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    M125 Builds for a Novice

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    ryaneirich

    Posts : 5
    Join date : 2016-03-14

    M125 Builds for a Novice

    Post by ryaneirich on Tue Mar 22, 2016 11:28 pm

    Like many probably have, I stared at these M125's for probably a year before pulling the trigger (after all $2,300+ is no small purchase).  While I did get them from someone on eBay that did not get around to building them, they were in Bob's original packaging, which was very good packaging to say the least.  The first thing I did was catalog every item against the parts list to make sure they were all there, and that's when I started to see the attention to detail that Bob puts into these kits.  I should also comment out front how accessible and helpful Bob is.  I have contacted him many times prior to buying these amps, and a couple times during the build and it's amazing how quick and helpful he is.

    I have done a basic rebuild on an ST70 (all capacitors replaced, new tubes, replaced selenium rectifier with diode), and rebuilt the power supply on a PAT-4, and that's really about it.  So it's fair enough to call myself a novice when it comes to soldering and working on amplifiers in general.  

    I kind of laugh at the 8-10 hours per amplifier because I know I have way more time into them than that.  But I think the key here is that if you can practice patience, you can go a long way towards mitigating your lack of experience.  

    I really appreciate the clean look of a Dynaco ST70/120 and likewise these M125's. I wanted to put my touch on them without ruining that simple, classic look.  In the end, I played off a simple vision of subtly recreating a retro automotive look; some acorn nuts and wrinkle finish paint on the transformers (think M/T valve covers).  I briefly thought about drilling the nuts and safety wiring them, but thought that it would be a bit too much... though I wouldn't mind seeing it on someone else's if they wanted to do it!   Not that this ties in with that look, but I was also discontent with the Weber rectifiers, so I started sanding them with 800 grit sandpaper before finally finishing with 3000 grit and then some Nevr-Dull.  I am a fan of big bottles, so I didn't have the choice of getting anything but the KT-120's.  I could not believe how big they were!

    Some of my notes on the build:

    1. Roll the solid strand wire under a book to get it straightened out (that one's pretty obvious, but doesn't hurt to mention)
    2. The output transformer covers were not the most straight.  Take the time to get them straight, particularly the feet and the small sides of the 90 degree bends. Between a vise and some scrap steel and a hammer I was able to get these completely straight.  Of course test fit them against the transformer one last time before painting them to make sure the fit is still good.
    3. Take the time to build the stand.  Bob's instructions say to lay it on something soft like a towel, but I would never do a build of this care and detail without taking some time to make a good stand.  A hint: Not a bad idea to add a tray.  I made a little magnetic parts tray that I glued to my stand... very glad I did that.
    4. When prepping the transformers for paint: The lamination of the casing does a great job of catching and ripping fibers off your cloth.  If you don't have a tack-cloth get one, it handles it better than anything else.  A blast of compressed air doesn't hurt either.
    5. Make a non-marring screwdriver tip out of something like a spare piece of brass. It helps keep the screw heads clean looking; it doesn't take much to notice even the smallest deformation on the edges of the screw slots.  

    I have yet to finish the 2nd one (although it's close to complete), but I got the first one hooked up and got a few hours on it.  I can't believe how much power there is!  Even in triode, the amplifier has way more power than my old Pioneer speaker could handle.  I like to listen loud, so I think that I am going to start looking for some new speakers that can take advantage of at least most of the power on tap!  I can't comment too much on the sound because I was going through my laptop, to the aforementioned PAT-4, and into a single speaker.  However, I should have a better idea when I at least get some stereo sound coming from my turntable.

    Next step with these amps: Build stock-style covers for them. I have already found a perforated steel sheet online that mimicks the hole pattern of the ST-70 covers very closely.  That will be quite the project in itself, but one thing at a time!

    I thought I'd post some pictures of the build process for you all to see.  I am not getting them to show up in the preview; I am pasting the "Email and IM" URL from the photobucket site into the "Insert Image" option on this website.  Looks like the link works, but anybody know why the pictures aren't showing up?




    AWESOME KITS!!!

    ryaneirich

    Posts : 5
    Join date : 2016-03-14

    Re: M125 Builds for a Novice

    Post by ryaneirich on Wed Mar 23, 2016 12:51 am

    Well here's the link to the pictures, but I'm still looking for any advice on why they won't show up in the thread:

    http://s1360.photobucket.com/user/ryaneirich/library/

    Ryan, hope you don't mind, I inserted one of the better photos here (tubes4hifi)


    Tube Nube

    Posts : 603
    Join date : 2008-12-06
    Age : 53
    Location : Calgary, AB

    Re: M125 Builds for a Novice

    Post by Tube Nube on Wed Mar 23, 2016 1:06 am

    Lovely job, Ryan.

