Is it really safe to run 6L6 tubes on a DRRI amp?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by javierj, Jun 28, 2020 at 3:12 AM.

  1. VICOwner

    VICOwner Silver Supporting Member

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    Yep, pin 2 and 7 on the power tubes.
     
  2. javierj

    javierj Member

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    Thanks a lot guys!

    Does it matter which one I probe with the multi meter black or red probe?
     
  3. Dan40

    Dan40 Supporting Member

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    In this case it does not matter which probe goes to each pin.
     
  4. VICOwner

    VICOwner Silver Supporting Member

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    It safer to check it at the preamp sockets because you are not right next to plate voltage of a power tube.
     
  5. javierj

    javierj Member

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    Oh okay, so will it be the same result that will give me that information, by doing it on a preamp socket, any preamp socket? Yes actually I was a little afraid to probe around on the power tube's pin 2 and 7, since they have the high voltages and are kind of close together.

    I have being biasing my amp with no problems, I take all the safety precautions when I probe to measure the plate voltage of the power tube. I have done it many times and I feel comfortable to do it, but checking the heater voltage on pin 2 and 7, that I am not sure if I am comfortable. So maybe I can do it on the preamp socket and feel more comfortable or safer. So it will give me the same result heater voltage than if I were to do it on the power tube?
     
  6. javierj

    javierj Member

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    And what will be the safest way to take this measurement, using one aligator clip to get one of the socket contacts and the other pin use a sharp meter probe, so that I only use one hand at the live voltage???
     
  7. drewl

    drewl Member

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    Just measure heater voltage at the pilot light.
    Nowhere near any high voltage.
     
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  8. pdf64

    pdf64 Member

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    To get the characteristics specific to that winding, I suggest to remove all tubes and note the mains and heater Vac. Then repeat with all stock 6.3V tubes fitted. Then repeat with the 6L6 in place of the 6V6.
    A drop of 10% or less, no tubes to 6L6, would be encouraging; but that that wouldn’t eliminate the possibility that the total, full load on the transformer wouldn’t cause a problematic temperature rise at some internal point in the windings. Hence the suggestion to avoid pushing it into power amp overdrive for prolonged periods.
     
    Last edited: Jun 30, 2020 at 7:44 PM
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  9. bluesky636

    bluesky636 Member

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    Neal Young runs 6L6GCs in his 5E3 Deluxe. He also has fans blowing on the PT to keep it from melting. :omg
     
  10. javierj

    javierj Member

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    Yeah in the end, I am gonna roll with a matched pair of JJ 6V6S that I had. They sound good, after I re bias them to my taste. The pair of 6L6 that I had are rattling, they sound really good though, except for the rattle (tube rings dampers did not fix it). The JJ 6V6 don't sound as full as the RCA black plate 6L6, but still I like how it sounds as well with the JJ 6V6. The 6L6 had much better low end and clarity. The JJ 6V6 have good enough clarity and low end, but not as good as the RCA blackplates 6L6GC.

    I know my friend and other people run 6L6 on their DRRI, but I feel like it could be putting too much strain on the power supply. Even though some people can get away with it, without modifying the power supply, I feel like is may not be worth it...

    Honestly I don't think that Fender "over engineered" the power supply on the DRRI in order to safely run 6L6 power tubes that take double the heater current. I have a Swart amp Champ clone that does say you can run a 6L6 tube, but that one was build like that, with a more powerful transformer and that is why they say you can do that on their website.
     
    Last edited: Jun 30, 2020 at 10:38 PM
  11. mc5nrg

    mc5nrg Member

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    I played through a SF DLX RVB for a few years that had 6L6GC power tubes. However it was a bandmate's amp and he was not technically oriented and couldn't explain what else may have been involved when his friend worked on the amp.Note that over the period of time I used the amp I pulled preamp tubes that weren't actually being used.
     
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  12. rusty

    rusty Supporting Member

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  13. javierj

    javierj Member

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    Yes that is exactly what I experienced on my DRRI with 6L6, much better and bigger bass response and fuller overall sound.

    I don't care too much about the SS rectifier for extra voltage, I like the sound better of a 5AR4.

    I was thinking, would it help with the extra heater current that the 6L6 pulls, if I take out the V1 preamp tube of the Normal channel that I never use and maybe I could also sacrifice the tremolo effect by removing that V5 tube, would that help out in any way to free up some stress on the power supply?
     
  14. rusty

    rusty Supporting Member

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    We need @HotBluePlates insight, but I don't THINK those tube pulls would do anything significant...
     
  15. HotBluePlates

    HotBluePlates Member

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    A 6V6 draws 0.45A of heater current per tube, while a 6L6 draws 0.9A per tube. So the swap tries to pull 0.9A more than the stock setup.

    A 12AX7 wired for 6.3vac draws 0.3A per tube, so you'd free up 0.6A by pulling 2 tubes. Better, but not all the way there.

    Still, you haven't replied as having measured the heater voltage with 6L6s in place, so we don't know if it's really needed to be yanking preamp tubes. (And personally, I'm not inclined to experiment on my vintage DR)
     
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  16. javierj

    javierj Member

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    Okay I will give that a try later...

    Should I measure this in V AC or V DC? How much aproximately should I be getting to know it is good?

    Does it matter the bias adjustment setting or just pop them in and measure the heater voltage? Should I do this with the amp on or stand by, with or without input, volume knob all the way down?
     
    Last edited: Jul 1, 2020 at 4:21 PM
  17. javierj

    javierj Member

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    In this case I actually plan to bias them quite cool. Anyway, when I tried the RCA 6L6GC on this amp, the max iddle I could get from the bias pot with the stock resistor was like 26mA...

    Does biasing the amp cooler with 6L6 help put less stress to the power transformer?
     
  18. javierj

    javierj Member

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    So I got the measurements, I did it on stand by mode. Do these measurements work even if I did them in stand by mode?

    Here they are:

    Heater voltage measured in volts AC from pin 4-5 (with black alligator clip) to pin 9 with sharp red probe after aprox. 2 to 3 minutes in stand by mode:

    JJ 6V6S

    [​IMG]

    RCA 6L6GC blackplates:

    [​IMG]

    Let me know what you think about these measurements!!!

    Thanks a lot!
     
    Last edited: Jul 1, 2020 at 4:59 PM
  19. javierj

    javierj Member

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    I take this back, this actually happened with an older Fender re branded 5AR4 tube that I had that came with the amp new 5 years ago. Now I replaced it with a new JJ GZ34 and now I have normal plate voltages and I can supply up to like 45 mA to a 6L6 if I wanted to, with the stock bias pot and resistor value.
     

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