1W JTM build

[TriodeLuvr]

How about the .022uf coupling caps from PI to the 12AU7? I could try .01uf there? The tremolo TMB has that value there...

Sure, that will roll off the bass at those grids. You could also reduce the value of the 0.47uF at the input jack. That might be a good place to experiment with a .01uF.

Are the grid stoppers actually 100K? I know a 12AU7 doesn't have much Miller capacitance, but that seems excessively high to me. I need to throw that in the simulator later to see how much they're rolling off the highs.

Jack

[TriodeLuvr]

The simulator says the -1dB point is well above 20 kHz, so no worries there. The 100Ks are intended to prevent grid blocking when the amp is overdriven, but I wanted to see if they might be creating a response problem. It all looks fine to me.

Jack

[Bieworm]

The simulator says the -1dB point is well above 20 kHz, so no worries there. The 100Ks are intended to prevent grid blocking when the amp is overdriven, but I wanted to see if they might be creating a response problem. It all looks fine to me.

Jack

Thanks Jack

[Bieworm]

Need Help here... there is a sub octave note on the amp. Where do I look?
When I increase gain it's less audible. ( not on the recording)
I tried adding a parallel filter cap to get 55uf on the 1st filter, but no help in that..
It is on several different speakers, so that's not the problem.
Also tried different preamp tubes, but kot the 12AU7 power tube.. I don't have a spare

https://www.dropbox.com/s/i2jmhgc0wi1do ... 5.m4a?dl=0

20210616_213816.jpg

[Bieworm]

Tried parallelling all 3 filter caps with an extra 22uf.. no difference

[JMPGuitars]

You looking for a bandaid or the source of the problem? Try using your scope. Try chopsticking. You've got the skillz!

[Bieworm]

You looking for a bandaid or the source of the problem? Try using your scope. Try chopsticking. You've got the skillz!

I'm going to do that next.
What I've tried this far is:
- larger filtering capacitance, nothing, removed the extra caps
- 10k grid stoppers on all grids, one by one.. nothing , removed them except the one on V1a
- injected the signal from the preamp into the power amp of the tremolo TMB and vice versa. Though the preamps don't match I have the impression the problem is in the preamp of the jtm145.

Going for the scope next

BTW you were right about the muddiness, but it turns out to be something really wrong. In fact the problem got worse while tweaking.

[JMPGuitars]

Fix your input jack ground. Only the input jack should be grounded directly to the chassis there. The other preamp grounds should be connected at the input jack itself.

[TriodeLuvr]

Fix your input jack ground. Only the input jack should be grounded directly to the chassis there. The other preamp grounds should be connected at the input jack itself.

This needs to include the negative lead of the preamp B+ decoupling cap. Otherwise, grounding the jack in this way sets up the possibility of a ground loop due to current flowing across the chassis. It's perfectly acceptable not to do this (chassis ground at the jack) and to consider the jack merely an extension of the shielded input cable.

Jack

[JMPGuitars]

Fix your input jack ground. Only the input jack should be grounded directly to the chassis there. The other preamp grounds should be connected at the input jack itself.

This needs to include the negative lead of the preamp B+ decoupling cap. Otherwise, grounding the jack in this way sets up the possibility of a ground loop due to current flowing across the chassis. It's perfectly acceptable not to do this (chassis ground at the jack) and to consider the jack merely an extension of the shielded input cable.

Jack

Agreed. But as far as I can tell from his photo, that's included on the bus wire connection that needs to be moved to the jack.

Thanks,
Josh

[Bieworm]

I thought ground loops identify as buzz.. not ghosting?
BTW. Preamp ground go to the buss.. buss goes to the ground lug where the input is grounded

[TriodeLuvr]

That's a beautiful amp, BTW. I really like the black faceplate. Very cool!

Jack

[Bieworm]

That's a beautiful amp, BTW. I really like the black faceplate. Very cool!

Jack

Thx. I with it sounded healthy...dang!!!

[TriodeLuvr]

I thought ground loops identify as buzz.. not ghosting?
BTW. Preamp ground go to the buss.. buss goes to the ground lug where the input is grounded

Hum and buzz is usually the symptom. Why was that last clip so different from the first? Was it only a difference in playing level, or did you change the circuit?

Jack

[Bieworm]

I thought ground loops identify as buzz.. not ghosting?
BTW. Preamp ground go to the buss.. buss goes to the ground lug where the input is grounded

Hum and buzz is usually the symptom. Why was that last clip so different from the first? Was it only a difference in playing level, or did you change the circuit?

Jack

No, just playing level..