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36 watt variation - TMB tonestack with footswitchable cold clipper stage

Discussion in 'Shock Brother's DIY Amps' started by Fiat_cc, Nov 3, 2017.

  1. Fiat_cc

    Fiat_cc Tele-Holic

    Age:
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    Oct 8, 2016
    Melbourne, Australia
    Hi all.
    The next project is a 36 watt power section, with something akin to a Plexi preamp with a foot switchable cold clipper. I plan to put a pot in series with a resistor on the cold clipper stages cathode to run between maybe 8k (a little warmer than JCM 800) through to 40k or so (MESA and Soldano territory). No reverb or trem in this one. It’s being built for a mate who blew up his JTM 30, so I said I’d build something similar powered, but different into the chassis. I’m using 4 x EL84, 4 x 12ax7, a Hammond modern Vox AC30 PT, and a Hammond Fender style 35 watt OT with 4, 8 and 16 ohm (4000 ohm primary). I’ll post a schematic and layout once finished.

    One question... Tube rectifier... 5V4 or 5AR4? I want some sag, similar to a 5Y3 in an 18 watter...
     

  2. hoaryoldfox

    hoaryoldfox TDPRI Member

    Age:
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    Jul 27, 2017
    Mt Ommaney
    Not much sag in a 5AR4
     

  3. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

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    5U4 would be the obvious choice, I believe. You'd also be pushing the limits of 5V4 which is rated at 175mA.
     

  4. sds1

    sds1 Tele-Holic

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    Count me in.
     

  5. Fiat_cc

    Fiat_cc Tele-Holic

    Age:
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    I thought that too, but doesn’t a 5u4 want 3 amps of heater current? I’m using the Hammond 290MX Vox AC30 reissue replacement power transformer because it’s comparatively cheap, and can supply the right amount of HT and 6V heater current, but the 5V winding is for a 5AR4, so only 2A heater current.

    Edit: I just re-read the Hammond specs and there is 3A at 5V so 5U4 it is.
     
    Last edited: Nov 3, 2017

  6. Fiat_cc

    Fiat_cc Tele-Holic

    Age:
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    That’s what I thought, and I was planning to put a switchable solid state rectifier in too. My friend hasn’t played tube rectified amps, so I want him to feel the difference in playability, and for a few $ of components it’s not hard to implement. However, it’s not really worth it with a 5AR4.
     
    Last edited: Nov 3, 2017

  7. Fiat_cc

    Fiat_cc Tele-Holic

    Age:
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    Oct 8, 2016
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    Quick question, @robrob has diagrams for a switchable cold clipper cathode resistance, but if not seen it adjustable on a pot. Is that due to DC on the pot, and therefore a scratchy pot when adjusted, another specific reason, or is it just no one has done it?
     
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  8. Fiat_cc

    Fiat_cc Tele-Holic

    Age:
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    Oct 8, 2016
    Melbourne, Australia
    OK folks.
    Here is my 1st draft schematic. This is an odd bastard child of SLO100, JCM800, Plexi, 18 watt, Vox AC30, with a few ideas pinched from Dumble ODS.

    It's a whole lot more gain than I've ever dealt with before, so please, those more in the know than me, point out my mistakes. Some component values are guesstimates based on other circuits, but I plan to go through this design and draw the load lines for all the preamp stages, and tweak if I need to. Likewise the voltage dropping resistors in the power section. I will better calculate those when it's not midnight. Some things like the switchable parallel/cascade V1 triodes, NFB switch and tone stack lift (raw switch), are for experimental purposes and may get removed if they don't work, or don't help/add anything. I plan to make the parallel/cascade switch, and the clean(er)/distortion switch foot switchable via relay.

    Will the overdrive circuit be stable do you think, or do I need more attenuation somewhere?
    Also, do you think I could get away with a solid state/tube rectifier switch, or will the SS rectifier be putting out too much voltage for the EL84's to deal with?

    If you think I'm in the ball park, I'll start a layout drawing. Well two actually. One for the JTM30 chassis I'm putting the first prototype into for my mate, then another for a more typical layout if I want to build another in the future.
     

    Attached Files:

    Last edited: Nov 7, 2017

  9. sds1

    sds1 Tele-Holic

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    For sake of discussion, I was wondering if a solid state rectifier and/or negative feedback would be worth considering in this high gain application?
     

  10. Fiat_cc

    Fiat_cc Tele-Holic

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    The schematic does have negative feedback (switchable in or out of the circuit). I am interested in experimenting with values.
    As far as SS rectifier, I would like to make it switchable SS/tube, but I am a little scared that the SS rectifier will put too much voltage on to the power tubes. I'm hoping someone might chime in and let me know if it's safe to try.
     

  11. sds1

    sds1 Tele-Holic

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    Wow it's like I didn't read your post or look at the diagram at all. I was looking for NFB at the top of the diagram. Sorry.

    Man I didn't realize the rectifier tubes you were using. Looks like SS rectifier would add 50VDC to your B+. How did we end up so similar on our voltage charts? Our PT HT secondaries I think were within 10V of each other.
     

  12. Fiat_cc

    Fiat_cc Tele-Holic

    Age:
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    Different transformers will account for some of the difference (or lack thereof). The 18 watters I've built use the VHT Chinese PT's that I got cheap off the nweb with the chassis' I picked up too. The power transformer for this project is a Hammond Vox AC30 PT, rated for 300V-0V-300V. I think Vox used a GZ34 rectifier, so I might get away with SS, as it's not going to be a massive difference in voltage from SS to GZ34. It will definitely be a difference to the 5U4GB though.
     

