1W JTM build

Forum to discuss amp circuits besides the 18 Watters and their ilk.

Moderators: JMPGuitars, CurtissRobin, colossal, zaphod_phil, Daviedawg, Graydon

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 545
Joined: Tue 10/16/18 11:19 am

Re: 1W JTM build

Post by TriodeLuvr »

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
3 x

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 545
Joined: Tue 10/16/18 11:19 am

Re: 1W JTM build

Post by 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
1 x

User avatar
Bieworm
Verbose Wizard of Oz
Verbose Wizard of Oz
Posts: 1507
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: 1W JTM build

Post by Bieworm »

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
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
Bieworm
Verbose Wizard of Oz
Verbose Wizard of Oz
Posts: 1507
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: 1W JTM build

Post by 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
You do not have the required permissions to view the files attached to this post.
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
Bieworm
Verbose Wizard of Oz
Verbose Wizard of Oz
Posts: 1507
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: 1W JTM build

Post by Bieworm »

Tried parallelling all 3 filter caps with an extra 22uf.. no difference
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3104
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: 1W JTM build

Post by JMPGuitars »

You looking for a bandaid or the source of the problem? Try using your scope. Try chopsticking. You've got the skillz! ;)
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

User avatar
Bieworm
Verbose Wizard of Oz
Verbose Wizard of Oz
Posts: 1507
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: 1W JTM build

Post by Bieworm »

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.
1 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3104
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: 1W JTM build

Post by 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.
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 545
Joined: Tue 10/16/18 11:19 am

Re: 1W JTM build

Post by TriodeLuvr »

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
1 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3104
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: 1W JTM build

Post by JMPGuitars »

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
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

User avatar
Bieworm
Verbose Wizard of Oz
Verbose Wizard of Oz
Posts: 1507
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: 1W JTM build

Post by 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
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 545
Joined: Tue 10/16/18 11:19 am

Re: 1W JTM build

Post by TriodeLuvr »

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

Jack
0 x

User avatar
Bieworm
Verbose Wizard of Oz
Verbose Wizard of Oz
Posts: 1507
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: 1W JTM build

Post by Bieworm »

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!!!
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 545
Joined: Tue 10/16/18 11:19 am

Re: 1W JTM build

Post by TriodeLuvr »

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
0 x

User avatar
Bieworm
Verbose Wizard of Oz
Verbose Wizard of Oz
Posts: 1507
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: 1W JTM build

Post by Bieworm »

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..
0 x
"THIS should be played at high volume..preferably in a residential area"

Post Reply