Building 1974 style 18W

[JMPGuitars]

More questions on the Modern Classic layout and schematic:
- Is the 2A Slow Blow fuse for 110-120V operation?
- Any recommendations for 230-240V? 1A Slow Blow

Yup.

[BlueX]

Is there any chassis template (like drill template) for Modern Classic as a head? Main question is where to put the 32+32 uF cap can.

By the way, is there any search function for the forum? I can only find "Search this topic".

[Bieworm]

Is there any chassis template (like drill template) for Modern Classic as a head? Main question is where to put the 32+32 uF cap can.

By the way, is there any search function for the forum? I can only find "Search this topic".

You can’t search outside the topic when inside the topic. But on the home page you can..or when you click forums

[JMPGuitars]

Bieworm is correct. You can click the breadcrumbs above to go to Forum and search there, or under quick links, it will take you to a broader search page.

Thanks,
Josh

[BlueX]

Finally got almost all parts. My next step will be to dry-fit to check possible layout, and what else I need (like fasteners, etc.)

Most parts are from Modulus Amplification (UK), some from TAD (DE), and Mouser.

Parts.jpg

[BlueX]

Any comments on chassis layout before I start drilling? I need to place reservoir cap and OT (and of course fasteners for board, ground lugs, etc.). This will be a head version (not combo).

Most straight forward seems to be to place
- Reservoir cap in the unused "noval hole" between power amp tubes and rectifier tube
- OT between the power amp tubes (V4 and V5)

I have seen other placements for OT (closer to PT), and reservoir cap (at the end of the chassis, both on top of or inside chassis).

Chassis came pre-drilled on front and back for in-/outlets, controls, and fuses, and on top for laydown PT and 7 noval sockets (of which I will use 6).

18W chassis layout 1.jpg

18W chassis layout 1b.jpg

18W chassis layout 2.jpg

18W chassis layout 2b.jpg

18W chassis layout 3.jpg

18W chassis layout 4.jpg

[JMPGuitars]

Reverse your tube sockets. You want the gap between pins 1 and 9 away from the chassis wall.

[BlueX]

Reverse your tube sockets. You want the gap between pins 1 and 9 away from the chassis wall.

Thanks! I'll keep that in mind when it's time to solder.

For V4 and V5 it seems obvious to reach pins 4 and 5 from the chassis wall.

For 12A*7 tube sockets I've seen comments to orient the gap to the wall, to reach pins 4 and 5 through the gap between pins 1 and 9. I guess it's better to be consistent and orient all sockets the same way (as you suggest).

[chaliapin]

I don't think the AI bots are going to take over the world just yet...

[BlueX]

Hello, I'm back with a couple of questions:

In the 1st photo, I put a jack on the printed layout. Is this the correct way to orient the jack when reading the layout? (Looks like that when I look at the schematics, to get switched and unswitched lugs correct.)

In the layout, the ground wire for the pots is drawn as a straight connector across all pots, with a "T" connecting to the board ground bus at the 1M resistor. However, I was adviced not to put a bus bar there (see comment below).
- Is it OK to have separate ground wires for each volume pot, connecting to board ground bus in the closest location, and to connect ground for tremolo speed and intensity pots with one wire going to closest location on board bus?
- It looks like tone pots are not grounded
- See 2nd photo for my actual layout (so far).

"Do NOT put a bus bar on the pots, that's a huge opportunity for noise issues and ground loops." (from post #2)

Input jacks.jpg

Pot ground.jpg

[JMPGuitars]

The jack is correct, you can see the lugs that get lifted when a jack is inserted.

The bus bar you're referring to is not the same thing as daisy chaining your ground wires. Some people follow terrible diagrams from other sources that actually solder a bare wire bus bar across the back of the pots.

The layout you're looking at is good, follow it and you should get a nice amp.

