More “Mostly” Mojotone 5e3, version 2! (The Yellow Jupiter build)

Discussion in 'Shock Brother's DIY Amps' started by BobSmith, Jun 17, 2019.

  1. BobSmith

    BobSmith Tele-Meister

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    Polished stainless sounds good. I guess that’s where I’m going with the next build. (Yeah, there will probably be a third build)
     
  2. BobSmith

    BobSmith Tele-Meister

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    I was planning to use metal oxide between the filter caps.

    Do you have some favorite places to use CC’s? I was strongly advised on the metropolous forum to avoid CC’s on the inputs. Here on TDPRI, no one seems to make a big deal about it.
     
  3. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Great question... often starts component wars though. None of that here, gentlemen!!!

    Many top experts (Merlin, Aiken) say CCs just cause noise and drift and so they (and quite a few top DIY builders here) replace them all with MF. There's sometimes a 'style thing' to retain the 'carbon' part and use carbon films, but I can't find any published reason to do this. If CCs do have mojo, it's likely the 'composition,' not the 'carbon.'

    OTOH many top commercial and boutique shops go all CC (on the board that is); quite a few good homebuilders do this too. Some reasons they cite: Their customer wants it, it's seen as 'true vintage', they figure the mojo is maybe not understood by science, so they'll keep all the fairy dust just in case. A good *simple* reason is you're building a small simple amp and you want it to feel, look, and sound vintage -- right down to the CC hiss, which won't be too bad in a small circuit.

    One *very* simple and logical compromise, even on small simple amps, is to put MF at least in the input chain. The hiss coming from your V1 grid stoppers and input resistors is amplified much more than any other signal in the amp. For the same reason, you'll see folks step up to 1w or 2w here since the bigger resistors hiss much less.

    Is there an intelligent, logical compromise? I end up liking the mix of mojo and applied science used by Mr. Keen. He has a few simple guidelines as to where CC mojo (sweeter distortion, actually) can happen. It's a pretty limited set of spots in most amps, especially NFB amps. Read the whole thing, it's great, but I'll ladle out the gravy...

    "So where do they work best? Where can we use CC's for their soft distortion, and where can we sidestep them to lower noise?

    First, they do no good and lots of noisy bad where the signal level is small and the following amplification is high - a classical description of an input stage. The input to an amp should probably have a metal film plate resistor to minimize noise. Grid resistors in all but output stages also do no good, because the signal level is typically too low. A 12AX7 can be driven from cutoff to positive grid voltage with a couple of volts of signal, so the grid resistor never has a big enough signal to be distorted appreciably.

    Cathode resistors are another poor use of CC. They typically only have a few volts across them, and they're often decoupled with a capacitor, both of which would minimize the resistor distortion. In cathode followers, there can be substantial DC and signal voltage across a cathode resistor, but in this case, the resistor is driven by the low impedance of the cathode, and the voltage across the resistor is controlled by the grid voltage very tightly, so the exact resistor value doesn't matter much - there won't be significant distortion.

    The place to use CC's is where there's big signal - plate resistors, and ideally the stage just before the phase inverter. The phase inverter would otherwise be ideal, with plate resistors carrying the highest signal voltage in the amp, but phase inverters are often enclosed in a feedback loop. The feedback minimizes the distortion the resistor generates."

    A 5e3, it turns out, is an easy and useful place to apply his guidelines!!!
     
    Last edited: Jul 3, 2019
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  4. BobSmith

    BobSmith Tele-Meister

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    Wow! That’s an excellent summary!

    I also think that smaller wattage amps (like champ, princeton or even deluxe) can get away with packing more CC than design considerations would theoretically justify. I think after this amp, I’m going to modernize my build to compare.
     
  5. BobSmith

    BobSmith Tele-Meister

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    Would you care to comment on Dale resistors? I see in other threads you seem to like them.
     
  6. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Another good question! Half a sec, I gotta see what I said about this once before.
     
  7. King Fan

    King Fan Friend of Leo's Ad Free Member

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    First, metal film. Second, milspec. Third, 1% tolerance. You grab a 68.1k resistor (the decimal point is a clue) and measure it -- you get pretty much 68.1k. Finally, not expensive.

