New JMP 50 style build in JTM45 chassis

[Bob S]

OK Miles wins in overtime - using a duck in a box traded for breast implant material or sexual favors.
Makes sense - pass the wine bottle.

[gktamps]

Back to the build...
Parts ordered, except waiting on Marstran PT to get back into stock.
Decided to avoid the embarrassment and ridicule of using a tube recto, so am moving one can cap up top (the chassis I have would have mounted them both inside).
Had to reinforce the unwelded corners with aluminum angle pieces I made. Drilled and countersunk for rivets, chassis is now much stiffer. Big iron is pretty hard on soft aluminum chassis.
Decided I couldn't wait for, and really didn't want to pay for, high-end coupling caps, so went with CDE (Mallory type) 150. Also, no CC resistors, just CF and MF. Tell me if I'm blowing it somehow...
Rectifier diodes will go on the board, and I still haven't decided whether to mount the preamp stage filtering over the board as a can cap or on the board.
Planning on an MV, and may take Miles' advice to another poster about using both Pre and Post PI MVs and see how it goes. Not planning on using VVR at this point.
Will have NFB select switch for 2 different values.
After it's built, I'll play around with some of the great ideas for modding and voicing that Miles and many others have shared here recently and in the past.
Now it's hurry up and wait...
Thanks for the help so far,
Greg

[M Fowler]

You could have used aluma-weld on the corners. I haven't had any problems with unwelded chassis though.

I'm puzzled why your re-inventing this amp it has been built many times over with tried and true layout all over the web.

Also, if your going to go with expensive Marstran transformers why not make the amp as original as you can by using correct resistors and coupling caps sourcing NOS mustard caps?

When I go cheap I go all the way, Magnetic Component transformers, mojo chassis/plates, NOS cc resisters, M150 coupling caps, etc.

[gktamps]

Hi Mark,

Forgot about aluma-weld, thank you.

I hear you about reinventing the layout when it isn't necessary. I studied all of the various examples on Brian's spreadsheet of historical data of different models, and found that there are many variations on the 50 watt layout, including everything I am considering, except for the aluminum chassis which went to steel after awhile. They are all over the board - two can caps on top, tube recto, ss, filter can cap on the chassis over the board, caps on the board, etc. Looking at kits and published layouts of other builds, they are almost as varied.

http://www.marstran.com/Historic%20Data%20Base.htm

I was just trying to pick the version of Marshall layout that appealed to me, not too concerned about whether it is available as a kit, but using the historical examples as my basis. My last four amps have not been kits and all have been modded to my plans; I already had most of the parts for the build and make all my own boards, so thought I'd continue in that vein.

But you are making a good point that maybe I am going far enough away from what is commonly built and not going top shelf or correct with coupling capacitors and resistors, so maybe the expensive transformers are just not the best choice.

Thanks for your input, Mark.

[gktamps]

Here's what I'm trying to avoid by reinforcing unwelded chassis. This was shipped well packed but the box dumped on end, so weight of transformers did its thing...

[M Fowler]

I was just looking at a brand new Splawn Quick Rod with leaning OT from shipment. I know Scott Splawn switched to laydown PT because of that reason maybe he needs to go to using a laydown OT now.

[JazzGuitarGimp]

I was just looking at a brand new Splawn Quick Rod with leaning OT from shipment. I know Scott Splawn switched to laydown PT because of that reason maybe he needs to go to using a laydown OT now.

Or a stronger chassis!

[Richie]

I don't think it matters a lot. I've seen steel chassis a lot worse than those pics. And like fenders, since they are mounted with transformers hanging down, The weight of the transformers pulls the chassis, which make a rounded,instead of flat chassis. Then that makes the tubes lean. And many times the whole top fall in,chassis still mounted to the board. The sides of the cab spreads apart.

[Reeltarded]

A major is made of angle iron. You should see a dropped one's OT. 45° haha

Not here. Someone else. Swear.

[gktamps]

Waiting on parts, with a 2+ week delay on the PT, so only got the board cut, drilled, turretted, and a few components placed, plus a little work on the chassis. Should have planned ahead, so all parts could have been on hand...

[statorvane]

Wow - you planned that layout very well. Looking great.

[gktamps]

Thank you, but I can't take credit for the board layout, just the implementation (it's nice to have a lathe/mill to drill and stake). I used the Tubedepot layout with a few minor changes to accommodate the rectifier diodes and hv hookups. Made it easier than my last two builds - there I did have to lay out the boards to accommodate my circuit changes.

[gktamps]

Delete

[gktamps]

If I had done a better job putting together my BOM, I'd have all my parts by now, except the out of stock PT, so progress is slow.

Here's where I am now, though. Figured I'd build it pretty straight to start, then tweak and mod one step at a time. Starting with a 3H choke, but I drilled the chassis for a 10H swap if I feel like it.

Went with Classic Tone transformers for this build, and used up some NOS film caps and on-hand electrolytics in parallel combos to arrive at stock V1a/b cathode cap values. Didn't have a PT chassis bracket, so made one out of aluminum for good reinforcement.

I've been a fan of star grounding for awhile, so will do away with the more common bus bar, and place grounds at each node.

Hopefully the PT will arrive in a week, then I can finish this up!

[gktamps]

Finally getting more work done - had to wait 4 weeks for the PT. Spent that time voicing 3 other amps and starting another build, but now I'm moving along with this one.

Grounds will follow the Larry scheme, or closely at least.

Had to reposition the OT and choke positions because the PT was quite a bit larger than the JTM45 version the chassis was drilled for. Isn't going to be the prettiest or most professional looking build from the outside, but it's what's inside that counts, right?