Sc20/sv20 Power Tubes...what Is The Deal Exactly?

Discussion in 'The Workbench' started by tallcoolone, Apr 18, 2019.

  1. santiall

    santiall Well-Known Member

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    I did design the 20W amp that was used in the mini-Jubilee, the original Origins and some other amps that did not see the light. That design DOES work with any octal tube, even 6V6. I don't know what circuit is in the Studio series but I'd be very surprised if it isn't just the same as the mini-Jubilee with the preamp modified accordingly. In fact those amps were already planned when I was still in Marshall and that's what was being done, just a preamp change (which is actually the same if you think about the 100W amps, same power amp, different pre).

    Regardless of circuit changes, the amps will work with the other tubes. Actually the EL34 is harder on the transformers than tetrodes like the 6L6, 5881 and so on. The KT88 and 6550 draw a bit more of heater current but that was taken into account when specifying the power transformer.

    In Marshall's defense it is normal that if the amps come with EL34 they told you to only use those but that doesn't mean it won't work or will be dangerous to use other tubes. I'm sure if you ask if you can use a 12AT7 in the preamp they'll tell you it is designed for 12AX7 too.


    Hmmm, a KT66 should work in the SL-5, perhaps it runs a bit cold and it is not optimal but definitely should work.
     
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  2. Kinkless Tetrode

    Kinkless Tetrode Well-Known Member

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    I put KT66s in the MJ and ran them there for quite awhile. It made the Jube sound more vintage and less modern to my ear. I liked it for my playing. I did need to remove the bear trap style tube retainers and I had to notch the mounting stripping for the rear panel to fit them into the head shell.

    I have not tried KT88s.
     
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  3. BanditPanda

    BanditPanda Well-Known Member

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    I presume any warranty would be voided ?
    BP
     
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  4. Solid State

    Solid State Well-Known Member

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    It's really not hard to understand. I sell hardware also. People ask me all the time "Can I put another stick of RAM in this thing?" and I can't say "Yes" even though it'll fit. If I say "Yes" in any way shape or form and some dumbass doesn't install it correctly and ends up down a terminal or completely bricking the thing they'll call me up and scream "WELL YOU SAID!! and now you owe me a new system plus compensation for my downtime" - There are a LOT of people willing to toss a company under the bus for perceived bad advice.
     
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  5. scozz

    scozz Well-Known Member Gold Supporting Member

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    Yes, that’s for sure. Were you thinking about trying something different than EL34s @tallcoolone?
     
  6. mickeydg5

    mickeydg5 Well-Known Member

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    I think as mentioned already the amlifier was designed with specific power tube in mind. Sure it will run harder with 6CA7/EL34, a little less with 6550/KT88 and even less with 6L6GC/5881/KT66. That is all part of the tube type charactersitic. But the cathode resistor is a fixed value and is not optimal for all power tubes. Therefore each tube type will have a different effeciency along with its designated amplification factor and internal resistances in the state given for opertion which affects its overall character.

    So while you can use various tubes and they will still operate, which ones sound the best and do what you want?
    Its a personal preference.
     
  7. mickeydg5

    mickeydg5 Well-Known Member

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    I am thinking the same if the owner tries undesignated power tubes.
     
    Last edited: Apr 23, 2019
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  8. RLW59

    RLW59 Well-Known Member

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    Any failure would be due to defective parts that were going to fail anyway, even with the "correct" tubes. So since the tubes weren't the cause of the problem, there are no ethical issues with just putting the stock tubes back in and never telling them you had different types of tubes in it.
    ----------------------
    As a general legal thing, mods only void the warranty if they might have caused the failure.

    Buy a new car, slam the suspension so the car scrapes the ground.

    You've definitely voided the warranty for all the suspension components, and possibly drivetrain components, even if the failure was due to a defective factory part and not caused by the crazy geometry you created. But if the air conditioning breaks, it's still covered by the warranty.

    So if the tone controls on your amp stop working due to a bad solder joint or a failed cap or resistor, it doesn't matter what power tubes are in the amp. Still covered by warranty.
    ----------------------
    But if it makes you nervous, don't do it.

