Safe to pull two output tubes?

Discussion in 'The Workbench' started by Matthews Guitars, Aug 24, 2019.

  1. Matthews Guitars

    Matthews Guitars Active Member

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    Informed opinions wanted.

    I have a 100 watt JMP 2203. In order to reduce the volume a little, save on tubes, and generate a little less heat, I COULD pull one pair of output tubes and just run as a 50 watt amp.

    I believe this would be safe for the amplifier. Two tubes will provide adequate loading for the power supply. The amount of current going through the output transformer would be reduced, as would current draw in the power supply as well.

    I don't see any mechanism by which this could be harmful as long as I pull a pair that leaves one tube in circuit on each half of the output transformer.

    Looking at the schematic, the tubes on either side of the OT are wired exactly in parallel. The amp doesn't know how many output tubes are installed.

    Is there any flaw in my logic?
     
  2. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Yes the amplifier does know.
    Yes you have flaws.

    :)

    But on the good side, there is a way to do it but it aint going to be that much less loud or cooler.
     
    Last edited: Aug 24, 2019
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  3. anitoli

    anitoli Well-Known Member

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    Does the amp have a master volume? Pulling tubes is not going to make a "too loud" amp just right, it will still be loud depending on where the master volume is set or if it is a NMV you would be better off attenuating it.
     
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  4. Chris-in-LA

    Chris-in-LA Well-Known Member

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    If you pull two tubes you have to change your impedance selector, half the tubes means you need half the load. I should have set my ohms selector to 8 to run my 16 cab but I didn’t. I was blowing pre-amp tubes on more than one occasion, I believe it was because I pulled two tubes out of my 2203 and did not change my input selector. BTW, it didn’t really lose any volume but it did lose some tone, which was obvious after putting back the other tubes.
     
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  5. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Oh O.
    Can of worms just opened. Hurricane ballbaggery on the way.

    Although no one discusses it, there is an affect on tone because it was not designed to operate that way.
     
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  6. anitoli

    anitoli Well-Known Member

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    This is exactly how i could immediately tell when the 6100's output protect fuse would shit due to a tube/pair failure, not by the volume loss but the tone went south.
     
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  7. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Think about it. You have a pair of tubes and then half the reactive resistance/impedance. Of course it is going to affect the output frequencies.
     
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  8. RickyLee

    RickyLee Well-Known Member VIP Member

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    If you pull two tubes but do not double the cab load, then you will defeat the process of turning it into 50 watt amp by another important factor as well . . . . the rise in the running voltages and now a hotter bias!

    When I have taken any amp model and ran it set at 16 ohms but the cab load was 8 ohms (half the load it should have seen regardless), I noticed the amp had a touch more high end sheen and seemed more punchy for sure. So the impedance matching is important.

    I have been running my JVM 410H with two tubes pulled with good results. First off, I run my speaker load double to what the amp is set to. Then I have my mains voltage reduced to keep it the same or even less than where it was running the four EL34's. And then the other important one is to now set your bias idle up to these parameters.

    Yeah, it is not a big volume drop, but enough to notice and be able to push the master volume slightly higher. Clarity seems a touch better. And then, I am only burning up a duet of EL34's instead of a quartet. Amp seems to be running cooler as well.
     
    Last edited: Aug 24, 2019
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  9. john hammond

    john hammond Well-Known Member

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    4 tubes = 1.7k load
    2 tubes = 3.5k load.

    if you double the impedance, you double the speaker load.

    so if you pop two tubes out, you must double the speaker if you're trying to match the sound of the amp with four tubes.
    ( 4ohms from amp into 8 ohm speaker.)

    in regards to frequency response...if you pull two tubes and double the speaker (load)...the amp will still put out the same frequencies, sort of.
    Theoretically, the 'curves' that the tubes were making before, are still being made.

    However the issue with the amp sounding the same after you pop two tubes out and then double the speaker load is that suddenly the output transformer and power supply have just become ( a lot) more efficient.
    there will be much less sag in the power AND output stages of the amp, your preamp voltages will rise as a result of all this extra available power. ( this alone doesn't sound good to my ears )
    The output transformer will be much happier in that it will work ' better' at converting high voltage to low impedance ( and music) because the core of the output transformer will be much less likely to saturate...there will be more headroom, less dirt.
    the amp will sound more spikey, and as a result of there being little or less saturation in the output transformer, less mids, less pleasing.( subjective)
    When i hear an output transformer sag, I hear mids.

    - let the ball baggery flow

    edit- i'd re-bias after popping two tubes out if you're trying to ' match' the sound of the amp as it was with four tubes.

    - fascinating field...guitar amp makers were always skimping on transformers, ( except hiwatt and one or two more that sound like ass sandwich..or like six scrotums tearing at the same time) because they represent the highest cost to them to manufacture amps. these cheap power transformers in particular simply cant provide the output tubes with enough current for them to produce the frequencies they're meant to produce ...their published ' curves', at anything near max volume. You can see these curves on the tubes' data pages. In short, guitar amps are good because they're bad.

    further edit- the above bollock regarding cheap transformers.. frequency response only applies to ' dirty' amps. Clean amps can sound magnificent with high quality iron and efficient power supplies.
     
