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    Prototype 125 watt monoblock tube amplifier KIT

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    Bob Latino
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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by Bob Latino on Thu Jan 20, 2011 9:41 pm

    I had a request by Email for a photograph of the interior wiring of the M-125 amp. Below is a photograph of the inside of the M-125's interior wiring taken by the amp builder.

    This amp is quieter than either the ST-70 or the ST-120 and those two amps (if built correctly) are pretty quiet. I had a friend with some test equipment who lives near Boston run both prototype amps through some tests. He came up with about a 95 dB signal to noise ratio. Some reason's why ..

    1. Stainless steel chassis
    2. Only one channel on the amp
    3. The use of an ST-70 type power supply and grounding system rather than a Mark III
    4. Lower gain level of the M-125 amp vs the ST-70/ST-120 amps

    I recommend to each amp builder that he use screened low noise 12BH7A driver tubes especially for the front voltage amplifier tube. For the rear 12BH7A phase splitter/phase inverter tube the use of a low noise tube is not as important.

    Bob

    NOTE - The photo below is of an earlier VTA M-125. The newer CCS M-125's as of Jan 2014 have a very slightly different wiring of the driver board. The rest of the wiring is the same as the wiring shown in the photo below ..




    Last edited by Bob Latino on Mon Aug 01, 2016 4:59 pm; edited 4 times in total

    RockyAM

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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by RockyAM on Wed Mar 02, 2011 11:59 am

    Hey Bob, I was looking online for 100 watt+ tube monoblock amp kits and I couldn't find any. Is your M-125 kit the only one around? RockyAM

    Bob Latino
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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by Bob Latino on Wed Mar 02, 2011 12:58 pm

    Actually Transcendent Sound has an OTL (output transformer less) monoblock tube amp kit called "The Beast". It is rated as 160 watts per monoblock into 8 ohms. You better whip out your (hopefully) fat wallet to get a pair though. One pair of "The Beast" monoblock kits with tubes is $4500. In comparison, two M-125 monoblock kits with tubes is about $2000. The thing that makes the Beast expensive are the tubes. Each "Beast" monoblock uses SIXTEEN EL34 output tubes or THIRTY-TWO (!) EL34 output tubes for a pair of Beasts.

    Interestingly enough, because of its circuit design, the Beast only puts out 110 watts per monoblock into a 4 ohm load while the VTA M-125 monoblock will still make its spec of 125 watts into a 4 ohm load.

    ONE Beast monoblock amp below ..



    Bob



    tubes4hifi
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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by tubes4hifi on Wed Mar 02, 2011 10:31 pm

    yes, Bob is correct, AND, I'll bet that "beast" uses at LEAST 10 amps of wall current (I was looking for the actual spec but it's not listed), about 3-4 times as much as our ST120 amp does. Keep that amp on for an hour and watch you electric meter go around crazy fast, also works as a great room heater during the winter and REQUIRES air conditioning in the summertime!! Let's see, 32 tubes times 1.5 amps of filament heat for each tube, that's 48 amps of filament heat, plus about 20-25 watts of pure heat from each tube, pretty close to a 1000 watt space heater, maybe more! If you make a wiring error, I think you can expect a MELTDOWN !!

    scombridae

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    M-125 vs Mcintosh M275

    Post by scombridae on Sun Mar 20, 2011 2:49 pm

    Hello,

    I was wondering if anyone has heard both and could describe the differences. I understand the M125 have quite a bit more power but I was wondering more about the character and quality of the sound. I had read reviews of the ST-120 vs MC240 stating that the former had more clarity and depth. It this a product of the ST-120 just having more power? I also realize the Mcintosh uses a

    "Unity Coupled Circuit, one of the bifilar primary windings is connected through the power supply to the plate and cathode of one of the output tubes. The other bifilar winding is connected to the other tube in the same manner. This unity coupled output virtually eliminates notch distortion.

    The MC275 used an advanced design of this technology by incorporating a trifilar wound primary to improve the power bandwidth. The third winding is connected to the plates of the cathode follower driver. Groups of interleaved primary and secondary windings improve coupling and bandwidth of over 100,000 cycles is achieved."

