2204 ST1 board build. How am I doing?

Discussion in 'Building the Classics' started by LSDtigers, May 11, 2020.

  1. LSDtigers

    LSDtigers New Member

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    Hello all,
    New guy here. I'm attempting my first build, and I'm seeking some advice. I'm no stranger to amplifier circuits and how they function, but this is my first scratch build of an amp. I chose to build a 2204 circuit in a short box using the Granger Amps replica of the ST1 iss. 202 PCB.
    Yes, I saw the many hand wired layouts. But for authenticity's sake, I liked the challenge of trying to build it the way Marshall built it first before delving into modifications, hand wiring, etc.

    Sourcing transformers during the covid crisis was a REAL challenge. So here's a rundown of what I've managed to assemble:

    Heyboer Drake-wind OT
    Classictone Choke
    Mojotone Low voltage JTM45 PT 300-0-300

    Yes, I'm aware those PT secondary voltages are low for this circuit but its what I could get on short notice. Call it an experiment I guess.

    The problem I'm having is I haven't been able to locate a good layout for using this board in a scratch build. Yes, I know it is much like all the hand wired layouts in form, but if there were a visual reference I could use to assist in connections between the board, the tubes, caps and transformers, I'd feel more confident about what I'm doing. I can read and understand the schematic, however translating that schematic into a functional layout is not one of my strengths. If anyone has a cheat sheet for wiring up an original ST1 board, I'll owe you one.

    I spent the better part of Saturday populating the board by cross referencing the schematic with gut shots of old JMP 2204's and looking at hand wired layouts. The real ST1 board has component values and indicators as to what model they correspond to silkscreened on it. The reproduction does not. So it took a little longer than I had expected. Once that was done I installed the transformers and caps, wired the heaters and decided to stop until I could get around to making this post.

    So please, if you could just take a look at what assembled so far. Is there anything that sticks out as wired improperly? I'm 95% sure what I've done is correct, but you guys have the experience here. If I screwed something up, I'd much rather address it now.

    IMG_9174 (1).JPG

    Ok, lets talk specific questions.

    1. Grounding. The Metro amps 50w build guide has ground points that seem to be all over the amp. Bus wire on the pots, grounding lugs on individual cap can mounts, all that.

    I've read a bit about Star grounding everything to single point, and also the Larry Method, which seems to make a lot of sense. But again, being new, I felt I should inquire about personal experience you guys have with the pros and cons of various methods of grounding. What works best for a quiet amp that sounds great?

    2. Presence pot value. The 2204 schematic has a 25k pot and a resistor in the NFB network, while other Marshall amps used a 5k pot. I've read this can leave an amount of DC on the pot, causing it to be scratchy when turned. I don't really turn my presence pot while I'm playing. So... in your opinion, which presence circuit sounds better?

    Thats all for now. I'm sure there will be more questions. Thank you for taking the time to read this. Advice is welcome and much appreciated.
     
  2. South Park

    South Park Well-Known Member

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    That low voltage might not work right . The tube load lines are set for a higher voltage . The star grounding is what you want separate groups and straight to the center tap. When the board Is put in test all voltages Before tubes go in . I also test things with a meg meter for shorts . Look good
     
  3. LSDtigers

    LSDtigers New Member

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    You’re referring to the B+ line? I was planning to address that by making some lower value dropping resistor substitutions along the filter capacitor string. Is there an issue with doing that?
     
  4. South Park

    South Park Well-Known Member

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    A 2204 has about 480 volts dc on the plates of the drivers 300 will not get you thar . You are trying to do to much
     
  5. LSDtigers

    LSDtigers New Member

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    Oh, I misunderstood where you were headed. Yeah I know what stock runs. I knew I’d have to deal with somewhat lower plate voltage when I picked up this transformer. I’m shooting for ~400v on the plates. I did the math, under load I’m pretty sure I can hit ~395-405v.
     
  6. South Park

    South Park Well-Known Member

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    The voltage will not increase under load . Find out what amp that transformer went On and compare The values on the preamp
     
  7. LSDtigers

    LSDtigers New Member

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    Check it out, here’s the calculator.

    https://thesubjectmatter.com/calcptcurrent.html

    These are the values I’m using. Am I missing something?

    300-0-300 power transformer
    Diode rectifier
    2 EL34 power tubes
    3 preamp tubes
    3250 ohm primary resistance output transformer
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    Is that an aluminum chassis?
    If so, you need to use the Larry method.
    Don't use the chassis as a circuit path on an aluminum chassis.

