JTM45 / 50 Build

Help Support GroupDIY:

Spencerleehorton

Well-known member
Joined
May 12, 2012
Messages
3,251
Location
Felixstowe, Suffolk, UK
I'm working on the fact that there is 125ma current and this wont change.
So adjusting the caps and resistor dividers up until there is current draw to pull the voltage down?
 

Attachments

  • 20210318_083617.jpg
    20210318_083617.jpg
    443.7 KB · Views: 10

john12ax7

Well-known member
Joined
Oct 14, 2010
Messages
1,851
Location
California, US
Just to be clear in terminology,  what you had originally posted is a layout, not a schematic

The resistor values are not critical,  just make sure they have sufficient power handling.  220k is fine,  470k is fine.

I'm not sure about the 20k, are you not using a choke?  Most of the current in an amp will go to the power tube plates.  Not that much to the screens and preamp tubes.

Attached is a jtm45 schematic.
 

Attachments

  • JTM45Classic2b.pdf
    474.1 KB · Views: 11

Spencerleehorton

Well-known member
Joined
May 12, 2012
Messages
3,251
Location
Felixstowe, Suffolk, UK
The only thing I'm worried about is if its 520v dc after the voltage divider it would still be over the 500v rating of the dual cap 500v which is before the choke, that's why I did the second drawing.

So if I understand this correctly putting in the voltage divider in place of the dual cap 32uf 500v before the choke?

Would I then have to adjust anything else?

Is there a drawing or image which I could see which is the correct way?
 

john12ax7

Well-known member
Joined
Oct 14, 2010
Messages
1,851
Location
California, US
Looks fine. Some notes,  which you already might be planning on.  You also want to fuse the live.  The earth ground should be it's own dedicated chassis connection.  The 220k should be at least 0.5W, really all tube amp resistors should be 0.5W or more.

You will have high plate and screen voltages,  so you need to pick power tubes that can handle it. The preamp tubes will be high voltage as well. Overall this will end up a rather clean amp with headroom.  Cleaner than a typical jtm45.
 

Spencerleehorton

Well-known member
Joined
May 12, 2012
Messages
3,251
Location
Felixstowe, Suffolk, UK
Yes sorry, mains fuse and HT fuse will be used.
Yes to earth connection at IEC socket on chassis.
I'm using all 2w resistors apart from the ones needed to be higher.
I'm using EL34 power tubes, I think they are good up to 800v.
I'm interested in how it will become cleaner than normal jtm45, as I was hoping this would distort nicely.
Could you explain why it would be cleaner?
Is it because of less sag?
 

Spencerleehorton

Well-known member
Joined
May 12, 2012
Messages
3,251
Location
Felixstowe, Suffolk, UK
I've done some calculations, dont know if they are correct but they do seem consistent to other documented voltages.
B+1 = 518v to CT of output txf
B+2 = 516v to EL34 screens
B+3 = 396v to V3
B+4 = 356v to V2
B+5 = 336v to V1
 

john12ax7

Well-known member
Joined
Oct 14, 2010
Messages
1,851
Location
California, US
Some of the 6L6 variants are limited to 500V plate voltage or less.  EL34 will be ok,  but be mindful of the screen voltage which still might be limited to 450V, so you need appropriate resistors feeding the screens.

Generally a lower B+ voltage will have earlier breakup.  A typical jtm45 might use 400V, so 500V will have later breakup.
 

Walrus

Well-known member
Joined
Jul 18, 2006
Messages
780
Location
London, UK
Spencerleehorton said:
I think I've located a fairly local company who can wind me a new mains txf, output txf and choke for a good price.
Sounds interesting! Can you name names?  :)
 

Squeaky

Well-known member
Joined
Feb 7, 2014
Messages
237
Location
New Zealand
I have built one of these amps (from an JTM50 schematic that was a GZ34 valve rectified version). I wouldn't call it a clean amp, it is a pretty classic Marshall sounding amp and rocks if you like that sound. It is probably a bit crunchier that a JTM45 if that makes any sense. Nothing like a Hiwatt though. I think the JTM50 has really high V1 gain (big value bypass cap)? The one I built was compared to an original and the sound of my amp was pretty spot on so I was happy with that.

I am glad to see that you decided to use a fit for purpose power transformer. The power transformer that I used had three different HT taps so you had a choice which voltage you wanted to go for. It was a 50W Marshall type power transformer that tried to cover off on a range of the different HT secondary voltages that had been employed over the years. The company unfortunately shut down recently. I wish I could still play this amp (too loud for me now and I don't gig anymore). It sits around gathering dust.

The only problem I had, which I believe might also have been true to character of the originals, was some feedback/microphony problems when the presence setting was up pretty high. I fixed that using a sheet of aluminium, as a shield, attached to the cabinet (inside) underneath the chassis. The fact that I used a massive cap dangling from the pot with long leads probably didn't help. I think you also need to watch the run of the feedback wire.

As you can see I like using WMF capacitors. These are film/foil polyester capacitors. The same sort of caps would have been found in the originals (probably Mullards).

Looks like I used a veritable lolly scramble of different resistor types. I had a thing for 1W CC for a while. Doesn't seem to be much rhyme or reason to it.
 

Attachments

  • JTM50-3.jpg
    JTM50-3.jpg
    375 KB · Views: 25

Spencerleehorton

Well-known member
Joined
May 12, 2012
Messages
3,251
Location
Felixstowe, Suffolk, UK
Thanks for sharing your build, ive just cut the chassis and hopefully getting a price on these transformers from Demeter windings, im hopefully going to start with this JTM 50 plexi 68 lead style build but may experiment with another turret board and mod up to something like a chupa!!
 

Squeaky

Well-known member
Joined
Feb 7, 2014
Messages
237
Location
New Zealand
Great to see you are even building the chassis. I wish I had the wherewithal to build chassis. However if I did, I'd probably have many more projects lying around! I read earlier in this thread consideration of whether to use large film coupling caps for cathode bypass. I bought this capacitor years ago for that purpose. It is only 20uF, but 80mm x 40mm x 50mm in dimension. Close enough in value to Vox amps though. More of a gimmick than anything else really, who knows what is inside the steel case.
 

Attachments

  • 20uf.jpg
    20uf.jpg
    1.8 MB · Views: 15
Top