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KT120 Oddblocks
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=3089
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Author:  ozapaydin [ 23 May 2017, 06:24 ]
Post subject:  Re: KT120 Oddblocks

In KT120 specs, I see 3K plate resistance for single tube. For push-pull design, what should be primary impedance of the output transformers? I couldn't find any information about it.

Author:  poty [ 23 May 2017, 10:59 ]
Post subject:  Re: KT120 Oddblocks

Bruce uses 3.5K p-p.
Usually there is a compromise between power and distortions.

Author:  ozapaydin [ 23 May 2017, 13:34 ]
Post subject:  Re: KT120 Oddblocks

I believe 4.2K will be a better match looking at the ratios of other tubes such as 6L6 which has 4.5K single tube and 6.6K push-pull ratings.

Author:  gofar99 [ 23 May 2017, 15:21 ]
Post subject:  Re: KT120 Oddblocks

Hi, I have found that either value will do fine. A compromise on output vs distortion. Both are nearly equal. You can lower the distortion slightly by going to 5.6K. It will cost you some power though. Since the distortion is quite low already my preference is the lower values. The "load specifications" that are published vary a lot from company to company and are often determined based on a particular design they were using. I consider them moderately good guidelines and a starting point. Simulations can also be used, but I find they don't translate well into what a final build will do. Close sometimes, but with tube designs I have found it necessary to tweak them using test gear. A distortion analyzer is a must in this area as are scopes that will do spectral analysis. All my projects get "run through the testing mill" repeatedly until they do what I wanted them to do. Some projects (notably the Forewatt preamp) have a whole range of suitable values for components. Sometimes just a change from one brand of tube to another will make one set of component values better than another. This is part of the reason I indicate which brand of tubes I used in a build. In the KT120 amps you can really only change the drivers as unless you want to go big dollars there is only one game in the KT120 town. Unfortunately I have some concerns about their quality at present (see another thread on that).

EDIT: Part of the reason that either 3.5 or 4.2 will work well is that the speaker impedance end of the business is not always as the company says it is. 4 ohm ones often swing between 3.2 and 6 ohms and in some cases a much wider range and 8 ohm ones often dip to the 5 ohm range and peak at three times that much. This plays havoc with some tube amps, but the Oddwatts are quite happy with it. My personal speakers are Martin Logan ESLs and they are nominally 4 ohms. However they act like huge capacitors and drop to 1.2 ohms at 20KHZ. The amps are not bothered by this at all.

Good listening
Bruce

Author:  poty [ 24 May 2017, 02:59 ]
Post subject:  Re: KT120 Oddblocks

ozapaydin wrote:
I believe 4.2K will be a better match looking at the ratios of other tubes such as 6L6 which has 4.5K single tube and 6.6K push-pull ratings.
You may try, no problem. The schematic allows this kind of changes.

Author:  gofar99 [ 24 May 2017, 09:45 ]
Post subject:  Re: KT120 Oddblocks

Hi, For a 6L6GC I would use 8K. That is the load that I used on the EL34s (at 425-450B+). The power output (remember this is class A U/L) is going to be about 15-18 watts rms clean (under 2%..likely better than that) and max at about 20-22 at about 5%. I originally designed the amps around the 6L6GC (with 8K) but decided that the KT77 and EL34 sounded better, and with only an increase in the current, KT88s worked fine there as well. The 6L6 version seemed grainy compared to the others. (nice for guitar use though) The KT88 version turned out to be even better than the others IMO for overall sound quality and power (25wrms max at 33 about) The EL34 version does seem to be better suited for light jazz and such as it has to my thinking a slightly crisper mid range than the KT77 or KT88. The posted KT88 project is the end result of the experiments with the 6L6GC and subsequent tubes. Again in all the designs I traded off a little output power for higher quality sound.

Good listening
Bruce

Author:  ozapaydin [ 24 May 2017, 13:16 ]
Post subject:  Re: KT120 Oddblocks

Bruce,
What is the RMS ac voltage in the primary winding of output transformer?

Author:  gofar99 [ 24 May 2017, 16:02 ]
Post subject:  Re: KT120 Oddblocks

Hi, I have never measured it. Based on the output power and impedance it could be calculated. If I can get the math right ...For the KT88s at 8K primary Z and 25 watts RMS it ought to be about 440 VAC RMS. That is based on a turns ratio of 31.6. with the kt120s at 45 watts and 3.5K primary it will be just a tad lower at about 400 vac. As I mentioned earlier efficiency was not a goal in the amps, sound quality was and the impedance values were first approximated and then finalized by experiment with scope and distortion analyzer.

Good listening
Bruce

Author:  ozapaydin [ 25 May 2017, 03:28 ]
Post subject:  Re: KT120 Oddblocks

What frequency can KT120 monoblocks go down to at full power?

Author:  poty [ 25 May 2017, 06:38 ]
Post subject:  Re: KT120 Oddblocks

In the first message of the topic there is a link to the amp build. There are many answers to your questions. Your last question has the answer there as well:
Quote:
... typical response puts the -3 dB point below 10 Hz.

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