Cannibalizing a DSL 50 to make a JTM 50

[warioblast]

Hi,
Disclosure; I'm a total noob in building amps, so far I have only built pedals. I can read schematics, and lately I have been reading articles and watching videos to have a little insight, learn about the terminology...

That being said, I have this DSL 50 head, which has been faulty for months now. I'm a heavy procrastinator....
A few days ago it occured to me that rather than getting the amp fixed and sell it, I might as well cannibalize it. The DSL has been repaired a couple of years ago, the channel switching  and reverb went down. I think the repair guy charged me something like 120€. I could sell the amp for 450€ once it get fixed but the I'm guessing the repair would cost me at least another 120€.

Then came the idea of cannibalizing it. A head cab for the JMP would cost at least 230€, the chassis 50€, a set of tubes 60€.

My biggest question is could the DSL50 power transformer be used for the JMP50. Not that I'm willing to do it but for my amp education. 
I found those specs on TAD website:

Mains transf. Marshall original for JCM2000 50/60W models
Original Marshall® MAINS transformer for JCM2000 50/60W Models like DSL50, TSL60, TSL601, TSL602

primary for 115/230V

Primary:
0V #2
115V #5

0V #8
115V #11

Secondary:
335V #20
0V #19

20V #18
0V #17
20V #16

3,15V #15
0V #14
3,15V #13

I found this article today http://www.valvewizard.co.uk/bridge.html so I guess it's doable.

What about the layout ? Would you guys move stuff around ?

This one is the DSL100

[tubenit]

I did something similar maybe 8-9 yrs ago with a friend's JCM900 amp.  He didn't like the tone of that amp and wanted a "Plexi 50" type tone.  I think the conversion costs $125 in parts to make it happen. It turned out really good tone wise.  I didn't change the layout much and overall worked around the original layout.

He didn't tell his bandmates that he had gutted the amp and had it rewired and they were all commenting how "amazing" his amp sounded that night when they played at a small club.  However, he is a remarkable player and opened up for Dave Matthews and on another occasion for Derek Trucks.

Here is a photo of what I did.  He didn't even want to buy new pots so I even used the PCB ones.

http://s28.photobucket.com/user/tubenit/media/CPlexi50chassisinteriordescription.jpg.html?sort=3&o=39

I will say that this is somewhat an ambitious build for a noob in amp building.

With respect, Tubenit

[kagliostro]

My 2c

if your decision is to gut the amp and rebuild, dismount the PCB with components and keep as they are

when you finish your Turret Board conversion and all is OK, you can try to sell the boards, may be for a not very high price

but I think you can get some bucks for it also if not 100% OK

Franco

[warioblast]

I will say that this is somewhat an ambitious build for a noob in amp building.

Maybe, but I feel confident  Plexis are well documented and you guys are here to help me if it goes sour 
My intention is to build the circuit as stock as possible and maybe start doing mods when the amp is already running.
I see you put a reverb in your build. What kind of circuit did you use ?

My 2c

if your decision is to gut the amp and rebuild, dismount the PCB with components and keep as they are

when you finish your Turret Board conversion and all is OK, you can try to sell the boards, may be for a not very high price

but I think you can get some bucks for it also if not 100% OK

Franco

This is an option I thought about.

I could also try to fix the amp before selling it but I feel way less confident. That thing is flawed.

[drew]

What problems are you having with it?

[tubenit]

I see you put a reverb in your build. What kind of circuit did you use ?

Look at this thread in ARCHIVES about the one tube reverb.  Go down to reply #25.  There is a Tweed Bassman schematic there with one tube reverb added.   That is essentially what I ended up doing with this Plexi 50w build with reverb. 

http://el34world.com/Forum/index.php?topic=7957.0

[warioblast]

Look at this thread in ARCHIVES about the one tube reverb.  Go down to reply #25.  There is a Tweed Bassman schematic there with one tube reverb added.   That is essentially what I ended up doing with this Plexi 50w build with reverb. 
http://el34world.com/Forum/index.php?topic=7957.0

Ok, I was hoping you had used the JCM tank. I did a quick research but it looks like it would be too much work tweaking a reverb circuit for the 4BB3C1B tank used in the DSL.

What problems are you having with it?

Channel A hums louder than channel B. It used to be really quiet. And now the pedals I used are noisier too, whether they are dirt pedals or modulation. I also have a popping problem with them. Once again it used to be fine.
I sometimes got the sizzling, frying bacon sound.
And not too long ago the amp showed symptoms of a bias drift. And since the first bias drift manifestation, I noticed that when the channel A is engaged, the LED of the channel B is slightly lit.

