Building my non traditional JTM style combo amp

Discussion in 'Building the Classics' started by SnickSound, Jun 1, 2020.

  1. SnickSound

    SnickSound Member

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    I'm streamlining my rig. I have my homebrew 2204 head+cab for dirty tones, and I don't like what is currently my clean amp (it started life as a Supro ,then got Blackface'd, but there are too many limitations with its basic architecture so I'm starting over again).

    So I'm rebuilding my clean amp, which has a single Greenback in it, and the transformer set is good for a pair of 6V6, and I like JTM style cleans and light crunch. So I'm going for something similar to a JTM50 (I prefer a bit more filtering and 470k mix resistors, based on my experience with a 6V6 JMP build). 32uF for plates, screens and PI, and two 22uF nodes for the preamp.

    KT66s are not an option due to the filament current handling of the PT (3A)

    Transformers are all from Hammond: 290BX PT, 194A choke and 1750PA. So that's Deluxe Reverb PT and choke, and Marshall 18W output.

    So already, a bit non traditional but should be a fun amp.
    Other non traditional thing is the use of radial caps for the preamp. Because you get a LOT more for your money with those (rated for 12 000 hours at 105c, very low ESR and a LOT more ripple current handling that they will ever see, they should outlast the amp). That changes the layout a bit as you will see.

    Also, there will be a split/shared cathode switch on V1, variable NFB, and a PPIMV.

    Checking my printed layout (done with DIYLC) against the real components to see if the dimensions are right:
    [​IMG]

    Drilling the board:
    [​IMG]

    As you can see the layout is MOSTLY similar to a regular JTM layout, except the radial caps are located as close as possible to their respective plate resistors (the V2 node of course feeds the cathode follower's plate directly).

    Also have a few little tricks like bias sensing resistors on the board (this being a combo, taking the back panel out to measure and adjust bias is pretty simple), also screen resistors on the board because why not.

    The bias circuit is based on the Deluxe Reverb scheme due to using a Deluxe Reverb PT and its associated bias tap (plus it's proven to work well enough for 6V6's).

    The "normal channel" cathode resistor is a 1.2k, so that when in Shared mode I just parallel with the 2.7k and get 830R. The NFB resistor is also not on the board due to the variable NFB switch that will go on the front panel.

    [​IMG]

    The populated board. I miscalculated the size of the bias pot a little, and it's casing is touching the junction of the bias sensing resistors. But that goes straight to the ground buss anyway, so worst case the pot casing will be grounded which is actually a good thing.

    [​IMG]

    Oh yeah, there's a ground buss on the board. I always build that way, laying out everything so that each node is grounded together. Then I connect to the chassis only at the input end. PT center tap will go straight to the main reservoir cap (a can cap) which will then ground straight to the turrent at the top left. This prevents any ground loop and keeps large currents away from the more sensitive early stages.

    The chassis I will use, a Hammond 1444-1763 (same as what's already in the cabinet):

    [​IMG]

    [​IMG]

    The cabinet (with the old amp chassis still in). It doesn't look like much, but I love this cabinet. I often use it with my head cause it sounds really good as a normal speaker cabinet. I will likely clean up the routing of the venting holes a bit, and might go ahead and tolex it up (I've never done tolex before).

    As mentioned, it's loaded with a 25W Greenback. Maybe not ideal for a "clean" amp, but at least my clean tone doesn't clash with my dirty tone too much. I actually like how it sounds clean.
     
  2. pleximaster

    pleximaster Well-Known Member

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    811CFAC7-A402-4193-9344-674E056F20AC.jpeg I will follow your project with great interest!

    by the way just for history telling

    Marshall did offer a Jtm45 layout (non tremolo) in 1964 as in this old newspaper ad.

    Combined speaker and amp unit.... 811CFAC7-A402-4193-9344-674E056F20AC.jpeg
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    Too bad about using the aluminum chassis.
     
  4. SnickSound

    SnickSound Member

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    Yes, I've heard of non-Bluesbreaker JTM combos before. I think I've seen one used live once, but dude was running a Zoom effects unit for his drive sound which made me sad lol.

    The "non traditional" part is mostly about the fact that I'm using my own circuit layout, also 6V6 power tubes and a mix of Deluxe Reverb and 18W transformers (it's what I have on hand and will do fine for the intended use). Basically, I have no illusion of this sounding like a real JTM, but should have the overall tonal signature. I won't be driving it hard which is where the bigger transformers and KT66 tubes would start showing their color.
     
  5. pleximaster

    pleximaster Well-Known Member

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    Your build is very professional looking

    locking forward to sound clips!

    plexi
     
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  6. Big Mike

    Big Mike Member

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    Why is that bad? Nothing wrong with AL chassis IMO.

    Nice layout! Interested to see the finished work.
     
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  7. Travis398

    Travis398 Well-Known Member

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    I haven't heard anyone complain about aluminum chassis for a couple weeks now, It was more common to hear about it a few weeks ago. Back then I kept my aluminum chassis in the closet, I wasn't interested in the lecture.

    Seems like we can come out of the closet now.
     
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  8. Big Mike

    Big Mike Member

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    ha! Never had an issue myself, was curious. I've built (2) JTM45's and converted both to JMP50 Bass (changed OT, new board, etc). But obviously didn't bother with the chassis. One has been lead and bass back and forth, smaller OT for 6v6's, JTM iron, etc. It's classictone JMP50 OT now, and with it at lead specs, it sounds about the same as my regular steel chassis Lead spec amp (Though that one sounds better, I attribute that to the merren Iron).

    When it comes to late 60's marshall, really I just go with Germino's lead. Greg's brilliant, nice cat, amazing amps. All his 50's and most of his 100's were on Aluminum.

    Too much hair splitting. but what do I know...I also don't see the point of vintage mustards when the Sozos and Mallory caps sound just fine.
    Plus I hate silver mica caps...and use Old ceramic disc...why old? Found a big stash of them in mom's garage from my Dearly departed Dad's ham radio parts stash.

    Here's where I really piss folks off...I don't think NOS Powertubes are worth it in most cases for the plexi's. Spend the money in teh preamp.

    Ill go stand in the corner now...:scream:
     

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