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    AB for M125?

    LeGrace
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    Post by LeGrace on Sun Jun 30, 2019 8:21 pm

    Is latest revision ready for prime time? Jumpy bias on these amps drives me nuts, curious if these are are reliable enough now to assume control?
    bluemeanies
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    Post by bluemeanies on Sun Jun 30, 2019 11:42 pm

    I have the m125's without the AB control and I am using the Genalex Gold Lion KT88's..no jumpy bias here..Jim McShane..I highly recommend.
    Had the Sovtek KT88's and the bias was jumping constantly.
    So based on this I have to assume quality matched tubes are necessary.
    peterh
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    Post by peterh on Mon Jul 01, 2019 1:58 am

    LeGrace wrote:Is latest revision ready for prime time? Jumpy bias on these amps drives me nuts, curious if these are are reliable enough now to assume control?    
    It's tubes that is jumpy, not the amp.
    Get a matched quad from a reputable dealer and you will avoid "jumpy bias"
    Bob Latino
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    Post by Bob Latino on Mon Jul 01, 2019 7:25 am

    LeGrace wrote:Is latest revision ready for prime time? Jumpy bias on these amps drives me nuts, curious if these are are reliable enough now to assume control?    

    A steady bias reading is predicated by a steady line voltage. If your line voltage changes even a 1/4 of a volt, then the bias will change noticeably. In some parts of the USA and Canada the line voltage changes quite a bit during the day. As line voltage goes UP so will the bias. A rock steady bias voltage is not absolutely necessary for good sound. Output tubes have a "range" over which they will operate efficiently. The output tubes should be operated in the lower part of that range for good sound AND reasonably long tube life.

    Bob
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    Post by Guest on Mon Jul 01, 2019 9:43 am

    LeGrace wrote:Is latest revision ready for prime time? Jumpy bias on these amps drives me nuts, curious if these are are reliable enough now to assume control?    

    just sent you a private message
    LeGrace
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    Post by LeGrace on Mon Jul 01, 2019 6:48 pm

    Thanks for the pm.
    tubes4hifi
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    Post by tubes4hifi on Wed Jul 03, 2019 1:58 pm

    I've never had any trouble with steady bias on EL34 tubes, but many many times have had trouble keeping steady bias with KT88 tubes.
    Make sure your bias grid resistors are 100K or less. If you ever run KT120 or KT150 those resistors need to be 51K. (R29-32 on any VTA board)
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    KSMITH

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    Post by KSMITH on Fri Jul 05, 2019 9:45 pm

    tubes4hifi wrote:I've never had any trouble with steady bias on EL34 tubes, but many many times have had trouble keeping steady bias with KT88 tubes.
    Make sure your bias grid resistors are 100K or less.   If you ever run KT120 or KT150 those resistors need to be 51K.  (R29-32 on any VTA board)

    M-125 monos bias nicely with KT120 with no resistor change necessary.  KT150s are a different story IME.
    LeGrace
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    Post by LeGrace on Sat Jul 06, 2019 8:03 pm

    KSMITH wrote:
    tubes4hifi wrote:I've never had any trouble with steady bias on EL34 tubes, but many many times have had trouble keeping steady bias with KT88 tubes.
    Make sure your bias grid resistors are 100K or less.   If you ever run KT120 or KT150 those resistors need to be 51K.  (R29-32 on any VTA board)

    M-125 monos bias nicely with KT120 with no resistor change necessary.  KT150s are a different story IME.

    Wish I could say the same. Since installing KT120's my M125 are hypersensitive. My target is 0.6 per tube. Quite common I'll turn the amps on and see a low reading on the order of 0.3. I'll barely move the pots and recheck and they jump to 0.8 to 0.9. Move them back a whisker and bias falls way too low again. Dialing them in is a nightmare. Doesn't help that my line voltage is up and down like a toilet seat.

    The AB boards I'm hoping can be my salvation. Surprised though that no one has chimed in yet and said I've done this mod and they are great.
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    KSMITH

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    Post by KSMITH on Mon Jul 15, 2019 8:48 pm

    My house has its own transformer. I do run a variac to reduce but my voltage is fairly steady throughout the day and night. The utility LTC seems to be set at 122 VAC with a deadband of +/- 3 VAC from my observations. Bottom line is my line voltage is very clean and solid. From what I read that is not always the case.
    deepee99
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    Post by deepee99 on Wed Jul 17, 2019 10:05 am

    KSMITH wrote:My house has its own transformer.  I do run a variac to reduce but my voltage is fairly steady throughout the day and night.  The utility LTC seems to be set at 122 VAC with a deadband of +/- 3 VAC from my observations. Bottom line is my line voltage is very clean and solid.  From what I read that is not always the case.  
    I thought that only existed in fairy tales Smile
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    costerdock

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    Post by costerdock on Thu Jul 18, 2019 7:02 pm

    I have no problem with bias - mine have been running strong for close to 8 years?
    deepee99
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    Post by deepee99 on Thu Jul 18, 2019 7:07 pm

    costerdock wrote:I have no problem with bias - mine have been running strong for close to 8 years?
    Wow, eight years, same tubes? Where's your bias set?
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    Post by costerdock on Thu Jul 18, 2019 7:31 pm

    deepee99 wrote:
    costerdock wrote:I have no problem with bias - mine have been running strong for close to 8 years?
    Wow, eight years, same tubes? Where's your bias set?


