I moved them in the layout to keep things "cleaner". 

Anyways, hopefully some of you experienced guys can look these over and let me know if there are any errors you can find. Below is a link to a schematic from Mark Huss' website on the JTM45 as a reference.
https://mhuss.com/amps/JTM45/index.html
Thanks!

[PharmRock]

I did find a mistake in how I had the bias trim pot wired, so that is now changed.  I had assumed the rightmost front "leg" was the wiper, but in fact both legs are the initial and final ends of the resistor, and the metal base under the pot amounts to the wiper.   I also changed the layout to update the placement of CC resistors.

The board layout, for the most part, follows Hoffman's Plexi board, except the power rail is on the tube side vs the control side.  I also have it spaced for Mojo-Dijon caps vs the Xicon polypropylene caps.

The bias range resistor and elevated heater voltage divider network will be adjusted accordingly once I go through the start up procedures, so those resistor values will likely change a little. 

Again, if any of you experts out there could give this a thumbs up or point out any problems, I sure would appreciate it!  the schematic remains unchanged from the previous post.

[sluckey]

Looking at the board... The big red "C" should be on the left side of that 10K resistor anywhere along that long blue jumper. My choice would be the turret that the PI plate resistors are connected.

[pdf64]

You could use the spare section of the side can cap for the heater elevation bypass.
Wouldn't it be nicer to use a DPST mains switch? eg it could avoid running 2 neutral wires from the IEC inlet.

[PharmRock]

Looking at the board... The big red "C" should be on the left side of that 10K resistor anywhere along that long blue jumper. My choice would be the turret that the PI plate resistors are connected.

Thanks for catching that.  I actually have a spot for that wire access hole on the correct side of that 10K resistor on my drill template for the board...I don't know why I had "C" labeled on the other side of the resistor by the preamp cap when I did the chassis layout.  I'm glad you caught that though as I could easily see myself wiring it up the wrong way.  I did go ahead and move the connection for the PI filter cap to the PI plate load resistor turret as you suggested.
 

You could use the spare section of the side can cap for the heater elevation bypass.
Wouldn't it be nicer to use a DPST mains switch? eg it could avoid running 2 neutral wires from the IEC inlet.

Thanks PDF....I was thinking of leaving that unused side of the side-mounted dual 16+16 cap for possibly beefing up the screen filtering or the reservoir cap if I ever wanted to do so.
As for the DPST switch...that is a great idea...I'll see if I have one in the parts drawer.

[PharmRock]

found another error...power tube coupling caps should be 0.1uF.  I have that value in the schematic but I have 0.022uF on the board layout.  So that will make it a little crowded down at that end of the board if I stick with the larger diameter Mojo Dijon caps.  I do have some "regular" Mallory caps that are substantially smaller diameter, so I will go with those.

[PharmRock]

Hope at least one of you experts out there could give this a good look-over, especially the voltage chart and schematic.  The layout is sort of a mess with all the wires, but I am trying to get that straightened out.  Not good with graphical stuff.

The amp is working, no issues, biased at ~68% max plate dissipation (~17W).

My main concerns are the screens and the plates on the power tubes being the exact same voltage (is this even an issue?), and the relatively high voltages for the preamp and PI.  The build I based this on (Metro Amp) used a 325-0-325 PT....mine is 345-0-345.  Seems like I could increase the value of the resistor feeding the screens (currently 1K), and also the voltage dropping resistor going to the PI (currently 10K).  A 5V4GA was suggested in another thread as a replacement for the GZ34 to drop a few more volts.

I appreciate all the help I've received on this forum with this project and moving me along the learning curve...still a ways to go.  Once I get the voltages squared away, I'll move forward with tone tweaking.

Thanks again.

[sluckey]

Voltages look good, including the screens.

[pdf64]

Agree, all look pretty similar to my JTM45 type build, and close to the real thing http://raw-sewage.net/images/jtm45orig-schm1.jpg
It seems to be a widely held misapprehension that idle screen grid voltage must be below its anode voltage.
As I mentioned earlier, idle voltages are largely irrelevant. With signal, anode voltage can swing down to maybe 50V, up to 800V.
And to a much less degree, screen grid voltage will follow suit.

