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Pulling 2 tubes from Twin Reverb (or any amp) - is it safe?

Discussion in 'Amp Tech Center' started by itsGiusto, Jan 22, 2021.

  1. itsGiusto

    itsGiusto Tele-Holic

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    The conventional wisdom is, if you want to cut the power in half for any amp with 4 power tubes, pull two tubes, double the impedance of the speaker you hook up, and rebias the amp. However, a thought occurred to me.

    By doing this, there's significantly less current draw on the PT, meaning the B+ will rise considerably. In an amp like a TR, could this result in voltages being too high for the filter caps?
     
  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    I don't think the voltages will rise *that* much. You could always measure it, but there's lotsa little preamp tubes in there drawing current too.

    It's important that you either pull the center two or the outer two to keep one in each half of the OT so the amp functions properly.
     
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  3. itsGiusto

    itsGiusto Tele-Holic

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    You can't pull tubes 1 and 3 or 2 and 4? ;)
     
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  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Yeah, but that's way harder to type out :D
     
  5. Dacious

    Dacious Poster Extraordinaire

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    Remember for long term operation to change the output transformer reflected impedance. Two 6L6s want to see 4000 ohms Instead of 2000ohms-4 ohms.

    So you need to disconnect one speaker but leave the other plugged into the main speaker jack. That way the two tubes will 'see' 4000 ohms reflected from the one 8 ohm speaker plugged into the 4 ohm jack

    That also cuts volume.
     
  6. TwoBear

    TwoBear Tele-Meister

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    Not if you’re counting from the preamp tubes! Ha!
     
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  7. TwoBear

    TwoBear Tele-Meister

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    Have you thought of experimenting with those substitube sockets from yellow jackets or Pittman’s? I held onto a pair of each of those, I used to try them out when I had a lot of amps. Super reverb and pro reverb I also had some old super leads but I most like them in my 1987 plexi
     
  8. studio

    studio Poster Extraordinaire

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    Are those the ones that used to advertise on this forum?
     
  9. Chupacabra

    Chupacabra Tele-Meister

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    upload_2021-1-23_3-47-54.png upload_2021-1-23_3-47-54.png
     
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  10. TwoBear

    TwoBear Tele-Meister

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    I apologize for the crappy picture, I guess if you tap it it will enlarge.
    I’m not sure if they used to or not. But these are they, at least a couple versions I’m not sure of others. I got them when our store closed and I’ve seen that they are sort of pricey but thinking about it it seems pretty easy to build your own.
    image.jpg
     
  11. Jon Snell

    Jon Snell Tele-Holic

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    One thing that never stops amazing me is the little knowledge people have when it comes to valve biasing.
    In a fixed bias amplifier, the bias voltage is taken from, (in a well designed amplifier) the mains transformer HT tapping or dedicated interwoven winding. What that means is if the HT rises, the -ve bias voltage also rises, countering the thermal runaway effect of over cooking the anodes or changing states from Class AB to Class AB2 or even Class A.
    So, in a nut shell, to remove two valves from a quad or four valves from a sextet, will auto correct the bias but the output impedance will be wrong affecting the efficiency of the power transfer between anode power and loudspeaker. This has the added affect of reducing further the power produced and will have no adverse affect upon the output valves that are left in circuit assuming the valves are sat in possition 2 & 3 or 2 & 3 & 4 & 5 in the case of a sextet.
    A few amplifier manufacturers, including myself, use this method with a selector switch on the rear panel effectively disconnecting pair/s of cathodes from ground. And that system works very well.
    Some designers come up with a stabilized bias supply ... think about that for a moment ... when the mains voltage rises, the HT rises but the -ve bias voltage stays the same meaning the output valves are instantly over cooking themselves or the bias is wrong.
    I can't understand why they haven't thought about this known issue!

    As a note, halving the power doesn't make much difference because that is only a 3dB drop on maybe a sea of 110dB ceiling of sound pressure hitting your ears. To shunt between the phases of the grids in the output stage has a more marked affect, like VOX do and leaves the valves working partially in phase causing breakup without shattering ones ears.
     
    Last edited: Jan 23, 2021
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  12. Paul G.

    Paul G. Friend of Leo's

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    Others have weighed in on the safety and operation. I'll just mention that there will be a barely perceptible difference in volume, and if you're looking for "early breakup", well, you need to define the word "early".

