And here's the output transformer

JTM/JMP switchable Plexi?

peterleroux wrote: JTM/JMP switchable Plexi?

?

4 big bottles of pain though... How are you going to reign her in?

psyx wrote: 4 big bottles of pain though... How are you going to reign her in?

Only 2 big bottles and they will be KT66. The other 2 big holes in the chassis is for a can capacitor and the valve rectifier (GZ34). I haven't finished the schematic yet but it'll have switchable tonestack between JTM and JMP and switchable rectifier. The reigning in will be done with a reactive attenuator with a line out so I can make it quieter or silent and use software cabinet simulation.

ez wrote:

psyx wrote: 4 big bottles of pain though... How are you going to reign her in?

Only 2 big bottles and they will be KT66. The other 2 big holes in the chassis is for a can capacitor and the valve rectifier (GZ34). I haven't finished the schematic yet but it'll have switchable tonestack between JTM and JMP and switchable rectifier. The reigning in will be done with a reactive attenuator with a line out so I can make it quieter or silent and use software cabinet simulation.

Sounds like a party 8)

Pardon my ignorance. How many watts will the JTM be? What does the switchable rectifier do?

Around 35-40 watt. The rectifier changes the wall's AC power to DC, which is what the amp needs. You can make a rectifier very simply with 4 diodes (for like 20c) or you can use a valve (for many ZAR). Now, wheny you crank the amp a bit and you hit the strings hard, the amp needs more current than when you play softly. The valve rectifier is worse at providing this current than the diodes, they give a bit of sag. The diodes don't really. So the difference between the two is how 'sudden' the attack of the notes feel. Most modern amps have SS rectifiers. Old school amps will have valve rectifiers. In the case of the amp I'm building here, the JTM45 had a valve rectifier and the JMP (model 1987x) had diodes, so I'm putting in both ?

ez wrote: Around 35-40 watt. The rectifier changes the wall's AC power to DC, which is what the amp needs. You can make a rectifier very simply with 4 diodes (for like 20c) or you can use a valve (for many ZAR). Now, wheny you crank the amp a bit and you hit the strings hard, the amp needs more current than when you play softly. The valve rectifier is worse at providing this current than the diodes, they give a bit of sag. The diodes don't really. So the difference between the two is how 'sudden' the attack of the notes feel. Most modern amps have SS rectifiers. Old school amps will have valve rectifiers. In the case of the amp I'm building here, the JTM45 had a valve rectifier and the JMP (model 1987x) had diodes, so I'm putting in both ?

Like with some Mesa Boogies too... you have the option to have a tight attack or some sag

Exactly. It makes the build a bit tricker since the biasing circuit needs a tweak between the SS and Valve rectifier. The power supply rail to the rest of the amp runs 10-20V higher on solid state, so the switch must be a triple-pole dual throw (or on-off-on) so that I can switch that bias resistor also.

psyx wrote:

ez wrote: Around 35-40 watt. The rectifier changes the wall's AC power to DC, which is what the amp needs. You can make a rectifier very simply with 4 diodes (for like 20c) or you can use a valve (for many ZAR). Now, wheny you crank the amp a bit and you hit the strings hard, the amp needs more current than when you play softly. The valve rectifier is worse at providing this current than the diodes, they give a bit of sag. The diodes don't really. So the difference between the two is how 'sudden' the attack of the notes feel. Most modern amps have SS rectifiers. Old school amps will have valve rectifiers. In the case of the amp I'm building here, the JTM45 had a valve rectifier and the JMP (model 1987x) had diodes, so I'm putting in both ?

Like with some Mesa Boogies too... you have the option to have a tight attack or some sag

Thanks guys. It makes more sense now. I guess you are referring to the Mesa Boogie Dual Rectifier.

...one of the half dozen patents Mesa hold.

This looks like a great build. Good luck!

A couple of questions...

Are you doing the designs of the circuit tweaks yourself? Are you self taught or do you have a technical background? I've got a basic understanding in a paint by numbers type of vibe, but I'd love to get a better understanding so that I can hopefully put together something "unique" in the future...

Are you building off a basic kit or sourcing components individually?

Rikus wrote: This looks like a great build. Good luck!

A couple of questions...

