ST-120 Kit Finished

Finished ST-120 wiring yesterday. This is a great kit and enjoyed building it.  The chassis is something special.  I added an IEC connector and it took two saw blades and a couple of drill bits to get it done.  This is one tough chassis.  

The ST-120 sounds great.  There is no comparison to the AV receiver I have been using even with only a few hours of run in.  Good sense of rhythm and pace, reduced harshness, and extremely low background noise.  All is good!  

I also made a few layout changes to the triode/ ultralinear switches to save wire. I wanted twisted pairs of the plate and screen wires off the switches/ terminal blocks to match the ouput transformer colors.  So I saved the green and blue striped wire cuttings from the transformers to route back to the V2 and V7.  They were just barely long enough to make the run.  The regular green and blue wire transformer cuttings weren't long enough but I had a few spare feet of those colors.  This change sure made verification of wiring very simple.

There are also some other wire color changes that attempted to match the pictoral drawing where possible.  Also used some shielded cable off the stepped attentuator.  And a special nod to Tech Spray solder flux remover. It does the job well but you have to spend some time with cotton swaps to remove the last trace. But it gets it all and leaves you with a clean, professional looking board.  

I appreciate Bob's attention to detail and his help. This is a great kit and parts are top notch.  Regards, Scott





Edit: Latest mods  Caps, Attenuator

Thank you Bob. Great kit and sounding better everyday. I do have a small problem in that the stepped attenuator is wired backward. (operator error). Big jump in volume on first step and small steps as the knob is maxed. So need that corrected.

Just remarkable how quiet this amp is and I do have the chassis grounded through the IEC connector (potential ground loop). When the TDR kicks in I hear nothing from the speakers until the music starts to play. Thank you again, Scott

arledgsc

Solder flux can be easily removed with a combination of isopropyl alcohol and hydrogen peroxide.

Mix equal parts and use cotton for cleaning. And don't forget to use gloves.

Solder flux can be easily removed with a combination of isopropyl alcohol and hydrogen peroxide.

Thanks for that tip. Tech Spray is quite volatile and ventilation is mandatory. Also fairly costly.

In the world of metal working cutting oil helps when drilling and tapping. You really don't need any special oil unless you are machining exotic and tough materials such as titanium or Inconel, though stainless can be tough too. A few drops of regular motor oil works wonders. WD40 or 3-in-1 will work too but they are actually a little on the thin side. Standard 10W-30 or 10W-40 will work fine. No need for synthetics either though if that's all you have it will be better than nothing.

Scott, as a fan of cap rolling I have to ask: How is your amp with those V-Caps compared to the Russian caps? I'd love to get a set of those CuTF caps but that would cost nearly as much as the amp itself!

Bob Latino wrote:I hand tap the the four 6-32 bottom cover screw holes that the stainless 6-32 screws go in. The taps wear out very quickly.
Bob

What kind of cutting oil do you use? I recommend Ridgid 74012 Extreme Performance Stainless Steel Thread Cutting Oil:
http://www.toolup.com/ridgid_74012_extreme-performance-stainless-steel-thread-cutting-oil-1-gallon.aspx

Also look into getting a solid tungsten carbide tap, they're expensive but worth the extra $$:
NEW # 6-32 SOLID CARBIDE TAP 2 FLUTE

"Scott, as a fan of cap rolling I have to ask: How is your amp with those V-Caps compared to the Russian caps? I'd love to get a set of those CuTF caps but that would cost nearly as much as the amp itself"

I have no regrets but yes the cap changes cost about as much as the entire amp kit.  I recapped in June 2013 and the CUTFs supposedly take upwards of 400-500 hrs to settle in.  At the same time as the cap change I rolled in some Psvane KT-88s and 12AU7s which also have a notoriously long break-in cycle.  I thought if I am to wait several months I may as well kill two birds with one stone.  Sounded dreadfully thin with strange harmonics initially but after about a month sounded a lot better.  So I now have about 300 hrs on the package and I don't even remember what the PIOs caps sounded like LOL.  But it seems the CUTFs are not really a holy grail change but rather give better refinement and I say that as the silibance seems in better control.  The tubes, of course, play a roll in the amp's voice as well but the amp does sound great with a ton of controlled bass, silky highs, and nice vocal mid-range.  The harmonic detail is warm with excellent resolution and WIDE soundstage.  I really do enjoy the ST-120 (a lot) but the changes are taking the slow train to sonic nirvana but feel I am well over the hump at this point.... Scott

Edit - In the past two weeks I've made some changes to source input to the DAC and its output fed straight into the amp's the Gold Point attenuator.  This has reduced the last bit of excessive sibilance and glare.  And have done some critical listening which I really haven't done in the past several months.  Have to say the amp sounds great!  I am wanting for nothing but still can't remember what the original caps sounded like.