    2. There's a better way to do the filament wiring for less noise also. But you have to change the layout.
     
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  9. LSDtigers

    LSDtigers New Member

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    It’s a steel chassis

    Please tell me more about the filament wiring.
     
  10. coolidge56

    coolidge56 Well-Known Member

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    Here's a 2204 I'm building now. For lower noise (filament wiring AC hum) keep the filament wires away from the signal wires. Here I'm arching up and over to the tube socket pin. The filament wiring doesn't cross or touch any other wires.

    Tip: You may notice I have a tight twist in my filament wiring but it keeps things neat and tidy and quiet. It's easy to do...cut two lengths of filament wire maybe 20 inches long. Tape the two wires together at one end and chuck them in a cordless drill. Clamp the other ends to your workbench. Stretch the wires out snug and let the cordless drill twist the wires slowly.

    Tip: I'm using PTFE (Teflon) insulated wire that's plated (aka top coat, silver plated, tin plated, etc.). This makes the wire somewhat stiff, if you straighten or bend the wire it retains that shape. The curl in the insulation doesn't cause it to spring back to the curled (spooled) shape. PTFE is also heat resistant so the insulation doesn't melt and draw back from the solder point. PTFE is also much thinner than say vinyl or PVC wire insulation so its less bulky, though they do make plated wire using other less expensive insulations that also hold their shape well. One suggestion, if you plan to bend this wire around a turret put a tiny dab of solder on the very end of the wire first to keep the strands from fraying.

    Tip: Note the shielded wire I'm using for V1 pins 2 and 7, this can also lower noise. Historically I HATED the available shielded wire, almost always insulated with stiff as a board thick bulky PVC insulation. Then TubeDepot started stocking this highly flexible Mogami insulated wire. It's only 1/8 inch in diameter.

    Tip: To straighten the solid buss wire which always ships mashed into a bent up mess :realmad: cut a 16 inch length. Now grab each end with a pair of pliers and pull. Put your back into it PULL when you feel the wire give slightly abracadabra perfectly straightened buss wire!


    heaterwiring.jpg
     
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  11. neikeel

    neikeel Well-Known Member

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    [​IMG]

    The grounding I use is to have bus wire on volumes, treble, and mid pots and I run the v2cathode and preamp can to these and a wire to the preamp ground point near the jacks.
    Have a dedicated ground for the inputs and run the heater centre tap and ground it at the inputs.
    I run a wire back from the bias caps to the output tube ground lugs.
    Take a dedicated wire from the presence pot ground and run that to the second (PI) filter cap ground.
    Mains in has its own dedicatd lug and the HT, mains filter cap have their own combined lug.
    To summarise:
    Preamp has its own ground
    Power amp has its own ground
    HT has its own ground
    Mains has its own ground
    That works for me
     
  12. South Park

    South Park Well-Known Member

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    Cables with RCA jacks have a shield and are flexible for the in put .
     
  13. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Here’s a pic of my real 2204 for reference. Note the heater wires, not real tight and the amp is very quiet. But notice that the yellow and green wires are raised up and away from the heater wires.

    upload_2020-5-19_3-18-34.jpeg
     
  14. _Steve

    _Steve New Member

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    Hey not sure if you're still interested but I have a recently purchased '76 2204 open on my bench right now. Let me know if you want me to look at anything specifically. (note mine looks to be missing its bright cap)

    20200521_095312 (2).jpg
     
  15. South Park

    South Park Well-Known Member

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    Never buy a board without a layout . If thar is no info on it then don’t do it . If want that amp to work ptp circuit board
     
  16. LSDtigers

    LSDtigers New Member

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    But the challenge is half the fun!

    I poured over the schematic, read lots of forum posts here and on the metro forum, studied every layout I could find and compared it all with the traces on my board.

    Happy to report that I finished the amp yesterday and it sounds AWESOME. Dead quiet too, which surprised me a little.

    Thank you guys for all the advice. Here’s a couple shots.

    7412E3DB-091E-48C5-999F-2B7F54B3C8D3.jpeg 879F924D-1B5C-41F6-BA34-CEB92E61FDD0.jpeg
     
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  17. neikeel

    neikeel Well-Known Member

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    Well done:cool:.

    Glad it sounds good and there is no extra noise.

    Ideally you should look closely on how you route the wires off the board to the pots and tubes to keep them as short as possible (without tension) try to avoid crossing other signal wires, and where you do ensure they cross at right angles.

    But if it all works all the way up to 10 on gain and master then all is good and leave it alone - maybe for your next build;)
     
  18. _Steve

    _Steve New Member

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    congrats it looks great!
     

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