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    M-125's With Variable Transformer

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    Leigh W

    Posts : 47
    Join date : 2013-04-02

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    Post by Leigh W on Thu Nov 07, 2019 2:44 am

    Hi everyone. I haven't posted here in a long time, but could use some reassurance on my plans.
     I built a pair of M-125 monoblocks in December of 2017. They were to be my weekend amps where I could just sit down and listen. However with 3 kids, weekends are just as busy as weekdays, so they probably have less than 30 hours on them. They both consist of K120 output tubes, Nos Mullard signal tubes, 5ar rectifier, the yellow page rectifier diode mod and the time delay relay board. I check the bias whenever I manage to find time to listen and its always been rock steady and have never had any issues with the amps. They have also never been played at high volumes as it's always been late night listening. I now am retired and plan to listen a lot more. Our line voltage varies here from anywhere between 122-125v so have always been careful to listen when the voltage is on our low end. So here is what I am in the process of:

    I have on order a pair of Weber WS1 rectifiers with thyrister.
    A gfci installed to shut the amps down permanently when a power interruption occurs.
    A 2000 watt peak variable transformer that came today.

    I have some questions regarding the variable transformer if I may. I set the output tonight at 118 volts and verified it with my volt meter. So will I need to recheck the transformers output at every listing session as the line in voltage varies to maintain the set 118v out? Or will the variable transformer maintain pretty much 118v on the output regardless of my varying line voltage? Is it safe to leave the VT turned on all the time at my dialed in voltage when not using the amps, or should it be dialed back to zero and then turned off? Can it be shut down safely at the 118v setting without dialing it back to zero and then turned on again at a later date? I have no issue shutting it off but have read that it should be dialed back down to zero beforehand and it seems tedious to to have to dial in 118v every time before using the amps. When using the WS1 is it ok to leave everything as is (diode mod, supply voltage to rectifier socket etc...) in place and just do a swap out for the tube rectifier and re-bias the output tubes? The time delay on my amps is roughly 14 seconds`I am hoping I have all bases covered here to protect these beauties but any input to educate me further would be greatly appreciated.

    Cheers,
    Leigh
    Bob Latino
    Bob Latino
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    Join date : 2008-11-26
    Location : Massachusetts

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    Post by Bob Latino on Thu Nov 07, 2019 8:55 am

    Leigh W wrote:Hi everyone. I haven't posted here in a long time, but could use some reassurance on my plans.
     I built a pair of M-125 monoblocks in December of 2017. They were to be my weekend amps where I could just sit down and listen. However with 3 kids, weekends are just as busy as weekdays, so they probably have less than 30 hours on them. They both consist of K120 output tubes, Nos Mullard signal tubes, 5ar rectifier, the yellow page rectifier diode mod and the time delay relay board. I check the bias whenever I manage to find time to listen and its always been rock steady and have never had any issues with the amps. They have also never been played at high volumes as it's always been late night listening. I now am retired and plan to listen a lot more. Our line voltage varies here from anywhere between 122-125v so have always been careful to listen when the voltage is on our low end. So here is what I am in the process of:

    I have on order a pair of Weber WS1 rectifiers with thyrister.
    A gfci installed to shut the amps down permanently when a power interruption occurs.
    A 2000 watt peak variable transformer that came today.

    I have some questions regarding the variable transformer if I may. I set the output tonight at 118 volts and verified it with my volt meter. So will I need to recheck the transformers output at every listing session as the line in voltage varies to maintain the set 118v out? Or will the variable transformer maintain pretty much 118v on the output regardless of my varying line voltage? Is it safe to leave the VT turned on all the time at my dialed in voltage when not using the amps, or should it be dialed back to zero and then turned off? Can it be shut down safely at the 118v setting without dialing it back to zero and then turned on again at a later date? I have no issue shutting it off but have read that it should be dialed back down to zero beforehand and it seems tedious to to have to dial in 118v every time before using the amps. When using the WS1 is it ok to leave everything as is (diode mod, supply voltage to rectifier socket etc...) in place and just do a swap out for the tube rectifier and re-bias the output tubes? The time delay on my amps is roughly 14 seconds`I am hoping I have all bases covered here to protect these beauties but any input to educate me further would be greatly appreciated.

    Cheers,
    Leigh

    Hi Leigh,

    A variable transformer (variac) will not respond on its own to changes in your incoming line voltage. What you should do first is determine the rise and fall of your incoming AC line voltage over a few days. When your line voltage is at its highest (lets say 123 VAC), you then set the variac to give 118 VAC on the variac's AC outlet. As your wall outlet line voltage drops, the voltage will also drop at the variac's AC outlet to maybe 115 - 117 VAC but that is OK. The amp's bias will also drop a little but that is OK also. The amp will play and the AC and DC voltages within the amp will be at safe levels.

