New Member (and VTA ST-120 owner)

I just finished building and biasing my new VTA ST-120 yesterday evening, and wanted to thank Bob for all his time and effort spent in the creation of such a fine product. The instructions and pictorials were clear and concise, and reflected a high attention to detail. I've built many kits but I found this one to be the most fun to assemble. I think I probably drug the assembly process out a bit, realizing "I'll never get to assemble this again." What to do? I dunno, maybe assemble a stack of these amps for my home theater :: grin ::

I went ahead and did the diode rectifier mod after reading the thread about it. Thanks to all of you for posting that info. I used some Vishay GP15s instead of 1N4007s, mainly because I had the GP15s on hand.

Congrats on your successful ST-120 build and welcome!  The fun is not over as it will require several months of run-in plus tube rolling to find what you like best.  Also if you are "jonesing" for the smell of solder flux check out the tubes4hifi preamps or a couple of mono block amp kits.

TFM55X,
Welcome to Tommy's Holiday Camp. Never mind the weather, the holiday's forever . I built a pair of Bob's VTA M-125 mono-blocks and he was hugely helpful while I stumbled through the process and pestered him nearly to the breaking point. No flaws in the directions, just brain-phartes on this end.
Yes, as arledgsc says, you'll want to roll a few tubes through your 120 till you find just the right flavour. But leave things be for a month or two, let the capacitors settle in, and keep an eye on the bias. Assuming you've got Russian high-quality Gold Lion KT88s or Tung-Sol KT120s for outputs, they will serve you well. So when you get OCD, try different driver tubes out. IMHO, they have more influence on the sound than the power tubes do.
And when you begin suffering from lead-fume withdrawal, indeed build an SP-13 or -14 line preamp from Roy at tubes4hifi.
Have fun. That's what this is all about.
Cheerio,
DeePee

Indeed, welcome. Though the joy of the build is behind you, the fun is only just begun! Even if youre far from a novice, I wager you'll learn a lot here. "Dealer support" and involvement here is outstanding, and there are quite a few members with all kinds of expertise to contribute.

I second DeePee Dave's recommendation about the sp14 pre amp. Without exaggeration, it is achingly good. But before going there, there are tubes to roll, and recommendations on dozens of subjects to be considered, discussed, implemented, cursed, complained about back here of course, and then refined. Like how to position your speakers, high pass your mains from your subs, etc.

Plus, this is a fun forum to hang out. Dont think I've ever seen an unkind word or snarky post. So get stuck in.

tfm55x wrote: I've built many kids but I found this one to be the most fun to assemble. I think I probably drug the assembly process out a bit, realizing "I'll never get to assemble this again."

Nice typo ... I think of my little projects as children as well ...

My ST120 is well beyond newborn, but still one of my favorite things in the world ... you can look forward to many happy years of listening pleasure ...

One question: how warm should the power transformer get (either whittle idling or while playing?) This is the longest I've had the amp on since completing assembly, and the transformer seems awfully warm. Bias is spot-on for all tubes, and I'm not seeing any red-plating.

tfm55x wrote:One question: how warm should the power transformer get (either whittle idling or while playing?) This is the longest I've had the amp on since completing assembly, and the transformer seems awfully warm. Bias is spot-on for all tubes, and I'm not seeing any red-plating.

They can get pretty hot after an hour or three. That's quite normal. Lotsa variables, like the speaker load, but if they're not blowing fuses or emitting any magic smoke, you're OK.

Thanks, Bob. I wanted to clarify that I was talking about the power transformer in particular. I wasn't terribly worried, since the amp overall was behaving perfectly and sounding terrific, but I wanted to double check, since the transformer was approaching the almost uncomfortably hot to the touch" range.

Just read your referenced post. Thanks for the context!

Bob photo with temps on it shows 122 degrees, that like pretty warm but not what I would call too hot to touch.
About 2/3 of that heat is actually heat the transformer is absorbing from the tubes all around it. Consider the amp a 150w heater unit.
Great in the north winter, not so great in Phoenix in the summer . . .

tubes4hifi wrote: Consider the amp a 150w heater unit.
Great in the north winter, not so great in Phoenix in the summer . . .

In a note of irony, my M-125's have actually been making the house cold this winter.  One amp is situated a few feet from my thermostat.  It generates enough heat that the furnace tends to not run because air near thermostat is several degrees warmer than in rest of house.

I use a larger aluminum Hammond box for my ST120.  Spreading the tubes out helps cool things down a bit.  The right power tube is cooler than the others because it's dying.