    No harm in doing it, so give it a whack, but don't expect to turn your Twin Reverb or SuperLead into a creamy distortion machine at less than earsplitting volumes.
     
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  13. PCollen

    PCollen Friend of Leo's

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    Are those the sub-sockets that adapt EL84's to be run in amps originally having 6L6's or EL34's ?
     
  14. itsGiusto

    itsGiusto Tele-Holic

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    That all makes sense. But my question wasn't primarily about problems affecting the bias from pulling two tubes, it was primarily about problems that might affect the filter caps or other components, like the plates of other tubes. The idea being that the lower current draw from having 2 less tubes would result in higher voltage from the PT and affecting everything else, and whether that's okay for sustained operation.
    Yes, I'm well-aware that half-power doesn't make a big difference volume-wise. But remember, even if it doesn't sound that different, it still is half-power, which can make a great difference to downstream components, like speakers and attenuators! I primarily want to do this so that I can connect my attenuator between the TR's output and the speaker. A TR is rated for 85-100w, but could probably produce 150-200w, or maybe more if driven really hard! I don't think my attenuator would be able to properly handle the full output of the TR without heating up and smoking, but it can definitely handle the 45-~100w if the power is halved!
     
    Last edited: Jan 23, 2021
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  15. Jon Snell

    Jon Snell Tele-Holic

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    There is no added stress to other components, it is quite safe to do. All that will happen is reduced power and increased THD due to the mismatch on the transformer. There is no advantage to plug an 8 Ohms speaker into the 4 Ohms outlet in my view.
     
  16. TwoBear

    TwoBear Tele-Meister

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    Yup. I’ve since downsized all my amps To a 30 W Fender Bantam Bass-2 6L6s tube rectified in a 1964 Overstocked super reverb chassis and Cabinet (cabinet might be built later closer to 69 When the amp was made-it’s getting insane the amount of information that is passed around and I feel I need to touch on before someone Flames me with ‘you’re wrong those cabinets were made on the second week of the 39th day after the 20 year Anniversary of the falling of the Reich, and they are an 8th inch difference from the originals in 64) Along with a ruby amp a Crate limo 50, and Roland Street cube. I’ve got the parts and the Bantam is due for new caps, and I’m trying to decide whether I want to use these which do get pretty hot or maybe make it switchable to a cathode bias six V6. I should probably just leave it stock and build up a small maybe single ended little Kranker for fun and recording use.
     
    Last edited: Jan 23, 2021
  17. TwoBear

    TwoBear Tele-Meister

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    I keep hearing half power doesn’t make a lot of difference volume wise but having owned 72 and 73 super leads and playing them through the same 4x12 blackback 30’s Checkerboard cabinet, There is a definite noticed difference felt in the back of your legs, and the urgency of the note, and the Rustling of the wind that is unmistakable, When changing over to my 1987 plexi 45–5Owatt Oh yeah and a 1987 plexi 45/50 will send 100W 900 scurrying looking for some aqua net ultra hold to keep it from shaking apart.
     
    Last edited: Jan 23, 2021
  18. TwoBear

    TwoBear Tele-Meister

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    In the case of the four power tubes, I didn’t see any difference in pulling the two inside or outside tubes? And I am reminded of a Carvin V16 Amplifier I had with two EL 84s. It had a Pentode triode switch that gave The amp a markedly different feel in volume and sound. It also used the output trans from their 30 W amplifier with them cutting the correct impedance wind of the 30 and usingThe I assume four ohm tap for the eight R speaker, which I was lucky enough to get a stock greenback made in England don’t you know, haha, that came stock before they downsized to some cheaper model.
     
  19. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    It may not make a dramatic difference in volume, but it will make a difference in headroom.
     
  20. SoK66

    SoK66 Tele-Afflicted

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    I ran a bought new '69 Twin Reverb on just the two outer tubes for five years of regular gigging with not a single issue. We just ran it at 8 ohms, never worried about the OT, never had an issue. I traded it to a steel player who ran it that way until he passed away in 2002. Only thing ever done to is was a series of re-tubeings and a cap job back in the 90s. So, pull the two outers or the two inners, check & reset bias, you're good to go. It doesn;t cut the power in half but the reduction in output is noticeable and can make the difference between acceptable gigging tone and having a very heavy doorstop.
     
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