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Building a Tweed Bassman out of a JTM45 Kit?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Heinzi, Oct 10, 2018 at 1:32 AM.

  1. Heinzi

    Heinzi Member

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    Hey Folks,

    please forgive me my technical ignorance - in germany it´s quite hard to find a kit for a 5F6A Tweed Bassman but there are plenty of JTM45 Kits out there.

    Since they have quite a lot of similarities - do you think it would be possible to use a JTM45 Kit as a basis for a Tweed Bassman (Head) by changing transformers, tubes and neccessary parts?

    I´m not a technician but know a guy who could do that. What are the essential parts to change and to take care of to your opinion to get as close as possible? Also any recommendations for transformers?
    (I know there are similiar amps like the EHX MIG50, but the MIG is not available in Germany...)
    Thanks in advance!
     
  2. 79Stone1

    79Stone1 Member

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    Kinda depends on the kit but there aren't many parts that you'd have to change. Decide which power tubes you want to run and select appropriate transformers. One of the main differences is the Bassman was a 4x10 often back combo, while the jtm45 is typically a 2x12 closed back cab paired with a head. Both amps used different speakers and that contributes to much of the tone difference but the circuits are nearly interchangeable and they make a fantastic platform for modification so the are heaps of "one off" amps out the. Most of them sound great. I'd suggest building whichever circuit blows your skirt up and then season to taste.
     
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  3. Heinzi

    Heinzi Member

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    Yes i thought that it would be possible. Maybe switch to 6L6/5881 and 12AY7 Tubes and the transformer also make a huge part of the sound I´d guess.

    You´re right with the cab, but I´d go for a head anyway
     
  4. pdf64

    pdf64 Member

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    TAD offer a 5F6A kit, as do Modulus in the UK (we're still in the EU!).
    But if you want a head format, the tweed style chassis layout seems a bad basis; a Marshall type chassis would altogether be much better layout and an easier, less cramped build.
    Plenty of vendors in the EU offering JTM45 kits, and I'm sure if you enquire, they would be happy to provide the transformer spec of your choice, or supply without transformers to allow you to source your own.
    The PT chassis cut out dimensions, and mounting holes of all transformers being the main problems that might arise.
    Also I strongly suggest not to go down the road of obsessively building an exact clone; the chassis must have a mains ground fitted to current standards (hopefully any kit info will inform / advise about that), and power tube grid stoppers, silicon diode in bias supply, bias adjustment, a HT fuse, 1 ohm current sensing resistors on the power tube cathodes, and a re-arrangement of standby to avoid 'hot switching' are all highly beneficial mods to the stock circuit.
    Another benefit of a JTM45 type kit is that it should have a circuit board made from a proper electrical grade insulator, not that awful black fibre board which is comically unsuited to high impedance / high voltage circuits.
    https://www.thegearpage.net/board/i...-noise-with-pedals-and-guitar-volume.1981066/
    https://www.thegearpage.net/board/i...-bassman-6g6b-problems.1936459/#post-26359627
     
    Last edited: Oct 11, 2018 at 2:15 PM
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  5. Heinzi

    Heinzi Member

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    Cool thanks - any suggestions concerning good transformers for the bassman?
     
  6. Humble Texan Fan

    Humble Texan Fan Member

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    In Europe Hammond transformers comes thirteen by the dozen, really. Great sounding and rugged trannies.

    Otherwise I'd recommend Classic Tone, but these you'd have to order from the States like I did. Not too costy with all VAT and shipping, I think. More compression and overtones here.

    You probably want a paper layer wound, anyroads, particularly for an OT. Most modern transformers are nylon bobin. None are better or worse, but paper just has a more vintage sound to it. It's not so much in the actual paper but the spacing of the winds inherent in the winding technique.
     
    Last edited: Oct 11, 2018 at 4:39 PM
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  7. Heinzi

    Heinzi Member

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    Great thanks!
    I hear many people talk about the Mercury Magnetiics, and they´re supposed to be very good, I just thought there might be some alternatives
     
  8. 79Stone1

    79Stone1 Member

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    I'd recommend Classic Tone or Hammond over Mercury without hesitation. Here in the U.S. Mercury is very expensive and I don't think they sound any better or worse than other brands.
     
