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The Stupid Idea Vol 3: "Stupid Gets Real"

Discussion in 'Amp Input - Normal or Bright' started by GunMonkeyINTL, Dec 19, 2017.

  1. GunMonkeyINTL

    GunMonkeyINTL Senior Stratmaster

    Feb 5, 2015
    So, it turns out that the Stupid Idea has, without a doubt, been the most fun I've ever had with a hobby project in my entire life. I usually end up with some unused vacation time to burn at the end of the year, and I like to do some sort of project. Last year, I recovered a Mustang IIIv2 in Blonde/Oxblood:

    This year, I decided to take up trying to build an amp. The Stupid Idea came about because I decided that one of those Marshall Solid State mini-stacks was exactly the form factor I wanted, I found a good deal on one, and decided to gut it and build something in it:

    One of my first decision points was whether to do a kit, or start from scratch. I decided to go the scratch route, and found a modified antique sports PA system that originally ran on 12ax7s and el84s.

    Over the course of the last thread, and some personal emails, I let @Robins talk me into building a Friedman Pink Taco clone. Robin was HUGELY generous with his time over these last couple months, and I cannot thank him enough. I'm sure that those of you who have interacted with him appreciate what a resource he is, but I hope that the Forum Admin/ @Strat-Talk know what an asset having a member like that is to their board. Over the course of October and November, I probably put a good dent in his bandwidth allowance, sending sketches, images, and such back and forth with him, sorting out the build plan, and he never once acted anything less than eager to be helping me out. Solid guy, of the first-order.

    So, it turns out that building one from just a schematic takes A LOT of planning - at least if it's your first. After considering Robin's advice, and doing some research on the Pink Taco, I decided it was the right amp to build, but the Taco is SS rectified, my salvaged PT had a tube rectifier heater secondary, and my OCD wouldn't let me wrap up those two wires and pretend they weren't there, so I decided to make it as a switchable rectifier.

    My first step was to make take the Pink Taco schematic and a photo of the guts and make up a layout document, like comes with most kits, to see if I could figure it out. After that, I had to figure out how to make the rectifier switching work, and modify the schematic to suit. Then I had to make another layout document, this time to fit the chassis space I had, and the added parts. This is what I'm building from:

    I call it the Fish Taco.

    All told, I probably have 50 hours into this thing so far. Being at a total novice level with electronics, figuring out the schematics, reading enough to make some sense of them, drafting a build document, then modifying the schematic, making another build doc, then making chassis schematics took more time than I expected. Heck, just designing and making the chassis probably took 5 hours. But, it's been a great learning experience, and, as I said, I've been enjoying the heck out of it. The only downside, is that it's 50 hours that I could/should have spent practicing guitar (God knows I need it).

    I'm almost finished with the build. I had intended to start a thread, and update as I went, but, once I had everything drafted and collected, the build has been going really quickly, and I've had a hard time tearing myself away from it. I'll post some of the progress photos below.
  2. GunMonkeyINTL

    GunMonkeyINTL Senior Stratmaster

    Feb 5, 2015
    So, here is everything collected up. I tried to be spend-conscious, and get as much as I could in as few orders as possible, but found that, if you decide on some specific components you want to use, there doesn't seem to be one single vendor that you can get it all from. It probably took a week to figure out where I could get everything from, track it on a BOM, and get it all ordered.

    I was able to get the chassis blank from a local sheet-metal shop. It turned out that it was cheaper to get them to cut and bend a piece of their stock to my specifications than it would have been just to order the stock and try to bend it myself. I had them make it of 1/8" thick material so that I could just drill and tap and screw my parts directly to it, rather than having to use nuts/bolt and/or rivets. The actual drilling/tapping/cutting only took about an hour, once I had the plan sorted and sketched out.

    fattboyzz, Staypuft1652 and Robins like this.
  3. GunMonkeyINTL

    GunMonkeyINTL Senior Stratmaster

    Feb 5, 2015
    Here are the transformers mounted, and the heater circuit in place.

    fattboyzz, Staypuft1652 and Robins like this.
  4. GunMonkeyINTL

    GunMonkeyINTL Senior Stratmaster

    Feb 5, 2015
    Here is the control/tone-stack wiring. I did it on the outside of the chassis, so that I had more room to move, and then flipped it around when I was done.


    And the circuit board. I decided to do the ground bus on the back side, as opposed to how it appears to be on the Pink Taco. I may end up regretting it, but I was worried about exposed leads shorting on it and causing me problems. It was totally an arbitrary decision to use eyelets instead of turrets, but I think I will try turrets with my next build - the eyelets do appear to have their limitations.

