JTM 45 build - in an old combo amp

[JeffBro]

hey Fellas. I was thinking of gutting my Marshall Valvestate 8080 80w and putting in a JTM 45 (which I guess is the same as a 5F4 circuit or Super Tweed). I was looking at MojoTone for a basic kit and instructions to build it.

Few things I was thinking of doing(probably mistake)
-Dual speakers (right now there's a single 15" in there) 22" across.
-maybe use the same transformer(it is a solid-state amp) and reverb tank
-use the same knobs(potentiometers), faceplate, AC plug
-Single input (not sure why they have 4 inputs in the Super Tweed or JTM 45)

the chassis of the Marshall 8080 is 21.5x9x3.5"
it has a strange tilt to it, so part of the it is deeper that the other section(seen in the last photo)

any comments are welcome.

thanks!

 

[Paul-T]

Fairly certain the Valvestate transformer won't work; even if the secondary voltage is sufficient there won't be enough of a heater supply as you'll be running a lot of tubes. When I checked there's a shortage of those Valvestate autotransformers as they blow often so you might be able to raise some cash selling it.

AFAIR the JTM45 is basically a 5f6a circuit with ahigher gain preamp valve - 12AX7/ECC83 - and slightly different negative feedback value. Rob Robinette's pages should tell you pretty much all you need

 

[JeffBro]

Fairly certain the Valvestate transformer won't work; even if the secondary voltage is sufficient there won't be enough of a heater supply as you'll be running a lot of tubes. When I checked there's a shortage of those Valvestate autotransformers as they blow often so you might be able to raise some cash selling it.

Thanks for the info. I'll budget for a transformer and see if I can sell it on Reverb.

You ever heard of someone drilling into the chassis to make modifications?

 

[dougsta]

The chassis can be used but yoy will have to do some metalwork.

I did a conversion recently - an old 75W solid state 12" combo amp updated with a full high voltage tube plexi type pre-amp, Ch I Ch II (with jumpering and cascading) Bass/Mid/Treble, the tube pre-amp output gets fed into the solid state amp clean channel with its 4 band eq/effects send/return built in reverb/master volume still working.
Detailed thread with the final sound/video here:

She won't win no beauty pageants but let's get her sounding good!

So I saw an add for a 75W solid state amp, 1x12" combo inc reverb tank, 1980's vintage, working but some pots have issues, free to a good home... my kind of price and only a 5 min drive :) At minimum it's got a speaker in a cab, 75W of pedal platform, good sized power transformer I could rewind...

www.tdpri.com

Doug

 

[dougsta]

Here's some photos of what you can do with solid state amp chassis/cabs. This Fender Showman 212R needed a lot of creative metal work to get it tube ready, the original chassis isn't rectangular at its base, it slopes front to back.

Doug

 

[tubedude]

Here's some photos of what you can do with solid state amp chassis/cabs. This Fender Showman 212R needed a lot of creative metal work to get it tube ready, the original chassis isn't rectangular at its base, it slopes front to back.

Doug

When people hear that amp I'm sure they're confused!
Hopefully they won't go out and buy one!

 

[dougsta]

When people hear that amp I'm sure they're confused!
Hopefully they won't go out and buy one!

I'll print a label to stick over the Showman 212R on the front panel, I've yet to wind the choke/transformers for it, will be a 50W vibrolux type circuit with 2x12" to shift lots of air.

 

[tubedude]

Winding your own iron? Man that's a skill to have. Power xfmrs also? That must take some patience.

 

[dougsta]

Winding your own iron? Man that's a skill to have. Power xfmrs also? That must take some patience.

It does take a while, put some good music on, take a coffee/tea break...

Not too hard to do, start with an old power transformer, take it apart. Here's a video I did on how to get the 1st lamination out (1st one is always a PITA) and a cheapo turns counter.



Here's a $1 counter, cheapo calculator, micro switch and a DIY cam made from a washer:



Doug

 

[tubedude]

It does take a while, put some good music on, take a coffee/tea break...

