changing classic 18 watt to 18 watt modern classic

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Bieworm
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changing classic 18 watt to 18 watt modern classic

Post by Bieworm »

been busy last few days on turning my classic 18 watt to the new modern classic 18...

I like the normal channel as is on the standard, so I reviewed the schematic and layout to achieve that.
one remark: by lack of a .0047uf cap (in my former build) I used a .01uf cap between the plates of V1 and the first lug on the vol pot. I did the same on this one, except I joined the wires from tone pot and volume pot after the .01uf cap instead of giving both a separate .01uf cap.

I also made it a 2 input amp... one for the normal channel and one for the trem channel. The grid stoppers are 22k instead of 10k. Didn't have 10k, but my former build had the .22k too...

Previously I did a 1 input version where the channels were switchable by a toggle switch. But that was no good.. when I switched channels the other channel gave me hum like a grounding issue. When I turned the volume from the unused channel down to zero the hum was gone. But I added the effects loop on that build and when I put a pedal in between I got a seriously loud buzz (kinda like buzz from a guitar cable that has no guitar plugged into it and the jack in your hand) so I removed that FX loop. It was build exactly like the layout on the EF86 extra, so it must have worked... but I think it is somehow related to the 1 input switchable channel... I was suspicious that the hum problem had something to do with it.

I added the FX loop again on my new coversion to the modern classic 18 watt.
I only have to wire the pots and the cap can to the board and I'm set to fire it up. Of course with the lightbulb current limiter (I learned my lesson earlier the hard way and blew my power transformer)

The standard schematic has a 100R screen resistor on V5, with a jumper to V4. The modern classic has 2x 1k for each EL84 tube. Is it a problem if I copy the standard on this one?
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Last edited by Bieworm on Mon 05/04/20 5:23 am, edited 2 times in total.
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Re: changing classic 18 watt to 18 watt modern classic

Post by crgfrench »

Shared 100 is OK but 2 1ks is safer. Either way will work.
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Re: changing classic 18 watt to 18 watt modern classic

Post by Bieworm »

crgfrench wrote:
Mon 05/04/20 3:18 am
Shared 100 is OK but 2 1ks is safer. Either way will work.
it'll have to do... not changing the layout no more ;)
I worked on the original turretboard after dismantling it and cleaning all turrets. Had to add a turret here and there, but it's ok now.
18W classic.jpg
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Re: changing classic 18 watt to 18 watt modern classic

Post by Bieworm »

I really hope it will run fine the way I wired it. Lots of things can go wrong:
1. it's a new schematic, new layout... thus never built before
2. I changed stuff on the design, so there could be hidden problems

But all in all I've checked the schematics (standard and new) and it seems pretty OK for that matter...
Just hope the inputs are wired like they should (see layout).

If you could look at what I've done and see anything suspicious please inform me about it :)
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Re: changing classic 18 watt to 18 watt modern classic

Post by JMPGuitars »

Bieworm wrote:
Mon 05/04/20 5:30 am
I really hope it will run fine the way I wired it. Lots of things can go wrong:
1. it's a new schematic, new layout... thus never built before
2. I changed stuff on the design, so there could be hidden problems

But all in all I've checked the schematics (standard and new) and it seems pretty OK for that matter...
Just hope the inputs are wired like they should (see layout).

If you could look at what I've done and see anything suspicious please inform me about it :)
1. Wouldn't worry about this one. It's essentially the same but with improvements and changes that have been tested in a gazillion builds.

2. This is where some of your problems are from.

For example, your single input switching doesn't work because you need to ground the input of the unused channel. This is possible with correctly wiring the switch, but tricky if you're trying to use both an onboard switch and a foot switch.

You can replace the shared 100R resistor with a shared 1K resistor and that will still be an improvement.
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Re: changing classic 18 watt to 18 watt modern classic

Post by Bieworm »

JMPGuitars wrote:
Mon 05/04/20 7:19 am
Bieworm wrote:
Mon 05/04/20 5:30 am
I really hope it will run fine the way I wired it. Lots of things can go wrong:
1. it's a new schematic, new layout... thus never built before
2. I changed stuff on the design, so there could be hidden problems

But all in all I've checked the schematics (standard and new) and it seems pretty OK for that matter...
Just hope the inputs are wired like they should (see layout).

If you could look at what I've done and see anything suspicious please inform me about it :)
1. Wouldn't worry about this one. It's essentially the same but with improvements and changes that have been tested in a gazillion builds.

2. This is where some of your problems are from.

For example, your single input switching doesn't work because you need to ground the input of the unused channel. This is possible with correctly wiring the switch, but tricky if you're trying to use both an onboard switch and a foot switch.