    Staypuft1652 and Robins like this.
  5. GunMonkeyINTL

    GunMonkeyINTL Senior Stratmaster

    Feb 5, 2015
    Last but not least; here is the switching group. It's not quite as big a rats' nest as the build doc indicates, but still more complicated than any sane person should allow their amp to be. But, here's the deal: The Pink Taco uses a center-tapped HT secondary and two diodes for rectification, and my PT supported that, so I wanted to keep that. Also, my salvaged PT had a tube rectifier heater secondary, and the donor amp came with a decent vintage GE 5y3, so I wanted to use that, too. Not to mention, being a learning experience, I figured it'd be cool to have both rectifier types so I could measure and hear what the actual differences were.

    I found some pretty simple rectifier switching schematics online, but I had a few things I wanted mine to do. Specifically:

    1 - I didn't want either the heaters or the anodes of the 5y3 to be powered when it was in SS mode.
    2 - I wanted to be able to control the indicator scheme with the same switch.
    3 - I wanted to be able to change the bias of the power tubes, or some other 'tuning' function for each mode.

    That all required at least a 4p switch. After finding switches in a reasonable size/price, I worked out a schematic that in theory works like this:

    There are three switches, one for power on/off, one to select the rectifier, and one for play/mute. There are two LED indicators, one for power on/off and one for play/mute.

    The LEDs are common cathode, bi-polar, and powered off of the (AC) rectifier heater secondary. When the amp is in SS rectifier mode, the On and Play indicators will light blue. When it is in Tube rectifier mode, they will light amber.

    The fourth function of the switch is currently set up to switch the power tube cathode resistor, but that may change after I've had a chance to test and measure. Currently, I only have one cathode resistor in place, but planned the board so that I can make one "channel" hotter/cooler, or equal them out. That will all be part of the learning process once it is running and I can get to testing and measuring.


    Attached Files:

  6. s5tuart

    s5tuart My Dad used to say.... Strat-Talk Supporter

    Nice job so far!!!!!!! Thumbs up! :D
    GunMonkeyINTL and Omar like this.
  7. Stormy Monday

    Stormy Monday Most Honored Senior Member Strat-Talk Supporter

    Jan 19, 2011
  8. Raiders757

    Raiders757 Senior Stratmaster

    Sep 2, 2016
    Wow, this is just wicked awesome. I can't wait to see this when it's finished. It looks like a huge undertaking.
    GunMonkeyINTL likes this.
  9. Bodean

    Bodean Rock N Roll Ain't Noise Pollution Strat-Talk Supporter

    Aug 23, 2014
    South Carolina
  10. Dare1

    Dare1 Senior Stratmaster

    Feb 16, 2011
    Way more than I could do, I'll happily watch from the sidelines here
  11. Tone Guru

    Tone Guru Senior Stratmaster

    Dec 13, 2011
    Music City TN
    Hopefully you won't get much induced hum from the power to output transformer orientation.
    Have you verified this before committing to the locations ?
    I ask because it appears that the PT is a flat mount design that is flipped on its side.
    Normally the transformers are at 90° to each other.
    GunMonkeyINTL likes this.
  12. GunMonkeyINTL

    GunMonkeyINTL Senior Stratmaster

    Feb 5, 2015
    You are correct about the PT. I made brackets to mount it above the chassis, for space concerns.

    I made a paper and cardboard mock-up of all the chassis top-side parts, to figure out where I could put everything, and @Robins saw a problem and suggested that I power the PT, hook a headphone to the OT, and see what I got.

    I did the experiment, came up with roughly the arrangement I’m using here, showed him that layout, and he confirmed it was right, and then had me make one adjustment, from his experience, to account for the pre-amp tubes.

    Again, he’s been an immeasurable help. He’s got a bit of Mr. Miyagi in his style. He’d let me know I was about to screw up, point me in the right direction, and let me figure out the solution myself.
  13. carver

    carver The East Coast Strangler Strat-Talk Supporter

    damn, that is looking CLEAN.

    excellent work so far.

    and yeah @Robins is a damn good man
    GunMonkeyINTL and Robins like this.
  14. Robins

    Robins Dr. von Loudster Strat-Talk Supporter

    Dec 22, 2010
    Absolutely awesome job so far. It can´t be better than that. It was a real pleasure for me and it still is. You are such a strong learner and your power and your enthusiasm is huge and I really welcome that.
    To be honest building a Taco"clone" is long on my list but as always I am a bit limited finding time because of doing things on other amps (repair and changing parts/modding) and programming (main job).
    I would be happy to take the layout and all the work went into @GunMonkeyINTL ´s project and build one for me as well.
    To make clear - @GunMonkeyINTL did all the layout by himself including taking just the schematics and make it into a real circuit board for his amp. He really had a lot of work to do and he was a very fast learner and doing that as his first big build it is just stunning.
    The board itself looks perfect the way it is and to me it looks as good as the real one (maybe even better).

