Very upgraded ST-70 kit/build w/ 6550 tubes - diyAudio
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Old 26th January 2013, 05:54 PM   #1
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Default Very upgraded ST-70 kit/build w/ 6550 tubes

I just received Kevin Carter's (K&K Audio) latest upgrade kit for the Dynaco ST-70. I'm building this on a powder coated new chassis from DynaPartKits, larger than original Mains transformer from TiodeElectronics, original Dynaco A-420 OTP's, Tung Sol 6550 tubes and misc connectors, wire, etc from PartsConnexion.

This is a new offering by K & K Audio, not on their website yet.

I ordered the upgraded parts offered by Kevin, Lundahl LL1544A transformers and Jensen copper foil/paper in oil coupling caps. Also part of my kit is the new shunt regulator kit.

I'm a newbie, only my third tube project, so I don't have all the technical knowledge to describe all the aspects of this kit, but I was really impressed with the kit, as delivered. It came with a bound instruction manual and a CD with pics. The board is a lot larger than the original and the heat sinks are huge. Going to be fun putting this together in the tiny chassis!

I'm painting the A-420's today and starting to put it together. I'll post back as it comes together.

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Old 27th January 2013, 06:00 PM   #2
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Default Pic of the prototype of the new kit

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Old 28th January 2013, 03:45 PM   #3
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Default Completed the main board

I completed the main board last night, great complete build manual, all the parts as detailed were included and a couple of photos were included to help identify assembly. Really fun putting together so far.

The new main heat sink is massive;

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Old 28th January 2013, 04:02 PM   #4
MikeR is offline MikeR  United States
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Nice to see your pictures. Looks like you are working your way through it quickly.

I built the prior version of the K&K kit with the HV shunt regulators and it has proven to be a killer amp. It's a radical departure from the ST70 and that may put some people off but well worth it IMHO.

Keep posting as you work your way through it.
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Old 29th January 2013, 07:13 AM   #5
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Quote:
Originally Posted by MikeR View Post
Nice to see your pictures. Looks like you are working your way through it quickly.

I built the prior version of the K&K kit with the HV shunt regulators and it has proven to be a killer amp. It's a radical departure from the ST70 and that may put some people off but well worth it IMHO.

Keep posting as you work your way through it.
MikeR, thanks for the comments! I'm new to stereo, been always focused on 7-channel for movies. I'm enjoying the process of building, learning and listening. I have high hopes for this build, especially after Kevin told me that him and an audiophile friend compared the new modded ST-70 to a ARC VT-100 amplifier, he said that they "both
agreed after the first few bars that the little amp was very clearly the
better purveyor of music and even sounded more dynamic!"

Just finished putting the shunt regulator kit portion together tonight;

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Old 29th January 2013, 07:42 AM   #6
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Nice work that kit looks awesome, really well packaged.
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hoping to pick up some things.
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Old 6th February 2013, 10:54 PM   #7
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Originally Posted by sq225917 View Post
Nice work that kit looks awesome, really well packaged.
SQ225917 thanks for commenting!

I finished stuffing the chassis and soldering the last wire. Grabbed the instruction manual and reviewed the start up process. Very well documented, BTW.

Got everything ready to power up and test, but forgot about the fuse ;(
Had to wait another day.

Picked up some fuses yesterday (3 amp) and ready to try again. Even though the manual doesn't required powering up slowly with a variac, I did. Started at 50%, first off no smoke or fuse blowing, good first sign! Checked the tranny outputs for 50% power, all within 10%, brought it up to full power, still good.

Then you have to adjust the input and output tubes. Kevin has added designated test points designed to hold the leads of your meter, nice touch since the tubes and heat sink are so close together, lets you leave the leads connected while you concentrate on the meter reading while you make the required adjustments.

Here is where the manual falls a little, just a little mind you. The manual was written before this board with the designated test points, so the references are not accurate. But even for being a novice it was easy to figure out and correctly adjust.

Completed the adjustments, no issues. So I felt safe to start hooking up equipment. Started with the speakers, powered back up, no hum, static, noise, nothing. Started worrying I messed up something, should some hum, was hoping for very low, but nothing. Checked all the test points again, all still good. So I powered down and decided to take my chances and connected the preamp. Hooked my iPod up, and powered everything back up. Note: the amp takes 45 seconds to power up. All of a sudden music starts coming out of the speakers, hurrah!

Anyway, I sat and listed for about an hour, no issues. The amp sounds absolutely fantastic. I'm still building the rest of my system so it's not even as good as it gets yet! Can't wait to get my other speakers finished and my DAC comes in this weekend.

I will say even with this HUGE heat sink even after about 30 minutes it's hot enough that you can't hold your hang on it long. I'm running 6550 tubes adjusted to 58 mV's.

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Old 7th February 2013, 03:19 PM   #8
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Super and congrats on a great DIY project. as an fyi, the Ip range of 6550 tubes range from 20 to 55 ma's. This effects how low you can adjust your bias with out circut modifications. At 58mv, you are slightly above the bias for long service life of the tubes. Its a subjective matter in trying to figure out how much longer tubes would last anywhere along the 20 to 55 ma range.
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Old 7th February 2013, 03:56 PM   #9
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Originally Posted by speakerfritz View Post
Super and congrats on a great DIY project. as an fyi, the Ip range of 6550 tubes range from 20 to 55 ma's. This effects how low you can adjust your bias with out circut modifications. At 58mv, you are slightly above the bias for long service life of the tubes. Its a subjective matter in trying to figure out how much longer tubes would last anywhere along the 20 to 55 ma range.
Speakerfritz, thanks for commenting!

On start up I kept bringing it up to push the limit. After a few more hours of breaking in I'll start working down to a final adjustment based on what I hear. Still, learning so I don't know if I will even be able to "hear" a difference at a lower setting. So lower would be better for tube life.

I'll post some more pics soon of the daylight front, rear and underside.
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Old 7th February 2013, 04:43 PM   #10
rmyauck is offline rmyauck  Canada
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Maybe go as low as you can go where it still sounds good as new production usually aren't as tough as NOS. Less heat and better reliability also!

Randy
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