[warioblast]

I decided I'm going to try using the DSL PT & OT and see out it goes.
Am I getting it right ?
I'm not settled on the circuit; 1987, 1986 or SS JTM50.

[sluckey]

That bias circuit will not work with a full wave bridge. Copy the bias circuit from the JCM900 instead.

[warioblast]

Do you mean this circuit ?

[sluckey]

yes

[warioblast]

Ok thanks. 
The circuits don't look very different, what's the skinny ?

[kagliostro]

Have you seen a capacitor that is between the PT and the Bias circuit ?

That is

Franco

[warioblast]

Have you seen a capacitor that is between the PT and the Bias circuit ?

That is

Franco

I have. That's why I want to understand why this 47n cap and 56k res to ground combo is better than the original 220k. I guess it allows more current.

[sluckey]

It's not a matter of whether it is better or not. Without that cap the bias circuit cannot work. Look at page 6 of this pdf for an explanation...

     http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[kagliostro]

Good explanation Steve

Thanks

Franco

[warioblast]

It's not a matter of whether it is better or not. Without that cap the bias circuit cannot work. Look at page 6 of this pdf for an explanation...

     http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

That 340VAC with 0V baseline is a bit confusing but I get the idea. Thanks.

[warioblast]

I ended up choosing the JTM 50. I received my parts and made a 'ghetto' amp cradle.

I didn't give it much thoughts; I ordered slow blow fuses. Did I make a bad choice ? From what I read, afterwards,  most people use SB fuses for the mains and FB for the HT. I also read that once the amp is up and running you can switch the FB fuse of the HT for a SB.

Does the placement / orientation of the choke matter ?

[sluckey]

Does the placement / orientation of the choke matter ?

Yes. Don't put it near the input jacks. Look at some pics of the JMP 50. Try to make it look like that.

[warioblast]

The makeover has begun.
I may adjust the center plate to get the tubes closer to the edge. Cutting the plate and soldering it flat to the chassis, this way I will not have to play with 2- level standoffs. Will see...
I did a few quick layout mockups.
This JTM50 is going to be my reference.

Looking at the DSL50 schematic I see another class X cap. Do I really need one in my JTM50 build ? Does it have to do with the dual primary transformer ?

[sluckey]

Looking at the DSL50 schematic I see another class X cap. Do I really need one in my JTM50 build ? Does it have to do with the dual primary transformer ?

That cap is being used as a line filter. It is connected directly across the AC line. Anytime you place a cap across the AC line it should have a "Class X" rating. This is a safety issue.

You do not need this cap in your rebuild.

[warioblast]

Once again many thanks. 

[drew]

Two level standoffs are easier then metal work . . .

[kagliostro]

Cutting the plate and soldering it flat to the chassis, this way I will not have to play with 2- level standoffs.

You are not bound to solder something to the chassis to have a flat surface, cut an adequate aluminium sheet (2mm thick) and fix it on the top of the chassis using rivets, it is very simple and sturdy


Franco

[warioblast]

Cutting the plate and soldering it flat to the chassis, this way I will not have to play with 2- level standoffs.

You are not bound to solder something to the chassis to have a flat surface, cut an adequate aluminium sheet (2mm thick) and fix it on the top of the chassis using rivets, it is very simple and sturdy


Franco

This is a great & simple idea, thanks. I was focusing on the center plate and how it was handy that the holes were already drilled, plus one of my uncle is pretty good at arc welding. And I always borrowed tools from him. Anyhow, I figured a step drill would be amortized so I ordered one.

Two level standoffs are easier then metal work . . .

In the meantime I ordered taller standoffs. Though I think the board would be too close from the pots.

[warioblast]

Hi guys.
Can I hook up the OT secondary common to the close ground lug or must I solder it to the speaker jacks ground ?

[pompeiisneaks]

That needs to connect to the speaker jack ground or the speaker won't have continuity to the OT.   

[Willabe]

Wire/solder it directly to the speaker jack ground.

Twist any/all OT secondary 2/4/8/16 ohm tap fly lead wires with the common/ground fly lead wire going to the speaker jack(s) and impedance selector.   

You have a Marshall with plastic body (Cliff) jacks. That type of jack has no direct ground connection with the chassis.

Even with metal body (Switchcraft) jacks you still should wire/solder the OT common/ground wire/fly lead to the jack ground.

The OT secondary has a lot of current so you don't want to use the chassis as a wire. That could/will cause problems.   