    I watch this thread with joy and man how I love that stuff,
  15. GunMonkeyINTL

    GunMonkeyINTL Senior Stratmaster

    Feb 5, 2015
    @Robins you are too kind.

    I got the transformers finished up, and the switching group installed last night. It was kind of a beast to wrestle into place, but building it outside the chassis was definitely the way to go.


    After install, I plugged it in and tested, and the switching group appears to work as planned.

    The Play/Mute switch kills the HT, and, in Play mode, the two phases of 330 vac HT shows up either at the 5y3’s anodes or the SS diode inputs, as dictated by the rectifier switch.

    The LED indicator scheme works (seal clap), and the 5y3 filaments only get powered when the Play/Mute switch is on ‘Play’.

    I had a moment of confusion when I was first testing it, though:

    With my DMM leads connected to pins 2 & 8 of the 5y3, with the tube rectifier selected, and the amp in ‘Play’, I got the 5.8 vac I was supposed to have. But, when I switched it to ‘Mute’ or ‘SS’, the voltage at my 5y3 filaments jumped to 8.9 vac, instead of dropping to 0/near-0. Obviously, I got concerned that my tube’s filaments would risk burning out.

    I poured over the schematic, my build drawing, and the actual wiring, and could not find how it could be getting any significant voltage.

    On a whim, I tried it with my other meters. I have a second, cheap DMM from Harbor Freight, and an analog one from Radio Shack. Neither of those showed the mystery voltage.

    In fact, when I hooked my main DMM and my analog one to the same tube socket pins, and powered on, they both showed 5.8 vac when the pins were supposed to have it, but, when I switched to Mute or SS, the DMM would show 8.9 vac, while the analog one showed 0.

    Still not sure what that was about. I put “ghost ac voltage” into Google, and it turns out that’s actually a thing, but I’m not sure, with the little bit of reading I did, if it’s even possible on the scale of my little tube amp. Either way, I’m going to double check it with the tube in place today- hopefully I don’t fry the filaments.
    Last edited: Dec 21, 2017
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  16. Robins

    Robins Dr. von Loudster Strat-Talk Supporter

    Dec 22, 2010
    Well, you are running AC high voltage wires and low voltage wires nearby. But also check for bad solder points and "ground" issues.
    You always need to put away low voltage cables from high voltage cables esp. if they run AC.
    Some times it can be a bad thing twisting cables (not in your case) esp. cables running from the output transformer.

    All the best,
  17. GunMonkeyINTL

    GunMonkeyINTL Senior Stratmaster

    Feb 5, 2015
    Twisting the OT leads would be a bad idea?

    It seems like the primary leads, coming from out of phase power tubes, it would be a good idea to twist them- albeit they shouldn’t have much current in them.

    The secondary wires would all be in-phase (I think), so it shouldn’t matter, but they always seem to be twisted in others’ builds.
    Bodean likes this.
  18. Robins

    Robins Dr. von Loudster Strat-Talk Supporter

    Dec 22, 2010
    OT -
    Drilling cables mean you have to use longer cables - not always a good idea. Running them in parallel should be nearly as good and you can use shorter cables.
    Drilling low ohm carrying cables do not have any effect at all. A speaker cable is not twisted and not shielded.
    But twisted wires look neat and are a good way to make an amp and the layout look clean.

    All the best,
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  19. GunMonkeyINTL

    GunMonkeyINTL Senior Stratmaster

    Feb 5, 2015
    And, as always, that totally makes sense when you say it. Thanks!

    My understanding to this point has been: out-of-phase AC lines get twisted. All others only get twisted if it supports neatness.

    I did (hand) twist both the primary and secondary leads of the OT. I twisted the primary lines because they were coming from the out-of-phase outputs of the power tubes (although they are low current, like you point out), and I loosely twisted the secondary leads just for neatness.

    My run from the OT to the impedance selector is only about 2.5”, and I didn’t have to extend them, so I didn’t give up more than 1/2” or so that I could have trimmed back- hopefully not too much lost.

    ....I put the 5y3 in tonight and confirmed that the voltage my one DMM is showing is not real. The filaments light up in Tube mode, and not in SS mode- and they didn’t burn out in SS mode.

    I also measured the rectifier output and its at 352 vdc. Will have the SS rectifier hooked up tomorrow and will get to measure the difference.
    Robins likes this.
  20. sumran

    sumran Fan of Leo

    Mar 7, 2010
    Gainesville, FL
    An inspiring job! I need to build one of those to compliment my Fender style amps. Your work is first rate. You are learning from a great mentor! I know you will enjoy the ride and the result. Thanks for sharing.
    Robins and GunMonkeyINTL like this.