Supro model 24

Discussion in 'Shock Brother's DIY Amps' started by Rock_Glenn, Jul 29, 2018.

  1. Rock_Glenn

    Rock_Glenn TDPRI Member

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    I have a vox cambridge chassis and cabinet that I'd love to turn into a supro 24. I have a set of classic tone 18w transformers also kicking around. PT secondary is about 15 volts low on each leg and the OT primary impedance is abour 2k ohms high depending on secondary impedance.

    I'm planning on playing with voltage dropping resistors in the power supply, and probably using the 4 ohm tap secondary of the OT. I'm not terribly concerned that this be an exact recreation of a 24 but would like it close sound wise.

    Should I attempt the build with the 18w trannies? Any pointers? I'm mostly concerned that the OT impedance will be too high for the 6973 tubes.
     
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  2. Nickfl

    Nickfl Tele-Afflicted

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    I think you should easily be ok on the PT, 15v either way isn't all that big a deal and its easy enough to bump it up with a different rectifier tube or bring it down with some zener diodes.

    As for the OT, it kind of depends, what is your target primary impedance? A 2k deviation is one thing if you target is 11K and quite another if you need 4K... Also, where did you get that figure? I googled it and it was easy enough to find a supro 24 schematic but it had no detailed info on the OT. It amazes me how often schematics leave out such an important piece of information... If you are going off of the 6973 datasheet, thats a good starting point, but not necessarily what Valco would have used. From what I've seen it was not uncommon for amp manufacturers to deviate from the recommended primary impedance on the datasheet, especially if they were running higher than standard plate voltages.

    One way or another I bet you can make it work. You may have to deviate from the original in some ways, but thats not necessarily a bad thing.
     
  3. GeorgeG

    GeorgeG TDPRI Member

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    If you mean the Supro 6424, that used a pair of 6973's at 350V plate voltage with a 6487 output transformer which crosses to a Classictone 40-18063 which has a primary impedance of 7.5K with an 8ohm secondary, or a Mojotone 7029965 which has an *k primary impedance, plate to plate.
     
  4. Rock_Glenn

    Rock_Glenn TDPRI Member

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    The 6424 is the model. I see the 18063 is 7.5k at 8 ohms and apparently only rated at 10 watts. Maybe a stressed OT is part of the sound?

    The 18w is the 18067, 9.2k at 8 ohms. Its actually closer than i thought. But the valco 18063 is pretty inexpensive so maybe i'll just grab that.

    The PT however is kind of expensive.

    Supro is 305 on each leg of secondary a.c.. the 18w is 290 on each leg. A simple rectifier tube swap could make this 100 percent moot. So this could work, right?

    Again noticed the PT is rated at 10 watts according to classic tone. I know these ratings are less than scientific but what's the deal with that?
     
  5. GeorgeG

    GeorgeG TDPRI Member

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    You can play with rectifier tubes and dropping resistors in the power supply filtering chain to tweak the B+ voltages. I use a variac in my test bed set up and even though varying B+ does change volume and tone, its a small impact when your tweaking plate voltage +/- 10 volts or so. As far as transformer specs are concerned, its like amplifier power ratings at best buy. Your much better off looking at the weight to gauge power output or handling. there is a reason why a 5x100 watt Sony receiver is $100 and weighs 20 lbs and 5 x Mark levinson 33h monobloc weighs over half a ton. Some of the "magic" in vintage tone are things that depart from the theoretical and would never be done in the ultra linear audiophile world. Undersized sagging power supplies with compression, undersized OT's for saturation, intentionally imbalanced phase inverters, pre amps biased cold or hot, wacky frequency shaping bypass and signal path capacitors ..... Half the fun is seeing which audiophile abomination is vintage guitar gold. I had a Supro build on my list but I was screwing around with all of these things in a SE 6v6/EL84/6973 build with a $29 8" speaker and BAM, out came LZ1. I spent the night playing 45 year old songs like Communication Breakdown with my plate of spagetti rats nest and I didn't even go find my Telecaster. I ran B+ rails from 250 to 350 volts and cathode bypass capacitors from zero to 100uF ....

    Happy tone chasing.
     
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  6. Meteorman

    Meteorman Tele-Holic

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  7. Rock_Glenn

    Rock_Glenn TDPRI Member

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    Thanks for the advice. I'm going to use what i have and just have fun with this one.
     
  8. Rock_Glenn

    Rock_Glenn TDPRI Member

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    I saw his layout while researching. His organ transformer pulls are out of spec in the opposite directions, but certainly show the 6973 aren't picky tubes. I like some of the mods steve did as well, like the channel jumper switch.
     
  9. GeorgeG

    GeorgeG TDPRI Member

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    If your going to tinker around then make sure you visit Rob Robinettes amp stuff, although I think he hangs out here so you may already have seen his resources.
     
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  10. Rock_Glenn

    Rock_Glenn TDPRI Member

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    Update on this, and help requested.

    I have everything built, B+1 spot on, B+2 and 3 were low, playing I had a great sounding clean guitar, would not go into overdrive even on 10, and no working tremolo.

    Pulled tubes, played with dropping resistors to get a perfect 355V, 350V, 290V B+ 1, 2, 3, respectively. Also got rid of the 100ohm resistor after b+2 to power tubes. (was losing 3-4 volts).

