I need help here. I've been staring at schematics and amp guts for days. I'm wanting to make the "bass instrument" channel of my AB165 bassman into a JTM-45 channel (or at least JTM-45-ish). I'm great with layouts but my brain and experience stops where I need to put a schematic into layout form or physical form
What components and jumper configuration changes do I make on the AB165 to get as close to this as possible to JTM-45 tones? If someone could draw changes over the AB165 layout it would be invaluable!
I'm only looking to mod the bass channel. If there is an easier way to get close to where I'm trying to go I'm open to suggestions.
It's not as simple as it might seem because the circuits have very different configurations.
The AB165 Bass channel is a typical Fender TMB circuit: Input triode > TMB tonestack > Volume > grid input to 2nd gain stage > 3rd triode (mixer) > 12AT7 LTP-PI.
The JTM45 circuit: Input triode > Volume > 2nd gain stage triode > 3rd triode with cathode follower TMB tonestack > 12AX7 LTP-PI.
So, first you've got to relocate the tonestack. Maybe not physically on the board, but surely where it connects and that may or may not be problematic with respect to the lead dress. I don't know but suspect it will be challenging.
The Fender tonestack has only treble and bass pots, with a fixed mid. If you want the Mid to be variable, you'll need to find a place for it, but it can be fixed like in the Fender. You'll be removing the deep switch, but I don't know if that spot will work for a Mid pot (25KB). You're still short one spot for a pot.
This isn't complete, but you'll need to swap the Fender Bass pot for a 1MA, three caps change from .1 to .02. The triodes are baised differently (cathodes). Where do you intend to mix the two channels or will you just feed one to one side of the LTP and one to the other side of the LTP?
Are you intending to convert the NFB to Presence? That's a few more parts, too.
Are you sure you want to get started on this? I don't think this is for the faint of heart.
Edit:
BTW, I see this is your first post here. Allow me to say welcome to TAG. I'm not sure you recognize that asking someone to draw this for you may be a bit much. I suggest you put the two schematics side by side and draw it as best you can. Then post what you draw for comment.
Also, it will be helpful to you if you tell us about your experience with tube amps and electronics. It will help us shape our replies.
Some may disagree with me but aside from the component differences the output transformer is going to make a difference in the sound as well. I freshened a genuine '64 JTM45 not too long ago and it uses Radio Spares transformers and caps so that will make some difference.
There are a couple of major differences in the two schematics:
The JTM goes from input to one triode of V1 on each channel to a volume control (on each channel), then into the first triode of V2 which then goes right into the second triode of V2. Then it goes into the tone stack, then the PI circuit.
The Fender goes from input to the first triode, into the tone stack, then into the volume control, then into the second triode of V1 but then the Bass channel goes into more caps & resistors and then into V3 before it goes into the PI.
The normal channel goes from input to V2A triode to tone stack to V2B triode but then it ties in with the bass channel at the front of the PI circuit.
Schematic to follow...
--Jim
"He's like a new set of strings, he just needs to be stretched a bit."
Plus10 for the longhorn mod. I put it in mine as well and am perfectly content with it. And I'm not very content with anything usually. It is a major mod but still reversible if u want to sell it. I never plan on selling mine though so not much of a concern for me. Also, If your worrief about messing with normal channel don't, cause it is a major improvement.
If Hendrix had lived would he have been as good as EDDIE? (Shaver that is)