    I match impedances, even though many sources (including Santiago) say mismatches are fine. I have no need or desire to mismatch , so I don't.
     
  9. tallcoolone

    tallcoolone Well-Known Member

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    Great info, thanks man! Pretty cool having a place like this where you can get questions answered. I love JMPs that are modded to take KT88 tubes—I will report back after trying this with the Studio
     
  10. K2JLX

    K2JLX Well-Known Member Gold Supporting Member

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    @santiall thanks for the info, def going to try some KT66’s in the studio vintage now.
     
  11. 67Mopar

    67Mopar Well-Known Member

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    I don't think the OT is biased for 6L6 tubes, but considering the lower current, I expect that they would operate just fine. I ran EL34's in an SLO for years, with nary an issue.
     
  12. Georgiatec

    Georgiatec Well-Known Member VIP Member

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    Looking for something I don't remember what.
    I did find one of my spare KT66's to be defective. It's possible when I was rolling valves in the SL-5 that I tried that one without realising. I'll give it another go with a tested good tube next time I have it open. :thumbs:
     
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  13. ampmadscientist

    ampmadscientist Well-Known Member

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    What circuit is this and how does it limit current of "any tube?"
    I mean forgive me for asking but this suggestion of "any tube" (without proper re-biasing) sounds pretty reckless.
    Cathode biased or grid biased, the current needs to be adjusted to make a tube run right.

    Well "any" amp works with "any" tube....it just burns up when the current is too much.
     
  14. Fender

    Fender Active Member

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    Precise bias current is overrated
     
  15. santiall

    santiall Well-Known Member

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    well, the main reason why I said that is because it was part of the design parameters and it has been tested.

    hmm, let's see if I can explain this. The working principle of cathode biasing is that if the current increases, the bias gets more negative (grid to cathode voltage) until it finds an equilibrium point. In short, the current limits itself and it just happens that the octal pentodes and tetrodes used in guitar amps all end working around the same zone. Should the current keep increasing, then the bias will try to stop it increasing, it is negative feedback. Obviously the circuit was optimised for a particular tube but it still works with others delivering more or less distortion/ power but it doesn't blow up.

    Also, these amps run at such low anode voltage that the margin is quite big, hence the flexibility. The only tube that runs near the maximum parameters in this circuit would be the 6V6. Had these amp had higher anode voltage then yes, I'd not have advised to change tubes. In a 50 or 100W amp, the tubes would be powered at 450V-550V or even more, in these amps around 'only' 280V-300V

    I had the same circuit running EL34s for certain series and 6L6/5881 for some other series that never saw the light perfectly without issues.

    IF it had fixed bias, then I'd have said you can use any tube but you need to re-bias accordingly like in the 100W amps. A JVM, 2555x, YJM, and pretty much any amp with enough bias range will also take "any tube" (apart from the 6V6 as the maximum voltage will be exceeded).

    Hope it is clearer, also, with "any tube" I mean any octal tube with pinout similar to the EL34 and a minimum anode voltage of at least 280V also know as 6L6, 6V6, 5881, KT66, KT77, KT88, KT90, EL34, 6CA7, etc. etc. just in case someone plugs in there an octal preamp pentode or something and it doesn't work but I guess this was pretty much clear...
     
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  16. santiall

    santiall Well-Known Member

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    yeah, let us know!
     
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  17. Matt_Krush

    Matt_Krush Well-Known Member

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    To be extra clear....All these tubes will work with no changes to screen/grid resistors?
    Just as long as there is enough range in the current bias?
     
  18. Fender

    Fender Active Member

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    That’s what he just said ! :redface:

    Just add : "Thank you Mr Santiago" :redface:
     
    Last edited: Apr 20, 2019
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  19. mickeydg5

    mickeydg5 Well-Known Member

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  20. santiall

    santiall Well-Known Member

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    no changes, pretty much the way to go go is to install the tubes and see if you like the result
     
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