changing classic 18 watt to 18 watt modern classic

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

Moderators: CurtissRobin, colossal, zaphod_phil, Daviedawg, Graydon

User avatar
Bieworm
Occasional poster
Occasional poster
Posts: 455
Joined: Mon 02/10/20 8:24 am

Re: changing classic 18 watt to 18 watt modern classic

Post by Bieworm »

JMPGuitars wrote:
Mon 05/04/20 4:29 pm
Most of the voltages look good, thought the PI is slightly off.

Did you use shielded wire on the inputs?

Get them chopsticks ready. ;)
Yes I used shielded wires.
How can I manipulate the grid voltages of the PI. I suspect the problem is somewhere after the preamp. Both channels hum.
The power section remains untouched from power transformer to cap can. Maybe the PI is somewhere faulty. Because of the thin sound?

But I will start with checking each tube..
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
Bieworm
Occasional poster
Occasional poster
Posts: 455
Joined: Mon 02/10/20 8:24 am

Re: changing classic 18 watt to 18 watt modern classic

Post by Bieworm »

Nope not the tubes. On both channels when I strum harder there is an ugly fizz overdrive, even on low volume
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 1977
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: changing classic 18 watt to 18 watt modern classic

Post by JMPGuitars »

Bieworm wrote:
Tue 05/05/20 10:37 am
Nope not the tubes. On both channels when I strum harder there is an ugly fizz overdrive, even on low volume
Did you try pulling out the preamp tubes 1 at a time to see how it affects the constant buzz?

Do a highlighter test with both the schematic and layout. Make sure your mods make sense too.
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

User avatar
Bieworm
Occasional poster
Occasional poster
Posts: 455
Joined: Mon 02/10/20 8:24 am

Re: changing classic 18 watt to 18 watt modern classic

Post by Bieworm »

JMPGuitars wrote:
Tue 05/05/20 11:05 am
Bieworm wrote:
Tue 05/05/20 10:37 am
Nope not the tubes. On both channels when I strum harder there is an ugly fizz overdrive, even on low volume
Did you try pulling out the preamp tubes 1 at a time to see how it affects the constant buzz?

Do a highlighter test with both the schematic and layout. Make sure your mods make sense too.
I will. Hum loud when all tubes are in. Hum loud with only v1 pulled. Same with only V2 pulled.
But the hum is in the background when v3 is pulled. So it's not the preamp. Got to be something with the PI, but I can't see what...
1 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 1977
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: changing classic 18 watt to 18 watt modern classic

Post by JMPGuitars »

Bieworm wrote:
Tue 05/05/20 11:26 am
I will. Hum loud when all tubes are in. Hum loud with only v1 pulled. Same with only V2 pulled.
But the hum is in the background when v3 is pulled. So it's not the preamp. Got to be something with the PI, but I can't see what...
Also look for frayed wires or anything that could be making a connection that shouldn't be there.

Sometimes buzz is related to a ground issue. Which could mean a bad ground connection, or something connecting to ground that shouldn't be.
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

User avatar
Bieworm
Occasional poster
Occasional poster
Posts: 455
Joined: Mon 02/10/20 8:24 am

Re: changing classic 18 watt to 18 watt modern classic

Post by Bieworm »

JMPGuitars wrote:
Tue 05/05/20 12:04 pm
Bieworm wrote:
Tue 05/05/20 11:26 am
I will. Hum loud when all tubes are in. Hum loud with only v1 pulled. Same with only V2 pulled.
But the hum is in the background when v3 is pulled. So it's not the preamp. Got to be something with the PI, but I can't see what...
Also look for frayed wires or anything that could be making a connection that shouldn't be there.

Sometimes buzz is related to a ground issue. Which could mean a bad ground connection, or something connecting to ground that shouldn't be.
Found the problem...
It were the wires from jack to point x and y for the fx loop. That mod is giving me nothing but trouble ... I removed them...
Amp is shining like a star now. Dead quiet!!! Noisewise that is... ;)

BUT found another issue.. the trem comes on very slow. It takes a few seconds before it starts...
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 1977
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: changing classic 18 watt to 18 watt modern classic

Post by JMPGuitars »

Bieworm wrote:
Tue 05/05/20 12:17 pm
Found the problem...
It were the wires from jack to point x and y for the fx loop. That mod is giving me nothing but trouble ... I removed them...
Amp is shining like a star now. Dead quiet!!! Noisewise that is... ;)

BUT found another issue.. the trem comes on very slow. It takes a few seconds before it starts...
Did you properly ground the jacks? I generally ground mine on the same ground line as the speaker outs. Probably more out of convenience than anything else.

Here's a recent build of mine with the FX loop:
_JMP7608.jpg

You can hear how clean that amp is here: https://youtu.be/tY9RUIO3KjY and here: https://youtu.be/3oSZVg2STXw

Remember that lead dress is always important. If you have a wire running the wrong way, it will get angry. If something's not grounded correctly, it will be even angrier.

Try swapping to a different tube for the trem. If that doesn't solve it, do the highlighter test for the trem section.

Thanks,
Josh
You do not have the required permissions to view the files attached to this post.
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

User avatar
Bieworm
Occasional poster
Occasional poster
Posts: 455
Joined: Mon 02/10/20 8:24 am

Re: changing classic 18 watt to 18 watt modern classic

Post by Bieworm »

JMPGuitars wrote:
Tue 05/05/20 12:51 pm
Bieworm wrote:
Tue 05/05/20 12:17 pm
Found the problem...
It were the wires from jack to point x and y for the fx loop. That mod is giving me nothing but trouble ... I removed them...
Amp is shining like a star now. Dead quiet!!! Noisewise that is... ;)

BUT found another issue.. the trem comes on very slow. It takes a few seconds before it starts...
Did you properly ground the jacks? I generally ground mine on the same ground line as the speaker outs. Probably more out of convenience than anything else.

