Help with my Supro 1624t clone

Discussion in 'Shock Brother's DIY Amps' started by Prophetsnake, Mar 4, 2020.

  1. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

    Posts:
    433
    Joined:
    Aug 2, 2016
    Location:
    ireland
    Hi all, I built this thing about a year and a half ago. It uses 6973 tubes.
    It works, but it isn't blowing me away.
    Here are the issues I'm having:
    No matter what type of guitar I play through it, it sounds a little brittle as I go up the neck. If I play a single coil guitar through it, this gets noticeably worse unless the playing is very aggressive. With the Tele in particular, fingerboard noise is very noticeable. By that I mean that the usual finger squeaks and strings slapping on the frets are amplified to a ridiculous level. Sounds like a hammer on an anvil.
    By the way, this same guitar sounds great through any other amp it's played through.

    When I researched the amp before the build, I came across something that suggested that part of the amp's character was the relatively small 10W OT. I've since read that they only used those on the later 6V6 models, and that the early ones used something else.
    Apart from that, I'm thinking caps, but really, I don't know, and I don't want to diagnose simply by buying new parts and trying them, though this may be the only way in the end.

    Any suggestions greatly appreciated!
     
  2. BigDaddy23

    BigDaddy23 Tele-Holic

    Age:
    46
    Posts:
    504
    Joined:
    Jan 4, 2019
    Location:
    Australia
    Can you post the schematic you used/ended up with?
     
  3. Badside

    Badside Tele-Meister

    Posts:
    430
    Joined:
    Jul 5, 2011
    Location:
    Montreal, Qc, Canada
    Did you use ceramic coupling caps? I didn't believe it at first but they are a big part of the sound.

    That said... I never really liked mine either. I ended up blackfacing the preamp which made it glorious.
     
  4. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

    Posts:
    433
    Joined:
    Aug 2, 2016
    Location:
    ireland

    This is the circuit there. Are you talking about the .005s after the first stage? Then no, they're orange drops. You're saying that they should be ceramic? And by blackfacing, I assume you mean that you just cut and pasted an entire blackface circuit all the way through the PI?
     

    Attached Files:

  5. wabashslim

    wabashslim Tele-Afflicted

    Age:
    68
    Posts:
    1,212
    Joined:
    Nov 26, 2005
    Location:
    Sonorous Desert
    Speaker...
     
  6. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

    Posts:
    433
    Joined:
    Aug 2, 2016
    Location:
    ireland
    Is a Weber 12A125. I've piped other amps into it and it's fine. I've also swapped tubes and chopsticked the amp.
    It does have one other issue. The trem doesn't work at all and never has. I've investigated but come up with nothing. The amp sounds the same with or without it switched on or even with or without the tube in that position installed.
     
  7. sjoko

    sjoko TDPRI Member

    Posts:
    54
    Joined:
    Mar 4, 2020
    Location:
    The Netherlands , Amsterdam
    This is a very simple build amplifier , but....
    If you want to get it fixed , you need an oscilloscope , tone generator and dummy-load , to check the output wave-form
    and check why the oscillator is not working properly

    If you don't have this equipment , then it's more like gambling
     
    Last edited: Mar 4, 2020
    Prophetsnake likes this.
  8. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

    Posts:
    433
    Joined:
    Aug 2, 2016
    Location:
    ireland
    Haven't got an oscilloscope, so gambling it may be. Not really that pushed about the tremolo. but a good tone would be nice.
    No takers on the OT being the culprit? It's crossed my mind that because it's only 10W and the rest of the contraption is more like 14 it might be simply saturated. On the other hand, the problem is at lower volumes. It sounds better when it's loud.
     
  9. Badside

    Badside Tele-Meister

    Posts:
    430
    Joined:
    Jul 5, 2011
    Location:
    Montreal, Qc, Canada
    Yes, all the .005uF caps "should" be ceramic for the real deal. It's part of the sound of the amp. Then again, there are various types of ceramics, some that would be as clean as film caps. I don't remember what I used, they were 0.15$ a pop at the local surplus store and they messed up the sound beautifully.

