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 Post subject: Re: KT120 Oddblocks
PostPosted: 02 Apr 2011, 21:50 
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mwhouston wrote:
I'm not convinced electrostatic speaker and valve amps are a good match.


I dunno... I've worked on and modded Acoustats that did very well (sweep tubes in SRPP).

Cheers!

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 Post subject: Re: KT120 Oddblocks
PostPosted: 03 Apr 2011, 00:27 
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gofar99 wrote:
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.

For the heat sinks issues how about a small CPU (muffin) fan?

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 Post subject: Re: KT120 Oddblocks
PostPosted: 03 Apr 2011, 00:34 
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Geek wrote:
mwhouston wrote:
I'm not convinced electrostatic speaker and valve amps are a good match.


I dunno... I've worked on and modded Acoustats that did very well (sweep tubes in SRPP).

Cheers!

I just read an article in Stereophile where the reviewer mentions negative feed-back and electrostatic speakers in the one sentence. Implying that they work together. The tube amps you know work well with electros do they have negative feed-back and let's talk global at this point. I think only global, which takes it's signal from the secondary of the OPT to ealier parts of the cct. will work here.

I really feel this is the answer?

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 Post subject: Re: KT120 Oddblocks
PostPosted: 03 Apr 2011, 07:07 
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Yeah, actually the Acoustats are a hotrodded op-amp. The sweep tubes and their DC potential of 2,5KV at the centre of the SRPP was even coupled DC to the op-amp input (through lotsa resisitor, of course).

I did try some Hafler step-up transformers for use with SS amps.... but the Acoustat blew it away, bigtime 8-)

Cheers!

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 Post subject: Re: KT120 Oddblocks
PostPosted: 05 Apr 2011, 21:48 
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Location: Arizona, USA
Hi , A small amount of global - about 5-6 db. U/L mode is a form of NFB as well. It may account for them working well into virtually any kind of load. They are not really sensitive to mismatched loads. The biggest consequence of a mismatch is slightly reduced power output. On the bench the response and distortion are within statistical limits for a wide range of impedance loads and apparently crossover components and types of speaker. :?

I have run into a quirk on the KT120 version I have now up and running. The DC voltage across the LM317 increases with output when you start to push the tubes. I believe it has to do with the use of 100K "grid leak" resistors. The tubes in this design seem to work like they have fixed bias - even though it is cathode derived. The max value for the grid resistor in a KT120 for FB is 51K. I'll try it out this week and see what happens. Clearly, one should read the specs before warming up the soldering iron. :(

Good listening,
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 07 Apr 2011, 06:49 
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gofar99 wrote:
I have run into a quirk on the KT120 version I have now up and running. The DC voltage across the LM317 increases with output when you start to push the tubes. I believe it has to do with the use of 100K "grid leak" resistors. The tubes in this design seem to work like they have fixed bias - even though it is cathode derived. The max value for the grid resistor in a KT120 for FB is 51K.

You can save your time just to measure the grid-to-ground potential of the tubes without actually lowering the grid resistor value. If it is the proposed source of increasing the CCS voltage - then the potential would grow, if not - stay the same.
My guess - the grid circuit of the "slave" tube. How it is connected to ground now? Do you add some positive potential from the B+ (you mentioned that you were going to add additional 5W in the way you do in other OddWatts - and this is the positive voltage additions to grid). Maybe the circuit reflects some external changing and rises the potential?


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 Post subject: Re: KT120 Oddblocks
PostPosted: 07 Apr 2011, 11:00 
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Hi Poty, Good thoughts. When I apply a positive bias to the outputs it is to both tubes from a common source (eliminates noise as it will be in common mode). However, I believe I found the problem in a rather unlikely source. :idea: The output transformers I have in the prototypes would seem to be quite sufficient. However when I contacted Edcor regarding the max balanced dc current rating and their design for them it was lower than what I am using. The CXPP60-MS-4.2K are only rated 100 ma per plate. :| Even I can figure that at 155-160 per plate there could be problems. The good news is that the CXPP70-MS-3.5K is good for 220ma per plate. I have two on order now and will report the results after I get them installed. (allow about 3 weeks). The slightly lower impedance may also help as some of my testing indicated that the 4.2K value is slightly high.

Based on the specs of the 60 watters.... I can envision all sorts of problems when you want more than about 20 watts. A good lesson to re-learn..... Class A amps have very different properties and make quite different demands on some components. :D Check the specs on key components even if they seem to be OK. Saves, times and $$.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 08 Apr 2011, 06:58 
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Location: Northern VA
That's interesting, I'm curious as to what powers numbers you'll be getting with the new Edcors.


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 Post subject: Re: KT120 Oddblocks
PostPosted: 10 Apr 2011, 20:32 
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Joined: 26 May 2009, 18:51
Posts: 130
Location: Lilburn, Georgia USA
While I wait for Bruce`s 120 design I am sitting here playing my KT88 Oddblocks playing Pink Floyd Dark Side of The Moon and I love it. I have JBL ND310ii speakers which are 91db capable of 250 watt They call them Rock Star speakers and they rock even at 25 watts. I can`t wait to build a set of KT 120 Odd Block amps. While I wait for Bruce to finalize the 120s I am building a Pete Millett Jonokuchi Amp. It uses a pair of 13EM7 odd dual triode octal tubes. Of course you have to wait for your Edcor iron as usual. One day I will have to show pictures of my huge investment of Edcor Blue. Anyway I love how it all sounds. My friends think I`m crazy but all I know is I`m crazy about tubes! Robert Parzych.


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 Post subject: Re: KT120 Oddblocks
PostPosted: 10 Apr 2011, 21:30 
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Location: US Pacific Northwest
Quote:
It uses a pair of 13EM7 odd dual triode octal tubes.
I like the *EM7/*EA7 family (6EM7, 10EM7, 13EM7) for small computer/iPod amps. Section 2 will give about 2.1W very clean with a 5K load and section 1 is a great preamp (especially with the cathode unbypassed). And the tubes are really cheap. Combined with the AES generic 5k:8Ω output transformer ($13.95/ea., in stock) you can put together a really inexpensive amp that actually has pretty good sound. Two channels, two tubes (plus a rectifier for purists).

Maybe I'll start a separate thread. :idea:

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