18Watt.com

Bieworm wrote: ↑

Sun 06/13/21 1:50 am

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

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

TriodeLuvr wrote: ↑

Sun 06/13/21 8:36 pm

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

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

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

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

JMPGuitars wrote: ↑

Thu 06/17/21 2:12 pm

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.

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.

JMPGuitars wrote: ↑

Thu 06/17/21 4:48 pm

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

TriodeLuvr wrote: ↑

Fri 06/18/21 11:00 am

JMPGuitars wrote: ↑

Thu 06/17/21 4:48 pm

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

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

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

Jack

TriodeLuvr wrote: ↑

Fri 06/18/21 1:14 pm

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

Jack

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

Bieworm wrote: ↑

Fri 06/18/21 1:10 pm

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

TriodeLuvr wrote: ↑

Fri 06/18/21 1:48 pm

Bieworm wrote: ↑

Fri 06/18/21 1:10 pm

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..

Bieworm wrote: ↑

Fri 06/18/21 3:46 pm

TriodeLuvr wrote: ↑

Fri 06/18/21 1:48 pm

Bieworm wrote: ↑

Fri 06/18/21 1:10 pm

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..

Well, it might be due to blocking. That's where overdrive causes bias level(s) somewhere to shift in a way that obscures half the waveform. The remaining energy is half-frequency. This probably needs a scope to troubleshoot, but OTOH, there is such a thing as simply having too much signal. Maybe the gain control needs a governor.

Jack

I don't think the grounding is the problem in this amp.
I scoped yesterday and I can get perfect sine waves all through the amp with low volume and gain settings. I'm not sure that counts for the output...so maybe the problem is the OT or that totally unuseable .1W switching. I'm going to try connecting the OT secondaries directly to the output jacks and evaluate that.
Jack may be onto something too with the too much gain thing. But this being a 1W and my hearing I'm not sure the amp dialed to clean and audibility is even possible...
One other thing that may be important.. when I connected the preamp from the jtm145 to the output of the tremolo TMB the sound was really thin and sharp.. that would indicate the fatness in the jtm145 is created in it's power section? No?
Well.. that would mean the power section there is something overcompensating things?

TriodeLuvr wrote: ↑

Fri 06/18/21 5:41 pm

Bieworm wrote: ↑

Fri 06/18/21 3:46 pm

TriodeLuvr wrote: ↑

Fri 06/18/21 1:48 pm

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..

Well, it might be due to blocking. That's where overdrive causes bias level(s) somewhere to shift in a way that obscures half the waveform. The remaining energy is half-frequency. This probably needs a scope to troubleshoot, but OTOH, there is such a thing as simply having too much signal. Maybe the gain control needs a governor.

Jack

Jack, if I understand this correctly.. too much signal comes from the preamp and hits the power stage. That might be the problem.. or at least part of it. I noticed when I injected the preamp output signal from the tremolo TMB into the power stage of the jtm145 that it was much more quiet than with the jtm145 preamp. Does that explain things to some degree?

Bieworm wrote: ↑

Sat 06/19/21 8:38 am

when I connected the preamp from the jtm145 to the output of the tremolo TMB the sound was really thin and sharp.. that would indicate the fatness in the jtm145 is created in it's power section?

I noticed when I injected the preamp output signal from the tremolo TMB into the power stage of the jtm145 that it was much more quiet than with the jtm145 preamp. Does that explain things to some degree?

The second test is somewhat inconclusive because the level from the tremolo TMB preamp isn't known. The first test seems to indicate that it's the power stage that is being overloaded. And honestly, I would expect a 1W output stage to overload easily. You could confirm it by pushing the amp like you did in the second sound clip and scoping it under those conditions.

Do you have access to a Marshall 1W that you could test with your guitar? It would be interesting to know how it sounds in comparison to your amp at high gain settings. Also, is there any reason to think the 12AU7 might be a little tired? Just thinking out loud here. That could also contribute to early overload.

Incidentally, I really like the tone of that first clip. Be sure to document your changes as you work through this. Don't want to lose that combination!

Jack

TriodeLuvr wrote: ↑

Sat 06/19/21 10:54 am

Bieworm wrote: ↑

Sat 06/19/21 8:38 am

when I connected the preamp from the jtm145 to the output of the tremolo TMB the sound was really thin and sharp.. that would indicate the fatness in the jtm145 is created in it's power section?

I noticed when I injected the preamp output signal from the tremolo TMB into the power stage of the jtm145 that it was much more quiet than with the jtm145 preamp. Does that explain things to some degree?

The second test is somewhat inconclusive because the level from the tremolo TMB preamp isn't known. The first test seems to indicate that it's the power stage that is being overloaded. And honestly, I would expect a 1W output stage to overload easily. You could confirm it by pushing the amp like you did in the second sound clip and scoping it under those conditions.

Do you have access to a Marshall 1W that you could test with your guitar? It would be interesting to know how it sounds in comparison to your amp at high gain settings. Also, is there any reason to think the 12AU7 might be a little tired? Just thinking out loud here. That could also contribute to early overload.

Incidentally, I really like the tone of that first clip. Be sure to document your changes as you work through this. Don't want to lose that combination!

Jack

I don't have access to another 1W... I never came across one either. So that's a no-go
The 12AU7 is a NOS GE JAN 6189W... would be a shame that one is a goner. I will have to buy and try another. Tried a 12at7 but that's uncontrollably OD