5F1 Output Issues

Discussion in 'Shock Brother's DIY Amps' started by TATomlinson, Feb 17, 2020.

  1. TATomlinson

    TATomlinson TDPRI Member

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    So, I've reached the end of my very limited skill set fellas. I just finished a Mojotone 5F1 build. I had the squeal issue that many experience as a result of the primaries being installed backwards off the OT. I was able to address that after seeing posts on this forum.

    I found another issue with the 22k ohm resistor. It metered well bellow so I had to replace the one that came with the kit with the correct value.

    My latest issue is this. I can get sound out of both channels. Channel 2 is noticeably louder than Channel 1. My meter readings off the 100k resistors are very high compared to the chart. Most other readings are at or near the mark.

    Here's what I'm working with:
    5Y3
    Pin 2 378 v
    Pin 4 -33 mv
    Pin 6 -33 mv
    Pin 8 375 v

    6V6
    Pin 1 20.12 v
    Pin 2 -.1 mv
    Pin 3 367 v
    Pin 4 324 v
    Pin 5 15 mv
    Pin 6 N/A
    Pin 7 -.1 mv
    Pin 8 20 v

    12AX7
    Pin 1 168 v
    Pin 2 .5 mv
    Pin 3 1.5 v
    Pin 4 -.1 mv
    Pin 5 -.1 mv
    Pin 6
    Pin 7
    Pin 8
    Pin 9

    16 U Cap 370 v
    8 U Cap 324 v
    8 U Cap 300 v
    500 U Cap 20.5 v
    1.5k Resistor .7mv - schematic calls for 1.65v
    100k Resistor 295 v - schematic calls for 165v
    100k Resistor 200 v - schematic calls for 165v
    1.5k Resistor 1.5k

    All measurements were taken with the speaker plugged in a max volume for consistency. I'm not sure if that's the correct way to meter but I noticed all the values changed on the preamp section if I metered at a lower volume.

    Thanks in advance for any suggestions you have. I'll post pictures shortly.

    Tony
     
  2. TATomlinson

    TATomlinson TDPRI Member

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    I forgot to include the rest of the preamp readings
    12AX7 at max volume:
    Pin 1 201 v
    Pin 2 -.1 mv
    Pin 3 1.5 v
    Pin 4 0 v
    Pin 5 0 v
    Pin 6 296 v
    Pin 7 1 v
    Pin 8 4.4 v
    Pin 9 -.1 mv
     
  3. TATomlinson

    TATomlinson TDPRI Member

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    09660A65-1308-415E-925F-9A50AEDA1009.jpeg 8022F249-AA41-4C49-99C7-722116588522.jpeg 1E0F2B55-4727-4618-88CC-21985B907DCF.jpeg 364FEB4D-0B73-4D02-9470-A988139BB4A8.jpeg
     
  4. TATomlinson

    TATomlinson TDPRI Member

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    E2DB9452-FA06-437B-B181-A1773C576800.jpeg
     
  5. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    A very short post in the middle of the night...

    The two inputs should have a difference in volume.
    Pin 1 of a 6V6 has no function according to the datasheet.

    Did you measure at the correct sides of the 100k and 1.5k resistors?

    You get sound. Is it OK? Is there a problem? If your voltage reading are done at the correct places, I don't see how you could get any (decent) volume, as they indicate no current is flowing through the input tubes.
     
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  6. JohnnyCrash

    JohnnyCrash Doctor of Teleocity Gold Supporter

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    I’m only chiming in to ask why V1’s cathode isn’t bypassed.

    Should be a 25 uF electrolytic parallel with that 1.5k to ground.

    That’d definitely boost your volume, but I doubt that it’s your main problem.
     
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  7. TATomlinson

    TATomlinson TDPRI Member

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    I'm getting sound from both inputs. Just very low. I misspoke above. Input 1 is louder than input 2. The amp is just very quiet compared to my other amp I have that is similar wattage.
     
  8. TATomlinson

    TATomlinson TDPRI Member

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    Right. The kit didn't come with one. Not sure why.
     
  9. JohnnyCrash

    JohnnyCrash Doctor of Teleocity Gold Supporter

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    After installing that cap, I’d double check your input jack wiring. Double check the resistor values (simply read the color bands). Visually double check the jack lugs/terminals and the tip connector.

    Next, check the soldering at the tube socket. First check the input grids. Is the first one fed by the input jacks/grid resistors? Next double check the cathode. Is it the proper value resistor and bypass cap? Then check the plate resistor and wiring to the socket. Is it a 100k resistor? Is the coupling cap here the proper size/value?

