Tubedepot Jtm45+ Conversion To A Jtm45

Discussion in 'Building the Classics' started by Garrett, Mar 16, 2017.

  1. Garrett

    Garrett Active Member

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    Built this kit a few years ago and it's a nice sounding amp and solid pedal platform, but it is not a JTM45. Not even close.

    Recently purchased a Budda SD30 so the high gain territory is covered.

    So I got looking at the TubeDepot JTM45 schematic versus the original Marshall circuit. Wow, not even close. The TubeDepot design is a mix of a JTM50 and some later component values here and there. The only thing JTM45'ish about Robert Hull's design is the tube rectifier.

    When I have a bit more time I'll list what the TubeDepot kit used and what the real JTM value was. Purchased some nice components from Valvestorm, need a few more but for the most part it's almost there.

    The original build;
    [​IMG]

    Newly installed components;
    [​IMG]

    The kit was designed for EL34's. But the problem with this kit is the high B+ voltage at ~490V. The assembly manual stated it should be 480 V but most people that have built this kit state it's a bit over 490V. The original running KT66's are around 450V from what I've seen on line.

    This kit will eat EL34's with the stock 470 Ohm grid resistors at about a rate of once a year with moderate home playing. The 470's get stressed and have to be changed as well. Reading online, many kits use 1K resistors to alleviate the issue. The g2 voltage on Pin 4 is pretty high for modern tubes.

    Will be trying a set of 6CA7 tubes this time as they are suppose to be a hybrid of the EL34 and 6L6.

    If I like what I'm hearing I may get an OT for KT66's and make the full changeover.

    Will update this weekend.

    Would also appreciate any experiences other people have had with their TubeDepot JTM45 kits.
     
    Last edited: Mar 17, 2017
  2. Matt_Krush

    Matt_Krush Well-Known Member

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    470 ohm resistors are for 5881/6L6s....or the KT66's

    If you want to run EL34's/KT77s or 6CA7s, you need to change from 470ohm to 1k-1.5kohm.

    El34's can handle the 490vDC plate voltage without issue.

    That's the difference between the JTM45 (5881/6L6) & JTM50 (EL34/6CA7).

    You may have to change the bias resistor if you can't get enough variance through the range.
     
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  3. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    One step further; KT66.

    Still you have a modern day JTM45 unless all old stock parts are sourced. That will make a difference in final sound as well.
     
  4. jensvonbustenskjol

    jensvonbustenskjol Active Member

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    Very interested indeed.
    I don't know if the kits out there has other circuits due to copyrights or something. I do know that the outcome of your new amp is very interesting. Can't wait to see the complete component list, measured values and finally sound :)

    Good job, and good luck!
     
  5. slater529

    slater529 New Member

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    As a Tube Depot JTM45+ owner, I'll be watching this thread!
     
  6. Garrett

    Garrett Active Member

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    Had to take the wife to the ER for a torn up knee Friday, haven't had time to update.

    This forum has plenty of people versed far better than I in the history of the JTM's. Correct me if I'm wrong but from looking around on the internet the first few JTM's built were basically the Bassman circuit using 6L6's. Then they were changed to KT66's. And some would suggest that the last few built were used EL34's. I have no idea of the validity of that information.

    NOS Allen Bradley resistors and Royal Mustard's replaced metal oxides and Sozo's. I have left the 50uF x2 filters in the amp due to the better filtering compared to the original.

    I referenced this schematic for the JTM;
    http://raw-sewage.net/images/jtm45-readable.jpg

    This is the TubeDepot schematic;
    https://s3.amazonaws.com/tubedepot-...d_files/jtm45plus_schematic_v4.pdf?1391458123

    Plate voltage has dropped since the original build. The B+ with the tube rectifier is stated as 490Vdc in the assembly manual, but this time I measure 470Vdc. Nice that it's lower but not sure why it changed that much since the initial build. Didn't have another GZ34 on hand to swap out and verify.

    Ran the amp with EL34's and 6CA7's. The 6CA7's are exactly as the descriptions depict. Lower bottom end that isn't spongy with nice highs. A mix between the two tubes but with higher cleaner headroom. Running through a 4 x 12 with WGS ET-65's and Vintage 30's.

    The amp is what you'd expect with very, very nice cleans. Not Fender Bassman 3D cleans but very nice.

    Takes pedals well as the circuit is known for and a great pedal platform. The amp now sounds rounder, warmer and with a thicker bottom end. No surprise when the original coupling cap values are used and the bottom end holds together nicely unless the bass is pushed.

    Using just an OCD and Bluedriver-2 the amp the amp sounded great from old Metallica to Audioslave. And of course AC/DC sounds glorious, that is the voice this circuit does so very well.

    Natural breakup is nice but this amp has to be cranked to get there and it's painful when you get there. Sounds great but sickening loud. I have a PPIMV on it but don't care for it, does choke it down. Usable but don't expect AC/DC at bedroom volumes. Forum member Harleytech suggested trying the Rich PEC mod.

    In the end it sounds like a Marshall. I personally prefer the tone of the amp now versus the TubeDepot design, it's fuller and richer but with less gain. There's nothing wrong with the circuit mods they did as many were found in later Marshall designs. But TubeDepot is not selling anything close to the JTM45, they should call it a JTM50+.

    Still have much to learn about amps but am enjoying the process. Also have a Tubedepot 5e3 Deluxe and that amp sounds great but will have to verify it's component values as well. May change it may not.

    A couple more itches to scratch for sure. A Bluesbreaker build will happen one day but first will be the 5f1!
     
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  7. neikeel

    neikeel Well-Known Member

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    Agreed on the correct name for the TD version, nothing like a JTM45.
    The JTM45s never had EL34s. The JTM50 introduced the 784-128OT when the EL34s were introduced and they had different mixers and slightly less NFB. There is an odd JMP50 that I have a version of (the one with block end alloy chassis and EL34s that some people confuse with a JTM45).
    I have a couple of KT66 JTM45s with B+ over 450v and they are stunning with GEC KT66s. Might be a good project for you next (most of the BB Trem clones don't work very well btw).
     
  8. Garrett

    Garrett Active Member

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    Sad news on BB clones.

    I like the sound of 6L6's, friend has a late 60's Basemen, huge clean and overdrives nicely. But I do like the Marshall for grind. Does the Marshall tone still come through with KT66's?
     
  9. neikeel

    neikeel Well-Known Member

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    Well set up KT66 sound very nice. At lower volumes they have nice chimney cleans and have smooth break up and plenty of grind when cranked. I have GE 6L6s in my offset clone and
    I'm not sure if it is the amp being new but my 64 and 65 45s are a lot more complex and interesting. The offset does what it is supposed to but sounds a bit modern.
     

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