    Not same tubes - I got about 6 years out of my Sovtek KT88s - my favorite tubes.

    I've since used KT88 Gold Lions - lost one - then replaced all with Sovtek 6550 which I'm very happy with.

    I run two tubes in each amp with the bias at 500.
    deepee99
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    Post by deepee99 on Thu Jul 18, 2019 8:34 pm

    costerdock wrote:
    deepee99 wrote:
    costerdock wrote:I have no problem with bias - mine have been running strong for close to 8 years?
    Wow, eight years, same tubes? Where's your bias set?


    Not same tubes - I got about 6 years out of my Sovtek KT88s - my favorite tubes.

    I've since used KT88 Gold Lions - lost one - then replaced all with Sovtek 6550 which I'm very happy with.

    I run two tubes in each amp with the bias at 500.
    I think that bias setting is your secret. I keep mine at .475 and can't hear a bit of difference between that and the .62 maximum allowed (at least in VTA amps. Hopefully the payoff will be in longer tube and transformer life.
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    costerdock

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    Post by costerdock on Thu Jul 18, 2019 8:40 pm

    deepee99 wrote:
    costerdock wrote:
    deepee99 wrote:
    costerdock wrote:I have no problem with bias - mine have been running strong for close to 8 years?
    Wow, eight years, same tubes? Where's your bias set?


    Not same tubes - I got about 6 years out of my Sovtek KT88s - my favorite tubes.

    I've since used KT88 Gold Lions - lost one - then replaced all with Sovtek 6550 which I'm very happy with.

    I run two tubes in each amp with the bias at 500.
    I think that bias setting is your secret. I keep mine at .475 and can't hear a bit of difference between that and the .62 maximum allowed (at least in VTA amps. Hopefully the payoff will be in longer tube and transformer life.

    Yeah - I used to do closer to .600 - then backed off from that - and was glad to hear Bob recommending 1.0 and .5 vs 1.2 and .6 (for M125 4 tube setup vs 2 tube setup - I prefer the 2 tube setup.)

    I also had less rectifier issues.
    deepee99
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    Post by deepee99 on Thu Jul 18, 2019 8:58 pm

    Good thinking, if great minds think alike.
    I ran my TS 6550s (old US-made) down to .450; anything below that and you could hear a little mud. .475 seems to be their 'company notch.'
    I think I read somewhere that running output tubes at too-low of a bias is just as hard on them as over-biasing. Anyone else hear anything like that and explain why?
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    Post by KSMITH on Fri Jul 19, 2019 6:01 pm

    deepee99 wrote:
    KSMITH wrote:My house has its own transformer.  I do run a variac to reduce but my voltage is fairly steady throughout the day and night.  The utility LTC seems to be set at 122 VAC with a deadband of +/- 3 VAC from my observations. Bottom line is my line voltage is very clean and solid.  From what I read that is not always the case.  
    I thought that only existed in fairy tales Smile

    400 amp service at the end of a dirt road.... Laughing
    corndog71
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    Post by corndog71 on Fri Jul 19, 2019 7:24 pm

    deepee99 wrote:Good thinking, if great minds think alike.
    I ran my TS 6550s (old US-made) down to .450; anything below that and you could hear a little mud. .475 seems to be their 'company notch.'
    I think I read somewhere that running output tubes at too-low of a bias is just as hard on them as over-biasing. Anyone else hear anything like that and explain why?

    I’ve been told that for longer life, power tubes should be run at half their max dissipation.
    LeGrace
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    Post by LeGrace on Sat Jul 20, 2019 2:53 pm

    Just received from Erhard-Audio:

    https://www.amazon.ca/photos/share/6yJisLnNKx4fVlrFjxiHHhQJo6yPz3dPsgS74xJxgSu

    The AB boards appear high quality.

    Edit: Let me know if link works
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    New2Tubez

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    Post by New2Tubez on Sat Jul 20, 2019 6:03 pm

    LeGrace wrote:Just received from Erhard-Audio:

    https://www.amazon.ca/photos/share/6yJisLnNKx4fVlrFjxiHHhQJo6yPz3dPsgS74xJxgSu

    The AB boards appear high quality.

    Edit: Let me know if link works

    Link works (picture of 2 AB boards w/ caps, resistors and standoffs)
    LeGrace
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    Post by LeGrace on Sat Jul 20, 2019 6:10 pm

    New2Tubez wrote:
    LeGrace wrote:Just received from Erhard-Audio:

    https://www.amazon.ca/photos/share/6yJisLnNKx4fVlrFjxiHHhQJo6yPz3dPsgS74xJxgSu

    The AB boards appear high quality.

    Edit: Let me know if link works

    Link works (picture of 2 AB boards w/ caps, resistors and standoffs)

    Thanks for confirming. First time using Amazon photos. Tried to paste pic directly into post but didn't work out so this will have to do until I can figure it out.
    deepee99
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    Post by deepee99 on Sat Jul 20, 2019 6:13 pm

    No joy here on pics but they are nicely built.
    LeGrace
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    Post by LeGrace on Thu Jul 25, 2019 4:20 pm

    This weekend planning to start the build. Nervous but hopeful as well.
    deepee99
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    Post by deepee99 on Thu Jul 25, 2019 5:47 pm

    LeGrace wrote:This weekend planning to start the build. Nervous but hopeful as well.
    You'll do fine. Not speaking on his behalf, but Holger @ Earhart Audio is very handy with these things and do it right. Enjoy!

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