    From what I understand the unity coupled circuit does not need the output tubes to be matched or biased. There has to be some drawbacks to this.

    What are the drawbacks besides loss of volume and that it is very expensive?

    thanks to all that reply.

    Bob Latino
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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by Bob Latino on Sun Mar 20, 2011 4:19 pm

    Hi,

     The character and quality of the VTA ST-120 and the VTA M-125 are the same. The ST-120 and the M-125 use exactly the same driver circuit. The components (the values of the resistors and capacitors), however, within the drive circuit of both amps are not the same. The M-125 has slightly lower gain than the ST-120 and a very slightly wider bandwidth. The ST-120 uses 12AT7 driver tubes (NOTE - As of 4/1/12 all VTA ST-70 and VTA ST-120 amps now use 12AU7 or 12BH7 tubes and have a lower gain circuit like the M-125's) and the M-125 uses 12BH7A driver tubes. The 12BH7A driver tubes in the M-125 were chosen because they give somewhat higher voltage swings that this amp requires. Anything said about the sound of the ST-120 is applicable to the M-125 also.

    The McIntosh MC-275 is a stereo amp and uses a much more complex circuit than the VTA M-125. The McIntosh amps may not need to be biased but the M-125 does need to be biased. Biasing takes about 2 minutes per amp and then you just check the bias once a month. The McIntosh MC275 is 75 WPC. Each M-125 monoblock is a 125 watt amp that may be operated in either ultralinear or triode mode. The McIntosh amp does not operate in ultralinear mode but operates in their own proprietary "unity coupled output" mode.

    The drawback of the  McIntosh unity coupled circuit is that it is very complex and expensive to implement.

    Bob


    Last edited by Bob Latino on Sun Nov 17, 2013 11:11 am; edited 3 times in total

    scombridae

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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by scombridae on Sun Mar 20, 2011 7:00 pm

    I'm sorry. I was asking about the sound differences between a "Unity Coupled Circuit" and the push pull circuit of a Dynaco amp. I realize I may be comparing apples and oranges, especially when all other components, type of music, CD, turntable, preamp, and speakers are added to the mix. I was curious of each design having their "musical" strengths or weaknesses.

    Gregg357

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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by Gregg357 on Sun Apr 03, 2011 11:08 pm

    Bob,
    I'm working on my M-125 build and am confused about the method of securing the quad cap to the chassis. The manual says to twist the tabs 1/4 turn. I feel if I were to twist them they would tear. They don't look like they could endure a 1/4 turn twist. It looks like they should just be bent back up toward the chassis, at a 90 degree angle to the quad cap itself. Please give me your thoughts. Lastly, what's your preferred method for me to ask my questions as I go through the build, here on the forum or direct via email? Thank you.
    Gregg

    Luddite

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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by Luddite on Mon Apr 04, 2011 12:24 am

    Gregg357 wrote:Bob,
    I'm working on my M-125 build and am confused about the method of securing the quad cap to the chassis. The manual says to twist the tabs 1/4 turn. I feel if I were to twist them they would tear. They don't look like they could endure a 1/4 turn twist. It looks like they should just be bent back up toward the chassis, at a 90 degree angle to the quad cap itself. Please give me your thoughts. Lastly, what's your preferred method for me to ask my questions as I go through the build, here on the forum or direct via email? Thank you.
    Gregg

    Gregg,

    The tabs are designed by the manufacturer to be twisted, and they will not break. Bending the tabs over will not secure the capacitor properly to the chassis.

    Best Regards,
    Charlie

    Bob Latino
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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by Bob Latino on Mon Apr 04, 2011 7:08 am

    Gregg,

    Charlie is correct and so is the manual. If you just give those tabs maybe a 1/8 to a 1/4 turn they will usually lock to the chassis securely. If the quad cap seems loose OR anyone has had to remove and reinstall a quad cap, you can also place a small dab of epoxy over each tab after the twist.

    If you have a minor issue with a kit instruction, you are probably better off to Email me directly for a quicker response.

