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 Post subject: Re: KT120 Oddblocks
PostPosted: 31 Mar 2011, 16:26 
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Joined: 02 Mar 2009, 12:41
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Location: Vänersborg, Sweden
My KT120 are on the way. 8-) And I have just ordered some small few components for the driver which will be a Russian tetrode - 6E5P.
I hope to be able to get something working til DIY show in Gothenburg, Sweden in mid May.

The tetrode will be hooked up with 5K6 ... 6K2 on anode, 56 ... 62R on cathode, decoupled by 470uF. On the second grid, 50uF to ground and 39K to B+.

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 Post subject: Re: KT120 Oddblocks
PostPosted: 31 Mar 2011, 16:52 
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Joined: 04 Jun 2008, 20:59
Posts: 3552
Location: Arizona, USA
Cool, :thumbsup: once you have it up and running I would scope it to see if the drive is symmetrical and enough to drive the tubes. You may have to adjust the anode load / cathode bias resistors to get the driver in its most linear range.

I've been putting a few hours at a time on the OB KT120 amps and they seem to be getting more refined as time goes by, still relatively few - maybe only 75 hours on the them. Getting close to what I like to hear now. If nothing interferes I'll be running a full set of tests on Saturday.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 01 Apr 2011, 07:07 
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Joined: 06 Jun 2008, 18:23
Posts: 4834
Location: Australia
gofar99 wrote:
once you have it up and running I would scope it to see if the drive is symmetrical and enough to drive the tubes. You may have to adjust the anode load / cathode bias resistors to get the driver in its most linear range.

I've been putting a few hours at a time on the OB KT120 amps and they seem to be getting more refined as time goes by, still relatively few - maybe only 75 hours on the them. Getting close to what I like to hear now. If nothing interferes I'll be running a full set of tests on Saturday.

Bruce: To think you are playing with a new tube which has no history. You could really set some bench marks here. The world awaits!

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "Mimic Carbon" - carbon resistors and PIO caps. MM phono preamp
Website: retro-thermionic


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 Post subject: Re: KT120 Oddblocks
PostPosted: 01 Apr 2011, 07:30 
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Joined: 26 Jul 2010, 03:37
Posts: 11
Hi,

AR is already shipping amplifiers with KT120. Have a look at the following link

http://www.arcdb.ws/news.html

cheers!

--Rodrigo.


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 Post subject: Re: KT120 Oddblocks
PostPosted: 01 Apr 2011, 16:10 
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Joined: 14 Feb 2010, 13:13
Posts: 663
Location: Lisbon, Portugal
rockdrigo wrote:
http://www.arcdb.ws/news.html

WoW!!! Those are extremely pricey! :o

Miguel


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 Post subject: Re: KT120 Oddblocks
PostPosted: 01 Apr 2011, 16:40 
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Joined: 08 Aug 2009, 03:11
Posts: 2161
Location: Chilliwack, BC
rockdrigo wrote:
AR is already shipping amplifiers with KT120. Have a look at the following link

http://www.arcdb.ws/news.html

Oh, goody! Never has an amplifier crossed my bench so many times as an Audio Research... daddy needs a new Mercedes :smoking:

Cheers!

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 Post subject: Re: KT120 Oddblocks
PostPosted: 02 Apr 2011, 15:10 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Yes they are sending them out, I wonder if they need an Arizona based service center. (my bad - sorry :bawling: ). I bench tested mine this AM. Not a complete set, but enough so I can move them a notch closer to what I like. I am using a rather conservative B+ at present (biggest trannies I had) at 400 Volts DC. Idle current is 310ma per pair of tubes. A consequence of the higher current is that the voltage across the LM317 is lower. It went from 35 (at 260ma) to 30 (at 310ma). Dissipation is still within limits with a good heat sink - my new ones get rather warm, but you can touch them without getting fried. Output power was in the 33-35 watt range with these settings. I have a way to extract about 5 or so more and will try that shortly. Frequency response was 20-20K +/- 0.1db. Mostly 0.05 db. I actually had to add a small (500pf) cap to the NFB loop as there was initially a 0.25 db increasing slope from about 14K that extended into the 50KHZ region. This could be OK in many systems, but might cause ringing in some with odd crossovers at higher (say 8K and above) frequencies. They worked fine on my Martin Logan Vista ESLs that tend to be difficult to drive as nearly every thing above 1K HZ is increasingly capacitive in nature and the impedance drops to 1.2 ohms at 20K. Since the design is pretty much OK now (except the power boost bit) I'll post it as soon as I can tidy it up. :D

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 02 Apr 2011, 17:28 
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Location: Chilliwack, BC
gofar99 wrote:
Dissipation is still within limits with a good heat sink - my new ones get rather warm, but you can touch them without getting fried.

Heatsink rule of thumb I was trained by, "If you have to pull back within three seconds, it's too hot." Sounds fine.

When I design, I take what the math says I need, double it (triple it if going in a PAS or other lousy ventillated case) and call it a day :D

Cheers!

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* Ratings are for transistors - tubes have guidelines*
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 Post subject: Re: KT120 Oddblocks
PostPosted: 02 Apr 2011, 17:36 
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Joined: 06 Jun 2008, 18:23
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Location: Australia
I'm not convinced electrostatic speaker and valve amps are a good match. Generally tube amps don't like capacitive loads. But, Bruce, you sound like you have them all getting along. It maybe some global negative feed back helps here.

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 Post subject: Re: KT120 Oddblocks
PostPosted: 02 Apr 2011, 20:37 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, I can not really explain it, but these amps (all sizes) work well with the Martin Logan ESLs. They might not for other ESLs, but we have three ML dealers that bought them to match up with their speakers.

On the heat sinks, the biggest problem is finding ratings on them. What looks like a great heat sink, could be rather bad. The ones I got had a rating of 2.5-2.7 degrees C /watt. Most typical ones seem to be in the range of 4-7. 4 would probably be OK, 7 would fry the LM317. In the latter case the temperature rise (above ambient) would be around 70 C. The thermal shut down would kick in. Since it would shut off (no current flow), I doubt that any harm would occur to the tubes, but it certainly would mess up an evening of listening.

Good listening
Bruce

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