    RE: " but have read that it should be dialed back down to zero beforehand and it seems tedious to to have to dial in 118v every time before using the amps." You don't have to dial the amp down to 118 VAC every time you use the amps. These amps will operate fine at voltages from 115 VAC up to about 122 VAC. When your line voltage approaches 122 VAC, you should consider a variac. Line voltages in the 122 VAC range or above can cause shorter tube life.

    Bob
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    Leigh W

    Posts : 47
    Join date : 2013-04-02

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    Post by Leigh W on Thu Nov 07, 2019 11:30 am

    Thanks Bob for the reply. I will keep an eye on the line voltage for a few days and set the Variac appropriately.
    Leigh
    PeterCapo
    PeterCapo

    Posts : 738
    Join date : 2008-12-05

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    Post by PeterCapo on Thu Nov 07, 2019 12:48 pm

    One thing I would add about monitoring AC mains from the wall socket... I used to do the thing where you take readings a few times a day for a few days. But, when I purchased a line monitoring device that conveniently displays the level of AC mains 24/7, I found that it varied by more than was revealed by taking spot readings from time-to-time.

    I wonder if this might be the cause of difficulties some folks run into. Then again, my AC was wandering a bit too much, and I eventually was able to get in touch with the right person at my local electric company who ordered some adjustments. The line has since been steadier. But, I will continue to use my 24/7 monitor. These amps cost $ and measures should be taken to protect the investment.
    IEales
    IEales

    Posts : 12
    Join date : 2019-03-16
    Location : Florence OR

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    Post by IEales on Sun Nov 24, 2019 12:02 pm

    If the VT is Chinese, be leery of the power switch quality.
    Don't use the unit as a mains switch. Use the amp switches or use an upstream power switch.

    IMO a bucking transformer is a better option.
    VT have a big contact that can wear and become noisy.


    M-125's With Variable Transformer BuckTrans
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    stewdan

    Posts : 193
    Join date : 2010-03-07
    Age : 81
    Location : Houston Texas

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    Post by stewdan on Sun Nov 24, 2019 12:31 pm

    IEales ..... is the Bucking Transformer something you built or is it a commercial product? Details??
    Thanks!
    IEales
    IEales

    Posts : 12
    Join date : 2019-03-16
    Location : Florence OR

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    Post by IEales on Sun Nov 24, 2019 4:57 pm

    It's a unit I designed & built.

    Full information ieLogical Bucking Transformer
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    eickmewg

    Posts : 103
    Join date : 2014-08-29

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    Post by eickmewg on Mon Nov 25, 2019 9:27 am

    IEales,
    Your bucking transformer puts my DIY unit to shame! I need one!

    Bill
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    69Cutlass

    Posts : 18
    Join date : 2019-04-28

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    Post by 69Cutlass on Mon Nov 25, 2019 10:21 am

    eickmewg wrote:IEales,
         Your bucking transformer puts my DIY unit to shame!  I need one!

    Bill

    I have been very happy with one of these for my ST-120:

    AmpRX BrownBox Tube Amplifier Input Voltage Attenuator
    aguaazul
    aguaazul

    Posts : 114
    Join date : 2012-07-08
    Age : 59
    Location : Livermore, CA

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    Post by aguaazul on Mon Nov 25, 2019 10:21 pm

    69Cutlass wrote:

    I have been very happy with one of these for my ST-120:

    AmpRX BrownBox Tube Amplifier Input Voltage Attenuator

    '69

    I was checking out your SP11 Preamp Just Arrived! post and saw what looked like a stomp-box on the floor by your rig.
    Thanks for pointing out what that little gizmo is.

    Looks like it can do the job with many variables for different input and outputs.

    Cheers,

    Bluewater
    aguaazul
    aguaazul

    Posts : 114
    Join date : 2012-07-08
    Age : 59
    Location : Livermore, CA

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    Post by aguaazul on Mon Nov 25, 2019 10:59 pm

    IEales wrote:It's a unit I designed & built.

    Full information ieLogical Bucking Transformer


    Hey IEales,
    I must say, this project looks really nice. We live close to the Lawrence Livermore Lab in California, they have a mile long underground particle accelerator that needs a bit more than your everyday 117v. I think we get super strong voltage for being their neighbor. Our power is hot, up to 126v.

    We started out with a Kill-O-Watt matched with a Circuit Specialists Variac. We have a few variacs laying around now, never had a problem with them. Then we picked up a Furman SPR-20i Line conditioner. That keeps the voltage at max of 121v.

    Our main stereo listening setup is an SP14 (Don Sachs DS2) with Bob & Roy's ST-120. Both of these can handle 121v.

    But our backup gear is a little older and would be very happy at 117v. We have a VTA board upgraded pre 1970 Dynaco ST-70 & PAS3.
    I see one of your units in our future!

    Thanks for posting your bucking project.  

    Cheers,

    Bluewater

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