    But in the end, while it was a fun amp at home, it needed to be too loud to not sound thin. It had a very limited use for me. So yes, basically an AB763 front end sans trem sans reverb (input stage, tone stack, recovery stage, then into PI). Also added negative feedback. Made it a real fun amp due to the paraphase inverter. But a friend needed a clean amp that would take distortion pedals well and this didn't (good with overdrive, but all mushy with heavier sounds) so I switched to a LTPI. Long story short, I now have a simplified Deluxe Reverb with 6973 tubes. Had to leave the 5Y3 rectifier in to not fry them so it's a lower voltage DR.

    It's a very useful amp now, but it's also a very generic amp.

    For reference, I use Princeton Reverb iron (Hammond 290BX and 1750E).

    Either way, you can give the ceramic coupling caps a try. Helps getting that Supro tone. But ultimately, it's an amp that has a very narrow useable range IMHO, and doesn't have nearly as much mojo as a 5E3
     
  10. jsnwhite619

    jsnwhite619 Friend of Leo's

    Age:
    35
    Posts:
    2,933
    Joined:
    Sep 10, 2013
    Location:
    Georgia
    I don't know anything about Supro's, but looking at the schematic, I can suggest one thing to maybe try. The filter caps, or at least the preamp node. I've built a couple Harvard/Vibrolux circuits, and built one last year to experiment with. I used a 10uf cap on the last preamp node because I noticed several other 6v6 amps that had that, so I tried it, but the first two caps were both 22uf. It sounded thin, bright, painful, and just bad. I tried two different secondary taps, ran the voltage 75-80v higher & lower, changed bypass caps, coupling caps, anything that could add back some bass. Finally, I swapped the 10uf for another 22uf filter cap and BOOM. There it was and everything that I didn't like about it was solved.

    If you followed that schematic exactly, I'd swap those 10uf's for 20-22uf's across all three caps and see what that does.
     
    Wally likes this.
  11. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

    Posts:
    433
    Joined:
    Aug 2, 2016
    Location:
    ireland
    Interesting. I'd assumed I'd done something wrong, but I guess this is just the nature of the beast. I have a couple of tweed amps that I've built, but I was hoping this would be more useful than it is.
    I will definitely try swapping the caps for starters. I also have a 1750e in my shop that's going into a friend's build, so I can tack that in and see if it makes a difference.
    Thanks for that, very useful info!
     
  12. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

    Posts:
    433
    Joined:
    Aug 2, 2016
    Location:
    ireland
    Interesting. I thought the larger caps would only quell humm!
    Unfortunately, they're already all 22uf, so that's not likely a problem. But I did learn something today, so thank you!
     
    jsnwhite619 likes this.
  13. knavel

    knavel Tele-Meister

    Posts:
    217
    Joined:
    Jul 21, 2014
    Location:
    London
    I am not amp technical. But I do have what I believe is the Gretsch version of the amp (6156 Playboy), and it's 1965, not 6v6 (6973 valves) and factory Jensen 12''. It's my favorite amp. My recollection is that taking the chassis out is a pain but if you want photos of stuff I can access for reference let me know. This kind of poor photo is all I have of any transformer.
     

    Attached Files:

  14. Snfoilhat

    Snfoilhat Tele-Afflicted

    Age:
    39
    Posts:
    1,396
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    Is this brittle tone and high-frequency related volume stuff the same in both channels?


    2 things jump out at me from the schematic -- some of the coupling cap sizes look very small at first glance, so maybe this thing's preamp really was designed to be tilted toward treble and in that frequency range your instrument is putting out a ton of finger noise.

    Second, that phase inverter is strange to me and I wonder if it being out of balance could account for your frequency-dependent tone issues.

    Last question, did you build you amp to that schematic above precisely, or are there any substitutions?
     
    D'tar likes this.
  15. Badside

    Badside Tele-Meister

    Posts:
    430
    Joined:
    Jul 5, 2011
    Location:
    Montreal, Qc, Canada
    The phase inverter is strange indeed (http://www.valvewizard.co.uk/paraphase.html), but it's a big part of the Supro sound, as are the small caps (which were all ceramics in the original too).

