Nfb and lack of presence jtm 45

[Snake5150]

Been a while since I’ve been on here as life has got busy. Moved , had a kid, but finally got time to look at the amp again. Built a triode jtm50 black flag clone, sounds great, learned a ton thanks in part to yall.

The presence control does like nothing at any volume. I flipped the amp today and discovered that I put the 16 ohm ot tap on the nfb resistor downstream, and only use the 8ohm. THERES THE LACK OF PRESENCE DUMB A**. So am I correct in thinking, remove the 16 ohm tap, and wire from the nfb resistor to the output 8ohm tip.

And if it squeal the primary are reversed. Thanks fellas!

[sluckey]

The first real JTM45s had the NFB connected to the 16Ω tap. The 16Ω tap should give you more Presence control than the 8Ω tap. This Presence control is kinda subtle and will sound scratchy as the pot is turned.

Is the 5K pot wired correctly? Is the NFB resistor 27K?

[Snake5150]

The first real JTM45s had the NFB connected to the 16Ω tap. The 16Ω tap should give you more Presence control than the 8Ω tap. This Presence control is kinda subtle and will sound scratchy as the pot is turned.

It sounds like nothing. From zero - 10 on the pot at even moderately loud volumes. Would it hurt the amp to connect the 8 to the nfb resistor, my layout actually shows this but for some reason 2 years ago I didn’t do it.

[sluckey]

You can try the 8Ω tap but I don't expect that to change anything. Are you sure the speaker jack is wired correctly? I ask because it's unusual to see a yellow common wire from the OT. Also check the 27K NFB resistor as well as the presence pot.

[Snake5150]

You can try the 8Ω tap but I don't expect that to change anything. Are you sure the speaker jack is wired correctly? I ask because it's unusual to see a yellow common wire from the OT. Also check the 27K NFB resistor as well as the presence pot.

The pot and jack wiring seem correct. The layout shows the common to the jack. The resistor reads weird but has the 16 tap still attached

[sluckey]

The layout shows the common to the jack.

I see a yellow OT wire connected to the jack sleeve. But is the yellow lead really the OT common? And where is the jack sleeve actually connected to chassis?

The resistor reads weird but has the 16 tap still attached

You would have to disconnect one of the resistor leads in order to get an accurate reading.
 

[Snake5150]

The layout shows the common to the jack.

I see a yellow OT wire connected to the jack sleeve. But is the yellow lead really the OT common? And where is the jack sleeve actually connected to chassis?

The resistor reads weird but has the 16 tap still attached

You would have to disconnect one of the resistor leads in order to get an accurate reading.

The yellow wire is common. The ground goes to the chassis star ground,The pot has pin1 ground and 3 nfb in, that may be wrong