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    New Forum Member, Recent M-125 Monoblock Builds

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    gktamps

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    Join date : 2017-07-30
    Location : Santa Rosa, CA

    New Forum Member, Recent M-125 Monoblock Builds

    Post by gktamps on Mon Jan 15, 2018 5:58 pm

    Hello, Amp Builders,

    New forum member here, but a member of several guitar amp building boards, and thought I'd add a description and photos my recent M-125 builds.

    Prior to building these, my first audio amps, I'd built 22 guitar amps, and repaired a half dozen, so I figured it was time to add some nice tube amps to our audio system. After several weeks of online research, I selected these VTA M-125s and the time delay boards. I built the first one without benefit of a chassis stand, and made a stand to build the second amp, which had two sides, two top straps, and a prop for when I wanted to sit it on end - very handy.

    As you'll see, I've made a few changes to suit my own preferences, add safety, and potentially eliminate ac hum:

    Tighter tolerance, upgraded/uprated resistors in some locations, and teflon-coated, silver-tinned copper stranded wire;

    Moved power switch from front panel to beside the pentode/triode mode switch (new hole), reducing wire length, and re-routing away from preamp board

    Fused the AC load wire (not neutral as designed), and connected AC load wire to tip, not sleeve of fuse holder;

    Tacked down wires with silicone (holdover from my guitar amp building - doesn't look nice, but helps keep wires where intended);

    Shielded coax input cable;

    Moved red and green leds from time delay board to front panel power switch hole;

    Insulator paper between TDR board and chassis, and between speaker terminals and transformer bell;

    Painted transformer bells and lams;

    Constructed solid state rectifier using NTC and UF5408s, added UF5408s to socket in case I want to run rectifier tube instead;

    Moved input ground to dedicated screw/nut, rather than transformer mounting screw;

    Replaced power/mode switches to Switchcraft, and installed using Marshall-style mounting rings for a nice appearance;

    Thick aluminum washers for OT mounting screws.

    I had originally wired in a sag resistor that would be out of circuit when a rectifier tube was plugged in, but in circuit with my SS rectifier (different pinout), but I found the chassis-mounted resistor got quite hot, and I didn't want that heat inside the chassis. I considered making taller SS rectifier cans with room for sag resistors like the Weber, but decided I liked the higher voltage and no sag with the KT-120s I'm running. Photos show the resistor still installed, but I pulled them subsequently.

    Also, I had originally built a full-wave rectifier board for the bias circuit, but it bumped up the voltage too much, and I didn't want to play around with resistor values to drop it back down.

    The amps sound beautiful, with no hum or hiss, a beautiful frequency response, tight, responsive bass, and of course, they look beautiful with their well-designed SS chassis. One downside is the PTs get hot with KT-120s, so I added fans to a heat shield I built for my stand. Another downside is watching the service panel meter spin like a top when these are running, but hey, that's why we have solar panels! Having 8 KT-120s cooking away also helps heat up the amp corner of the room :-).

    My compliments to VTA for the design, planning, packing, etc., involved in making these kits available.

    Cheers,

    Greg Tatarian











    Last edited by gktamps on Tue Jan 16, 2018 11:20 am; edited 1 time in total
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    Peter W.

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    Location : Melrose Park, PA

    Re: New Forum Member, Recent M-125 Monoblock Builds

    Post by Peter W. on Mon Jan 15, 2018 7:44 pm

    Excellent. Further, you are an example of the dynamic of an excellent kit meeting an equally excellent fabricator!

    Congratulations!
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    gktamps

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    Join date : 2017-07-30
    Location : Santa Rosa, CA

    Re: New Forum Member, Recent M-125 Monoblock Builds

    Post by gktamps on Mon Jan 15, 2018 7:45 pm

    Why thank you, Peter!
    Best,
    Greg
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    Kentley

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    Location : Worcester, MA

    Re: New Forum Member, Recent M-125 Monoblock Builds

    Post by Kentley on Sat Jan 20, 2018 12:37 am

    May I add to the above compliments.
    You are a thoughtful and resourceful constructor. I am particularly impressed with your acumen regarding the compromises of rectification.
    And, finally, you are an excellent and provocative photographer. Were you sufficiently courageous and foolhardy, you could land a job in {I cringe to say} advertising.
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    gktamps

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    Re: New Forum Member, Recent M-125 Monoblock Builds

    Post by gktamps on Sat Jan 20, 2018 2:01 pm

    Thank you for the very kind comments! As it turns out, I have been into photography since the age of 7 or 8, and had a part-time photography business back in the days of film. I've since let my skills lapse, and put very little energy into good photographs these days, but I do appreciate your compliments.