    And welcome aboard!

    daveshel

    Posts : 148
    Join date : 2011-11-06
    Location : Tucson AZ USA

    Re: M125 Builds for a Novice

    Post by daveshel on Wed Mar 23, 2016 1:32 am

    Nice build.

    nerpissad

    Posts : 58
    Join date : 2012-09-16
    Age : 43
    Location : amsterdam

    Re: M125 Builds for a Novice

    Post by nerpissad on Wed Mar 23, 2016 6:53 am

    I really like the color you did on the transformers. Nice build and I think you may need a higher post count in order to post pictures directly.


    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: M125 Builds for a Novice

    Post by tubes4hifi on Wed Mar 23, 2016 2:17 pm

    Ryan, on your Photobucket page, when looking at photos, on the right side of the page is a "Share This Photo" section.
    You have to copy the part where it says IMG (the bottom row) into your post, then it works.
    Here's another of the photos from your page . . . great job on the amps, and I love the tranny paint!

    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: M125 Builds for a Novice

    Post by tubes4hifi on Wed Mar 23, 2016 2:22 pm

    One more thing, and I hope Bob reads this also - I see you are using KT120s on the amp.
    KT120s need small value grid resistors, Jim McShane (our tube guru) has posted on this a couple times.
    Change the 150K resistors (R30 & R32 on the PCB) to 47K resistors, and that will likely require a tweak of the bias.

    ryaneirich

    Posts : 5
    Join date : 2016-03-14

    Re: M125 Builds for a Novice

    Post by ryaneirich on Wed Mar 23, 2016 3:03 pm

    Okay I will swap out those resistors. What wattage should they be? In fact if you could tell me exactly which ones you would use (manufacturer and P/N), I would appreciate it.

    Just curious though; what risk is there with my current configuration?

    Here's my running list of differences so far between the build instructions and what's been suggested by you or Bob to change:

    1. The routing of the yellow output x-former wire down the middle of the chassis instead of along the side of chassis.
    2. The deletion of the AC bias pot (This was a design change, and not suggested for me to change, but Bob let me know. My amp was originally purchased by the guy on ebay in Feb 2015, so it was prior to this change).
    3. Changing out these aforementioned resistors.

    Anything else that rings a bell that has changed on these amps that may not have made its way to the ones I purchased (or their instructions?).

    Thanks!

    ryaneirich

    Posts : 5
    Join date : 2016-03-14

    Re: M125 Builds for a Novice

    Post by ryaneirich on Wed Mar 23, 2016 3:06 pm

    tubes4hifi wrote:Ryan, on your Photobucket page, when looking at photos, on the right side of the page is a "Share This Photo" section.
    You have to copy the part where it says IMG (the bottom row) into your post, then it works.
    Here's another of the photos from your page . . .  great job on the amps, and I love the tranny paint!

    Okay that might be where I went wrong: I was posting the link for the "Library". So it looks like I have to insert 1 link per picture?

    Bob Latino
    Admin

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    Location : Massachusetts

    Re: M125 Builds for a Novice

    Post by Bob Latino on Wed Mar 23, 2016 3:17 pm

    tubes4hifi wrote:One more thing, and I hope Bob reads this also - I see you are using KT120s on the amp.
    KT120s need small value grid resistors, Jim McShane (our tube guru) has posted on this a couple times.
    Change the 150K resistors (R30 & R32 on the PCB) to 47K resistors, and that will likely require a tweak of the bias.

    The VTA M-125's use 100K resistors in R30 and R32. 100K grid resistors work fine in the M-125's with either KT88, KT120 or 6550 output tubes. I have used all three of these output tubes my own M-125's since 2010 and have never had a problem with runaway bias or shorter tube life. No customer has ever reported a "runaway output tube" using the 100K grid resistors. Although Jim may mention the need for 47K grid resistors, in practice the need for the 47K's is (IMHO) not necessary.

    Bob

    Bob Latino
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    Location : Massachusetts

    Re: M125 Builds for a Novice

    Post by Bob Latino on Wed Mar 23, 2016 3:33 pm

    ryaneirich wrote:  I can't believe how much power there is!  Even in triode, the amplifier has way more power than my old Pioneer speaker could handle.  I like to listen loud, so I think that I am going to start looking for some new speakers that can take advantage of at least most of the power on tap!

    The VTA M-125's have slightly more power than the stated 125 watts per monoblock. This chart below has been up on another post. During M-125 testing in 2010, I had an audio friend who had the proper test gear test the power output of the M-125's with both the Gold Lion KT88's and the Tung-Sol KT120's. The results are below in a graph. The amp will put out 140+ watts over most of the audio range without clipping.

    Bob



    ryaneirich

    Posts : 5
    Join date : 2016-03-14

    Re: M125 Builds for a Novice

    Post by ryaneirich on Sun Mar 27, 2016 11:24 pm

    If there's no point swapping these resistors, then I won't bother. But before going any further, I thought I'd dwell on that question a little bit longer. Is leaving the current resistors in there a risk (however low)?

    Thanks!

    Ryan

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