    Bob

    kevco

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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by kevco on Thu Apr 07, 2011 9:03 am

    Bob Latino wrote:Actually Transcendent Sound has an OTL (output transformer less) monoblock tube amp kit called "The Beast". It is rated as 160 watts per monoblock into 8 ohms. You better whip out your (hopefully) fat wallet to get a pair though. One pair of "The Beast" monoblock kits with tubes is $4500. In comparison, two M-125 monoblock kits with tubes is about $2000. The thing that makes the Beast expensive are the tubes. Each "Beast" monoblock uses SIXTEEN EL34 output tubes or THIRTY-TWO (!) EL34 output tubes for a pair of Beasts.

    Interestingly enough, because of its circuit design, the Beast only puts out 110 watts per monoblock into a 4 ohm load while the VTA M-125 monoblock will still make its spec of 125 watts into a 4 ohm load.

    ONE Beast monoblock amp below ..



    Bob


    Actually the "Beast" uses octal EL509 tubes at like $78 a pair!!! They are only made by JJ as far as I know and when they decide to stop making them because nobody buys them except Beast owners, those amps will gather dust...Not the best type of amp for a kit IMHO.

    Bob Latino
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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by Bob Latino on Wed Apr 13, 2011 11:41 am



    I had someone Email me and request a closer look at the M-125 driver boards so I thought it would be a good idea to just post a photo of a pair of the PRODUCTION driver boards. They are of the same construction as the VTA ST-70 driver board with no open solder traces on either the top or bottom of the board. These boards have plated through holes and components may be place on either the top or bottom of the board.

    Bob


    Sutton
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    Picture of the back

    Post by Sutton on Sat Apr 23, 2011 11:59 am

    Hey Bob,

    How about a picture opf the back of the board, os we can see how the jumper wire and some of the other things are suppose to look. Thanks,

    Sutton

    tubes4hifi
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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by tubes4hifi on Fri May 06, 2011 1:00 am

    Bob and I had a special run of these PCBs made specifically for his M125 monoblocks,
    but they are identical to the MK3 mod boards that I also sell.
    But just to be clear, the MK3 boards (which you can use to build ANY P-P tube amp)
    are NOT silkscreened (the white parts outline) and solder masked (the green color)
    but are otherwise absolutely identical, even made by the same PCB supplier,
    here is a photo of the bottom side:

    Sutton

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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by Sutton on Mon May 16, 2011 11:11 am

    Just surfing eBay and saw that Captain Bob's Electronic Emporium is now offering a complete set of tubes for the VTA M-125 mono blocks. He is including a matched quad of Tung Sol KT-120 output tubes, a pair of Electron Harmonix 12BH7A gold pin driver tubes, and the Weber GZ68 Copper Rectifier for $279.97(now 265.97 on 5/19/11) per amp. This may save the shipping from all the different sources putting all the tubes together yourself (as I did). I bought my two quads of KT-120 tubes from Captain Bob, and am very happy with them. His seller ID is ak6r and the item number now is 380332534835 . I am in no way endorsing or have nothing to gain in reporting this. I am simply informing the owners and prospective owners of these wonderful amps of the option to get all the needed tubes and rectifier from one source.

    I have finished only one of my M-125 amps and it is very glorious. I just have to find the time to finish the other one. For those who are on the fence with regard to these mono blocks, I can whole heartedly give them a huge thumbs up. I own many Fishers, including an SA-1000, a Marantz 8B, a McShaned Harman Kardon Citation II, a Pair of Bogen MO-100's, a pair of Quicksilver 8417 mono blocks and many other vintage and "modern" solid state amps. The VTA M-125 is at the top of the crowd. For the money, you can not buy a better pair of mono blocks. Period. Get off the fence and get the soldering iron in hand. These really are THAT good.


    Last edited by Sutton on Thu May 19, 2011 11:51 am; edited 1 time in total (Reason for editing : updated pricing)

    Sutton

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    Any other Driver Tube Types

    Post by Sutton on Tue May 17, 2011 3:59 pm

    In my previous post I failed to thank Bob for all his help and hand holding during my build of the M-125. He is very gracious and patient with folks new to the kit thing, and I certainly asked some really dumb questions in hindsight. Thanks you, your support was invaluable. And I too, wonder when you sleep. Seemed you were always there to answer questions and steer me in the right direction. HATS OFF TO BOB!!!