You can replace the shared 100R resistor with a shared 1K resistor and that will still be an improvement.
Thanks Josh!
But now with the two separate inputs it should work I guess?
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Re: changing classic 18 watt to 18 watt modern classic

Post by JMPGuitars »

Bieworm wrote:
Mon 05/04/20 7:40 am
Thanks Josh!
But now with the two separate inputs it should work I guess?
Correct. The high/low inputs are not necessary. You would only be wiring in the "high" jacks.

However, for the first channel, if you're using only one input, I recommend running pins 2 and 7 in parallel. Or you can add a single/parallel switch to connect and disconnect pin 7 from pin 2.

22K is fine for the input grid resistors. Anywhere from 10K to 33K is fine.
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Re: changing classic 18 watt to 18 watt modern classic

Post by Bieworm »

JMPGuitars wrote:
Mon 05/04/20 8:55 am
Bieworm wrote:
Mon 05/04/20 7:40 am
Thanks Josh!
But now with the two separate inputs it should work I guess?
Correct. The high/low inputs are not necessary. You would only be wiring in the "high" jacks.

However, for the first channel, if you're using only one input, I recommend running pins 2 and 7 in parallel. Or you can add a single/parallel switch to connect and disconnect pin 7 from pin 2.

--> why is that?

22K is fine for the input grid resistors. Anywhere from 10K to 33K is fine.
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Re: changing classic 18 watt to 18 watt modern classic

Post by JMPGuitars »

Bieworm wrote:
Mon 05/04/20 8:58 am
JMPGuitars wrote:
Mon 05/04/20 8:55 am
However, for the first channel, if you're using only one input, I recommend running pins 2 and 7 in parallel. Or you can add a single/parallel switch to connect and disconnect pin 7 from pin 2.
--> why is that?
Try it and find out. ;)

...running in parallel thickens up the sound. Making it switchable would be more fun, cause I know you like switches and options and stuff.
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Re: changing classic 18 watt to 18 watt modern classic

Post by Bieworm »

JMPGuitars wrote:
Mon 05/04/20 9:04 am
Bieworm wrote:
Mon 05/04/20 8:58 am
JMPGuitars wrote:
Mon 05/04/20 8:55 am
However, for the first channel, if you're using only one input, I recommend running pins 2 and 7 in parallel. Or you can add a single/parallel switch to connect and disconnect pin 7 from pin 2.
--> why is that?
Try it and find out. ;)

...running in parallel thickens up the sound. Making it switchable would be more fun, cause I know you like switches and options and stuff.
So you mean running 2 separate 22k resistors from the single shielded wire terminal to each pin (2 +7) ?
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Re: changing classic 18 watt to 18 watt modern classic

Post by JMPGuitars »

Bieworm wrote:
Mon 05/04/20 9:45 am
JMPGuitars wrote:
Mon 05/04/20 9:04 am
...running in parallel thickens up the sound. Making it switchable would be more fun, cause I know you like switches and options and stuff.
So you mean running 2 separate 22k resistors from the single shielded wire terminal to each pin (2 +7) ?
No. You run 1 grid resistor to one of the grids, and run a jumper wire to the next grid.
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Re: changing classic 18 watt to 18 watt modern classic

Post by Bieworm »

JMPGuitars wrote:
Mon 05/04/20 9:54 am
Bieworm wrote:
Mon 05/04/20 9:45 am
JMPGuitars wrote:
Mon 05/04/20 9:04 am
...running in parallel thickens up the sound. Making it switchable would be more fun, cause I know you like switches and options and stuff.
So you mean running 2 separate 22k resistors from the single shielded wire terminal to each pin (2 +7) ?
No. You run 1 grid resistor to one of the grids, and run a jumper wire to the next grid.
Oh, I did that already.
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Re: changing classic 18 watt to 18 watt modern classic

Post by JMPGuitars »

Bieworm wrote:
Mon 05/04/20 10:54 am
JMPGuitars wrote:
Mon 05/04/20 9:54 am
Bieworm wrote:
Mon 05/04/20 9:45 am


So you mean running 2 separate 22k resistors from the single shielded wire terminal to each pin (2 +7) ?
No. You run 1 grid resistor to one of the grids, and run a jumper wire to the next grid.
Oh, I did that already.
Well done. ;)
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Re: changing classic 18 watt to 18 watt modern classic

Post by Bieworm »

Aaaaaaahhhh!!!
Will be troubleshooting tomorrow... the amp sounds kinda nasal and there is a constant buzz. But I'm tired now..
This is the voltage chart:
158862271258252823094248744450.jpg
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Re: changing classic 18 watt to 18 watt modern classic

Post by JMPGuitars »

Most of the voltages look good, thought the PI is slightly off.

Did you use shielded wire on the inputs?

Get them chopsticks ready. ;)
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