New JMP 50 style build in JTM45 chassis

[Blackburn]

Looks awesome!

[gktamps]

Thank you, Sir!

Getting to that point where it gets exciting and I get impatient to finish. Got to be methodical now that it's getting crowded inside.

[Reeltarded]

Very nice!! Wow even,

[gktamps]

Thanks, Miles! You're the one who inspired me in this build, in the first place.

Uh, I promise not to blame you if I feck it up, though...

[Reeltarded]

No way that amp will be a mess. These things are first the most forgiving circuits you can build, at least until you switch in a extra stage and start welding with the output section. hah

[gktamps]

Had to limp home with this - my Aoyue soldering pencil failed so I had to use a crummy soldering gun to finish up, without frying anything.

Next step is voltage checks without tubes, then if OK, put in some cheapos for testing. Can't find 100watt bulbs in stores anymore, so haven't built a bulb current limiter, but will use the variac. Thought I'd post pics before firing up first time (and before any smoke and tears).

Other than building the JMP50 in the JTM45 chassis, it's pretty stock right now, except for an NFB selector switch, 10-turn bias pot on deck, a Pre-PIMV and a PPIMV for kicks, and some different filter and bypass cap values.

This will go into a stock Marshall head cab, but I'm trying t figure out how to hypnotize Jeff12 into sending his beautiful new walnut/sapele/maple cab to me! https://tubeamparchive.com/viewtopic.php?t=22491

Question: is there a significant difference in using UF rectifier diodes as opposed to 1N?

Next post after voltage testing...

[gktamps]

Finished up this build, except for some tone tweaking, then probably some more mods for fun. I've only built 5 amps from scratch so far, but never had reversed OT leads until now. GOOD GOOGLY-BOOGLY, that's a loud howl!

Other than some fizz at high volume and high presence settings, sound is tight, tonal, and crunchy, though not insanely gainy. It's got PLENTY of treble, and other than fizz, no oscillation. I used 33k grid stoppers instead of 68k - not sure if that will be a problem, or is perhaps the source of my fizz?

Before I button it up for now, I'm going to fiddle with the PI fizz cap, which is already higher than stock at 100pf, unless the fizz is related to the lower grid stopper resistor value. Think I'll parallel a 47pf ceramic, or maybe polystyrene, which I've done with good results on other builds. Might also try a smoothing cap on the PI plate resistor. Fizz definitely rolls off with lower presence setting.

Might also adjust for a bit more saturation at lower volume. The combination of Pre-PIMV and Post-PIMV is great - they definitely have a different tonal effect. The Pre retains the treble and clarity, and the Post increases distortion but thins out the sound quite a bit. Nice to have both to twiddle.

I've been adding Merlin's CF protection to my recent builds, and will do so for this one: http://www.valvewizard.co.uk/dccf.html

Uh oh! I discovered one unanticipated problem from putting both master volume pots in front - can't run a channel jumper! If I want to parallel V1a and V1b I have to make a Y-cable. This started me thinking of a parallel/cascade switch mod, and making it a single-input panel, with the switch in the third hole.

It would be great to be able to switch from Ch 1, Ch 2, Ch 1 parallel Ch 2, and Ch 1 cascade Ch 2. Now I just have to research or figure out how do do this, unless you all tell me I'm crazy or it will take more effort than it's worth?

As I finish this one up, I'm starting an 18 watt build with a Radio Spares PT and a HiWatt Partridge style OT. Not sure of anything else at this point! (I'm taking suggestions).

[jeff12]

Very nice looking build. I had leads reversed first time on my last build,ouch.
Can't wait to see your next one. Hard to ever say it is finished, just one more
tweak. What is the little maple amp on top? Looks cool.

[statorvane]

Question: is there a significant difference in using UF rectifier diodes as opposed to 1N?

Somebody posted some sound clips with 1N4007, UF4007 and GX34/5AR4. Think it was the metro forum. Anyway, I could not really tell the difference, they all sounded the same - even the tube rectifier. I use 1N5408s now. Maybe overkill (3A) compared to 1N4007 (1A). I believe the UFs are supposed to be faster switching diodes - something to do with reducing rectifier "hash".

[jeff12]

[quote="gktamps"]

This will go into a stock Marshall head cab, but I'm trying t figure out how to hypnotize Jeff12 into sending his beautiful new walnut/sapele/maple cab to me!

I'm getting very sleepy, very very sleepy???" And all this time I thought it was the late nights I was working".
$$$$ also tends to make me send things in the Big Brown Truck.

[JazzGuitarGimp]

I like the big brown truck. The big brown truck is my friend..... Yeah, that's it. My friend....

[gktamps]

Thanks, Jeff,
Next one is on deck, so I'll start a thread soon.
The amp on top is a Fargen Mini Plex Mark II in a custom cabinet. Puts out some versatile, and gainy Marshall tones with less volume. Useful when the wife and dogs start giving me dirty looks!
You and I need to talk more about a cab like your last one. I'll pm you.
Cheers,
Greg

[gktamps]

Closing the loop on this thread, but must first say THANKS to everyone who has given me suggestions and guidance - you guys are really the best, and have helped me make every one of my builds better.

This amp ROCKS! No bad manners, no hum, buzz or other issues, but great tone. Cleans are tasty, dirt is uh, dirty, and crunch is, well, you know. Glad I went with SS rectification - I have plenty of tube recto amps - and went with moderate filtering, which sounds good.

After playing around with many different cathode cap and resistor combos, I've come to agree with Miles about the 2.7k/0.68uF combo on all stages. Also followed Miles' advice about a variable slope control, with a pot mounted on the rear panel, just tonight, so haven't even had time to play with that yet.

Added CF protection per Merlin, and 10k grid stoppers on V2 and PI just for good measure.

The amp sounds so good that any more tweaking is going to wait, including adding channel switching.

Voltage chart is attached.

Cheers,

Greg

[gktamps]

Oops, no voltage chart. Here it is.
Next post will be photos of the BEAUTIFUL head cabinet that Jeff (Jeff12) made for me. I used waterslide decals for the faceplates, then varnished over them.
Also, while the head was out and apart, I swapped the 25k mid pot for 50k, and swapped the PS diodes with UF5408s.

[gktamps]

Here is the cabinet Jeff made for my amp. I will be putting a brown leather handle on it. I made aluminum sheet plates for the bottom of the chassis, and as a heat shield under the top cab panel.
Still need to drill the bottom panel and bolt the chassis down, and add feet.
The image on the rear faceplate is a pallid bat (I am bat biologist).
Beautiful work, Jeff!
Cheers,
Greg