    Last edited: Aug 24, 2019
  10. Drinkingdeath01

    Drinkingdeath01 Well-Known Member

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    Umm, I'm not a tech but after reading all the other guys posts, I'm going to say No, don't run the Marshall with only two tubes. There, I just saved you a bunch of time.:cheers:
     
  11. john hammond

    john hammond Well-Known Member

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    - for anyone reading this that has a marshall amp with the four capacitors bunched like this image, if you pop two tubes and DONt change the speaker load ( 4 ohms from amp to 8 ohm speaker). you will blow your amp, I promise you..its just a matter of time.
    If your marshall doesn't have these four caps bunched like this ( the four blue things next to the transformer on the left), then i recommend you use whatever speaker out jack to whatever speaker sounds best as your amp doesn't have voltage doubling. Before the ball-bagging begins on this, there are plenty of good people here at this forum that have been doing this speaker mismatching for up to 20 years with no ill-effect as evidenced by posts on another thread.


    images (1).jpg
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor. free Mars!
    1. If you pull 2 tubes, you need to change the speaker load.
    This is because you are doubling the load on the remaining tubes.
    If you don't do this the sockets can arc and burn.
    I have seen it happen and repaired it many a time, in many amplifiers.

    Set amp on 4 ohms, connect 8 ohm speaker.
    Set amp on 8 ohms, connect 16 ohm speaker.
    Now you have cut the load on the tubes in half.
    Half the tubes ---needs half the load.

    "The amp doesn't know how many output tubes are installed."

    Yes it does Mr. Genius. It's called Ohm's Law. This is a law you cannot break. Yes your logic is thoroughly flawed.

    Boogie Amps: shuts off 2 tubes to make 1/2 power.
    I have seen Boogie amps arc and burn over and over again, and I have repaired them over and over again. This is because shutting off 2 tubes, Boogie does not adjust the speaker load....then the sockets start arcing.
    So much for "superior technology.":lol::lol::lol: "The transformers are very robust...blah blah blah..." :lol::lol::lol: Fact: It's just the same as any other amplifier.

    2. "Half Power makes the amp 1/2 as loud." Bullshit. It reduces the loudness very slightly, if at all. In fact most people cannot hear any change in loudness.
    This you can be sure of: wattage does not = loudness.
    1/2 the power does not mean 1/2 the loudness.
    So why bother to do this at all?
    I have news for you: pulling 2 tubes does not do what you think.

    3. You want to cut the loudness in half?
    Then you need to reduce the power to about 1/10th of original.
     
    Last edited: Aug 24, 2019
  13. Nik Henville

    Nik Henville Well-Known Member

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    .
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    I tried this 'trick' back in the day.
    Did it with several 4-valve amps...
    ...really did NOT drop the loudness by much at all
    .
    .
    .
     
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  14. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor. free Mars!
    Yeah I did the same thing. I was always trying to get a better sound at a lower loudness.
     
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  15. john hammond

    john hammond Well-Known Member

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    this is quite interesting information, I gather you're referring to the half power switch, not the ' simulclass' feature

    I have several of these amps and can confirm that both of these ' innovations' are absolute horseshit. randall truly enjoyed cockassing customers with marketing bling.
    One other bit of cockass is the ' tweed' setting...called something else i believe on the rectum frier models..its a tap that reduces all voltages aka EVH and his variac. I think this works on a plexi, not on a circuit that has more gain than a regular plexi it just doesn't sound good.
     
  16. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor. free Mars!
    "simulclass:" 2 of the output tubes burn up faster than the other 2. :lol::lol::lol: Now that's innovation.
     
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  17. john hammond

    john hammond Well-Known Member

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    this is confusing agreed, however..

    half the tubes means double the load.....or put another way
    half the tubes= double the impedance( of the tubes)
    and...double the impedance ( of the tubes ) = double the load ( speaker ohms)

    if you're trying to match things properly.
     
    Last edited: Aug 24, 2019
  18. Matthews Guitars

    Matthews Guitars Active Member

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    OK, I've received some good advice that was well presented and I've also received some that is bordering on abusive and I'm pretty thick skinned.

    I didn't bother to mention that of course I'd be increasing the speaker load because I DO know Ohm's Law. I've been a professional electronics technician and electronics hobbyist for over 30 years now. Not going to trot out my resume', but let's just say that I've spent more time as the chief technician in an electronics service shop than most people.
    However, the majority of my professional technician time has been spent in the RF side of things. My hands-on experience in audio tube circuits that are NOT designed for the most linear frequency response is relatively limited.

    I'm intimately familiar with dB measurements as well, both acoustic and electric. Doubling or halving the power creates a 3 dB change in volume. 10X the power makes a 10 dB change in volume.

    The other considerations that have been mentioned, reduced loading causing less sag in the power supply, had been taken into account. I expect that behavior.

    So, ultimately, I'm just looking to get a second opinion and be made aware of any hidden pitfalls before I pull two tubes and compensate by hooking up an 8 ohm cab instead of a 16 ohm cab, which was the original plan and still is.

    Even though the actual volume level change of 3 dB is not much, if the results are satisfactory then it means I'm burning two expensive output tubes at a time instead of four.

    And, yes, I would rebias for two.

    I ended up with a 2203 because I hadn't yet found a deal for a 2204. It seems that people are more likely to keep their 50 watt amps these days and sell off their 100 watters. Well, that's no surprise. You know things are bad when the band's volume is set to match the acoustic level of the drums and the manager is still telling the band to turn it down.
     
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  19. JM5010

    JM5010 Member

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    Maybe you can also try just running one EL34...very safe and will get you the brown sound at 1/4th of the vol!
     
  20. anitoli

    anitoli Well-Known Member

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    I get brown sound every day 1/2 hour after i wake up..............................
     

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