  13. Fiat_cc

    Fiat_cc Tele-Holic

    Age:
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    OK, here is a first draft layout. It was definitely harder than the 18 Watter to make this layout neat. If nobody spots any glaring issues, I'll tidy it up a bit (line up turrets a little better, tidy lead routing etc) and proceed to prototyping stage (i.e. actually building it). Note that the layout is more convoluted than necessary due to reusing my mates Marshall JTM30 chassis which has necessitated some unusual choices. If the amp builds well and sounds good, I'll rework the layout to suit a more conventional chassis arrangement.

    I'm still curious to hear anybodies thoughts on SS rectification. I'd like to add it, but I don't want to run the tubes too hard.
     

    Attached Files:

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  14. Fiat_cc

    Fiat_cc Tele-Holic

    Age:
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    Oct 8, 2016
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    Perhaps I’ll ask some specific questions...

    Will SW1 and SW2 pop.
    Can I do anything to stop them?

    Is there too much gain available in the preamp to be stable, or should it be ok?
     
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  15. Fiat_cc

    Fiat_cc Tele-Holic

    Age:
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    Oct 8, 2016
    Melbourne, Australia
    Howdy folks.
    So I’ve started on the build. This first prototype is going in to a Marshall JTM30 chassis that a mate donated (he gets the amp to road test and hopefully spruik to others)...
    First thing... This chassis is a nightmare for this conversion. There is very limited useful space due to the weird (stupid) layout and design of the original amp.
    Anyway, liberal use of the jig saw with hacksaw blade and some aluminium plate, and things were a little better. Don’t look too close at the cutouts, as the shape of the chassis made them hard to do.
    So, transformers and sockets mounted, and drilled and staked the board.
    I realised when populating the board, that I’ve made this layout a little cramped. I’ll need to revise it for more space and a few other adjustments before the next prototype. I think I’ll also do caps on a separate board, maybe Fender style outside the chassis.
    Once the board went into the chassis, the nightmare of this ridiculous chassis really began. It’s been really difficult to find real estate for everything, and even harder to put it in logical places! I’ve learned a lot so far, but I still haven’t finished wiring it up, and I’m a bit scared that some of the compromises will lead to noise or oscillations.

    Still, hoping to fire it up tomorrow... Fingers crossed.
     

    Attached Files:

    Last edited: Nov 25, 2017
    awasson and sds1 like this.

  16. sds1

    sds1 Tele-Holic

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    Yeah that chassis looks brutal, I definitely feel your pain. At least you're getting it done, I have an Ampeg chassis here I had to abandon completely -- it was that useless as a metal box.

    Very excited to see the results here sir, hope she doesn't give you too much trouble on the initial power-up. Looking forward to a few things on this build including the cathode pot and the relay setup.
     

  17. awasson

    awasson Friend of Leo's Gold Supporter

    Age:
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    Nov 18, 2010
    Vancouver
    Nice work on navigating that chassis. I didn’t realize what you were really up against until looking at the photos.

    Regarding the SS rectification. I expect you’ll be seeing a jump in voltage if you switch a SS full wave bridge but it should be possible to calculate the difference and put dropping resistor in to keep the tubes happy. I can’t see the schematic on my iPad so I’ll have a look later on a proper computer.

    Looking forward to seeing it all come together. It’s going to be pretty impressive once your done.
     

  18. sds1

    sds1 Tele-Holic

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    Wouldn't this add voltage sag though, and therefore sort of defeat the purpose of the SS rectifier in this application?
     

  19. awasson

    awasson Friend of Leo's Gold Supporter

    Age:
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    Nov 18, 2010
    Vancouver
    I don’t honestly know. I always thought that sag would be eliminated through the use of a silicon full wave rectifier because it responds much faster than tubes and that all you needed to ensure is that the voltage wasn’t too high as a result.


    The one I recently built had half wave rectification via a pair of diodes in the schematic and was supposed to put 325v at the first node of B+. When I used a full wave SS rectifier, the voltage at that node was 342v so I added a 150 ohm 5 Watt resistor to get it down to just about 326 volts. It’s only got half the tubes of this one so the power requirements are considerably less.
     

  20. Fiat_cc

    Fiat_cc Tele-Holic

    Age:
    35
    510
    Oct 8, 2016
    Melbourne, Australia
    The chassis is really frustrating. The double tiered thing. The stupid place Marshall originally put the tubes, then the daft vents/perforation they put above them to try to aid cooling. It looks roomy at a glance, but because of the angled bottom tier etc, you’re really limited in where transformers can go. As it was I cut most of the top tiers floor out and replaced it with aluminium plate. Still, nowhere for the relay boards to fit properly, the tube socket layout is less than ideal... Blergh. It’s horrid!

    Yes, I am expecting a voltage jump. I am working on the premise that it’s an AC30 PT with a similar tube compliment, and AC30 uses GZ34 rectifier which drops hardly any voltage. As such I’m hoping it’ll be safe in both operating conditions, and be nice and tight in SS mode, and nice and saggy/touch responsive with the 5U4GB rectifier.

    As mentioned above, I am really hoping I don’t need to drop any voltage. I’ll find out once I fire it up and see where the voltage settles.
     

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