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Radio to Guitar amp - 1st Schematic and Sound demo

Discussion in 'Shock Brother's DIY Amps' started by dougsta, Mar 9, 2020.

  1. dougsta

    dougsta Tele-Meister

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    This follows on from my project to use as much as possible from an old radio (project thread https://www.tdpri.com/threads/my-1st-project-a-1950-philco-radio-made-in-gt-britain.1010252/ )

    Here is my version 1 design, I've used the triodes from the 7C6 and 7S7 loctal tubes and the 6V6GT power tube. There will be a pentode input stage coming later using the 7B7.

    I've kept it simple and based it on a champ type amp just to get some noise out of it, it's also the 1st ever schematic I've drawn, feel free to comment on any mistakes etc. (I used Eagle 9 from Autodesk for the drawing)

    Sound clip to follow.


    Doug
     

    Attached Files:

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  2. dougsta

    dougsta Tele-Meister

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    And a photo of the prototype
     

    Attached Files:

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  3. dougsta

    dougsta Tele-Meister

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    Here's an mp3 sound clip, guitar straight into the amp.
    The mic is an old 1970s Sure Unisphere into a Focusrite Saffire firewire ($15 on ebay) on Windows 10.
    Windows 10 doesn't support the device, just need to disable driver signing to get it to work :)

    Recorded on on the free version of PreSonus Studio One v4


    Doug
     

    Attached Files:

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  4. Mr Ridesglide

    Mr Ridesglide Friend of Leo's Silver Supporter

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    Excellent. Sounds like Tom Dowd recording
     
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  5. FenderLover

    FenderLover Poster Extraordinaire

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    Cool!
    Do you find the response to feel a little spongy? I can't listen to the demo here at work. The first C-R filter before the B+ is essentially a pretty large sag resistor. (edit: but not really. See below) If you like it, of course it's fine.

    If you wanted to experiment a little, you could move the B+, Screen grid, and preamp connections upstream by one node each on your power supply and maybe get a little more punch out of it.
     
    Last edited: Mar 9, 2020
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  6. dougsta

    dougsta Tele-Meister

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    @ FenderLover, I used the caps and resistors that came with the amp, cap can looked like it had been replaced since new.
    I was just doin' some blues noodlin', didn't push the 6v6 much.

    Guitar is a tele I built (neck is from China tho) I wound the neck pickup with a 5k tap/8k tap, dpdt switches on the pots.


    Doug
     
    Last edited: Mar 9, 2020
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  7. ArcticWhite

    ArcticWhite Tele-Afflicted

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    Would that existing resistor be adding sag or reducing it?
    Also, I thought SE amps didn't have sag because they always run at full power..
     
  8. printer2

    printer2 Poster Extraordinaire

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    Didn't realize you had two triodes to work with, sounds fine.
     
  9. The Angle

    The Angle Tele-Holic

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    I'm actually more interested in preventing my amps from behaving like radios, since my house seems to be an RFI hot spot.

    But that conversion looks cool and sounds great.
     
  10. dougsta

    dougsta Tele-Meister

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    @printer2 yes, 2 triodes, the triode in the 7C6 has an amp factor of 100 like a half a 12ax7.
    The bonus triode in the 7S7 was designed to function as an rf oscillator but I just thought I'd give it a try and listen to how it sounds.

    I could add a 2nd input using the 7B7 pentode though it might be a bit hissy.
     
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  11. dougsta

    dougsta Tele-Meister

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    @ The Angle, thanks for the feedback. We don't see many Philco radios in the UK, the vintage AlNiCo speaker was worth the buy alone. Add in the PT/OT/6V6GT and chassis for a steal of a deal.
     
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  12. RoscoeElegante

    RoscoeElegante Friend of Leo's

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    Great stuff!
     
  13. FenderLover

    FenderLover Poster Extraordinaire

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    Looks like 470 ohms will bring down B+ 15-20V.
    You're right about the SE running at full power. We can get compression when creating square waves but that'll happen if the resistor is there or not.
     
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  14. ArcticWhite

    ArcticWhite Tele-Afflicted

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    So no sag either way then?
    Please Educate me. I only know enough to be dangerous. Just wanna know more.
     
  15. FenderLover

    FenderLover Poster Extraordinaire

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    If you're well filtered, like this one, sag is unlikely. Sag involves the attack on a note. If the reservoir cap is not large enough to hold a reasonable charge it could sag and/or cause ghost notes. This one is also biased much closer to center with the lower B+. A SE design (most) that use High B+ and then biased to 100% plate dissipation will not be center biased and will clip asymmetrically, creating a difference in current, and sag, but not nearly as pronounced as an AB amplifier. I edited my earlier post since you spotted it.
     
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  16. dougsta

    dougsta Tele-Meister

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    @FenderLover - Thanks for your feedback, this is my 1st non kit build and a great way to learn.

    The power section uses the components from the original radio with the exception of the rectifier which is an EZ80 in a noval-octal socket adapter I frankesteined together, the original 6X5GT was down 80v from the spec in the service guide I posted in the main thread for the radio->amp conversion. I added the final C-R stage, the 16uF is actually 2 new 33uF/200v caps in series.

    If I replace the old multi cap can rated at 350v and use diodes instead of the tube rectifier that would make for a lot more dc volts to start with, that should tighten it up a lot and be able to push more power into a larger external speaker.

    Doug.
     
  17. dougsta

    dougsta Tele-Meister

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    I tried the amp with SS diode rectification, here are the readings

    B+ 322v under load, starts at 365v at power up so 5% over the rating of the can caps but for only 10 secs while the heaters warm up to operating temp.
    B+1 308v
    B+2 300v
    B+3 287v (I tried a 10k resistor which gives 276v)

    tube voltages
    plate k
    7C6 212 1.3
    7S7 101 2.8
    6V6 280 18


    Sound clip attached

    Doug
     

    Attached Files:

    Last edited: Mar 10, 2020
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