[Bieworm]

Hello, I'm back with a couple of questions:

In the 1st photo, I put a jack on the printed layout. Is this the correct way to orient the jack when reading the layout? (Looks like that when I look at the schematics, to get switched and unswitched lugs correct.)

In the layout, the ground wire for the pots is drawn as a straight connector across all pots, with a "T" connecting to the board ground bus at the 1M resistor. However, I was adviced not to put a bus bar there (see comment below).
- Is it OK to have separate ground wires for each volume pot, connecting to board ground bus in the closest location, and to connect ground for tremolo speed and intensity pots with one wire going to closest location on board bus?
- It looks like tone pots are not grounded
- See 2nd photo for my actual layout (so far).

"Do NOT put a bus bar on the pots, that's a huge opportunity for noise issues and ground loops." (from post #2)


Input jacks.jpgPot ground.jpg

The tone pots are going to ground through the .0047uf cap. Gradually you dump more low frequency to ground through that cap while keeping the high frequencies.

[BlueX]

Just sharing some build pic's

18W board 2.jpg

18W soldering 1.jpg

18W soldering 2.jpg

18W soldering 3.jpg

18W soldering 4.jpg

18W soldering 5.jpg

18W soldering 6.jpg

18W soldering 7.jpg

[BlueX]

I have completed my build, but have one problem. There is a terrible noise as soon as I turn up that "tremolo volume" (regardless if guitar is plugged in or not). Frequency varies when I turn the Speed pot.

With the "tremolo volume" turned down I can play through each input on the "normal channel", with good sound.

I followed Robrobinette's startup procedure, and got reasonable voltage radings everywhere except for p1 and p6 on V2. Here I get varying voltage, with a frequency of about 4 sek: p1 140-160 V, p6 160-220 V.

Edit: Open/Closed footswitch does not change anything, and I've tested different 12AX7 tubes also and the problem remains.

Any known culprits? I plan to desolder everything on V2 socket, and redo that.

18W soldering 8.jpg

18W soldering 9.jpg

[JMPGuitars]

Before you do that, do a highlighter test. Your wiring is messed up somewhere, very likely as simple as 1 or 2 bad connections or solder joints.

You know this because you said the footswitch does nothing. If wired correctly, the footswitch will ground out (mute) the tremolo circuit entirely. That's like if the trem knobs are turned down, which you said is not an issue.

Also, with the tremolo muted (grounded) your voltage readings should be steady for V2. With the tremolo active, you should expect voltage fluctuation there.

[Bieworm]

It might be not related, but your footswitch input is wired at the wrong side of it. When you put in a jack it will never switch

[BlueX]

Before you do that, do a highlighter test. Your wiring is messed up somewhere, very likely as simple as 1 or 2 bad connections or solder joints.

You know this because you said the footswitch does nothing. If wired correctly, the footswitch will ground out (mute) the tremolo circuit entirely. That's like if the trem knobs are turned down, which you said is not an issue.

Also, with the tremolo muted (grounded) your voltage readings should be steady for V2. With the tremolo active, you should expect voltage fluctuation there.

Thanks! I will have a look at this before changing anything.

Voltage sheet says 110/150 V for V2-p1 so I guessed that means fluctuation. However, for V2-p6 it says just 160 V. Does that mean stable voltage on that side of the 220k resistor?

Does the footswich mute just the tremolo effect, or that channel completely?

Although I'm able to read and understand schematics there are still some things that are not obvious to me, like switching sockets.

[BlueX]

It might be not related, but your footswitch input is wired at the wrong side of it. When you put in a jack it will never switch

Thanks! I'll check that too.

[JMPGuitars]

Does the footswich mute just the tremolo effect, or that channel completely?

Although I'm able to read and understand schematics there are still some things that are not obvious to me, like switching sockets.

Review the position of the footswitch in the schematic and see if you can answer your own question. Ask questions if you need to understand the signal path better.

[Bieworm]

A layout can be confusing/misleading , but a schematic is less likely to be misinterpreted. Always wire your stuff with the schematic as a guide.