Surgery Time! (JTM45 Modding Content)

[LLL]

Re: Surgery Time! (JTM45 Modding Content)

update: Mouser parts have arrived (bout damn time).

- yesterday, replaced all remaining Mallorys (except pres. cap) with Sozo New Stock (or whatever they're called).
- tested again

The amp sounds night and day... fantastic. Improvement even over my last soldering run.
Sweet thick (not too thick) Marshall tone with clarity and grind on top!

A few more parts (putting new ceramic caps in there) to swap and should be done. .005uF bright cap test coming up too (500pF currently).

Side note: I decided to keep 47k NFB tied to 8 ohm tap.

After that, cap job on my EMU-1820m sound interface so I can resume my DAW fun.... and clips of the amp.

 

[LLL]

Re: Surgery Time! (JTM45 Modding Content)

Updated pic.

Amp is now a '68 model 1987 (50 watt) with tube rectifier running at 100v (variac'd).

The .005uF bright cap stays.

Glorious, grindy Marshall mids. :yeah:

 

[LLL]

Re: Surgery Time! (JTM45 Modding Content)

See this thread for a clip of the new guts (and compare with old clip):

https://forum.seymourduncan.com/showthread.php?325076-NAD-1968-50-Watt-Plexi-(clip)

 

[Joey Voltage]

Re: Surgery Time! (JTM45 Modding Content)

Side note: I decided to keep 47k NFB tied to 8 ohm tap.

May not be a bad plan if your trying to mimic the same amount of -feedback a 100W would have, especially one from the 60’s. It will have a direct effect on how the amp breaks up.

 

[LLL]

Re: Surgery Time! (JTM45 Modding Content)

May not be a bad plan if your trying to mimic the same amount of -feedback a 100W would have, especially one from the 60’s. It will have a direct effect on how the amp breaks up.

I think 100K + 4ohm tap... + .68uF V2a bypass is too much for a 1987. So I stuck with stock values.

 

[Darg1911]

Re: Surgery Time! (JTM45 Modding Content)

It's a Marshall, so again, may not want to be drilling holes. But, it's easy to make the feedback resistance switchable. I had switchable tube or SS rectification which also switched in another cap section when running SS. The wiring was kind of messy and I never switched from the tube rectifier so I removed the switch, diodes and wiring.

 

[Joey Voltage]

Re: Surgery Time! (JTM45 Modding Content)

I think 100K + 4ohm tap... + .68uF V2a bypass is too much for a 1987. So I stuck with stock values.

At 100w, there would be more through the 4R tap, at 50w that number drops a little so there is Slightly less being fed back for a 50w. Moving 100k to the 8r tap would give you roughly the same for a 50w as 100k to the 4 does for a 100w. The older Marshall’s had more feedback anyway. So I think you made the right choice.

Having more feedback gives you a more sudden transition into distortion which isn’t necessarily a bad thing in this case.

 

[LLL]

Re: Surgery Time! (JTM45 Modding Content)

It's a Marshall, so again, may not want to be drilling holes. But, it's easy to make the feedback resistance switchable. I had switchable tube or SS rectification which also switched in another cap section when running SS. The wiring was kind of messy and I never switched from the tube rectifier so I removed the switch, diodes and wiring.

Yeah I see what you're saying... naw, it sounds t!t$ now.

Only thing "switchable" I'd be interested in is another Weber CopperCap (I have their GZ34 module, but would like a "straight diode rectifier" version... if they still make these).

EDIT: Well whaddaya know? Weber still sells them!

Here's the GZ34 model: https://www.tedweber.com/wz34

Here's the just straight diode one: https://www.tedweber.com/ws1

They're great... just swap out like a tube.

 

[LLL]

Re: Surgery Time! (JTM45 Modding Content)

At 100w, there would be more through the 4R tap, at 50w that number drops a little so there is Slightly less being fed back for a 50w. Moving 100k to the 8r tap would give you roughly the same for a 50w as 100k to the 4 does for a 100w. The older Marshall’s had more feedback anyway.

Yeah, the JTM45 IIRC is 27k NFB resistor + 16ohm tap. I remember that amount of neg feedback well (wasn't my favorite). Very "spanky-clean".