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Old 25th August 2009, 09:02 PM   #1
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Default stereo SE kt88 build ... abdellah diyaudioprojects design

hi all,

I am collecting parts for a build of the se kt88 shown on diyaudioprojects.com that was originally of mikael abdellah's amp. The schematic I am basically following is one i think i found on this forum. It might be Porkchop's (porkchop since you just did a build like this one I am hoping you can give me some pointers!) anyway here is a link to the pic
Click the image to open in full size.

I already have a few things lined up... I have some EH kt88 tubes and 5u4g rectifier. Also have some james 6123hs OPT. My big questions right now are mostly about what type of caps or resistors people recommend. I also need to know more about how to make this power supply work for a stereo amp. Is the single hammond 193j fine for pushing into both channels? Also what are ASC motor run caps? I thought i needed some type of electrolytic in the power supply. I am a noob so forgive the dummy questions. thanks guys... really hoping you can help me out!
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Old 25th August 2009, 09:15 PM   #2
Salas is offline Salas  Greece
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Hello,

Please take out the .th part when you post links from Imageshack, because it only links a thumbnail. I edited this one for you. Only leave the .jpg
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Old 25th August 2009, 09:54 PM   #3
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I would suggest anode load of 6N1P 24K, and 360 Ohm in cathode of 6N1P, especially when you drive KT-88 in triode mode. You may use a 3.5V drop bright LED in cathode instead of 360 Ohm resistor.
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Old 26th August 2009, 11:54 AM   #4
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Quote:
Originally Posted by hoerath7 View Post
My big questions right now are mostly about what type of caps or resistors people recommend. I also need to know more about how to make this power supply work for a stereo amp. Is the single hammond 193j fine for pushing into both channels? Also what are ASC motor run caps? I thought i needed some type of electrolytic in the power supply. I am a noob so forgive the dummy questions. thanks guys... really hoping you can help me out!
Hi hoerath7-
I have pictures of the "turret board" type construction I used for my build showing all the caps/resistors I used. The only "audiophile" caps I used were in the coupling caps, Kimber caps from Parts Express.
http://webpages.charter.net/porkchop...8_SET_amp.html

The Hammond 273BX is what I used in my build for the power transformer, the 193J is the choke, both are fine for a "dual mono" build.

You can run electrolytics in the power supply, I just find the ASC motor run caps are VERY quiet (reduce ripple) which is very necessary for this type of amplifier design (and they're cheap)

I can supply more info as well as others who have built this amp.
Overall I'm very pleased with the sound of this design (and simplicity). I've made some modifications to mine to allow for switching bias resistors so I can run EL34, KT66, 6L6GC's etc.

Wavebourn-
I'm interested in what the 24k/360R bias change does for the 6N1P.
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Old 27th August 2009, 12:23 AM   #5
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porkchop ... you just used one hammond 273bx and one 193j for the whole thing right???? i was looking at your pics and it looked like you had two power trannys... i guess u only have one rectifier and one set of ASC's ... so maybe the other "tranny" is just a choke ...
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Old 27th August 2009, 12:59 AM   #6
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Quote:
Originally Posted by porkchop61 View Post
Wavebourn-
I'm interested in what the 24k/360R bias change does for the 6N1P.
It does increase it's working current so when loaded on Miller capacitance of triode strapped KT88 phase distortions are a bit lower.

Just try, listen and compare...
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Old 27th August 2009, 12:32 PM   #7
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Quote:
Originally Posted by hoerath7 View Post
porkchop ... you just used one hammond 273bx and one 193j for the whole thing right???? i was looking at your pics and it looked like you had two power trannys... i guess u only have one rectifier and one set of ASC's ... so maybe the other "tranny" is just a choke ...
Yes, the smaller one is the choke.
The Hammond 273bx has plenty of power for the stereo version of this build. I originally was going to build two monoblocks, but it's cheaper to build only one power supply for both channels (plus I can use both sides of the 6N1P).
Some purists may not like this approach, as you are using one side of the 6N1P for each channel, to each his own.

I didn't answer your original question about resistors. I used just plain metal film flameproof resistors for this build. Some may question the use of these types of resistors used in the 100R screen to plate connection for Triode mode. I've heard they should be non-inductive types.
I also added a grid resistor of 1k that wasn't in the original design.

Good luck!
Glenn
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Old 27th August 2009, 12:34 PM   #8
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Quote:
Originally Posted by Wavebourn View Post
It does increase it's working current so when loaded on Miller capacitance of triode strapped KT88 phase distortions are a bit lower.

Just try, listen and compare...
Ah, I see, probably worth a try.
How does this affect the KT88 in UL and Pentode modes?

Thanks
Glenn
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Old 2nd September 2009, 01:17 AM   #9
88man is offline 88man  United States
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I'd like to build this circuit at some point, but I had a question about Porchop's version: the 100-ohm resistor following the KT88 grid is active in all 3 modes - triode, ultra Linear, and pentode. In Alex Gendrano's version the resistor is active only the triode mode with a switch. And in Michael Abdellah's original circuit, the 100-ohm resistor is omitted.

Very curious, should the resistor be used in all 3 modes, only in triode, or none at all?...
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Old 11th September 2009, 12:08 PM   #10
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I added the 100R resistor per Tubelab.com's suggestion. I believe he commented that it didn't matter if this was left in place for all three modes of operation (correct me if I'm wrong George). I searched all postings by him to try and find his comments on this, but he's very prolific and I couldn't find his post.

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