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    M-125 Two tubes, or four?

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    j beede

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    M-125 Two tubes, or four?

    Post by j beede on Mon Mar 06, 2017 10:33 pm

    I pulled two of the four output pentodes out of my monoblocks today to see what it would be like to run them "half full". The power drop was not pronounced. Those of you doing this with your M-125s... can you hear a difference? Maybe I will scope the outputs to see if there is anything visible going on.

    ...J
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    10-E-C

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    Re: M-125 Two tubes, or four?

    Post by 10-E-C on Mon Mar 06, 2017 10:48 pm

    j beede wrote:I pulled two of the four output pentodes out of my monoblocks today to see what it would be like to run them "half full". The power drop was not pronounced. Those of you doing this with your M-125s... can you hear a difference? Maybe I will scope the outputs to see if there is anything visible going on.

    ...J


    I run only 2 tubes per mono block, my speakers are so efficient it's all I need.

    TM
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    pedrocols

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    Re: M-125 Two tubes, or four?

    Post by pedrocols on Mon Mar 06, 2017 11:25 pm

    After one of my kt120s started to show signs of wear I decided to run 2 tubes per amp. I do not listen loud so I really do not noticed a difference. Speakers are magnepans 1.7s. I do however prefer to run the amp in pentode mode with these speakers.
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    j beede

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    Re: M-125 Two tubes, or four?

    Post by j beede on Mon Mar 06, 2017 11:49 pm

    Yes, your amp's output will drop when running two tubes. I am curious to know if your amps sound different running two tubes. I notice a difference on close mic'ed vocals when running two pentodes.
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    sKiZo

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    Re: M-125 Two tubes, or four?

    Post by sKiZo on Tue Mar 07, 2017 12:13 am

    I'd expect some degradation or distortion, as you're only hearing one side of a push pull configuration?
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    Kentley

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    Re: M-125 Two tubes, or four?

    Post by Kentley on Tue Mar 07, 2017 1:10 am

    No No sKiZo {I've always wanted to say that!}. The amp was designed nicely to become effectively half-powered push-pull when configured thus. One simply hopes that Mr. j beede remembered to halve the bias settings when removing the excess power tubes!
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    j beede

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    Re: M-125 Two tubes, or four?

    Post by j beede on Tue Mar 07, 2017 2:20 am

    sKiZo wrote:I'd expect some degradation or distortion, as you're only hearing one side of a push pull configuration?

    Wink I suppose it depends on which two tubes you pull!
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    Bob Latino
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    Re: M-125 Two tubes, or four?

    Post by Bob Latino on Tue Mar 07, 2017 9:08 am



    This is one of the two prototype M-125 amps from 2010 set up to run in the two output tube mode. In the photo you can see the prototype yellow colored Mark III/M-125 driver board. The production M-125's have the green board with sealed solder traces and plated through holes. The switches on this amp are also the smaller 1/4 inch switches. The production models have the heavier duty 1/2 inch switches. In this photo the amp is using two 12BH7 driver tubes.

    When you use the two output tube mode, the FRONT two positions are probably the best choice. The two output tubes will be further away from the two transformers, have better air circulation and will run cooler. The bias is set for .500 VDC per side when using just two output tubes instead of the 1.00 VDC per side you should be using for four output tubes.

    I listened and had a few other people listen to the amps in two and four output tube mode. In my opinion the amps sound the same at lower volume levels. When you start listening at higher volume levels the fact that the four output tubes have more HEADROOM than the two output tubes comes into play. Bass notes will have more visceral impact at higher volume levels using four output tubes. Moral of the story > If your speakers are efficient and your room is not too large and you don't normally play rock music at high volume levels, using just two output tubes should be enough for you. Having the ability to use the four output tubes will also give you more future speaker choices. Speakers in the 84 to 88 dB efficiency range do need more power in for a given output.

    Bob


    Last edited by Bob Latino on Thu Mar 09, 2017 9:42 pm; edited 1 time in total
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    deepee99

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    Re: M-125 Two tubes, or four?

    Post by deepee99 on Tue Mar 07, 2017 5:47 pm

    What Bob said.
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    bluemeanies

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    Re: M-125 Two tubes, or four?

    Post by bluemeanies on Tue Mar 07, 2017 9:52 pm

    I'm using four output tubes but honestly except for the extra headroom the sonics are still there and only two tubes are necessary.
    Believe it or not I have not had the time to make the switch back to just two output tubes. Orginally that was how I rolled....two output tubes in the front positions.
    I have only used four tubes since December and while I do not play a lot of R&R like the classical and blues sound with a four tube setup.
    My room is not that big 25' long 8' wide at screen wall and 10.5' at rear of room with 6'.5" & 6'3" ceiling height.
    The room serves two slaves...Ht and 2channel

    turbotoy

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    Re: M-125 Two tubes, or four?

    Post by turbotoy on Tue Mar 07, 2017 9:55 pm

    I've been running only two KT-120s for the last year or so. I cannot say that I notice any difference in sound quality at normal listening levels, and this is in a 27'x36' room. Pushed hard, I'm sure the differences would become more apparent.
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    Tubes4ever

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    Re: M-125 Two tubes, or four?

