JTM45 clone mod questions

[casey73]

I built a JTM45 clone 7 years ago. It has too much bottom end for me. I keep the bass at 0. I thought I'd change out the 0.1 coupling caps to the OP tubes to .022 as a starting point to reduce low end. I've also always had KT66s in it and am going to replace them with current production Tungsol 5881s I have.

This is the only JTM45 I ever played so I don't have anything to compare it to. The only deviations I made from the early circuit was changing the 470R tube screen resistors to IK 5W and I replaced the original 32/32uf caps with 50/50uf caps because I had a real ghost note issue with the 32ufs.

I also considered replacing the tube rectifier with a solid state rectifier to bump the voltage a bit. I built this using the lower voltage (600VCT) Mojotone power transformer.

I'd welcome any suggestions from those familiar with JTM45s. Anything you would or wouldn't do from the above list?

[Colossal]

Those proposed changes all sound reasonable. I seem to prefer smallish output coupling caps in my own designs, but have a Bassman with 0.1uF output caps and it isn't too tubby. I run EL34s in that amp. Any reason you want to switch to 5881s from KT66s? Just too bassy? I don't have experience with the current production Tung Sol 5881s but had one of two TAD red base 5881s fail within 6 months of mild use at 450V.

[martin manning]

The other place to trim bass would be reducing the cathode bypass cap on V1.

[casey73]

Those proposed changes all sound reasonable. I seem to prefer smallish output coupling caps in my own designs, but have a Bassman with 0.1uF output caps and it isn't too tubby. I run EL34s in that amp. Any reason you want to switch to 5881s from KT66s? Just too bassy? I don't have experience with the current production Tung Sol 5881s but had one of two TAD red base 5881s fail within 6 months of mild use at 450V.

I want to change to 5881s just to see if there is a change in tone I might like. I watched a YouTube video of a guy doing an A/B comparison between the KT66 and 5881 in his JTM45 and it sounded like the 5881s were a bit brighter. But it's a youtube video on computer speakers so I thought I'd try in real time. I am aware of the lower voltage/dissipation limits on the 5881s and the fact that current productions 5881s aren't made to the same quality standards of NOS equivalents. That's why I'm on the fence about putting in the solid-state rectifier plug in. I'll measure the plate voltage on the KT66s before I pull them. It's been a while since I've opened it up and checked bias but I think plate voltage was around 430V.

[casey73]

The other place to trim bass would be reducing the cathode bypass cap on V1.

I hadn't thought about that but will give it a try. The circuit is built on a turret board so making changes is easy and reversable. I'm going to build a simple amp chassis stand using some good birch plywood I have so I can experiment and make changes on the fly.

[cdemike]

I seem to prefer smallish output coupling caps in my own designs, but have a Bassman with 0.1uF output caps and it isn't too tubby.

This has also been my experience with 100nF couplers coming out of the PI. IMO it really depends on where your amp is running into issues.

I agree with Martin about the V1 cathode bypass caps. I like 5uF since it'll trim the really low harmonics while keeping all fundamental frequencies for a guitar in standard tuning bypassed. Slightly reducing the V1 coupling caps to something like 15nF or 10nF also helps cut some of those super low harmonics while keeping it sounding like a bass-spec amp. The V2 gain stage's cathode also could be worth looking at. You'd obviously have the option of the same aggressive 680nF cap like the lead-spec amps, but based on this being a JTM45 not a JMP, I'm guessing that's not what you're going for. A cap between 100nF and 470nF would cover the very upper end of the midrange and higher.

Something I was surprised to find out lately was the role of low mids in making these amps sound too tubby. I was doing a recording project where someone suggested diming the bass to get more midrange clarity, which I was surprised worked. Fooling around on a tone stack simulator helps show why. Below are the frequency response curves for the same 56k/250pF tone stack with the only differences being control settings. Orange represents T-10, M-10, B-5, green represents all controls on 10, and black represents T-10, M-10, B-0. You'll note there's a significant dip in low mids with the bass control around 6.5 and higher which paradoxically leans out the sound. Might be worth a shot ahead of heating up the iron.

tone stack bass settings.PNG

[casey73]

I seem to prefer smallish output coupling caps in my own designs, but have a Bassman with 0.1uF output caps and it isn't too tubby.

This has also been my experience with 100nF couplers coming out of the PI. IMO it really depends on where your amp is running into issues.

I agree with Martin about the V1 cathode bypass caps. I like 5uF since it'll trim the really low harmonics while keeping all fundamental frequencies for a guitar in standard tuning bypassed. Slightly reducing the V1 coupling caps to something like 15nF or 10nF also helps cut some of those super low harmonics while keeping it sounding like a bass-spec amp. The V2 gain stage's cathode also could be worth looking at. You'd obviously have the option of the same aggressive 680nF cap like the lead-spec amps, but based on this being a JTM45 not a JMP, I'm guessing that's not what you're going for. A cap between 100nF and 470nF would cover the very upper end of the midrange and higher.

Something I was surprised to find out lately was the role of low mids in making these amps sound too tubby. I was doing a recording project where someone suggested diming the bass to get more midrange clarity, which I was surprised worked. Fooling around on a tone stack simulator helps show why. Below are the frequency response curves for the same 56k/250pF tone stack with the only differences being control settings. Orange represents T-10, M-10, B-5, green represents all controls on 10, and black represents T-10, M-10, B-0. You'll note there's a significant dip in low mids with the bass control around 6.5 and higher which paradoxically leans out the sound. Might be worth a shot ahead of heating up the iron.
tone stack bass settings.PNG

Thanks for the feedback. I'll play around with the tone stack before I dig in. I'm in no hurry. I have the chassis out of the head cab right now as I'm copying dimensions for my next project using the same sized chassis and similar layout. But I'll be using a top mounted PT that gives me more space inside the chassis to mount a board with individual filter caps instead of multi-section caps mounted to the chassis.

[Roe]

try 3.3nf at v1b, and 1nf from treble pot to phase inverter