I'd posted a while back about potential changes to my JTM45 clone to decrease bass. Since then, I made a few changes and am pleased with the results. A little background on this amp first.
In 2017 I was gifted $500 to my customer account at AES (Antique Electronic Supply). I live 4 miles from there and I was an employee there at the time. While I'd buy tubes and parts now and then I really didn't need $500 worth of merchandise, so it sat in my account for a while. That spring they got in a load of pre-punched JTM45 style chassis, including old style plexi front and back panels as well as premade JTM45 turret boards. I'd never seen let alone played a fabled JTM45 so I decided to use that money to build my own JTM45 head. I purchased almost all the parts for the amp at AES, except for the power transformer as they didn't have a drop in JTM5 style PT at the time (they do now). I purchased the PT at Mojotone, getting the lower voltage version (300-0-300) transformer as I thought it would be more authentic to the original amp.
A few weeks ago I replaced the shared 250UF cathode resistor on V1 with a 25UF cap. I discovered that for reasons I don't recall, I'd already installed 470K instead of the original 270K mix resistors when I built it so I left them in place. I replaced the Valve Art KT66 tubes with JJ 5881s. And I replaced the UK Mullard 5AR4 with a solid state plug in. The solid state rectifier gave me 10 more volts on the plates. The plate voltage is now 415 and I set the bias to 34ma per tube at idle. The amp has perked up considerably. Much less mushy, brighter sounding overdrive and more fun to play. And, this may be common knowledge, but I recently discovered that using an A/B/Y switch into the high treble and high normal jacks makes for a much more useful amp, working much better than the old jumper between channels. I favor the treble channel but it's nice to be able to combine them sometimes. Still getting used to the new set up, but I'm liking it alot.
JTM clone update
Moderators: pompeiisneaks, Colossal
Re: JTM clone update
Great to hear youve got an amp you get along with now. Ive only built 1 JTM45 in 25 or so years of building & I cant remember all the details I never did get along with it so I sold it. Rob Robinette has a lot of mods & tweaks for the 5f6a which most would apply to the 45 & at the bottom he does go into the 45. You got me wanting to give the 45 another try. . .
Resistors not Transistors !