    Here's a photo of the inside of the rectifiers I made for this amp. Please be aware that the pin connections are specific to how I have wired the socket to take either these units with an onboard dropping resistor, or a tube recto that bypassed the resistor.


    audiobill

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    Location : Philadelphia

    Re: New Forum Member, Recent M-125 Monoblock Builds

    Post by audiobill on Sat Jan 20, 2018 2:13 pm

    May I ask what your ac hum measures (in mv) from ground to each bias test point and from ground to output binding post?


    audiobill

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    Re: New Forum Member, Recent M-125 Monoblock Builds

    Post by audiobill on Sat Jan 20, 2018 2:16 pm

    Also, you should try running the yellow feedback wire straight up the chassis (not along the side) to the NFB connection on the board.
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    gktamps

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    Re: New Forum Member, Recent M-125 Monoblock Builds

    Post by gktamps on Sat Jan 20, 2018 5:24 pm

    I can get to that in a couple of days. Do you have some baseline measurements or a range you are thinking of as a comparison?

    audiobill wrote:May I ask what your ac hum measures (in mv) from ground to each bias test point and from ground to output binding post?

    On the NFB wire; why do you recommend re-routing? In some of my guitar amp builds, particularly Trainwreck style, position of the NFB wire is critical, sometimes to within a few mm, because the circuit is on the edge of stability (why I use silicone glue to hold down wires after all testing is done). No such concerns with this circuit, though, and no observed problems in operation. I considered three routes when building; 1) straight up, crossing the bundle of wires from the mode switch and the AC line wires to my relocated switch, 2) all the way up along the chassis from the input jack to the board, or 3) the chosen route. I discarded the first two as potentially causing some noise or stability issues. Can you expand on your rationale?

    audiobill wrote:Also, you should try running the yellow feedback wire straight up the chassis (not along the side) to the NFB connection on the board.

    audiobill

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    Re: New Forum Member, Recent M-125 Monoblock Builds

    Post by audiobill on Sat Jan 20, 2018 5:26 pm

    I suggest this routing to improve stability.

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    gktamps

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    Re: New Forum Member, Recent M-125 Monoblock Builds

    Post by gktamps on Sat Jan 20, 2018 6:22 pm

    OK, that's what I thought. No stability problems with my amps, but I'll keep it in mind.
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    Pillo69

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    Re: New Forum Member, Recent M-125 Monoblock Builds

    Post by Pillo69 on Sat Jan 20, 2018 7:27 pm

    gktamps wrote:Thank you for the very kind comments! As it turns out, I have been into photography since the age of 7 or 8, and had a part-time photography business back in the days of film. I've since let my skills lapse, and put very little energy into good photographs these days, but I do appreciate your compliments.

    Here's a photo of the inside of the rectifiers I made for this amp. Please be aware that the pin connections are specific to how I have wired the socket to take either these units with an onboard dropping resistor, or a tube recto that bypassed the resistor.


    I see two diodes and a thermistor in the photograph.
    Do you not think that a voltage limiting resistor for the output would be necessary?
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    gktamps

    Posts : 14
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    Re: New Forum Member, Recent M-125 Monoblock Builds

    Post by gktamps on Sat Jan 20, 2018 8:30 pm

    Hello,
    This is from my original post:
    "I had originally wired in a sag resistor that would be out of circuit when a rectifier tube was plugged in, but in circuit with my SS rectifier (different pinout), but I found the chassis-mounted resistor got quite hot, and I didn't want that heat inside the chassis. I considered making taller SS rectifier cans with room for sag resistors like the Weber, but decided I liked the higher voltage and no sag with the KT-120s I'm running. Photos show the resistor still installed, but I pulled them subsequently."
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    Bob Latino
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    Re: New Forum Member, Recent M-125 Monoblock Builds

    Post by Bob Latino on Sat Jan 20, 2018 10:51 pm

    audiobill wrote:Also, you should try running the yellow feedback wire straight up the chassis (not along the side) to the NFB connection on the board.

    Bill is correct here BUT > Since Greg used shielded cable on the wiring from the input jack to the driver board input, it probably precludes the wiring of the yellow wire up the center of the amp and away from the input jack wiring to the driver board. Below is a photo of the correct wiring of the YELLOW feedback line on each M-125 amp. Between the two chokes, then between the SCM and the triode/pentode ultralinear switch and around the Russian PIO caps to the NFB eyelet on the driver board.

    Bob


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    gktamps

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    Re: New Forum Member, Recent M-125 Monoblock Builds

    Post by gktamps on Sun Jan 21, 2018 6:55 pm

    Yes, Bob, that was one reason I chose to use coax; since I had decided to not cross the NFB wire over multiple wires and knew I'd be paralleling the input for some distance.

    Based on comments and other versions by folks on this forum, I also made a -5V bucking transformer and outlet to drop incoming AC voltage from ~122VAC here,  I considered using the 5V winding on the amp's power transformers, per comments by arledgsc, but decided to keep the versatility of being able to use either SS or tube rectifiers. Here are some photos.

    This one uses two 5VAC CT 8A filament transformers, each wired to its own outlet. The power transformers run much cooler than without the bucker.



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