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    M125s and 6L6 output tubes

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    Sadface

    Posts : 10
    Join date : 2013-05-03

    M125s and 6L6 output tubes

    Post by Sadface on Mon Oct 01, 2018 1:39 am

    Hey guys,

    Hailing all the way from Auckland NZ here.

    Would a set of 6L6 tubes be appropriate for use in a pair of M125 amps?

    It looks to me like the B+ on the M125 is within the range of a 6L6, all be it at the upper end.

    Also are there any other less obvious output tubes anybody has tried?
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    Bob Latino
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    Join date : 2008-11-26
    Location : Massachusetts

    Re: M125s and 6L6 output tubes

    Post by Bob Latino on Mon Oct 01, 2018 7:11 am

    Sadface wrote:Hey guys,

    Hailing all the way from Auckland NZ here.

    Would a set of 6L6 tubes be appropriate for use in a pair of M125 amps?

    It looks to me like the B+ on the M125 is within the range of a 6L6, all be it at the upper end.

    Also are there any other less obvious output tubes anybody has tried?

    I do not recommend that you use 6L6 output tubes in the M-125 amps for two reasons ..

    1. They may not bias properly

    2. 6L6 tubes are not made to handle the DC voltages found in the M-125 amps. If you could get them to bias, my feeling is that they would have a short life in the M-125's

    The only output tubes recommended for the M-125 amps are 6550, KT88 or KT120.

    Bob
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    peterh

    Posts : 1070
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: M125s and 6L6 output tubes

    Post by peterh on Mon Oct 01, 2018 8:00 am

    Sadface wrote:Hey guys,

    Hailing all the way from Auckland NZ here.

    Would a set of 6L6 tubes be appropriate for use in a pair of M125 amps?

    It looks to me like the B+ on the M125 is within the range of a 6L6, all be it at the upper end.

    Also are there any other less obvious output tubes anybody has tried?
    Why this ? There is no shortage of 6550-type tubes, and if one buys a top-class
    amp as the M125, why dilute the joy with possible inferior tubes ?
    avatar
    Peter W.

    Posts : 1036
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: M125s and 6L6 output tubes

    Post by Peter W. on Mon Oct 01, 2018 8:43 am

    Sadface wrote:Hey guys,

    Hailing all the way from Auckland NZ here.

    Would a set of 6L6 tubes be appropriate for use in a pair of M125 amps?

    It looks to me like the B+ on the M125 is within the range of a 6L6, all be it at the upper end.

    Also are there any other less obvious output tubes anybody has tried?

    A conventional glass 6L6G will definitely work, but not indefinitely. Life will be poor, nasty, brutish and short (with apologies to Thomas Hobbes).
    A metal 6L6, as above, but you may have to modify the socket to accept the additional pins.
    A 6L6 GT will have a slightly better life. Only slightly.

    A pre-blight 5881 will be your best bet for longevity, but cost you more than a current KT88/6550, which is where you should be in the first place.

    Horses for courses.

    Sadface

    Posts : 10
    Join date : 2013-05-03

    Re: M125s and 6L6 output tubes

    Post by Sadface on Mon Oct 01, 2018 11:55 pm

    Hi Guys,

    Thanks for answering my questions.

    This question mostly comes from a position of too much gain:

    I've ended up with a very small listening room and the M125s have turned out to be way to big for what I use them for.

    I purchased them in my mid 20's when I was living in what might be termed a party flat and thus required lots of noise.

    Now days I'm in a small unit with my fiance while we save up an Auckland sized house deposit. Being a small unit and not being in a noisy flat means I certainly don't need 100+ watts.

    I only run a pair tubes in these beauties, (never had a big enough room for a quad) but I still don't get to turn the volume dial past 1/5 unless I am listening from outside the room.

    Also local market conditions: I can get a matched quad of Russian 6P3S tubes (6L6 analogues) for less that a single KT88.

    Looks like that's how it is until I can afford myself a king sized listening room or replace my dynaudio speakers with something seriously inefficient.

    Regards
    Duncan
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    Bob Latino
    Admin

    Posts : 2713
    Join date : 2008-11-26
    Location : Massachusetts

    Re: M125s and 6L6 output tubes

    Post by Bob Latino on Tue Oct 02, 2018 7:23 am

    Sadface wrote:Hi Guys,

    Thanks for answering my questions.

    This question mostly comes from a position of too much gain:

    I've ended up with a very small listening room and the M125s have turned out to be way to big for what I use them for.

    I purchased them in my mid 20's when I was living in what might be termed a party flat and thus required lots of noise.

    Now days I'm in a small unit with my fiance while we save up an Auckland sized house deposit. Being a small unit and not being in a noisy flat means I certainly don't need 100+ watts.

    I only run a pair tubes in these beauties, (never had a big enough room for a quad) but I still don't get to turn the volume dial past 1/5 unless I am listening from outside the room.

    Also local market conditions: I can get a matched quad of Russian 6P3S tubes (6L6 analogues) for less that a single KT88.

    Looks like that's how it is until I can afford myself a king sized listening room or replace my dynaudio speakers with something seriously inefficient.

    Regards
    Duncan

    Duncan,

    If your room is small you and you don't need 125 watts per monoblock, you should use the M-125's in the TWO output tube mode. You use only one LEFT and one RIGHT output tube on each monoblock. The bias is then set at .500 volts DC per side instead of 1.00 volt per side. The amps set up like this will have about 65 watts per monoblock and give off less heat. They will sound the same but they will just have less power.

    Bob

    Sadface

    Posts : 10
    Join date : 2013-05-03

    Re: M125s and 6L6 output tubes

    Post by Sadface on Thu Oct 04, 2018 12:05 am

    Hi Bob,

    Sadly, I have only ever used the M-125's with two output tubes per channel. I feel like I'm wasting them to some degree but one day I hope to use them in anger.

    Some of the other choices I have made over the years haven't helped the gain situation either. I'm using one of Roy's fabulous SP14's which is fairly high gain for a preamp in my experience. I'm also using a Marchand Electronics active crossover so my Miller & Kriesel sub can do its job. The theory was that by removing LF from the power amp's duties it should make the mids and highs sweeter which it did but this seems to have the effect of making the M125s have more effective gain.

    I'd never noticed how much USA made electronics I have before now. I also just installed some NOS sylvannia 12au7s which put the new stock tung sols I was using the shame.

    I've ended up biasing my KT88s at .45 vdc on the advise of the tube supplier. I'm running matched Valve Art KT88s. I have tried biasing them up to .5 vdc but didn't notice any different in sound so I've settled on .45 vdc.

    mijohn

    Posts : 37
    Join date : 2013-06-19

    Re: M125s and 6L6 output tubes

    Post by mijohn on Thu Oct 04, 2018 5:06 am

    Hi Sadface,
    You could do what I did to my SP-14 when I found there was too much gain and that was fit a couple of resistors to the attenuator as covered in the linked article from Goldpoint. It includes tables for most attenuator values. I suggest using cheap resistors to experiment with then buy the best you can afford to attach permanently as you only need two per channel. It's a simple solution but works well.

    http://www.goldpt.com/mods.html

    Alternatively you could build a unity (no) gain buffer such as an Aikido ACF or Pass B1 Buffer:

    http://glass-ware.stores.yahoo.net/acf12stbu.html

    https://www.passdiy.com/project/preamplifiers/b1-buffer-preamp

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