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
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2380
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: 1W JTM build

Post by Bieworm »

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

geoff 1965
Verbose poster
Verbose poster
Posts: 1118
Joined: Mon 01/04/16 3:52 pm

Re: 1W JTM build

Post by geoff 1965 »

that sounds good bieworm,well done!
1 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 4037
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: 1W JTM build

Post by JMPGuitars »

Overall that sounds great! Are you using pedals too? There's a couple spots that might need attention:
1:00 to 1:20 is less articulate, and slightly muddied out.
1:35 to the end has a fast clicking noise. Is it a pedal or an oscillation? Or a fan?

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
Amplifier Tools & Parts Info


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

User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2380
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: 1W JTM build

Post by Bieworm »

JMPGuitars wrote:
Sat 06/12/21 10:18 am
Overall that sounds great! Are you using pedals too? There's a couple spots that might need attention:
1:00 to 1:20 is less articulate, and slightly muddied out.
1:35 to the end has a fast clicking noise. Is it a pedal or an oscillation? Or a fan?

Thanks,
Josh
The clicking is my wireless router in the next room. Sometimes it does it harder than other times... annoying, yeah..but I'm used to it.
Any suggestions about the muddiness?
Was thinking about cold clipper at V2a...
This is where I landed:
20210612_174320.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
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 4037
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: 1W JTM build

Post by JMPGuitars »

Bieworm wrote:
Sat 06/12/21 10:42 am
Any suggestions about the muddiness?
Sure. Experiment. ;)
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
Amplifier Tools & Parts Info


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

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

Re: 1W JTM build

Post by TriodeLuvr »

Nice work! Looks great, sounds great!

The way to figure bias is to first calculate the current through the cathode resistor: I = E ÷ R. In your case, it's 8.6 ÷ 470, or 0.0183 (18.3 mA). Next, use P = I X E to calculate the power dissipation in the tube. The voltage across the tube is 260 (anode voltage) minus 8.6 (cathode voltage), or 251.4V. Multiply by 0.0183, and the result is 4.6W, like Josh said. Dividing that by 2 gives the dissipation of each section of the 12AU7, or 2.3W. The tube is rated for 2.75W per section, so you're at roughly 84% of maximum dissipation.

This is very close to one of the "Typical Operation" scenarios in the Brimar datasheet. They show +250V anode, -8.5V on the grid (fixed bias) and current of 10.5mA. That's slightly over 2.6W, so you're right on the money.

Again, you can tweak the tonestack in software using Duncan Monroe's calculator. This will show you instantly what change to expect when any of the components are modified. His calculator is an indispensable tool for tweaking the tonal quality of the stack without spending days on the bench. The URL below contains a list of software available from the site. Note that this software is designed to run on older systems like Windows XP. Duncan's site is also where I go to look up datasheets when I'm too lazy to pull a tube manual off the shelf. :)

http://www.duncanamps.com/software.html

The muddiness Josh mentioned sounds to me like too much overdrive. I don't hear a frequency response issue.

Jack
2 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 4037
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: 1W JTM build

Post by JMPGuitars »

TriodeLuvr wrote:
Sat 06/12/21 11:53 am
The muddiness Josh mentioned sounds to me like too much overdrive.
I agree, but it could be either too much overdrive, or too much bass in the part of the circuit being over-overdriven. Given what we know about where the circuit started, I might guess the latter, but he needs to experiment anyway.
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
Amplifier Tools & Parts Info


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

User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2380
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: 1W JTM build

Post by Bieworm »

JMPGuitars wrote:
Sat 06/12/21 12:55 pm
TriodeLuvr wrote:
Sat 06/12/21 11:53 am
The muddiness Josh mentioned sounds to me like too much overdrive.
I agree, but it could be either too much overdrive, or too much bass in the part of the circuit being over-overdriven. Given what we know about where the circuit started, I might guess the latter, but he needs to experiment anyway.
Next on the list is the 10k cathode resistor on V2a , lift the cathode bypass cap on the PI and see where it takes things.
Or should I start with putting a .0047uf at the plate of V1a to V1b ?
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

Re: 1W JTM build

Post by TriodeLuvr »

Bieworm wrote:
Sat 06/12/21 1:53 pm
JMPGuitars wrote:
Sat 06/12/21 12:55 pm
TriodeLuvr wrote:
Sat 06/12/21 11:53 am
The muddiness Josh mentioned sounds to me like too much overdrive.
I agree, but it could be either too much overdrive, or too much bass in the part of the circuit being over-overdriven. Given what we know about where the circuit started, I might guess the latter, but he needs to experiment anyway.
Next on the list is the 10k cathode resistor on V2a , lift the cathode bypass cap on the PI and see where it takes things.
Or should I start with putting a .0047uf at the plate of V1a to V1b ?
The PI is already operating with gain below unity. Removing the bypass cap will mostly serve to unbalance it. I'd work on the bass a little more like Josh suggested, or reduce the gain somewhere. You could change the 470K just after V1A to 680K or 820K, and/or change the 10M in parallel with the gain pot to 2.2M or even 1M. Changing the V2A cathode resistor to 10K will probably make it impossible to ever play the amp clean. That puts the 12AX7 on a really shallow part of its curve.

Jack
1 x

User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2380
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: 1W JTM build

Post by Bieworm »

How about the .022uf coupling caps from PI to the 12AU7? I could try .01uf there? The tremolo TMB has that value there...
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
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: 616
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 Moderator
Verbose Moderator
Posts: 2380
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 Moderator
Verbose Moderator
Posts: 2380
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 Moderator
Verbose Moderator
Posts: 2380
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: 4037
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
Amplifier Tools & Parts Info


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

User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2380
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: 4037
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
Amplifier Tools & Parts Info


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

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
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: 4037
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
Amplifier Tools & Parts Info


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

Post Reply