18 Watt TMB Combo Build - Too middy..Solutions?

Discussion in 'The Workbench' started by Yamariv, Aug 20, 2020.

  1. Yamariv

    Yamariv Well-Known Member

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    Hey Guys,

    I've built an 18 Watt TMB Combo with a 25 Watt Greenback and find the amp to be very thin and middy, no real warm bass tones at all, just not my cup of sonic tea..I was hoping this amp would replace my DSL40C for my main gigging combo but as it sounds now that's not happening..

    Now, from what I've read, this middiness is a characteristic of the 18 Watt sound but I won't want to play it like it is right now..

    Any suggestions on how to change the voicing? I'll attach the schematic below for reference.

    - should I compensate with a less mid focused speaker? Would that make enough difference?
    - or should I change out some caps, like the .022uf's for .1's more like a Fender?

    Any and all suggestions appreciated! Cheers 18WattTMB.png
     
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  2. thetragichero

    thetragichero Well-Known Member

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    this occurs with both channels or just one?
    could it be a tube issue or biasing issue?
    could you maybe have used an incorrect coupling cap? 2n2 instead of 22nf, etc
    only other thought is that you could be using the wrong speaker
     
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  3. Yamariv

    Yamariv Well-Known Member

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    The regular channel is much more clean but it's nice and full sounding, the thiness is only on the TMB side so that's why I'm thinking a couple circuit changes might be the key. I'm just not sure what to change exactly as I am still new to circuit tweaks.

    Bias is perfect, caps are all proper and the tubes are all vintage and test great!

    Anyone have any suggestions or experience with different speakers? When I was planning on building the amp, most of my research pointed to the Greenback as being the ideal speaker for the 18 Watters.
     
  4. thetragichero

    thetragichero Well-Known Member

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    so the tmb channel is giving you issues? download Duncan's tone stack calculator and play around with some values
     
    Last edited: Aug 21, 2020
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  5. neikeel

    neikeel Well-Known Member

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    There is nothing wrong with a 25w GB in this amp (one of my 18w runs one and is as sweet as a nut since I swapped one of these speakers into it).
    Whose transformers did you use and what are the voltages on the plates of your EL84s (pin 7)
     
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  6. neikeel

    neikeel Well-Known Member

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    If you are happy with your normal channel then as tragic says you need to check for a wiring error. The thing that strikes me is those big pi output couplers I would think 0.022uF is more than enough and on my last build I used 0.01uF.
    How handy are you with a soldering iron?
    The recent TMB is the Stinger Tube Audio, 18w Superlite2b it is a fantastic little amp. Only things I did was 820R/4n7 on v1b cathode kept 100R screens 150R/250uF on EL84 cathodes and an EZ81 tube rectifier.
    Mine has original RS iron and is running Mullard YP tubes which may help. If it means anything I have three other 18w. An 18w trem clone (100% NOS except for elrctrolytics) in a 2x12 combo with pair of early 70s prerolas, an identical spec head version and a Ceriatone TMB18EF86 1x12 combo - now with modern celestion T1221. The latter has never quite cut it (had x4 différent speakers) and the EF86 channel is very picky - could do without it, intact turned off for now.
    if you want a gigging 18w to replace your other amp try the super lite on the TMB channel. You can keep the normal channel as is. Only thing is because of phase issues you will not be able to jumper channels.
     
    Last edited: Aug 21, 2020
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  7. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Replace the 22n PI coupling cap with a 10n and replace the PI tail resistor of 56K with a 47K to start
     
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  8. South Park

    South Park Well-Known Member

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    You can bypass the tone stack .
     
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  9. Yamariv

    Yamariv Well-Known Member

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    Ok, spent part of the morning playing the 18 Watt through various cabs, I tried Celestion Blues from my AC30, my 212 Geenback Cab and my DSL40C's 75 watt Creamback. My analysis from that would be this:

    - The normal channel sounds full and thick through all the speakers, no issues on that side of the amp. That channel is very Fendery and sounds great, crisp, clean and nice bass.

    - 212 Greeback cab sounded a little fuller but still thin, not much improvement
    - 212 Alnico, sounded a bit fuller but not much of an improvement on the TMB either
    - Single 75 Watt Creamback sounded much nicer and a little more full. What a great sounding speaker but still something is off..

    Just before I shut everything down I noticed that the Bass and Treble knobs when turned full either way do pretty much nothing [​IMG] Yes, how did I not notice this before..well I set them to where I like them and played with the middle knob most assuming that the Treble and Bass would be working..ugh! The middle knob varies the sound like crazy when turned full left and right.. I do have an effects loop inserted in the TMB but it seems to be working fine.

    So given I definitely have a tone stack issue if my Treble and Bass arent working, I've attached a couple pics of the build..maybe someone can spot the issue with a keen eye as I won't be able to open the amp up for a few days..

    18WattResize.jpg
    18 Watt2Resize.jpg
     
  10. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Verify your brown wires on your tone stack to make sure one is not crossed
     
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  11. Matthews Guitars

    Matthews Guitars Well-Known Member

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    I'm not familiar with a Marshall 18 watt TMB. What model would that have been? (Model number)

    I am somewhat familiar with the 18 watt model 1973, having had a chance to play with an original one on occasion, that's owned by a friend, and it's just magical.

