Going to build a JTM 45 - need advice

[kissfanps]

Hello all. I am going to be building a JTM 45 and need some advice. I'm just jonesing for some Beano and vintage AC/DC tones. I had a Marshall Vintage Modern (based on a JTM45) and loved it. After getting a JMP 50W clone from Curt Granger, however, I know that a true plexi circuit is the way to go.

So, I was clicking around youtube and forums and discovered that these amps are famous for their flubby/farty low-end. You listen to Greg V jam through a Retro-King 45 ( https://www.youtube.com/watch?v=3ShCv-AEFCQ ) and it sounds great, but its so loose. But then you listen to George jamming on his GPM45 ( https://www.youtube.com/watch?v=Z1SykFCz7o4 )and its just dripping with tone and the bottom is very tight. To the end, I've never heard any flub on AC/DC albums or on Beano.

So, what's the deal with this? I've read a bunch of threads here that discuss the different mods, but I don't get how George's build can be so tight while being true to spec or at the most a 32mf PI filtering cap.

Anyone? Thanks!

[stef]

go with the 1987 spec

[OnTheFritz]

Hello all. I am going to be building a JTM 45 and need some advice. I'm just jonesing for some Beano and vintage AC/DC tones. I had a Marshall Vintage Modern (based on a JTM45) and loved it. After getting a JMP 50W clone from Curt Granger, however, I know that a true plexi circuit is the way to go.

So, I was clicking around youtube and forums and discovered that these amps are famous for their flubby/farty low-end. You listen to Greg V jam through a Retro-King 45 ( https://www.youtube.com/watch?v=3ShCv-AEFCQ ) and it sounds great, but its so loose. But then you listen to George jamming on his GPM45 ( https://www.youtube.com/watch?v=Z1SykFCz7o4 )and its just dripping with tone and the bottom is very tight. To the end, I've never heard any flub on AC/DC albums or on Beano.

So, what's the deal with this? I've read a bunch of threads here that discuss the different mods, but I don't get how George's build can be so tight while being true to spec or at the most a 32mf PI filtering cap.

Anyone? Thanks!

Go for it, all the tones are there and you can easily tweak. KT66's sound awesome! To keep out the flub the bass control has to be way down, zero to two depending on preference and all the variables. I've build quite a few of these, and like using the lead tone stack values to make the bass control a bit more usable. 33k/500pf. Higher plate voltage and filtering of course will make it punchier. You build it, you can tweak it! 450vdc on the plates tube rectified is the target zone for the real deal.

[kissfanps]

I guess doing a few switches (shared/split cathode v1, on/off .68mf cap on v2a, and tone stack jtm45/lead) are easy enough.

Apart from bumping up the filtering, is there any reason to bump up the mix resistors to 470k or drop the PI coupling caps to .02mf? I mean, I don't want to get away from the JTM45. I already have a JMP.

[OnTheFritz]

Personally wouldn't do any switching on a JTM45. Was not what I was suggesting at all. Just perhaps changing the tone stack values, (resistor and cap). Keep it what it is, pure and simple. Filtering in the proper 45 range, shared cathode, etc.
That's the tone you're chasing.

[ivan H]

Hi, I thought I'd share this as there earlier talk of upping the filtering (I wouldn't). A 5AR4/GZ34 rectifier can handle a maximum of, IIRC, 60uf for the first filter. Any larger than this will overly stress the rectifier. I built a JMP50 circuit using a Marstran JTM45 PT. I used the Marstran PT as I believe it gives a higher B+ than the Metro spec PT. The EL34 plates have 446V on them when biased at 68% max dissipation. This amp thru a quad loaded with greenbacks & V30's is great for early AC/DC tones. Hope this helps. Cheers

[neikeel]

Hi, I thought I'd share this as there earlier talk of upping the filtering (I wouldn't). A 5AR4/GZ34 rectifier can handle a maximum of, IIRC, 60uf for the first filter. Any larger than this will overly stress the rectifier. I built a JMP50 circuit using a Marstran JTM45 PT. I used the Marstran PT as I believe it gives a higher B+ than the Metro spec PT. The EL34 plates have 446V on them when biased at 68% max dissipation. This amp thru a quad loaded with greenbacks & V30's is great for early AC/DC tones. Hope this helps. Cheers

+1

Stock JTM45, use a 32+16 can for approx 50uF mains, and usual 16/32 for PI/screens and 16 preamp. That and 450v on KT66 plates will be perfect!

[Roe]

agreed. Installing a 100-330k power resistor across the standby switch increases the lifespan of the gz34 since you get a softer start