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KT88 design questions

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diesel_tech

diesel_tech

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Joined 2006
#1
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.
 
#2
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.
 
#3
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.
 
#5
#6
diesel_tech said:
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.
Click to expand...

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.

An externally hosted image should be here but it was not working when we last tested it.


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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