Prototype 125 watt monoblock tube amplifier KIT

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 !!

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.

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.

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

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

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

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:

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.

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,

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?

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.

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.

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

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, as usual, thank-you for the quick info. It's a joy to meet you folks and this community.
-d-

Hey Bob, I had a friend listen to my M-125 monoblocks and he is interested in building a pair. He wanted to see what the inside wiring looked like. I lost your Email address so the only way I could contact you is through the forum here. Any way you could post a photo of the inside wiring? I guess I could have pulled the bottom cover off one of my amps but they were hot and he had to leave anyways. Thanks. RockyAM

That is one beautiful build. My compliments to the assembler, whose handle I cannot recall from another thread. Also, to die for were those two custom wood-framed mirror amps that fellow built in Texas or wherever. JT something or other . . . egads! stark beauty. I would not attempt the same at home.
Thank-you for the referral to Jim McShane for tubes. What a nice guy! He has some NOS GE 12BH7s for the front-end of the M-125s; I grabbed two sets but presume he has more if anybody's interested.
Last I could tell, you still lean toward the Tung-Sol K-120s. But you doubt you and the folks around here have rolled a lot of tubes through the M-125s since then. Jim likes the Genalex KT-88s in your amp.
Am in no rush for an answer as the kits won't get here till Monday and in my aulde age I'll probably be the summer putting them together, but are you still enamoured with the KT-120s? This question is informed with my desire to let the output tubes loaf a little bit rather than maxing them out.
Selah.

p.s. Your other recommendation, that I purchase new speaker wire from Blue Jeans Cable, also paid out. Got the roll of Canare wire almost before I ordered it. Good guys.
Last question I would toss up. Back in my good old days, you kept your coax short and your speaker ledes long. Now, it appears the opposite is true. Could someone enlighten?

Cheerio, and thank-you for bothering with these seemingly amateurish questions.