    Question--has anyone tried any other tubes in the driver board,? I have tried 12AU7 and 12BH7A, and I really liked the gain from the 12AU7, but I did not like the distortion and seeming lack of linearity. I love the 12BH7A tube, but it seems to be a little lacking in gain...seems like the M-125 is sitting there idling waiting for more to open up and do it's thing. Has anyone tried any other tubes, in either rposition in the driver board? How about the E80CC? Will it work in either position? Tungstram or Amperex? How about Russian tubes? Any suitable subs for the rectifier in Russian military tubes? Something from SkyLab or something?

    I'd love to hear from Bob and all users as to what tube types might work, and what your observations might be. Brands, years of manufacture, anything...

    Thanks again to Bob for a great amp at an affordable price.

    Bob Latino
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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by Bob Latino on Tue May 17, 2011 5:18 pm

    Hi Sutton,

    It is difficult to evaluate a single monoblock amp when the amp is supposed to be working with a "partner" in a stereo environment. The single amp may sound lacking in some respects playing alone since we are so used to the 3D stereo effect of two amps (or the two channels of a single stereo amp) playing together. When you complete the second amp and have them playing together you will really hear what these amps can do.

    As for the 12AU7 or 12BH7 tubes and which one you can use. You can use either but you probably won't hear much difference between them. The 12BH7 does have a little less gain but I can't really notice any difference at all in "gain" between the two tubes. The front voltage amplifier driver tube will have the most influence on the sound of the M-125. In that position a Mullard CV4003 (12AU7 equivalent) is my favorite tube so far for a very smooth and accurate mid range and vocals. The RCA "clear top" 12AU7 is also very good and may even offer a little more "air" to the top end of the amp than the Mullard. In the rear phase splitter/inverter tube (IMHO) you should use a 12BH7. A 12BH7 has larger plates and on paper can swing voltages better than a 12AU7. I haven't heard much difference in any of the 12BH7's (RCA, GE, ElectroHarmonix) that I have used in the rear position on the M-125.

    Bob

    Bob Latino
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    Better M-125 photos

    Post by Bob Latino on Tue May 24, 2011 5:21 pm

    I finally found the time to take some better photos of an M-125 amp with the green production driver boards. The other photos contained the lemon colored prototype driver boards. Photos below ...

    Bob







    davidness

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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by davidness on Wed May 25, 2011 9:06 am

    Bob,

    I notice in the pictures that you've paired the quad of KT-120's with the Weber WZ68. I believe you had good success using the Mullard GZ33 rectifier when used with the KT88's. Do you believe the GZ33 to be "too small" when used with the KT120's?


    Last edited by davidness on Wed May 25, 2011 11:19 am; edited 1 time in total

    Bob Latino
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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by Bob Latino on Wed May 25, 2011 9:20 am

    Hi David,

    The Weber WZ68 solid state rectifier is better for the M-125 if you play the amp loudly and if the speakers you have are inefficient and/or difficult to drive.

    The GZ33 and GZ34 are alternate (tube) rectifiers that you can also use with the M-125. They work better with more efficient speakers. The Mullard GZ33 works well in the M-125's but, as you know, these tubes are expensive. If you want to alternately use a tube rectifier in the M-125's, I recommend any GZ34 in place of the Weber WZ68.

    Bob

    bcinnamon

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    M-125 Amp Experience

    Post by bcinnamon on Mon Jun 20, 2011 9:10 pm

    Before I started this project I shopped around for a 100w+ tube amp. Even pre-owned amps were at least twice the price, so I took the plunge. I figured that even if my “project” worked OK it would sound better to me than anything else I could buy.

