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Old 23rd October 2008, 06:10 PM   #1
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Default Got a big output trafo I want to use.

Hey guys,


I now have in my possession an output transformer with a 4.5k primary capable of 100watts.
I want to take advantage of the full power available with this transformer but 100watt designs are very scarce so I am left with 50-60 watt designs. My goals are (in order) power output, clean sound, low cost. I dont have a budget as yet since I already have the most expensive part, but I will not be using exotic parts.

Now, there are sooooooo many designs to chose from I really cant decide. For about 3 weeks now I have been scouring the web and this site for ideas and clues but I have not found a lot that helps.So I have compiled a short list of possible builds.
If any of you have built any of those on my list please comment and let me know how stable the amp is and how it sounds.


http://www.drtube.com/schematics/radford/sta100.gif
http://www.audiofanatic.it/Schemi/Ti...550PP_6SN7.jpg
http://www.audiofanatic.it/Schemi/Ti...akit_mkIII.jpg
http://www.webace.com.au/~electron/t...s/achpwill.gif
http://www.triodeel.com/gec100w.gif
Not sure about the primary impedance of the output transformer for this: http://www.triodeel.com/m&atva1.gif


Comments and suggestions are very much welcome.
I really need some direction an which to build.




Thanks all


Lawrence
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Old 23rd October 2008, 06:36 PM   #2
kmaier is offline kmaier  United States
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It's true that you can push a pair of (real) KT-88s to 100 watts, but they take a beating and won't last long.... back in the day they were cheap to replace, not now. Suggest you revise your goals... reliability, sound quality, power, cost.

If you want 100-watts nowadays, you would be better to use a quad set of new issue T-S 6550 outputs, but you'll also need a lower P-P load to get the best from them. Just my $0.02.

As for schematics... I would suggest a LTP phase inverter... if funds allow, a high-quality current source on the common cathode pair. Would also provide for both AC and DC balance to get the best performance from it. As you're finding out... you have numerous options for circuitry. Some of the older ARC designs were quite good... a bit complex however.

Regards, KM
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Old 23rd October 2008, 09:27 PM   #3
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One more. I'm leaning more to this one than any other:
http://www.plitron.com/vtvkt88.asp
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Old 23rd October 2008, 10:02 PM   #4
kmaier is offline kmaier  United States
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Hi Badman,

The last schematic is almost identical to the modified williamson in your first post which is from the old Acrosound brochure... and was later printed in an old Dyna brochure sporting the latest (Dynaco) OPT from David Halfer, both dating back to the 50's.

I doubt there would be much difference sonically between them. I personally prefer a long-tail pair (LTP) phase inverter over the cathodyne used in the Williamson design. As you're not planning to build that soon, scan the net and look some for other designs. Here's one from Isao's site... uses an EF86/6267 as an input amp driving an ECC82/12AU7 configured as a LTP:

http://tube-amps.net/images/KT88_PP_..._Schematic.jpg

My old 50-watt design done back in the 80's used a 6072 as the input amp driving a 6FQ7 for the LTP and had both AC and DC balance on the output tubes and a custom current reference module in place of the LTP cathode resistor. Happy hunting.

Regards, KM
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Old 23rd October 2008, 10:12 PM   #5
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I saw that circuit, but I have heard that the 12A_7 tubes were no good for hi-fi, ture or not?

Are there sonic differences between the LTP and the cathodyne why you prefer the LTP?
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Old 23rd October 2008, 10:16 PM   #6
kmaier is offline kmaier  United States
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I found my old 50-watt plans and the DWG files in a directory... converted to a jpg, and linked it below. Hopefully you can read it and add it to your list of options.

Click the image to open in full size.

Hopefully you find it useful.

Regards, KM
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Old 23rd October 2008, 10:18 PM   #7
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link does not work.....
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Old 23rd October 2008, 10:19 PM   #8
kmaier is offline kmaier  United States
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Quote:
Originally posted by lawbadman
I saw that circuit, but I have heard that the 12A_7 tubes were no good for hi-fi, ture or not?

Are there sonic differences between the LTP and the cathodyne why you prefer the LTP?
Well, it would appear that many of the 12A*7 tubes have received a bad rap in the latter years. Amazingly enough, the recordings done 5 decades ago, which are still considered to be superb, were done with audio gear full of them, who knew. Despite public opinion, I still consider the 12AU7 an excellent tube and still design with it today.

Regards, KM
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Old 23rd October 2008, 10:27 PM   #9
kmaier is offline kmaier  United States
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Quote:
Originally posted by lawbadman
link does not work.....
Oops... edited. If you like, I can convert to PDF and email it. It will look much better. I also have the other 2 dwg files which are the power supply (all solid-state, hey, it was the 80's) and the even the current modules which were all custom designed with solid-state components available back then.

Regards, KM
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Old 24th October 2008, 02:21 PM   #10
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I think I like the Hashimoto circuit. The impedance of the output is 5k. Being that mine is 4.5k, would that put too much of a load on the KT88? What about adding an extra pair of KT88 so as to ease the load on just one pair and also to get more power???


KM, what current sources are those that you are using in your amp?
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