Are you doing the designs of the circuit tweaks yourself? Are you self taught or do you have a technical background? I've got a basic understanding in a paint by numbers type of vibe, but I'd love to get a better understanding so that I can hopefully put together something "unique" in the future...

Are you building off a basic kit or sourcing components individually?

I'm a very technically proficient person in general but I don't know much about circuits (besides the basic sort of stuff). So basically, I'm looking at the JTM45 and 1987x schematics, noting the differences between them then seeing what I need to do to put both flavours into a single amp. It turns out that there are only a small number of big changes between them. Of course, the 1987x (50 watt JMP Marshall) uses EL34s and the JTM45 uses KT66 so I had to choose the one or the other power section and since I could get a 8kΩ output transformer for cheap I went with KT66 so this amp will be a JTM45 (with a slightly stiffer power supply) with both the Bass-spec and Lead-spec tonestacks. There are other amps like this but they're few and far between and super expensive. Its going to be a Marshall and if I do this right, it'll sound like a late 60s plexi, I'm not aiming for something unique, I'm aiming for the holy grail of rock guitar amps ?

As to kits and parts. I bought the parts from Valvestorm, taking most of their basic JTM45 parts., i.e. sockets, pots, capacitors, switches, wire, nuts and bolts etc. and adding the extra ones I will need. Its not really a kit, nothing is pre-made. The chassis has been punched but that's it, I'll need to measure and cut and drill the turretboard, work out my own layout, have a box made etc.

I bought the turret material and turrets and standoffs from elsewhere and I still need to have faceplates made (I'll know what they need to look like once I've decided which switches I need and where to put them).

The trick is that you should buy as many of the parts from the same place, to save on shipping, plus I got friends to haul back everything from the states and the UK for me. This will probably save me around R4000 in shipping for the entire project (And with the exchange rate this helps since its already quite expensive)

And by "big bottles" I mean they're huge:



Edit: I made a mistake. Here's a JTM45 with KT66s. The little ones are 12ax7s.

Cool, thanks!

Looking forward to seeing it coming together.

Yeah, if you can avoid paying shipping for all the heavy magnet and iron bits that's a huge saving!

Jeez I've been drooling over a Marshall. I'll blow myself to pieces though if I attempt a DIY project. Maybe I need to fund my "Marshall JTM45/Bluesbreaker" project I've been thinking about...this thread is causing major GAS!!!

Out of curiosity, not that i've built an amp before, but looking from the perspective of someone who might do it one day,what is your opinion on this? http://www.marsamps.co.za/guitar-amp-kits.html
I see they also have a JTM45 kit, and it's local so no shipping. Any particular reason for not going that route?

If you would then like to modify it, you just alter the thing to what you want, depending largely on your intention for this specific amp I suppose.

Eujean wrote: Out of curiosity, not that i've built an amp before, but looking from the perspective of someone who might do it one day,what is your opinion on this? http://www.marsamps.co.za/guitar-amp-kits.html
I see they also have a JTM45 kit, and it's local so no shipping. Any particular reason for not going that route?

If you would then like to modify it, you just alter the thing to what you want, depending largely on your intention for this specific amp I suppose.

I have little knowledge of Karel Mars' amp kits. I know lots of people have built them and I know Karel is a very knowledgeable guy regarding tube amps. I have no idea if they come with instructions or not. If they don't and its your first amp, I would advise against something complicated like a JTM45 although I do understand that first building something simpler doesn't necessarily fit the budget. My first amp was quite complicated and I built it without instructions and miraculously it worked (and still works, 4 years later).

The Mars kits are slightly more pricey than what I'm paying since I'm avoiding paying for shipping (friends flying my stuff back in their luggage) plus I chose very specific parts based on a variety of highly technical specifications, like size and colour of the components (I'm serious). Also, the Mars kits would have incurred shipping costs on thir way here, so you are in fact paying for that (It makes a huge difference).

Another thing to keep in mind is that a JTM45 is basically a very loud clean amp without some technology to keep the volume down. You cannot overdrive them at any reasonable volume unless you install extra tomfoolery into the circuit that will make it behave in ways not entirely intended by the original designers or if you get an external device that will eat up the extra power. I opted in this case for an external device ?