    It is indeed a very treble heavy amp. It warms up at higher volume, but it's not for everyone.

    One can modify it with bigger coupling caps, and a more balanced PI, but at that point you're basically building a different amp.

    For reference, these couplings caps combined with the 500k volume controls have a -3dB point around 63Hz. They thin out clean tones, but they tighten up overdriven sounds.
     
    Snfoilhat likes this.
  16. D'tar

    D'tar Friend of Leo's

    Posts:
    2,585
    Joined:
    Jan 11, 2013
    Location:
    WNY
    I imagine that input could be easily miswired. I'm not certain what's even happening there at first glance...


    upload_2020-3-4_10-31-8.png
     
  17. Badside

    Badside Tele-Meister

    Posts:
    430
    Joined:
    Jul 5, 2011
    Location:
    Montreal, Qc, Canada
    The top input is the Treble input, the cap acts as a high-pass filter and in this mode you only get a 100k input impedance.
    (See EDIT) The bottom input is the Bass input, in this case the cap acts as a low-pass filter (it connects to ground via the unused Treble input). (See EDIT)
    If you jumper both inputs then you get a slightly treble lift courtesy of the 5nF cap in parallel with the 100k resistor.

    A fun way to use this is to connect to the Treble input of one channel and the Bass input of the other. Then you can use the 2 volume controls to balance it the way you prefer.

    EDIT: Got this wrong, didn't realize the top jack only grounds via the bottom jack. So the bottom input is actually full range (save for the effect of the 100k resistor that will for a low pass filter at a very high frequency).

    EDIT2: Top input should NOT ground via the bottom input. See the schematic I shared below.
     
    Last edited: Mar 4, 2020
    Prophetsnake and D'tar like this.
  18. D'tar

    D'tar Friend of Leo's

    Posts:
    2,585
    Joined:
    Jan 11, 2013
    Location:
    WNY
    Maybe its just me but it appears the lower input is shunted to ground at the tip via the top input sleeve, and the lower input sleeve would be connected to the 100k possibly requiring isolated input jacks?
     
  19. Badside

    Badside Tele-Meister

    Posts:
    430
    Joined:
    Jul 5, 2011
    Location:
    Montreal, Qc, Canada
    Oh god, I see what you mean now. Yeah... this is badly drawn. My memory of the circuit filled in and I skipped over this.

    This will make more sense:
    [​IMG]

    As you can see: top input has a high-pass filter and a low impedance, bottom one is full range with a higher input impedance.

    The top input doesn't sound very good on its own.

    (Note that the cathode resistor on the power tubes should be 250 ohms, not 250k ohms!)
     
    Last edited: Mar 4, 2020
    D'tar and Prophetsnake like this.
  20. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

    Posts:
    433
    Joined:
    Aug 2, 2016
    Location:
    ireland
    Thanks man. Useful gen.
    I have looked at pics of the innards of quite a few and since they are P2P I don't think it will tell me much. The pic of the transformer was, though, since Magnetic Components make that model and it's exactly what I have in mine. So I can probably scratch that off the list of possible culprits here.

    I'll have a look, but I think they're OK. There's no appreciable hum, and I imagine there would be hum and or squealing if I hadn't wired it up properly. The schematic was a bit sparse, so I had to make sure I knew what I was nailing together as I went along.

    Yeh, as I built it I was thinking that some of the inputs seemed a little odd, but I built it as per the info I had so as to keep the circuit as original as possible. As you can see, the inputs are often radically different, but oddly enough, the tone isn't affected that much by which you use. I have jumpered the channels together in all possible combinations, and the tone is more robust, but only a little.
    I have one recording on Soundcloud I did right after I built it. This is a Les Paul with Duncan JB/Jazz pups, and I was pleased, but not jumping for joy. With single coil pups ( good ones) it sounds very thin indeed. I'll do a quick iphone recording of that and post.

     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.