Project Tweed-O-Verb - squeezing a Vibroverb into a Tweed Deluxe...

Discussion in 'Shock Brother's DIY Amps' started by buddy_coles, Apr 12, 2021.

  1. 2L man

    2L man Tele-Holic

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    When doing a Star Ground to a tube amp every stage which has own power take (B2, B3, B4 etc) should have own wire to the PS last capacitor negative terminal. First all stage currents are lead to a Filter capacitor negative where wire continues to the PS. Just like all stage currents can be thought to begin after a voltage dropper resistor which usually connects to same Filter capacitor positive terminal.

    Only exception is/are power tubes where both Anode and Screen return current uses the same ground wire because currents meet already on tube cathode(s) so B+ = transformer and B1 = Screen has only one ground wire. Usually there is one ohm resistor on each tube cathode where ground begins. There also a thicker wire could be used. I have used one square mm which I think is about AWG18 but current there is quite small so perhaps it means nothing?
     
  2. buddy_coles

    buddy_coles TDPRI Member

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    I think for white lettering I would try printing to white self-adhesive vinyl and then give a good coat of lacquer on the top - I doubt the end result would be much different from painting the panel white and then using a clear waterslide decal followed by clear lacquer...? The finish I got printing on to self-adhesive vinyl on my last project was surprisingly good :)

    I am currently investigating a home etching kit for aluminium, where I can possibly etch the lettering to about 0.5mm and then fill with white paint before carefully sanding off the excess. And then lacquering. Seems everything I do ends up with a coat of clear lacquer at the moment!
     
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  3. buddy_coles

    buddy_coles TDPRI Member

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    Ground bus now attached... I tried to tin the thick copper wire but gave up after the first section - figure I can do this later on if needed, and I doubt it'll corrode and fail before I do! I used 30A twin and earth as the donor material. The earth strand is thinner so that's what attaches to the turrets, and one of the main strands is the thicker wire running the length of the board.

    20210610_075552.jpg
     
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  4. buddy_coles

    buddy_coles TDPRI Member

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    And a little bit more progress - I pondered how best to secure the reservoir caps, and finally decided on mounting them sideways in a hole on the board with a cable tie to hold them in place. But of a fiddly job but I think it'll work ok.
    It gives me a bit of space to connect B+ and grounds and fits neatly in the available space. Definitely going with PTFE board on next build though - that phenolic stuff takes some effort to cut and file!!

    20210615_132039.jpg 20210615_132157.jpg 20210615_154601.jpg 20210615_213032.jpg
     
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  5. buddy_coles

    buddy_coles TDPRI Member

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    Oh, and I used 220K resistors in place of the 470K ones in my schematic, as I found some in the drawer :D
     
  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    When using two caps in series the resistors are used to keep the voltages relatively equal on each cap.
    This one wasn't a stupid question. I am glad you asked this question because the old info I learned back when has been updated. Much higher values are now recommended. I like learning.

    To answer your question... if the value was too high the leakage of the cap may be more than the resistor could make up for and the caps would no longer have equal voltage on them which would make one cap work harder than the other shortening life. The good news is the 470K value is fine and even higher values could be chosen. So the advantage of using the 470K is the resistors will not get as hot and not as much power will be wasted. The only real advantage of using a smaller resistance is to use the balance resistors as bleed resistors to discharge the capacitors within a minute or three to make the amp safe for those of us that might put our hands in.
     
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  7. buddy_coles

    buddy_coles TDPRI Member

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    Thanks LLC, that's a very useful bit of info - and reassuring to know I wasn't too far off the mark as far as physics go!
    It seems a lot of the 'rules' around capacitors have changed in the 60 years or so since the Fender circuits were designed.
    In the end I've used 220K resistors - I'll try to find the calc I used to see what the projected discharge time will be using these. I can always swap the 470K resistors back in if needs be.

    It's all getting quite exciting now - most of the logistics are complete so I should be able to start wiring it all up in the next couple of weeks, as long as other commitments allow!
     
  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The 220K will be fine. The 440K total resistance as a bleeder will take a little longer to discharge the caps than I prefer but it will be half as long as the 470K resistors. Under normal circumstances the caps are discharged by the tubes so the bleeder resistors are more of a backup for safety. Many amps don't have bleeder resistors and it is not an issue.

    As balance resistors, either value will do the job so I am sure you would not notice any difference.
     