    Secret super power: no color bands. Instead, the values are printed, very clearly, on the resistor. No more grabbing a 470K instead of a 470. Anyone here who hasn't done that? Heck, even when I measure resistors, I've been known to miss the tiny 'K' next to the big numbers.

    Also, they come in a great array of values. I love 1-ohm resistors for bias test purposes, but finding tight tolerances (which seem kinda important there) can be a chore. These? I ordered 10 of 'em and each and every one measured, you guessed it, 1.0 ohms.
     
    Last edited: Jul 3, 2019
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  8. D'tar

    D'tar Friend of Leo's

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    Did we ever get sound clips of that lovely amp @KF?
     
  9. King Fan

    King Fan Friend of Leo's Ad Free Member

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    No, sigh, my simplistic recording methods don't seem to capture the magic. I kept planning to have my real-musician buddy over to do the recording and (heh) maybe play on it. Excuses, excuses...

    But you give me a chance to illustrate a very nice use of Dale resistors *and* R. G. Keen-style carbon comps... your lovely amp!

    dtar.jpg
     
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  10. D'tar

    D'tar Friend of Leo's

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    I have used them and can not imagine my amp sounding better with CC throughout. The only way for one to know would be to build 2x same circuit with only difference being resistor composition. I for one, am not going to bother building the same amp twice unless commissioned to do so or I get rid of the first one. I believe @SacDAve has been through this type of experimenting.

    If you change resistor comp... Now can you blame the good/bad on the resistor or yellow Jupiters?
     
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  11. SacDAve

    SacDAve Poster Extraordinaire

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    I dis one amp 5E3 with all metal film it just sounded to sterile for lack of better words, All CC sound good probably a mixture of both would be best. Next I'm going with all carbon film.
     
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  12. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Aha. I think Bob said he's planning at least one more 5e3 build. So we do all red + CC, then all yellow + CC, then 'play the winner' of the cap color contest with its CCs against a Keen-style (or all MF) version in the same cap color.... :D
     
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  13. BobSmith

    BobSmith Tele-Meister

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    That is correct. I may need a 3rd amp.
     
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  14. BobSmith

    BobSmith Tele-Meister

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    Of course you tested these 1 ohm resistors with the 4-wire method? PITA, but that’s really the only way to measure accurately in that range. That capability is beyond the capability of virtually all DMMs, even good ones.

    I may use the July 4th 15% off tube depot coupon to pick some of these dales up. For future experimenting.
     
  15. MasterEvan07

    MasterEvan07 Tele-Meister

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    I was thrilled with the noise floor of the Deluxe Reverb I (re)built using all CF 1W resistors - I wanted to absolutely minimize all contact noise possible. I've been really jonesing for a tweed build, a 5E3 being a top contender, but haven't pulled the trigger. I'm also thinking of using Jupiter and Dale or CF for it, so definitely interested in your results on this build.
     
  16. BobSmith

    BobSmith Tele-Meister

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    Thanks! I hope to pull this off in the next 2-3 months. I’ll definitely report back.

    Right now I want to see how red vs yellow turns out, then take that winner with the Dales. I already have all the parts on their way!
     
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  17. BobSmith

    BobSmith Tele-Meister

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    Thanks for posting this great photo. Nice neat work!
    Any particular reason for using the Carbon Comps in these 2 locations?
     
  18. King Fan

    King Fan Friend of Leo's Ad Free Member

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    I'd have to let @D'tar answer -- that beautifully neat build is his -- but I believe those are basically the locations R. G. Keen is talking about in the article above.

    I went with a slightly more naive or 'optimistic' interpretation of the article, sticking in a little more CC fairy dust. Mine has the faintest hiss and I figure it may be my V1 plate resistors.
     
    Last edited: Jul 6, 2019
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  19. BobSmith

    BobSmith Tele-Meister

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    I came across another stash sourced from Japan. If the Dales don’t work out, I think I’m set with A-B for at least a little bit.
     

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  20. King Fan

    King Fan Friend of Leo's Ad Free Member

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    By rights the yellow caps with A-B resistors will be your most 'vintage correct' 5e3. Sure, I'd be very surprised if version 3, with MF resistors everywhere (or almost everywhere) wasn't *quieter.* The question is if you (or any of us) can hear the advantages of one cap over another, or of CC distortion over MF. But no fear, I'm on the side of the fairy dust.
     
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