5C3 on the cheap

Discussion in 'Shock Brother's DIY Amps' started by Prophetsnake, Jan 15, 2020.

  1. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    Okay, my buddy and I have commenced the 5C3 build. The way it's working is that he is doing all of the work while I stand over his shoulder and annoy the bejezuz out of him.
    We fabricated a chassis out of .040 aluminium and that came out rather well for his first go at metal work.

    All of the little bits are good quality, but I got my hands on some very cheap transformers. I think the OT will work perfectly in this amp, but it's gonna be suck it and see with the PT, which came out of an old oscilloscope. the oval mounting holes and extra access holes for the PT are to provide for a proper transformer in the event that the intended one turns out to be a failure.

    We're in the process of doing the eyelet board now. I'll post some pics as and when.

    Please, please, if anyone has any tips or sees that we're doing anything stupid, by all means, pipe up!

    IMG_20200113_153224.jpg IMG_20200113_153216.jpg
     
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  2. Bob Mc

    Bob Mc Friend of Leo's

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    A true test of friendship, good luck!
     
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  3. Wally

    Wally Telefied Ad Free Member

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    Out of curiosity, what tubes were in that o’scope?
     
  4. Preacher

    Preacher Friend of Leo's

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    I think you are missing an input jack hole but other than that I have to say "well done!"

    highly recommend you look at @robrob web page for his NFB switch and incorporate that into your build somewhere since you are in the beginning.

    I built mine in a similar fashion on a serious budget.
    I think I have all told in it about $600 worth of parts and hours and hours of labor which I don't want to think about.
     
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  5. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    Thanks! The 5C3 only has three inputs, though. I put them in that sort of pattern to keep them down at one end and to provide a little room for a NFB switch.

    Two 6J5 and three 6SN7, so eight plates at 9ma each for a total of 72, plus the CRT plates, and I can't get a figure for those, though I think the demand from them is pretty light. The primary also supplied a second filament winding and a 1000 V winding for the CRT, so I'm thinking the primary is up to the job, just not 100% on the HT secondary. It produces about 350V with no load on either side of the centre tap, so I'm hoping when everything is lit up it will be about 330.
     
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  6. Nickfl

    Nickfl Tele-Afflicted

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    Are you using a 6sc7 for v1? If you haven't already acquired your tubes you might consider switching to 6sl7 for that position so you can split the cathodes of the input stages and do one cathode bias and one grid leak. Give it a little more variety plus 6sl7 are generally a little cheaper and easier to come by then 6sc7.
     
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  7. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    Thanks! Yeh, already gone down the 6SL7 route, and was actually considering doing something along those lines, but I was a little intimidated by the unusual inputs on the 5C3.
    So, if I were to make one channel cathode bias, could I pretty much just cut and paste a 5E3 or Champ input and leave the other side completely stock?
     
  8. Dacious

    Dacious Poster Extraordinaire

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    I would build it right to start with as a first build. Adding stuff before you flip the switch can take you down many rabbit holes trying to sort issues.

    Once you've verified it works and voltages correct and you have a stable amp with output, then incorporate changes.
     
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  9. Preacher

    Preacher Friend of Leo's

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    oops I had 5E3 in my head!!
     
  10. muchxs

    muchxs Doctor of Teleocity

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    Be careful with old 'scope transformers. They often have a high voltage winding with kilovolts on tap.
     
  11. Nickfl

    Nickfl Tele-Afflicted

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    Yep, that's basically what I did. Check out this thread. There are some things in my power supply that wouldn't apply to you since i was using a PT and field coil speaker from an old radio along with 6k6 power tubes, but the inputs would be exactly what you want.

    I did have some trouble with motorboating due to alterations to the power supply I made to work with my salvaged parts. See post #7 in that thread. I don't think you'd have to deal with that if yours is faithfully copying the original power supply, and its an easy enough thing to solve if it does crop up.
     
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  12. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    Thanks! Yeah, I have the schematic for the scope and it does indeed have a 1000V lead which I mapped as well. That will be carefully parked! The windings are very weird altogether, but the ones I want seen straightforward enough. It also has a dual rectifier setup which has me baffled.
    The windings I want all seem to be there, so I'll give it a whirl anyhow. I'll just wind it up gently and keep an eye on it all as the voltage increases.
     
  13. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    IMG_20200115_164542.jpg


    I think he has done a nice job on the board for a first go. The filament wiring isn't going as tidily as we were hoping, but it's certainly better than my first attempts. (or even my latest attempts)
     
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  14. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    OK, it's done and it works, but it wasn't completely smooth sailing. The PT was a bit of a disaster. With the amp finished, we powered it up through a current limiter and got a glowing bulb with no tubes installed at all. Ugh. It didn't take too long to determine that I'd used a pair of 5V leads that were attached to the CT. Never mind, there are two pairs of 5V leads and I swapped them. Better. The HT voltage was a bit his - about 360V - but We tried it with the 5Y3 installed and supplied it with 120V. The B+ went up to 540V! Well, continuing down that road seemed like a bad idea, so we abandoned all notions of using that tranny.
    I jury rigged a 275V toroidal transformer and bridge rectifier into it to see if the rest of the circuit was ok, and to make some noise. It did! And it sounds great too, thought there's a bit of hum, but we can fix that.
    We were tired, so we've shut it down for tonight and will check voltages and bias tomorrow.

    Well worth it to see the grin on my friend's face!
     

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    Last edited: Jan 19, 2020 at 5:10 PM
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  15. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    bob5c34.jpg bob5c33.jpg bob5c32.jpg Bob5c31.jpg
     
  16. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    New day and fresh eyes on the amp. We biased it and with a 470 ohm cathode resistor, we got 10 Watts on V3 and 10.3 on V4 with a pair of 6V6skis. (Russian 6P6S)
    I'm inclined to leave it like that as it sounds great!
    All of the voltages in the output section appear to be good.

    We altered channel 2 as per Nick's suggest, basically inserting a Champ input, and left Channel 1 as drawn. There's no grid stopper at all, just a .1 cap on the screen and already I can tell that this is a bad thing, so I'll try a couple of resistors in there. There's a good bit of hum throughout and it varies depending in which of the three controls is where, as one might expect with a Deluxe control, but I'm thinking that most is getting in through that one input.

    The voltages are a bit funny on the preamp tubes. Where I'd expect the two plates on one tube to be very close, they just aren't. A bit of tube swopping might shed a bit of light on this.

    NFB isn't installed yet. I'm going to temporarily put a 1M pot in the line and search for a setting that sounds good, then install a fixed resistor. When I get the noise under control, Bob will record something.
     
  17. Wally

    Wally Telefied Ad Free Member

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    I am curious as to why you are using such a large bias resistance on the 6V6s?
     
  18. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    I used a 250 on my 5E3 when I built it and was pulling 15 watts through each of my 6V6s. I had to up the cathode resistor to 560 ohm to get them down to something like 9.5. So, when we built this one I decided to install the 470 from the outset and hopefully get it into the ballpark.

    Did I do bad?
     
  19. Wally

    Wally Telefied Ad Free Member

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    Ballpark?? Each to his own, but a cathode biased Push/pull is best imho with the plate dissipation at or near 100%. You are biasing to a plate dissipation that most would use for a fixed bias circuit. There is a reason why Leo used a 250ohm there.
     
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  20. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    Okay. I was under the impression that the dissipation limit was to be treated as a limit, not a target, and I also thought that modern mains voltages made the old original cathode values obsolete. No?
     
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