    But I was blown away; I felt like that guy in the Memorex ad. I’ve been listening continuously to my collection of Jazz and rock — especially tracks with sax, trumpets and guitar. Even when I crank it up to 11 the sound remains pure. My teenage kids definitely preferred the tube amp sound -- and that is from the mp3-earbud crowd! Imaging is uncanny, the tone reminds me of a perfectly tuned bell, and I hear every nuance of brush strokes and guitar slides and audience coughs in the background (apologies to Keith Jarret in advance). The saxophone on “Jungleland” is especially haunting tonight ;(

    I'd attach some photos if I knew how. Suffice it to say that my pictures look just the same as Bob's. Most importantly, the whole build process went quickly and smoothly. Bob's instructions, amp design and suggestions are all top notch.

    Sutton

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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by Sutton on Sat Jun 25, 2011 9:56 am

    I have found that shopping eBay UK is the way to go to find less expensive GZ33 tubes. I recently bouht a brand new pair of Mullards shipped from London for $75.00. You will have to look and wait and perhaps correspond with the seller about shipping to the USA, but this particular tube is not as highly sought after in the UK and bargains CAN be had.

    RockyAM

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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by RockyAM on Thu Jun 30, 2011 1:15 pm

    Hi Bob, It was on the strength of building your VTA ST-70 kit about 2 years ago for my office system that I purchased a pair of your M-125 monoblock kits last month for my main system. Let me say right off - the assembly instructions for the M-125 were excellent. (as were the instuctions on the VTA ST-70) The pictorial was really helpful and there was no question of where a part or wire went. Everything fit perfectly and there were no missing parts or extra parts. The way that the amp is laid out you can tell that much thought went into the design. Both amps started up without issue although one of the 12BH7's that I had purchased was a little noisy. As per your suggestion by Email I swapped that tube with the one behind it and the noise went away. I Emailed the Ebay seller about the noisy tube and I am going to send that tube back to him and he will send me another. I also got the Tung-Sol KT120 output tubes from him. The rectifier is the Weber WZ68 that you recommended.

    The M-125's replaced an older Adcom GFA-555 200 watt per channel solid state amp that I had used since the 1990's. I don't know how the M-125's do it but they sound just as strong at 125 watts as the Adcom at 200 watts. With respect to the sound everything about the M-125's is better than the Adcom amp. The soundstage is wider and deeper on the M-125's and each instrument and vocalist is more pin point as to its location as compared to the Adcom amp. I have also tried the triode vs. the pentode configuration but have not really formed an opinion as yet as to which one I prefer. The amp is still breaking in and only has about 50 hours on it as I write this. I have noticed subtle improvements since the amps were new. In comparison to when the amps were new, the sound now seems smoother and more articulate. Vocalists and acoustic instruments like a sax or a trumpet just sound more like the real thing in comparison to my old Adcom amp. Thanks for offering this amp kit. I don't think that there is another 100+ watt monoblock tube amp kit out there with a better price/performance ratio. RockyAM

    deepee99

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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by deepee99 on Fri Jun 08, 2012 2:18 pm

    Hello all. Please forgive if this has been addressed in another thread and I missed it, but in casting about for tubes for the M-125s I am given the option of selecting (for quad-matched 6550 output tubes) either "early", "late" or "average" distortion. Could someone advise me on this? Many thanks.
    -dave-

    Bob Latino
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    Re: Prototype 125 watt monoblock tube amplifier KIT

    Post by Bob Latino on Fri Jun 08, 2012 3:18 pm

    deepee99 wrote:Hello all. Please forgive if this has been addressed in another thread and I missed it, but in casting about for tubes for the M-125s I am given the option of selecting (for quad-matched 6550 output tubes) either Could someone advise me on this? Many thanks.
    -dave-

    Hi Dave,

    That "early", "late" or "average" distortion is because you are looking at 6550 tubes on a GUITAR AMP web site. Those factors are important to users of guitar amps who use varying amounts of actual "distortion" in their amps to get a certain type of sound for their guitar amp. These early/late/average characteristics have no real bearing on stereo/hi-fi use. For good quality and well tested tubes for the M-125 check with Jim McShane at > jimmcshane@prodigy.net.

    You can also check > The Tubestore online

    or

    The Tube Depot online

    Bob

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