[PharmRock]

Agree, all look pretty similar to my JTM45 type build, and close to the real thing http://raw-sewage.net/images/jtm45orig-schm1.jpg
It seems to be a widely held misapprehension that idle screen grid voltage must be below its anode voltage.
As I mentioned earlier, idle voltages are largely irrelevant. With signal, anode voltage can swing down to maybe 50V, up to 800V.
And to a much less degree, screen grid voltage will follow suit.

Yes my build is largely the same as the original, except for implementing Merlin's "least worst way" standby, elevated heaters, and a couple of other minor changes.  I also implemented a Hoffman-style layout which accommodates the larger "Dijon" caps. 

I did a little more digging last night to understand the plate vs. screen voltages, and found this awesome thread: https://el34world.com/Forum/index.php?topic=12281.0.  HotBluePlates does a really nice job of explaining it. 
One thing I did see in that thread is when Doug mentioned he really liked a 3.9K screen grid resistor on his 50W Plexi builds:
"I can't do "Cork Sniffer" type descriptions, but

On my early 50 watt heads, I used 3.9K screen grid resistors which dropped the voltage to the screens from 477 B+ (plates) to 441 (screens)

The amps had a very creamy smooth type feel to them doing this."

I'll play the amp for a while and see how it settles in, and start tweaking as needed.  Bumping up the screen grid resistor may be worth trying to see how the sound/feel changes.

I also have the unused side of a 16+16 cap, so I could easily parallel that 16uF to the 32uF screen node cap.  I think some guys have this on a switch to change the amp's response.

[PharmRock]

At the request of a couple of forum members, I am posting pics of the completed JTM45.
The PT, OT and head cabinet were purchased from a seller on this forum.  I believe the cab is a Mojotone.  The OT was previously installed by the seller, and the secondary leads were too short for my build.  I had to extend them.  PT and cab were brand new.
The chassis, front/rear panels, and choke were sourced from Valvestorm.  I had originally gotten a "JTM" chassis and panels from an ebay seller but the laydown PT cutout was too small.
Board and turrets sourced from Hoffman.  I laid out and swaged the turrets using Hoffman's nifty little tool.
Components were sourced from Hoffman, Valvestorm and Tubedepot.
2-point grounding scheme used: buss bar includes mid pot, V1, V2, inputs, shields and preamp filter cap.  Bolted to chassis near inputs.
Power amp ground includes presence, PT-CT, bias supply, pin 8 (via 1-ohm resistors on board), elevated heater supply, reservoir, screen, and PI caps.
Schematic shows a 1K/5W resistor feeding the screen grids.  I currently have an 820R in there, and it will probably stay.
Amp sounds great.  Very little noise...just a slight bit of hiss when cranked, and that's with a furnace/HVAC 10 feet away and a fluorescent light overhead.  With the high treble volume around 5, and normal around 4, I get a really nice, full clean tone on the edge of break-up with a strat.  Anything higher than that adds some overdrive that nicely cleans up with the guitar volume.
I'll be tweaking over the next few weeks.  Definitely need to do something with the V1 shared cathode.  The 330uF in there now really flubs out the bass...I may go down to 10-25uF, and/or split the cathodes.  Also, there is no bright cap on the high treble volume pot at the moment.  I will experiment with that as well.

Thanks again to everyone for all the help along the way.  I know this is a pretty standard JTM45...nothing special as far as circuits go.  But building my own board and adding in a few changes to the layout made this a fun and challenging project.

[PharmRock]

a few more...

[Willabe]

Very nice work! 

[mresistor]

***Beautiful****       

[SILVERGUN]

That is a very professional looking job!
Very well done and congratulations on getting it done.
 

[Grees01]

Very very nicely done. Looks great PharmRock, both inside and out. Board looks great, wiring, solder work. Stellar on the outside as well. Like how the tube sockets are labeled. Very clean and professional as stated previously. Congratulations buddy. Hope it provides you many, many years of enjoyment. Are you posting a video of you playing thru it? I want to hear it!!!!

G.