    If I at some point decide to build a clone, I'll probably opt for a clone of the 1973.
     
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  12. neikeel

    neikeel Well-Known Member

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    There never was an 18w TMB so there is no model number.
    Clearly there is a wiring error in the tone stack (is it grounded properly?) an ungrounded tone stack can behave weirdly like this.
    Afraid those pics are at an angle and I can’t see where the ground on the mid pot is soldered to the preamp ground.
    Looking at your caps they actually appear to be 0.01uF which is normal for these and hence why your normal channel sounds ok.
    presume that is a master volume on the back next to the loop?
    Is that a triode kit?
     
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  13. myersbw

    myersbw Well-Known Member

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    There are quite a few variations of the 18W TMB channel...so, it's not, as noted by Neikeel, any standard "Marshall" comparison for correctness. That said, I can personally vouch for Mojo's 18W TMB schematic & diagram as I built a couple while they were in prototype stages and helped with a minor turret board issue (case of the missing turret! lol)

    Their results were repeatable three times for me. All with the same flavor of tone using their kits as a baseline then I tweaked a little, but nothing major.

    As for speakers, I love this flavor of amp in open-back combo form with either a Cannabis Rex or a CV75 speaker. Both yielded fabulous results. Almost forgot, an Alnico Gold was also an amazing match with this style of amp.

    I also used a 33K for the tonestack slope resistor and liked it. I'm also a strict hum bucker user, too.

    Brad
     
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  14. myersbw

    myersbw Well-Known Member

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    Ok, I just peeked at the schematic you used and refreshed myself with a quick look at the Mojotone version. I'd strongly suggest pulling Mojo's variation into the mix. I'm not a fan of a cathode-driven tone stack. Compare the two drawings and you'll see. It'll take just a little effort...not many changes at all...to rewire it. But, I think you'll find it so much worth it.

    For reference, the Mojo kit uses their 'Mojo Dijon' caps in the signal train and carbon comp where it counts. It's very harmonically rich and the noise level is way more than just 'acceptable'. As they were developing it, I liked the gain factor with just the three stages. They made an effort to minimize compression and give nothing but great raw tone.

    I really think you'll be happy if you change it up to that version. BTW - their kit uses Heyboer transformers, too. The only weak spot of their kit is the EL84 cathode resistor is way underrated. It will eventually bake-n-burn up if you don't up it to 10watts or greater. But, that's the only issue I've found in their version of it.

    EDIT - just saw you have a 5w, 200ohm cathode resistor in the power stage...no reason to change any of that. And, I think the clean channel of yours & the Mojo is about identical.

    Brad
     
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  15. Yamariv

    Yamariv Well-Known Member

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    No, it's not a kit, I sourced all the parts but used a layout that the guys on the 18 Watt Forum recommended. Yeah, there has to be a grounding issue somewhere in the stack, I'd be surprised if I completely missed a ground but agree, sounds like what a ground issue would do. I won't be able to get into it for a few days unfortunately.. Was mostly just curious if someone had a keen eye and could spot an issue before I opened her up. All good!
     
  16. Yamariv

    Yamariv Well-Known Member

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    Very interesting angle Brad, if you've built a few of these Mojo versions, I might just switch the TMB over to their specs then if you were that happy with them. Might just be easier to do a tried and tested layout while I'm in there..thanks for the suggestion!

    I'll just have to take a good look at both layouts and compare and hope I don't miss anything if I do convert :ohno:

    Interesting note about the recommended 130 Ohm Cathode resistor the layout originally recommended. I don't know if my vintage Mullard EL84's are crazy hot but I decided to measure the bias last time I was working on it and both tubes were way hot!! They were in the mid to high 40's for Ma and pushing 15-16 watts a piece! :facepalm: I couldn't see any redplating surprisingly but I promptly tested a bunch of resistors and settled on 200ohm with a 5 watt 100ohm Pot in series to fine tune the bias in perfectly.. :naughty:
     
  17. myersbw

    myersbw Well-Known Member

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    Yeah, when you push those EL84's to sweetness you'll burn through em pretty quick. I "think" I ended up with 270 with mine...?? Have to go back & see...I usually keep good pics of stuff I build.
     
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  18. Yamariv

    Yamariv Well-Known Member

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    This is interesting to me, why do all these amps recommend such a high bias for EL84's but not for any other type of power tubes "generally".. After seeing how high my bias was on my 18 Watt with the 130ohm resistor, I'm surprised they didn't implode! lol I'd be curious but scared to see what my AC30's stock bias is running at :ohno: I hear they are EL84 torture chambers but the tubes seem to last.."so far.."
     
  19. thetragichero

    thetragichero Well-Known Member

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    for one thing you're running cathode bias instead of a fixed negative voltage so to some extent it limits itself from redplating
    i know it's a different topology but check out how a fender champ is biased: generally super crispy!
     
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  20. Yamariv

    Yamariv Well-Known Member

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    Interesting!! I didn't know that you could run Cathode Biased hotter w/o redplating..I guess I thought it was pretty much the same as Fixed Biasing with the grid, just the other way around to get there
     

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