[warioblast]

Thanks for the quick answers. ;)

[warioblast]

So I finished the soldering part 
I mostly used the Metropoulos 50W guide, and a little help from the triode electronics and ceriatone layouts.
Still using the metro pdf for testing and biasing. (see attached pic)
With my cheap DMM I have zero reading on the mains switch.  It does lit up though.
I have 2.9 / 3.0 V AC on the heaters. Negative voltage on pin 5 of V4 & V5 is -32V DC.
I measured 600V DC at the ouput of the bridge rectifier. Pin 1 of V1 measured 535V DC.
All my reading were done with the standby switch off and with no tubes.
So this is where I'm at. I didn't try to put tubes without consulting you guys.

[sluckey]

Negative voltage on pin 5 of V4 & V5 is -32V DC.

Just as a precaution... adjust the bias pot for maximum negative voltage at pin 5. Later on you can set the bias properly.

[warioblast]

Negative voltage on pin 5 of V4 & V5 is -32V DC.

Just as a precaution... adjust the bias pot for maximum negative voltage at pin 5. Later on you can set the bias properly.

Ok, will do. I'm a bit concern by my high voltages readings. Is it safe to put tubes in ?

[sluckey]

Those voltages will come down once the tubes start drawing some current. Proceed with caution. What is your maximum negative bias voltage?

[warioblast]

-36V DC

[warioblast]

It's alive !! 
I put the preamp tubes then the power ones.
I played it for 90 minutes. No sparkles, no smoke. 
B+ is down to 480V DC; I set the bias to 37.3 for V4 & 36.2 mA for V5.
I measured 340V DC at pin 6 of V2. Is it risky ? That's the only voltage that is extreme.
PPIMV seems to work fine.
I do have a concern with the Bright volume. ( I have no experience with 4 holer ) When I'm plugged into the normal channel, the bright volume adds a fair amount of noise.
Tomorrow I'll swap / change the preamp tubes.

[sluckey]

I measured 340V DC at pin 6 of V2. Is it risky ?

No. That's a cathode follower.

[warioblast]

I changed the preamp tubes without results.

The bright channel is noisy. Normal channel is quiet until I turn up the bright volume. I'll try to move ground wires around. http://forum.metropoulos.net/viewtopic.php?t=16569
I looked at my wiring and realized I added an extra ground connexion on the normal channel. 

The action of the presence control is very subtle. You really need to listen carefully to hear a change. Although when the master volume is set low, turning the presence makes a quite audible 'washing' sound throughout the taper of the pot.

I attached the schematic I used. All stock values except for the power supply circuit.

Also I wish the amp had more gain. I was afraid the JMP50 would be to much. I think I'm going try the 1986 specs as I do have the few different resistors needed.

[sluckey]

The bright channel is noisy.

The only difference between channels is a 556pF cap. Remove that cap. Still noisy? If not, replace the cap. If still noisy, check your grounds and wiring for the bright inputs and volume control.

The action of the presence control is very subtle. You really need to listen carefully to hear a change. Although when the master volume is set low, turning the presence makes a quite audible 'washing' sound throughout the taper of the pot.

That's typical for that presence circuit. I don't see a master volume???

Also I wish the amp had more gain.

That's not a high gain amp. Neither is the 1986 Bass amp. If you want more gain, rewire the input jacks and V1 circuit like the JCM-800 2204. Then you'll have a choice of high gain or like it is now.

[warioblast]

The bright channel is noisy.

The only difference between channels is a 556pF cap. Remove that cap. Still noisy? If not, replace the cap. If still noisy, check your grounds and wiring for the bright inputs and volume control.

The action of the presence control is very subtle. You really need to listen carefully to hear a change. Although when the master volume is set low, turning the presence makes a quite audible 'washing' sound throughout the taper of the pot.

That's typical for that presence circuit. I don't see a master volume???

Also I wish the amp had more gain.

That's not a high gain amp. Neither is the 1986 Bass amp. If you want more gain, rewire the input jacks and V1 circuit like the JCM-800 2204. Then you'll have a choice of high gain or like it is now.

I forgot about the PPIMV I added.


I'm not into high gain and certainly not a '80s guy ; I'm a classic rock guy.
I don't have access to a lot of amps, so my amp education comes from recordings and internet. Like I said I was afraid the JMP50 would be too much; most of the videos I saw were made by 80's shredders. The JTM50 seemed like a good middle ground between the JTM45 and the Super plexis; I'm a big Hendrix fan. I also read that as the years went by, Marshall amps became gainier.
So I'm comparing my build to the few JTM50 videos on youtube, and my amp doesn't sound as gainier. I'd like a bit more gain but I'm pleased with what I'm hearing so far and I can say that my build has more Hendrix vibes than my DSL.