Not too hard to do, start with an old power transformer, take it apart. Here's a video I did on how to get the 1st lamination out (1st one is always a PITA) and a cheapo turns counter.



Here's a $1 counter, cheapo calculator, micro switch and a DIY cam made from a washer:



Doug

Interesting. Do you re-lacquer each lamination before re-inserting into the bobbin?

 

[dougsta]

Interesting. Do you re-lacquer each lamination before re-inserting into the bobbin?

Those old ones are quite rusty and that helps insulate the lams, I use a bit of WD40 if any are really stuck together.
Modern lams have really tough lacquer on them and doesn't come off.
After putting back together and testing them at design voltage and load I dip them in a tub of boat varnish and let them soak for a day then air dry for a week.

 

[JeffBro]

Here's some photos of what you can do with solid state amp chassis/cabs. This Fender Showman 212R needed a lot of creative metal work to get it tube ready, the original chassis isn't rectangular at its base, it slopes front to back.

Doug

yeah, that's how this is(I'm sure you've seen the photos).

What do most people do that don't have the tools to modify it? just buy a aluminum box and cut their own holes?

 

[dan40]

-use the same knobs(potentiometers), faceplate, AC plug
-Single input (not sure why they have 4 inputs in the Super Tweed or JTM 45

Most of my builds are done in repurposed chassis. It takes some very careful thought as to how to lay it out and drill everything so that it all fits well. Be sure to look at pics of original JTM's and try to follow that layout to a T. Changing things up and moving stuff around too much could result in a noisy amp. You will also need to invest in some hole punches or large step drill bits to make the holes for the sockets. You will also need some decent drill bits for all of the other holes. Metal chassis can be tough to drill and cut so I try to work with aluminum whenever possible.

It's doubtful that the potentiometers will be of the right values to work in a tube circuit but you may get lucky. Pots are cheap so I always start with new ones. The four inputs are there because these circuits have a bright channel and a normal channel. Each channel also has a high input and a low input which attenuates the signal a bit compared to the high input. If building a JTM 45, you will need at least one input jack for each channel. Most folks choose to use the "high" input jack if they are doing a single input build so you would have a high input for the normal channel and a high input for the bright channel.

If this is your first build, trying to repurpose a chassis like this can be quite challenging. Starting with a kit will teach you a lot and set you up for success when you are ready to tackle this build. Be sure to get a quality soldering station with temp control if possible. It will make soldering much easier for you.

Edit: I forgot to mention that you will want to pick one layout to work from and follow it closely. Trying to mix and match from different layouts can have pitfalls that will make the build more difficult.

 

[corliss1]

First question - does the current amp work? If so, you should sell it instead of gutting it because...

-maybe use the same transformer(it is a solid-state amp) and reverb tank

That transformer won't work for a tube amp

use the same knobs(potentiometers), faceplate, AC plug

those pots are PCB mounted, and not going to be something you want to try and take apart and use in a new, hand-wired build. The faceplate isn't going to match the new controls, so I don't see the point.

Single input (not sure why they have 4 inputs in the Super Tweed or JTM 45)

the multiple inputs go to different channels in the amp.

It sounds like this is your first build. You'll want to do some more homework before you start, and it will go MUCH better if you get parts that are already designed to work together, such as a pre-drilled chassis with holes that fit the transformer and pots. If you've never done this before, you'll soon realize you'll have as much time into the chassis work and cabinet as you will into the electronics.

A kit is a great way to go as everything will be included and will fit together. Mojotone kits are excellent.

 

[JeffBro]

I forgot to mention that you will want to pick one layout to work from and follow it closely. Trying to mix and match from different layouts can have pitfalls that will make the build more difficult.

Yeah I seen a few people submit schematics for new designs and people here shoot it down pretty easily.

Seems like a kit might be the best. do you have any recommendations as what kit to attempt?

reading on the MojoTone they say the Blackface Twin is the hardest to build. Do you know why it's the hardest to build or building a Dumble version of the fender circuit take a very high skill set or freidman has a Dirty Shirley modded version of the JTM45, would either of those be much more complicated?

