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Old 25th February 2013, 12:18 AM   #1
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Default KT88 design questions

I finally had time to sit down and have a cold one with the snow that is falling and draw up a schematic for a KT88 amp. Planning on using edcor x-formers. Power supply will use a 375-0-375 @200 or something close, a 10H choke, and 6.6K to 8ohm opt. Power supply will either be a 5U4, or 2-5Y3's, because that is what i have at the moment. DC filtered and regulated fillament supply. P-P KT88's, 6SN7 differential driver, and a LTP with NFB and a presesnce pot. Will this have enough gain for say an I-pod or CD player? Or would it be a good idea to use a 6J5 as a simpe voltage amp? The idea is to use octal tubes that i have, and build an amp that is about 30 watts.
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Old 25th February 2013, 01:42 AM   #2
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Diesel Tech,
A lot of your questions can be answered by looking at a few "standard" designs.
Good doco on this one:
http://plitron.com/wp-content/uploads/Atcl_8.pdf
Even ones which are hard to read can give you some ideas:
Monoblok KT88 PP amp

My own view is that a 6sn7 SRPP (or diff amp) driving a 6SN7 differental splitter driver driving KT88 in either pentode or ultralinear should be able to be easily driven from an IPod or CD player. There should be enough gain to apply NFB and add a prescence pot.

Cheers,
Ian

P.S.
Will follow with interest, I have Plitron VDV2100-CFB/H Output Trannies, Matching Plitron Power Trannies and Gold Lion Re-issue KT88s all sitting on the shelf ready for me to tackle a pair of PPP KT88 monoblocks.
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Old 25th February 2013, 09:59 AM   #3
roline is offline roline  United States
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Search the internet for Triode Dick Big Bill, there is an English site and Dutch site.
He uses 6N6P Russian ECC99 tubes in the LTP driving Kt88's. I did a similar PCB using 6SN7 as the first stage for tone flavor. Lots of 12SN7 and 6SN7's to choose from for character.
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Old 25th February 2013, 10:46 AM   #4
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Maybe you can take a look at this website.

Parks Audio - Eiclone DIY

PP KT88 with either fixed or cathode bias, 6SN7 as a longtail pair splitter, and can be built as monoblocks or in one large chassis (which is what I am leaning towards with my newly acquired boards). And a PCB to make it easier and cleaner.
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Old 25th February 2013, 04:01 PM   #5
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Quote:
Originally Posted by diesel_tech View Post
P-P KT88's, 6SN7 differential driver, and a LTP with NFB and a presesnce pot. Will this have enough gain for say an I-pod or CD player?

This thread should have lots of good info. The later posts talk about running KT88's either in UL or triode mode. It has 6SN7 diff driver, and a 6SN7 LTP with a CCS. Plenty of driver gain to run feedback if you want.

6L6GC AB2 Amp
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Old 25th February 2013, 05:43 PM   #6
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by diesel_tech View Post
I finally had time to sit down and have a cold one with the snow that is falling and draw up a schematic for a KT88 amp. Planning on using edcor x-formers. Power supply will use a 375-0-375 @200 or something close, a 10H choke, and 6.6K to 8ohm opt. Power supply will either be a 5U4, or 2-5Y3's, because that is what i have at the moment. DC filtered and regulated fillament supply. P-P KT88's, 6SN7 differential driver, and a LTP with NFB and a presesnce pot. Will this have enough gain for say an I-pod or CD player? Or would it be a good idea to use a 6J5 as a simpe voltage amp? The idea is to use octal tubes that i have, and build an amp that is about 30 watts.
First, I am not sure what you are really building, but a two-tube push-pull KT88 ultralinear circuit will work better with a 4.2 KΩ output transformer.

Edcor CXPP100-MS-4.2K will work perfectly. These run ice cold.

I use two Edcor XPWR-133 power transformers for my stereo amp. It is 400 VAC @ 250 mA. It has an 80 VAC tap for the bias and a 6.3 VAC @ 6 Amp filament winding.

This is just enough for one channel. I use two of these transformers so I have a truly independent power supply. They run warm, but within Edcor specs. It is like two mono blocks on one chassis.

The power supply is a solid state design since I feel modern diodes can be as quiet as anything. I used FRED diodes DigiKey # DSEI12-12A-ND

The power supply schematic can be seen HERE

And a CAD of the PCB HERE

I get about 50 Watts per channel and can easily handle line level inputs using 6SN7s for the input and splitter.

Click the image to open in full size.

The link below is a large JPEG file that illustrates how all the various PCBs are connected for my stereo amp. Hopefully you will find it helpful.

KT88 PCB Interconnects
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Old 25th February 2013, 06:03 PM   #7
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Very Cool Loren. Do you have a schematic of the amp circuit?
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Old 25th February 2013, 06:16 PM   #8
Loren42 is offline Loren42  United States
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Originally Posted by boywonder View Post
Very Cool Loren. Do you have a schematic of the amp circuit?
Yes, here it is:

Click the image to open in full size.

Here is a picture of the amp before I added balance line input transformers and XLR connectors:

Click the image to open in full size.
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