    Same problem, measure anodes on preamp tubes and v1 reads 130V.

    Change V1 to different tube and things get weird. Anode (pin1) now reads 270V.... and no sound at all.

    Change V1 to third tube and I'm right back at 130V on pin1 of v1. and when I did this B+3 now reads 270V... dropped 20 volts just changing a tube.

    Change V1 to a fourth tube and pin1 v1 reads 30V... yes 30.

    V2 and V3 voltages are perfect on both anodes, v3 oscillates and reads perfect 140V when oscillator is grounded.

    Cahode voltages are very low, like .6 volts on v1 and v3.

    I've gone over the schem 10 times, I've re-read every resistor and checked every cap for shorts. The only oddity is off of pin8, 2.2k resistor actually reads 1.9k.

    What is going on??? I don't understand the voltage swings, can someone point me in the right direction?
     
  11. mjcyates

    mjcyates Tele-Meister

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    I actually have the proper chassis for that amp that I don't need if you are interested in it let me know.
     
  12. Rock_Glenn

    Rock_Glenn TDPRI Member

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    It actually fit great in the chassis I have. I really just want to figure out these voltage issues, I'm hoping the symptoms will jump out at someone.
     
  13. Nickfl

    Nickfl Tele-Afflicted

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    Post some pictures, maybe someone will see something you missed. It sounds like maybe a a bad connection somewhere to me. Double check all the solder joints, especially on the v1 pins and reflow anything that looks suspect. Do you have lightbulb limiter? If so is it showing any sign of a short or excess current draw?
     
  14. Rock_Glenn

    Rock_Glenn TDPRI Member

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    I pulled v3 and now it overdrives nicely, when I get a minute I'm going to reflow every pin on v1 and v3 and take voltage measurements.

    No sign of shorts and currents are good. Reflowing cathode pins on tubes 1 and 3 gives me hope.

    It's also possible both tubes I tried in v3 were dodgy. Would explain cathode issues and lack of tremolo.

    Also volume seems low like between a tweed champ and deluxe. I know these are low output but seems like it could barely keep up with a heavy handed drummer on 10.
     
  15. Rock_Glenn

    Rock_Glenn TDPRI Member

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    I should add, I'm using a 5V4 rectifier tube to make up voltage out of the 18W PT.
    First pulldown resistor is currently a 560 ohm, second is a 33K.
    I replaced the 100 ohm resistor going to the 6973 grids, as I now understand why it was there.

    Re-flowed some solder, these voltages are with V3 pulled and a known good 12AX7 in v1:

    B+1 - 347
    B+2 - 343
    B+3 - 282

    V1
    1 - 107
    3 - .71
    4 - 3.13ac
    5 - 3.13ac
    6 - 125 (Crackles on probe contact)
    8 - .88 (~1khz hum on probe contact)
    9 - 3.13ac

    v2
    1 - 150 (Crackles on probe contact)
    3 - 1.1
    4 - 3.15ac
    5 - 3.15ac
    6 - 174 (Lots of crackle on probe contact)
    8 - Zero Volts
    9 - 3.15ac

    I will try to get pics up, but if you see a big red flag on any of these maybe I can narrow my search for the faulty connection/component.

    Thank you for your help.
     
  16. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    I've just built one of these ( well a 1624, but) and the 6424 schematic I have calls for 200V. The voltage on mine is 190 at that pin. What resistors have you installed there?
     

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  17. Rock_Glenn

    Rock_Glenn TDPRI Member

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    Off of V1pin1, I have a 270K > 100K > B+3

    There is a .047 cap to ground at the junction of the two resistors.

    The other direction off of v1pin1 I have a .0047 cap to a 270K resistor to ground and also off that cap I have another .0047 cap to volume for channel 1.

    All values verified with a Fluke 117, and caps checked for short/leaks, and ground connections checked forcontinuity.
     
  18. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    Now, don't assume that I have much of an idea what I'm doing. I'm very new to all of this. But what's the voltage between those two resistors and the power supply? And further up the line to the next node of the power supply?
     
    Last edited: Dec 27, 2018
  19. Rock_Glenn

    Rock_Glenn TDPRI Member

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    I'm with you, I've built a lot of amps. But beyond biasing it's been more of a paint by numbers than anything. This is my fourth build from scratch/leftover parts. It's the first to not work properly so it's frustrating.

    Voltage at the B+3 node where the 100K starts is 285V. at the end of the 100k where it meets the capacitor and the 270k it is 235V.

    On the backside of the 270k where it meets the .0047 cap and goes to pin 1 it is 105v and I have zero volts on the top side of the .0047 (towards the other .0047 and the 270k to ground.

    Is this cap (The .047) a good candidate to replace?

    I'll have to order one unfortunately.
     
  20. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    If it were mine, I would have thought so. It would seem to me that would be the best bet, unless there's a stray bit of solder or a little hair of wire shorting something out?

    I think you have already said that you've tried a different tube in that position?

    That reading at the cap doesn't make sense to me. Surely you'd have at least as much voltage as you're reading at the tube pin?

    .047 caps are something you will always use somewhere anyhow, so why not order a few and see?
     
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