  9. buddy_coles

    buddy_coles TDPRI Member

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    A slight detour - my adventures in electrolytic etching of aluminium :D

    So my quest to find a DIY way to get white lettering on a polished aluminium surface continues, and I thought I'd try etching. There were plenty of good Youtube vids to use as research, so I thought I'd try electrolytic etching as it uses safe substances and doesn't require any specialist kit. I wondered how good the resolution would be using photocopier/laserjet toner, and the answer is surprisingly good!

    Step 1 - print out the negative of what you want to etch. I tried glossy photo paper (worked better) and OHP transparency (didn't work so well).
    20210628_204413.jpg

    Step 2: take a shiny piece of aluminium, heat a bit with an (old) iron and press the transfer on to it. I used an older iron to avoid the possibility of getting toner all over my work shirts. If you don't have an old iron then it may be best to do this while your wife is out. Same goes for putting engine casings in the dishwasher, which acts as a surprisingly good degreaser...
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    Step 3: iron it on! I found I needed a lot of pressure and quite a bit of heat the get the toner to transfer. I found a good PCB etching post somewhere that described the behaviour of toner with respect to temperature, and it seems you don't want it to get too hot.
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    Step 4: cool it down and soak in water to remove the paper. The OHP transparency came off very easily but didn't transfer the toner very well. The photo paper was a bit of a pain to remove, but did a much better job with transferring toner. I may also try using the backing paper from sticker sheets, or magazine paper, as others seem to have had good results with these.
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    Step 5: mix up the medicine - white vinegar and as much table salt as it'll hold. I only mixed up about 50ml as I didn't need lots. It'll keep until the next etching experimentation session, and it is safe to be put straight into a drain.
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    Step 6: connect up the electrodes and etch! I used the 12V battery from my mower - positive goes to the plate to be etched, negative to the cotton bud which is saturated with the vinegar and salt solution. As you touch the plate you can hear a faint fizzing sound. This method is a bit time consuming but gives good control over which parts of the plate you want to etch. I understand there should be a 'line of sight' between anode and cathode, so the etching effect is greatest where the cotton bud tip is. That said I did get some overspill around the edges where the toner hadn't transferred very well.
    20210628_211859.jpg

    And the finished product, with a bit of white paint to show the depth of lettering. I was only doing this for experimentation so the finish at the end of the word VOLUME is not neat, but I was very pleased at the resolution in terms of accuracy of edges. Bear in mind the lettering is only 3mm tall, so this is precision work!

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    So here endeth the lesson - I was very pleased with the results, especially as this was a bit of a rough and ready experiment. Next attempt will use a cut-to size piece of brand new alu (which may have the advantage of being flat!) and I will be very careful to cover all areas where I don't want etching to occur. I think I will also find a tank about the size of the plate, and use a stainless steel plate of the same dimensions as a cathode, to sit 5mm or so away from the workpiece but still submerged - this should give a very uniform etch depth. I think also I'll go a bit deeper with the etching, and use the bold font for the numbers as these were tiny!

    All in all I consider this a great success, though it is now getting in the way of my finishing the wiring in the chassis.

    Thanks for reading, and I hope this was an interesting diversion!
     
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  10. dougsta

    dougsta Tele-Meister

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    Great work there.
    Do you plan to sand down the white (or whatever paint color you you use) down with very fine grit wen n dry paper and leave the paint in the etched parts?
     
  11. buddy_coles

    buddy_coles TDPRI Member

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    Hi Dougsta, yes that's the plan - think I'll need a deeper etch though. I was toying with the idea of powder coating, but just in the etched areas. Not sure how difficult that might be, but it could be a viable option as the rest of the front panel would still be masked off, assuming I powder coat before removing the toner... I fear this may be a long voyage of discovery :lol:
     
  12. dougsta

    dougsta Tele-Meister

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    Be good to see how it goes, keep pushing boundaries I say.

    You can get a nice shine on aluminium, here's a large multi capacitor can from '50s/60s that I did a 10 minute polish with stuff I use to get my chisels razor sharp, same stuff (wax impregnated with very fine grit, don't know it's name, probably just called stropping compound) that barbers use to get cut throat razors sharp on a leather strop. It does take the print off the can too though.

    IMG_8998.jpg
     
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  13. dougsta

    dougsta Tele-Meister

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    I forgot to add that you can remove the guts of these old can caps and fit modern caps inside.
     
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