Did some JTM45 clone mods

[sahlomonic]

-copied my post from GAB forum-

I got a Jet City JCA45 a few months ago, and did a little work to it. Still not done, but have come a long way from when it was stock. I may be going in over my head here since it's a JTM45 clone and not a 1959 or 1987 clone, but the mods I did has sure proven worthy so far.

I added an effects loop I bought from Jet City - the same pedal friendly one they put in the JCA20HV, changed the Channel 1 coupler from .022uf to .0022 uf (most noticeable change in gain), added a Lar-Mar style PPIMV, changed the NFB resistor from stock 27k to 47k (might also move it to the 4 ohm tap), converted the shared V1 cathode from shared to split (.68/1k), and have a 50 watt OT and solid state rectifier plugin on the way. Originally did a 'one wire mod' that cascaded the two channels like a JCM800 but that took away too much of the 'plexi' feel of the amp, so I put it back to stock before I did any more mods to it. Basically I'm building it to what is reported that EVH allegedly had on his original 12xxx series Plexi. And it sounds fucking fantastic right now. Still has that 'chirp' that I hear with plexis, yet enough gain to pull off EVH and not be drowned out in an insane amount of gain.

$600 for the amp, and not even $100 in parts and I'm happier than a pig in shit

I will try my best to get proper clips soon (I don't have clips of it stock though)

As it was delivered (stock

And currently:

Couple of shitty clips (BTW volume at Friday night bar & grill conversation levels
(Speakers are Eminence MIC 'Jet City Vintage')

 

[SpiderWars]

What a perfect, inexpensive platform for this. Very cool (I'll listen later at good speakers). With the laydown PT there isn't a ton of room like a big Marshall chassis, but just enough. I was just doing the opposite mod on my homebrew 50W smallbox this weekend (i.e. making it into a Plexi/Jose Mod fire breather).

As you probably know (but I'll mention it anyway), at anything but loud PPIMV levels your NFB is almost non-existent so your Presence won't work well if at all (and your NFB resistor won't matter much). Part of my modding this weekend was to install a pull-pot for the Depth control that switches the NFB resistor. But I'm using a Jose MV so the Presence/Depth always work.

 

[codyjarret]

Great job on the amp.
Sounds really good.

If you want a little more aggressive gain try a 100K NFB on a 4ohms tap.

 

[sahlomonic]

Great job on the amp.
Sounds really good.

If you want a little more aggressive gain try a 100K NFB on a 4ohms tap.

Thanks! I've been using your vids as a template for mine, to be honest

 

[mixn4him]

You'll want to change the tone stack as well, 250pf/56k to 500pf/33K slope resistor and those mix resistors 270/270/250pf to 470/470/500pf . The .0022 didn't add more gain it reduced the amount of bass moving forward in the circuit which lets the mids and highs seem a little gainier. LOTS of info and great help over on the Metro page if you need any.

 

[Rdodson]

Sounds killer.

 

[sahlomonic]

You'll want to change the tone stack as well, 500pf and 33K slope resistor. The .0022 didn't add more gain it reduced the amount of bass moving forward in the circuit. LOTS of info and great help over on the Metro page if you need any.

I know the .0022 cap is supposed to reduce the bass going forward, but when I swapped it there was a very noticeable increase in gain (or maybe that is just my perception of the bass being taken away?)

And I also plan on adjusting the tone stack once my parts get in (hopefully today!)

Thanks for the nod on the Metro forum.

 

[Kapo_Polenton]

Lots of gain in that bad boy sounds like! I mainly stick negative feedback 47 to 8 ohm tap but I am now thinking it is a little tame.. I need some more sizzle!

 

[jr21]

 

[jdel77]

Are you telling me the Jet City comes with a handwired ptp board?
Or did you do that yourself?

 

[SpiderWars]

Finally listened on good speakers and that sounds great. A lot of gain for a relatively stock JTM45. With a variac it would prob be even better. What OT you getting?

candletears, I was surprised when I saw that. A handwired JTM45 clone for $600 is a great bargain.

 

[psychodave]

Wow. I'd buy one of these for $600.

 

[Kapo_Polenton]

Just to clarify for everyone, pretty sure the under 100$ in parts is the turret board and caps/resistors. The amp doesn't ship with anything other than a PCB. Still good value for the money though. I think stock these amps sound pretty good.

 

[sahlomonic]

Amp is actually handwired from the factory (not PCB). The pic at the very top is how it came to me. $100 in parts included the ppimv kit, the caps and resistors I replaced, and the incoming OT. Jet City only licensed maybe a hundred or so of these.

 

[sahlomonic]

https://reverb.com/item/2183404-jet-cit ... mp-tribute