 

[Peegoo]

maybe use the same transformer

If you have a transformer from a solid state amp with adequate voltage for the rectifier but no secondary for the heater voltage, you can install a 2nd smaller transformer like a 9v or 12v and step it down to the 6.3v you need.

 

[JeffBro]

First question - does the current amp work? If so, you should sell it instead of gutting it because...

Yes. I don't like the sound, it's very mushy. I think it's worth $300. Seems like a cool fun project and I never play the amp.

the multiple inputs go to different channels in the amp.

The amp is 2-channels and each has a separate jack with a 1m resistor that lowers the volume/signal of input on those two jacks?

One channel is controlled by the normal volume and the other by the bright-volume?

It sounds like this is your first build. You'll want to do some more homework before you start, and it will go MUCH better if you get parts that are already designed to work together, such as a pre-drilled chassis with holes that fit the transformer and pots. If you've never done this before, you'll soon realize you'll have as much time into the chassis work and cabinet as you will into the electronics.

A kit is a great way to go as everything will be included and will fit together. Mojotone kits are excellent.

yeah, I've seen people on here recommend them as having the best kits. What's your opinion on using Sozo or Jupiter caps?

If I'm going to make a combo amp and the schematic shows speaker jack output can you disregard the jack and go straight to the speaker? and how to set the impedance of the output and does the impedance make a difference in the sound?

 

[JeffBro]

If you have a transformer from a solid state amp with adequate voltage for the rectifier but no secondary for the heater voltage, you can install a 2nd smaller transformer like a 9v or 12v and step it down to the 6.3v you need.

sounds like something that might get me electrocuted.

 

[corliss1]

When set properly, impedance doesn't change the sound.

You could go straight to the speaker, but it's still best to use a jack. That way the amp isn't hard-wired to the speaker in case you need to change things out, or want to play the amp through a different speaker without having to disassemble things.

Brand of caps doesn't really matter. The Mallory 150s that everyone uses are great. You're paying a bunch more for the big brand ones that "look" vintage.

 

[dan40]

reading on the MojoTone they say the Blackface Twin is the hardest to build. Do you know why it's the hardest to build or building a Dumble version of the fender circuit take a very high skill set or freidman has a Dirty Shirley modded version of the JTM45, would either of those be much more complicated?

The twin has ten tube sockets, a tremolo circuit and a lot of wires to keep track of. All of this adds up to a difficult build for the first time.

The amp is 2-channels and each has a separate jack with a 1m resistor that lowers the volume/signal of input on those two jacks?

The 1 meg does not lower the volume, it gives the grid a reference to ground. Each channel has it's own volume control. These amps are extremely loud and require a lot of volume before they start to overdrive. Many folks will add a master volume to the circuit to allow them to get some overdrive from the preamp without blasting the power amp which would be extremely loud. Most newer amp circuits come with master volumes for exactly this reason as the older amps were meant to be played at stadium volumes to get some breakup out of them.

If I'm going to make a combo amp and the schematic shows speaker jack output can you disregard the jack and go straight to the speaker?

Most output transformers for these amps come with the option for 4/8 and 16 ohm. As Corliss said, it would be better to install a jack and a 3-way switch for the different impedances. This gives you much more flexibility.

What's your opinion on using Sozo or Jupiter caps?

I have tried them all but I can't hear much difference between them and standard Mallory 150's or Vishay caps.

The small transformer that Peegoo mentioned doesn't deliver much voltage but you will be exposed to nearly 500v inside of a JTM 45 or other similar amp. During the build the amp will be safe but powering up and taking voltage readings after build will expose you to very high voltages. It's important to do your homework and learn to work safely around this much voltage. This is a great site to help you get started on understanding some of the basics. Our member Rob runs the site and it is chock full of great info.

How Amps Work

How Tube Guitar Amplifiers Work

robrobinette.com