In the chance that some of this might be helpful to another member here is what I have done so far to my JTM622:
1. Added a choke between R113 and D105/D106 I used this choke http://store.triodestore.com/tri-m100d.html
2. Removed the diode clippers from the circuit, as per the diode clipper mod.
3. changed C103 to a RIFA .047 PHE 426
4.C116 .0022 and C118 .0047 both to orange drops 16PS series
5. C119 .01 changed to a .02 WIMA MKS 4
6. C108 to a Solen .22 MKP-FC
7. C125, C126, and C128 all changed to WIMA .022 MKS 4
Does it sound like a JCM800? No. But it sounds so much better than it did stock. Big time chunk when palm muting and no buzzy distortion when the gain is cranked. This amp sounds great now.
For the heat problem I added a 3/16 piece of aluminum between the power tubes and chassis and isolated it from the chassis with fiber washers. Then added a vent on top and it doesn't get hot at all now. Just dont back it up against a wall.
Next I think I might try some vintage 30's in there. I played through an EV cab (2 X EV12L) and it sounded great but maybe a little to clean, plus the added weight might be the death of my back.
I hope someone finds this useful.
JTM60 improvements
Moderators: pompeiisneaks, Colossal
-
The Ballzz
- Posts: 369
- Joined: Thu Oct 24, 2013 7:22 pm
- Location: Las Vegas, NV
Re: JTM60 improvements
Jered,
What is this "diode clipper mod" you refer to? Did you simply lift the diodes or put something else in their place? I have a JTM30 and although different in some ways, the diode clippers appear to be used in a similar way. I tried simply lifting them and, IIRC, the involved cap also, and while it vastly improved the gain channel sound by eliminating that annoying "BUZZ SAW" sound of the diodes, the amp needed to be REALLY CRANKED and was quite LOUD.
Thanks,
Gene
What is this "diode clipper mod" you refer to? Did you simply lift the diodes or put something else in their place? I have a JTM30 and although different in some ways, the diode clippers appear to be used in a similar way. I tried simply lifting them and, IIRC, the involved cap also, and while it vastly improved the gain channel sound by eliminating that annoying "BUZZ SAW" sound of the diodes, the amp needed to be REALLY CRANKED and was quite LOUD.
Thanks,
Gene
-
Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: JTM60 improvements
the preamp section of a jtm60 could never sound like a 800 without a major overhaul if that's your goal!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
-
potatofarmer
- Posts: 50
- Joined: Fri Dec 02, 2011 2:54 am
Re: JTM60 improvements
Looking at the schematic on Dr Tube's site, that looks like it might be the wrong place for a choke. You want to put the choke between the plate supply and the screen supply - replace R113, basically. Or you could lift one leg of R113 and put the choke in series with it.
I haven't looked at your other mods but I just thought I should point this out as that choke isn't rated for the full plate current and it will be more than happy to fry without opening your HT fuse until it's too late!
I haven't looked at your other mods but I just thought I should point this out as that choke isn't rated for the full plate current and it will be more than happy to fry without opening your HT fuse until it's too late!
-
The Ballzz
- Posts: 369
- Joined: Thu Oct 24, 2013 7:22 pm
- Location: Las Vegas, NV
Re: JTM60 improvements
Jered,
Thanks for that link! It helps clear up some confusion that I now see was created by a couple or more different ways the clipper diodes were implemented in different versions of the JTM60/600. One way dumps the clippers to ground (nothing at all like the JTM30) and the other way uses the clipping diodes in a feedback loop back to the cathode of V101-B as in a JTM30 (in the JTM30 it's actually V101-A). The only two schematics I could find for the JTM60/600 had one with diodes to ground and the other actually in a feedback to V101-B, instead of V101-A, but the concept is the same as in the JTM 30. I'm not a rocket scientist and although the topography/layout/schematic of the JTM60/600 is substantially different than the JTM 30, I'm at least smart enough to extrapolate how one relates to the other. Just out of curiosity, which diode configuration did your amp have? Diodes to ground or in a feedback loop?
Thank You Sir,
Gene
Thanks for that link! It helps clear up some confusion that I now see was created by a couple or more different ways the clipper diodes were implemented in different versions of the JTM60/600. One way dumps the clippers to ground (nothing at all like the JTM30) and the other way uses the clipping diodes in a feedback loop back to the cathode of V101-B as in a JTM30 (in the JTM30 it's actually V101-A). The only two schematics I could find for the JTM60/600 had one with diodes to ground and the other actually in a feedback to V101-B, instead of V101-A, but the concept is the same as in the JTM 30. I'm not a rocket scientist and although the topography/layout/schematic of the JTM60/600 is substantially different than the JTM 30, I'm at least smart enough to extrapolate how one relates to the other. Just out of curiosity, which diode configuration did your amp have? Diodes to ground or in a feedback loop?
Thank You Sir,
Gene
Last edited by The Ballzz on Thu Sep 04, 2014 4:08 am, edited 1 time in total.
Re: JTM60 improvements
Stevem, not my goal, just trying to improve the sound of this amp.
Potatofarmer, thank you, i will do it right now
the ballz, mine went to ground
Thanks guys, I appreciate it
Potatofarmer, thank you, i will do it right now
the ballz, mine went to ground
Thanks guys, I appreciate it