BTW Sluckey I really appreciate you helping me. 

[warioblast]

I fixed my hum problem. 
The wire that I thought was an extra was actually my ground connexion to my buss wire I added because I couldn't solder the wire to the back of a pot. I had used the chassis ground lug that was closer to the board, and I also had soldered the ground from the 820r & 220uf cap to that lug.
So I moved the 820r & 220uf ground to the buss wire, unsoldered the ground connexion of my buss wire from the normal input jack. Pretty much like the page from the metropoulos forum I posted, I added a wire to the 'presence' side of the buss that I soldered to the ground lug where the rectifier was connected.

For future reference:
_ how the hell do you solder the buss wire to the back of pots ? I tried to scrape the back with a knife, I also tried sand paper. I heated my soldering iron up to 400°C / 752° F. I stopped trying before I ef' up my pot.

_ how do you avoid solder flux projections and keep the board clean ?

[pompeiisneaks]

For future reference:
_ how the hell do you solder the buss wire to the back of pots ? I tried to scrape the back with a knife, I also tried sand paper. I heated my soldering iron up to 400°C / 752° F. I stopped trying before I ef' up my pot.

You don't.  There's no need to solder them to the back, and it can damage some pots.  The ground connection should come from one of the connection points if the pot is supposed to be grounded.  I've read and been told here that soldering to the back of the pots is actually a really bad practice.

_ how do you avoid solder flux projections and keep the board clean ?

It's mostly about cleanup, you can't really avoid it a ton.  I always have it, and am sometimes pretty lazy about cleaning it up, but I asked this same question ona previous thread and was told that denatured alcohol is great for cleaning it up. 

Here is one thread about it:

http://el34world.com/Forum/index.php?topic=20380.msg214633#msg214633

but I think that part of what causes a lot of flux splatter is just keeping the tip on there too long and putting in too much solder.  I'm still learning that technique, but basically, wet the tip well, get good enough contact to start melting the solder and pull the tip away quickly as soon as the solder flows into the joint.  Then you don't over do it. 

I still over do it from time to time, but I'm getting better.

[whoops]

how do you avoid solder flux projections and keep the board clean ?

I don't avoid flux projections thats part of soldering but I clean them in the end with this product, it works great:



http://www.kontaktspray.hu/en/?Products:Cleaners_and_contact_maintainers:Kontakt_PCC_printed_circuit_board_cleaner


Congratulations on your build, it looks great.

I cannibalized and amplifier in the past and always regret it, I guess it's because I was never able to make the amp work properly, and then I missed my old amp, so when you started this thread I had a bad feeling that it would not turn out good, fortunately my feeling I was wrong and you did a great job. You made me realize that I need to Troubleshoot and Finish my canibalized amplifier.
 

[tubenit]

Also I wish the amp had more gain

Obviously switching to a JCM 800 where you have another gain stage would work.

Another option is to parallel the triodes of V1 tube which will give you about 30% more gain with little to no increase in idle noise.  If you added 3 mini-toggle switches, you could recreate something very similar to both original channels.

With respect, Tubenit

[warioblast]

Also I wish the amp had more gain

Obviously switching to a JCM 800 where you have another gain stage would work.

Another option is to parallel the triodes of V1 tube which will give you about 30% more gain with little to no increase in idle noise.  If you added 3 mini-toggle switches, you could recreate something very similar to both original channels.

With respect, Tubenit

That looks interesting, thanks. 

how do you avoid solder flux projections and keep the board clean ?

I don't avoid flux projections thats part of soldering but I clean them in the end with this product, it works great:



http://www.kontaktspray.hu/en/?Products:Cleaners_and_contact_maintainers:Kontakt_PCC_printed_circuit_board_cleaner


Congratulations on your build, it looks great.

I cannibalized and amplifier in the past and always regret it, I guess it's because I was never able to make the amp work properly, and then I missed my old amp, so when you started this thread I had a bad feeling that it would not turn out good, fortunately my feeling I was wrong and you did a great job. You made me realize that I need to Troubleshoot and Finish my canibalized amplifier.

Thanks for the kind words. I was lucky it was an easy fix. Good luck with your troubleshoot. With all the helpful and knowledgeable guys here you should make it.