    Post by Tubes4ever on Wed Mar 08, 2017 10:05 pm

    Bob,

    Doesn't using two tubes instead of four cause the OT impedance to be incorrect?

    Tim L.
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    Bob Latino
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    Re: M-125 Two tubes, or four?

    Post by Bob Latino on Thu Mar 09, 2017 4:30 pm

    Tubes4ever wrote:Bob,

    Doesn't using two tubes instead of four cause the OT impedance to be incorrect?

    Tim L.

    The output impedance on the M-125's is optimized for four output tubes The amps also work very well using just two output tubes with no real change in sound quality until volume is raised to the point where the greater headroom of using four output tubes comes into play.

    Bob
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    bluemeanies

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    Re: M-125 Two tubes, or four?

    Post by bluemeanies on Sun Mar 12, 2017 12:47 pm

    Bob,
    Currently I am using four output tubes (SOVTEK KT88's). Have been for about three or four months. I have the Mullard rectifier tubes.
    I am considering only using two output tubes...are the Mullard ok to use instead of the Webers' at a bias setting of .500

    Yes - The Mullards would work fine with just two output tubes ..

    Bob
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    bluemeanies

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    Re: M-125 Two tubes, or four?

    Post by bluemeanies on Sun Mar 12, 2017 12:50 pm

    OMG...that's service!
    Thanks Bob
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    deepee99

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    Re: M-125 Two tubes, or four?

    Post by deepee99 on Mon Mar 13, 2017 4:35 pm

    There's a cost consideration, too. A new octet of KT-88s or equivalent is quite the heart-stopper, whereas if you skip a car payment or not pay the gas bill for a month you can get a very fine quad Smile
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    j beede

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    Re: M-125 Two tubes, or four?

    Post by j beede on Thu Mar 16, 2017 1:01 pm

    For a ST-70, MkIV, MkIII or MkVI type topology, reducing the output drive (e.g. pulling two of the four outputs in a MkVI) should reduce the output at the 16 Ohm tap and the amount of negative feedback applied to the input stage for a given operating point of the 7199/6AN8/12A*7 input amplifier or phase splitter. NOTE: I do not know what the M-125 feedback path looks like, I don't think the schematic is published. M-125 may sound fine with two outputs tubes.
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    LeGrace

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    Re: M-125 Two tubes, or four?

    Post by LeGrace on Thu Mar 16, 2017 4:45 pm

    With what I have on hand I can run 4 x 6550, 2 x KT120, or 2 x KT88 in my M125's. Wondering if the differences are more then just a power output issue?
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    LeGrace

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    Re: M-125 Two tubes, or four?

    Post by LeGrace on Sun Mar 19, 2017 5:00 pm

    I started out with 4, went to 2, now I'm back to 4. 2 was sort of forced on me when one of my KT88's went kaput. Now that I'm back to 4 tubes per amp I am realizing this is the way they were meant to run! more depth.

    Steve G

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    Re: M-125 Two tubes, or four?

    Post by Steve G on Sun Mar 19, 2017 6:22 pm

    Even though I haven't received my amps yet, my intention was and still is to install 4 KT-88's in each amp. Like you said "that's the way they were meant to run".
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    LeGrace

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    Re: M-125 Two tubes, or four?

    Post by LeGrace on Sun Mar 19, 2017 6:49 pm

    The KT88s are really nice in these amps. Do you have prior experience with tube amps?

    Steve G

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    Re: M-125 Two tubes, or four?

    Post by Steve G on Sun Mar 19, 2017 7:35 pm

    Yes, I built two mono blocks about 12 years ago and originally configured them with 300B tubes then about 5 years ago reconfigured and installed 2A3's. I think that's why I went with KT-88 tubes even though the 300B was around 8 watts the lower power 2A3 at about 3 watts just sounded better.
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    pedrocols

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    Re: M-125 Two tubes, or four?

    Post by pedrocols on Sun Mar 19, 2017 7:48 pm

    Steve G wrote:Even though I haven't received my amps yet, my intention was and still is to install 4 KT-88's in each amp.  Like you said "that's the way they were meant to run".  
    They are meant to run with either 2 or 4 tubes as stated above.
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    deepee99

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    Re: M-125 Two tubes, or four?

    Post by deepee99 on Sun Mar 19, 2017 8:11 pm

    I have heard no difference between a pair and a quad of KT-88s in my M-125s, but I have reasonably efficient (91 dB) speakers so maybe no need for the extra headroom. Too many other variables to consider, too, i.e. room size and acoustics, etc.
    Interestingly, the drop in rated watts is about the same going from ultralinear to triode mode with four tubes as it is with two tubes in ultralinear. Anyone made that comparison?
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    LeGrace

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    Re: M-125 Two tubes, or four?

    Post by LeGrace on Sun Mar 19, 2017 8:20 pm

    They run fine either way, but such a waste not to be using them to their full capability.

    Reason I ask Steve I was a total tube newb when I started out. Main advice is do the yellow sheet mod right out of the gate. (ref diodes on the rectifier socket, see the sticky titled tube rectifier diode mod) I blew several rectifiers until I found out about this. Second buy good quality tubes from a reputable outfit. (ref my loose socket thread) Then enjoy! Keep us posted how things go.

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