New Amp Build Completion - JTM50-style

[GuitarGoat]

Here's another pandemic project...with a little more time at home, I've been working through the stack of projects I've got. For this one, I had built my first amp and made a combo cabinet for it - originally it started as a Crate VC508 chassis mounted in this cabinet, then that chassis got gutted and populated with a single-ended amplifier kind of like a Fender Champ but after a ton of modification, I felt like I should start from scratch on the amp. I toyed with the idea of another single-ended build, but ended up saying why not to a higher-powered amp. I settled on a JTM45-style build. The real estate on the front panel was very tight, so I couldn't do all the inputs and I wanted a few different options, so I started with that JTM45 schematic, added a switchable rectifier (tube and silicon diodes), changed the input stage to a parallel triode to take advantage of the extra half-12AX7 stage I had, a few voicing switches on the back to switch from shared cathode components on that input stage to split, a cathode bypass cap on the second stage, a master volume, and I ended up going with EL34 power tubes with the associated output transformer for that. It took a little bit of troubleshooting as I had the output transformer leads reversed, causing my negative feedback to turn into positive feedback and it was throwing the power tube bias off as that was happening. I tweaked the circuit a little bit to add a bit more brightness, but it sounds pretty good now, so perhaps I'll stop modifying too much. One of the tricky parts of this build was keeping space for the mounting locations...I ended up having to make sure my wires and boards weren't in the same place where the mounting screws needed to go. Overall, some of my wiring could be a bit neater, but it sounds nice and sounds quiet, so it's probably more of a lessons-learned thing at this point and I'll do a little more design work on the next amp to make sure I can run shorter and neater wiring.

Here was the chassis after drilling and mounting some of the components for fit:

Heater wiring completed:

Another shot of the heater wiring and parts coming together:

Turret board with turrets loaded:

Rear faceplate with some components holding it on:

More components mounted:

Most of the turret board components populated:

Another shot:

All together, time for troubleshooting and tweaking - the red knobs were an accident when I was ordering, but I kinda like them:

Rear panel after loaded into the combo cabinet:

Front panel after loaded into the combo cabinet:

All buttoned-up and ready to rock:

Can't wait to record a bit with this amp. It sounds really good with the gain down and the volume up...nice Marshally clean tones and the bright cap on the gain pot keeps the brightness up in these settings. Rolling the gain up and the master down provides crunchier tones, but I need to keep the bass down a bit for higher gain tones. The V1 shared cathode setting seems better for single-coils - I added a pretty big bypass cap on this setting which is causing that. The split setting sounds better with my humbuckers as the clarity is a bit better. I like the V2 bypass cap boost to get a little more gain. Overall sounds good to me. The selectable rectifier has been more in the silicon mode for me - the tube rectifier drops the voltage down by a bit so I'd really have to rebias to get my power tubes at the optimal setting as they end up pretty cold if I bias on silicon and switch to the tube rectifier. More of an option for me to fool around with than anything. Overall, I'm happy! Thanks for taking a look!

 

[SpiderWars]

Very cool! Nice clean work.

 

[glpg80]

Very cool features for the rectifier and split versus shared cathode on the front end. A modern take on a JTM-45 tone which can do quite a lot on its own already. Nice job on the build!

 

[GuitarGoat]

Very cool features for the rectifier and split versus shared cathode on the front end. A modern take on a JTM-45 tone which can do quite a lot on its own already. Nice job on the build!

Thanks! I love looking at your builds, so I appreciate it! I still might end up tweaking a few values the more I get used to it, but I’m pretty happy so far!

For the rectification, if I had a little more space and forethought on this one, I’d love to put circuitry to change the bias with the change in rectification. That way I could optimize the bias for either mode and not be super hot or super cold depending on which mode I bias it in and which I’m using.

 

[glpg80]

Very cool features for the rectifier and split versus shared cathode on the front end. A modern take on a JTM-45 tone which can do quite a lot on its own already. Nice job on the build!

Thanks! I love looking at your builds, so I appreciate it! I still might end up tweaking a few values the more I get used to it, but I’m pretty happy so far!

For the rectification, if I had a little more space and forethought on this one, I’d love to put circuitry to change the bias with the change in rectification. That way I could optimize the bias for either mode and not be super hot or super cold depending on which mode I bias it in and which I’m using.

That means a lot thank you for the comments! I had a ton of fun with my latest build. Still tweaking minor tonal changes but very excited to have it completed as I'm sure you can relate to the feeling.

I believe the way the SL68 switches bias for the lower B+ mode is that he uses two different potentiometers. What I would probably do, is use the two potentiometers in series and not in parallel and use one as a bias set for the full plate voltage with more negative voltage, and switch in the second potentiometer to reduce grid voltage a variable amount for a variac mode. That way you don't have a period where the grids aren't provided negative bias or if a switch fails, your tubes are still safe.

 

[GuitarGoat]

Very cool features for the rectifier and split versus shared cathode on the front end. A modern take on a JTM-45 tone which can do quite a lot on its own already. Nice job on the build!

Thanks! I love looking at your builds, so I appreciate it! I still might end up tweaking a few values the more I get used to it, but I’m pretty happy so far!

For the rectification, if I had a little more space and forethought on this one, I’d love to put circuitry to change the bias with the change in rectification. That way I could optimize the bias for either mode and not be super hot or super cold depending on which mode I bias it in and which I’m using.

That means a lot thank you for the comments! I had a ton of fun with my latest build. Still tweaking minor tonal changes but very excited to have it completed as I'm sure you can relate to the feeling.

I believe the way the SL68 switches bias for the lower B+ mode is that he uses two different potentiometers. What I would probably do, is use the two potentiometers in series and not in parallel and use one as a bias set for the full plate voltage with more negative voltage, and switch in the second potentiometer to reduce grid voltage a variable amount for a variac mode. That way you don't have a period where the grids aren't provided negative bias or if a switch fails, your tubes are still safe.

Yea, that's a good idea. I adopted the various bias protection circuitry from Merlin Blencowe's book (which worked wonderfully) and was thinking about switching in an additional resistor to the voltage divider for bias, but that wouldn't account for variation in rectifier tubes so I like your idea of an additional potentiometer in series.