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  9. pdf64

    pdf64 Member

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    Yes, Hammond stuff is generally excellent.
    FYI with their 'Tube Guitar' range, Material used & design specs. were kept as close as possible to the original part to preserve the stock "tone", and the OTs use paper to separate the layers, only the Vox RI types (whose originals use bobbins) aren't, see
    http://www.hammondmfg.com/guitarLineOT.htm
    The 1750B, which doesn't have the metal winding covers of the bigger OTs, doesn't look to be wound on a bobbin.

    [​IMG]

    Something that drives me nuts is PTs whose secondary voltages run too high, eg heaters way over 6.6V (6.3+5%) at full load and intended mains voltage; 7.1V being the record so far!
    So I suggest to actually measure your mains voltage and confirm with the PT vendor that the PT will work to spec with that.
     
    Last edited: Oct 12, 2018 at 3:07 PM
  10. Bob Arbogast

    Bob Arbogast Supporting Member

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    Yes. It puzzles me why old-school transformers might have an adjusted "lower B+" winding instead an adjusted primary winding that compensates for today's higher line voltage. More than the B+ winding needs to be adjusted! In the US, I would like to see PTs with a 125V primary winding.
     
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  11. Humble Texan Fan

    Humble Texan Fan Member

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    TAD and Tube-Town have some trannies marketed under their own names. The TADs I know are of british, american and german manufacture. The british ones are probably Dagnall, the american one... could infact be Magnetic Components/ Classic Tone, seeing they are made in Illinois and are paper wound, but the EIA code seems to be incorrect for them
    I'm aware that MM are quite popular in the US and elsewhere, but besides offering good quality products there's really nothing special about them to justify twice or even thrice the price of other brands.

    They use the same M6 laminations as other manufacturers and they're invariably bobbin wound, regardless of whether they're marketed as "Radio Spares", "Partridge" or what ever - not the british RS and Partridge of yesteryear, obviously.

    They come across to me as rather dark sounding to my ears, as if to hide the highend associated with bobbin wound transformers. If you're after the MM tonality specifically, then by all means go with them. However if you want more vintage tone or just something good quality you can rely on, others would do this for less hard earned money.
     
  12. 79Stone1

    79Stone1 Member

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    Haha I think they do this to cater to builders who use ss rectifier and don't understand how to simulate voltage drop that would normally accompany a tube rectifier
     
  13. pdf64

    pdf64 Member

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    The Hammond 200 series PTs have a 125V primary with a 115V tap http://www.hammondmfg.com/263.htm#263newpri
     
  14. Bob Arbogast

    Bob Arbogast Supporting Member

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    Thanks for this.I checked out the 273DZ, which would be the PT for a Deluxe Reverb. But I'm not so sure about it.

    Yes; the specs include a 125V primary. But the secondary is rated to make 350-0-350 under load. Even at a line voltage of 120V, that would still make for 335-0-335 on the secondary. Wouldn't that give about 450V B+ with a GZ34? That's hot!
     
  15. Heinzi

    Heinzi Member

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    I´m really glad, that MM are not the only good sounding OTs:) The would cost about 700€ (all three for the 5F6A) here.
    What do you guys think of the Mojotones?
     
  16. pdf64

    pdf64 Member

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    They seem to be very well made, and the OTs and chokes I've used are great, but (sigh) whilst others have been within spec, one PT had a way high heater voltage which I had to bring down with chassis mounted power resistors. If you can buy them locally (and so easily return in case of such issues) then fine, but I'd be wary of importing them yourself.
     
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  17. Heinzi

    Heinzi Member

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    Ok thanks, good to know. Man this is so difficult...there are so many Transformers and it´s impossible to make a fair comparison especially when you´re talking about an amp that is not even bulit:) But you´re right - it´s better to go for EU built oder at least distributors
    I´ve also stumbled over this italian manufacturer: http://www.inmadout.com/T-set.asp
     
  18. pdf64

    pdf64 Member

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    Hammond are my default choice; if they make something suitable, there needs to be a good reason to go with something else.
     
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  19. pdf64

    pdf64 Member

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    Yes, the HT on that one may be a bit high on voltage and low on current capacity.
    The 276X may be a better match, but that style of PT wouldn't suit a regular Fender BF type chassis anyway, as standard chassis have a cut out for a 'drop through' PT. http://www.hammondmfg.com/pdf/EDB276X.pdf
    Maybe send Hammond a message requesting a 200 series primary on the guitar range?
     
  20. Bob Arbogast

    Bob Arbogast Supporting Member

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    Good idea. I just sent them a note about the 290BX.
     

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