    From there I’d double check the volume pot wiring. Is the proper pot lug grounded? Are the solders done well (no cold solders)? Are the other audio lugs wired correctly? Now would also be a good time to ensure the power/AC switch-pot wiring is tightly twisted and routed away from (and perpendicular to, if possible) the pot’s audio end of things.

    This should cover the input/preamp half of things.
     
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  10. TATomlinson

    TATomlinson TDPRI Member

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    Also, pin 1 is jumped to pin 8, consistent with the layout mojotone provided. I'm reading 20 volts there. Looks like that's what it should be.
     
  11. FenderLover

    FenderLover Poster Extraordinaire

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    the output bypass cap looks like it could some solder too.
    Input 2 is 6dB lower than input 1. That's how it works.
     
  12. JohnnyCrash

    JohnnyCrash Doctor of Teleocity Gold Supporter

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    The original layout has no bypass cap. Some people argue that Leo intended it that way, others argue it was mistakenly omitted from the drawing.

    Either way, it significantly increases the gain and volume.

    I would, however, not use the typical Fender value. Tweed Champs are notoriously flabby in the bass and dark on the treble. I’d use something more like a 4.7uF or even 2.2uF. It reduces bass gain below guitar frequencies. They’re lower than the E string and they only flub/fart out the bass response.

    Leo was an engineer. He was aiming for full frequency response... like people would play bass or albums/records through these things?!

    So why not streamline the unused bottom frequencies out?

    To brighten the top end I always use very low watt AlNiCo speakers like the originals. They brighten up the darkness a lot. It makes sense why AlNiCos make the originals so creamy.

    The best is a Weber CVA8. The Sigs won’t fit the tweed cab, but it’s better to have a lower watt speaker in this application. The AlNiCo chime and bright-but-not-sharp treble make a more complex, organic, and warm sounding amp—clean or dirty.

    Plus, when AlNiCos are pushed to their max they compress the top end, making them still bright, but creamy when shoved into distortion. So low watts works in this circuit better than most.

    In addition to the Weber CVA8, I also sometimes solder a bright capacitor across the volume pot. It’s a really dark circuit to my ears.
     
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  13. TATomlinson

    TATomlinson TDPRI Member

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    I assume the 25 uf cap is polarized. Which way should the + end be installed? Toward the input jack or toward the tube socket similar to the other caps? Thanks for your help.
     
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  14. TATomlinson

    TATomlinson TDPRI Member

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    Thanks. I misspoke above. That's about right. I will check the solder. Thanks again.
     
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  15. JohnnyCrash

    JohnnyCrash Doctor of Teleocity Gold Supporter

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    The cap will be polarized. Negative end goes to ground along with the parallel cathode resistor. Positive end goes to the cathode’s lug on the tube socket.

    Again though, I’d recommend something much more reasonable for guitars: maybe a 4.7uF.

    Other similar amps in the sixties used smaller coupling caps to cut bass below guitar frequencies. Instead of stuff like 0.022uF, you’d see 0.002uF or 0.01uF couplers.

    This greatly improved the bass. Champs get a lot of heat for farting out at the speaker, but if you’re not throwing unused overtones at it, it can easily handle a guitar’s full range of frequencies.

    Instead of couplers, you could attenuate (though not entirely cut) lower frequencies through the cathode bypass. Ergo my suggestion of using a 4.7uF bypass cap.
     
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  16. JohnnyCrash

    JohnnyCrash Doctor of Teleocity Gold Supporter

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    You could plug values into the Cathode Bypass Calculator (Google it).

    Here’s the response from a 4.7 cap with a 1.5k cathode resistor:



    DBF59DD9-9195-44B3-B2AE-C637EF865512.png
     
  17. JohnnyCrash

    JohnnyCrash Doctor of Teleocity Gold Supporter

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    Keep in mind the low E string on a guitar is around 80 Hz.

    So a 4.7uF cap would start to roll off bass just around the low E and below. It’s a somewhat gentle roll off (see db side).
     
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  18. TATomlinson

    TATomlinson TDPRI Member

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    Thanks for that. Cool reference.
     
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  19. TATomlinson

    TATomlinson TDPRI Member

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    Well, I added a bypass capacitor and the gain improved. I'm still having an issue with volatage off the 100k resistors. I'm starting to think there's an issue with the volume pot. It's a little noisy from 1-3. There's no real sound until around 9 or 10. I've done the chopstick test and everything checks out ok. It's kicking my butt.
     
  20. CheesyCucumber

    CheesyCucumber TDPRI Member

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    364FEB4D-0B73-4D02-9470-A988139BB4A8.jpeg

    It looks like you have your NFB resistor (22k) and v1b cathode resistor (1.5k) swapped. That'd be a strong contender for low output since you'd basically have lots of negative feedback :lol:
     
    Last edited: Feb 20, 2020
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