Here's a recent build of mine with the FX loop:
_JMP7608.jpg


You can hear how clean that amp is here: https://youtu.be/tY9RUIO3KjY and here: https://youtu.be/3oSZVg2STXw

Remember that lead dress is always important. If you have a wire running the wrong way, it will get angry. If something's not grounded correctly, it will be even angrier.

Try swapping to a different tube for the trem. If that doesn't solve it, do the highlighter test for the trem section.

Thanks,
Josh
Aha! I didn't ground the jacks. There was no grounding on the layout of the EF86 extra... damn... that's probably it!

Here's the new voltage chart. PI even more off on the plates. What can I do with that?

As for the trem tube.. I tried 5 different tubes from different brands ...even a 12ax7h. The idd thing is that it was the same tube I had in there before the change-over from 18w classis to modern classic.
Anywayz...the trem oscillation is notorious in regard to the ideal voltages. Will look into that thrash online.. there are plenty of threads that treat this issue...
I had to do the snubber cap on the plates of the PI (150pf) to get rid of too much brightness that gave a lot of fizz. Doing the PR mod afterwards when I get the zeners...

Amp sounds awesome by the way!!!!
15887036049724190524317250561343.jpg
Tomorrow I'm gonna raise the B+ on the PT. Wire it for 230V instead of the 240V now.
My tremolo TMB is wired like that too. I like the extra volume ...it's a pretty significant difference. That will probably lift some voltages and maybe kickstart that tremolo sooner. It's worth a try... it's an easy reversible mod...
You do not have the required permissions to view the files attached to this post.
2 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 1977
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: changing classic 18 watt to 18 watt modern classic

Post by JMPGuitars »

lol! Man, how long until you read the schematic, and not just look at a layout? :lol: :lol:

Check out those cute triangles on the schematic:
FX-Loop.jpg

It's so much easier to understand what's going on with the FX loop looking at this schematic drawing, than guessing looking at the layout.
You do not have the required permissions to view the files attached to this post.
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

User avatar
Bieworm
Occasional poster
Occasional poster
Posts: 455
Joined: Mon 02/10/20 8:24 am

Re: changing classic 18 watt to 18 watt modern classic

Post by Bieworm »

JMPGuitars wrote:
Tue 05/05/20 4:23 pm
lol! Man, how long until you read the schematic, and not just look at a layout? :lol: :lol:

Check out those cute triangles on the schematic:
FX-Loop.jpg
I knew you were going to say that..

It's so much easier to understand what's going on with the FX loop looking at this schematic drawing, than guessing looking at the layout.
Any advice on the voltages?
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 1977
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: changing classic 18 watt to 18 watt modern classic

Post by JMPGuitars »

Bieworm wrote:
Tue 05/05/20 9:37 pm
Any advice on the voltages?
Why? It sounded like you had a decent plan switching the PT wire. See what happens. Test again. Check your PI connections are good, resistors are good, etc...
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

User avatar
Bieworm
Occasional poster
Occasional poster
Posts: 455
Joined: Mon 02/10/20 8:24 am

Re: changing classic 18 watt to 18 watt modern classic

Post by Bieworm »

JMPGuitars wrote:
Tue 05/05/20 10:00 pm
Bieworm wrote:
Tue 05/05/20 9:37 pm
Any advice on the voltages?
Why? It sounded like you had a decent plan switching the PT wire. See what happens. Test again. Check your PI connections are good, resistors are good, etc...
Indeed! But maybe I should first try lovering the 100k resistor from voltage C to pin 6 of V2? That should juice the voltage to the trem too, no?
By the way... I always wondered... why are there 2 voltages on the voltage chart of tremolo tube on pin 1? Are that the oscillation voltage extremes?
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 1977
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: changing classic 18 watt to 18 watt modern classic

Post by JMPGuitars »

Bieworm wrote:
Wed 05/06/20 7:55 am
Indeed! But maybe I should first try lovering the 100k resistor from voltage C to pin 6 of V2? That should juice the voltage to the trem too, no?
By the way... I always wondered... why are there 2 voltages on the voltage chart of tremolo tube on pin 1? Are that the oscillation voltage extremes?
Nah, stick with your plan. It already proved to be the better option for your other amp. Adjust V2 after changing the PT wiring if needed, but start with new voltages.

Yup.
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

User avatar
Bieworm
Occasional poster
Occasional poster
Posts: 455
Joined: Mon 02/10/20 8:24 am

Re: changing classic 18 watt to 18 watt modern classic

Post by Bieworm »

JMPGuitars wrote:
Wed 05/06/20 8:19 am
Bieworm wrote:
Wed 05/06/20 7:55 am
Indeed! But maybe I should first try lovering the 100k resistor from voltage C to pin 6 of V2? That should juice the voltage to the trem too, no?
By the way... I always wondered... why are there 2 voltages on the voltage chart of tremolo tube on pin 1? Are that the oscillation voltage extremes?
Nah, stick with your plan. It already proved to be the better option for your other amp. Adjust V2 after changing the PT wiring if needed, but start with new voltages.

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

User avatar
Bieworm
Occasional poster
Occasional poster
Posts: 455
Joined: Mon 02/10/20 8:24 am

Re: changing classic 18 watt to 18 watt modern classic

Post by Bieworm »

Did all the changes. 230V power trans tap. 100k resistor from point C to 91k (100k + 1M parallel)
Fx loop Works now!!!And I'm really satisfied!!! Muuuuuuch less digital noise ...neglectable in fact.

So this one is ready and shining:)

Here is the New chart. The green is the final state. Powertubes running at 98.3%, 11.8 watts. Should I change anything?
15887928909337086897979929496126.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"

Post Reply