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Old 3rd August 2004, 12:31 PM   #1
wasibi5 is offline wasibi5  Singapore
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Default kt88 power amplifier

Hi any friend there have the circuit for 4kt88 in push pull in a mono channel i like to buil one but dont know which the best. Thanks
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Old 3rd August 2004, 03:05 PM   #2
jogas is offline jogas  Italy
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HI Wasibi

look at this site:

http://ptsoundlab.com/sectubes/schemas/indexschem.htm

They have a lot of schematics including KT-88
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Old 6th August 2004, 09:38 PM   #3
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Hi wasibi5,

I trust you have a good source for an output transformer, for this single component will determine the quality of the finished amplifier. It needs to be bigger than many I see being purveyed, and thus personally I would rather go for class-A solid state. Yet there is something about those flourescing beamers that fire up the imagination.

I once had a Heathkit 2x KT88 amplifier - plain AB1 push-pull, it was better than plain pentode amps, but I was not impressed by it. I suspected the quality of the output transformer ! I also had a pair of Leak TL-50+ chassis, these really were very good disco amps, but not as good as my own chassis.

I built a 4x KT88 amp 30 years ago, and it remains one of the best amps I've ever heard running either quietly or loudly. It is still in the roofspace somewhere, and I don't know where my old circuit notebook would be.

It had a solid state series/parallel TV electrolytic power supply with 250mA choke smoothing. Fixed bias 40% ultra-linear parallel push-pull AB-1 output. One 12AU7 per push-pull half; 1/2 as a common cathode amplifier, 1/2 as a directly coupled cathode follower for the output grids, but it also bootstrapped the anode resistor for the first half. The 12AU7s were fed by a long tail transistor pair, possibly BF259s or BFY51s powered by the output bias supply. Global NFB from the output winding to the differential pair further reduced the already good open loop distortion figure, and it sounded fine with very highly reactive loudspeakers, approx 75-100W. There was no thermionic input stage hiss either !

Obtaining transformers was my problem for without grained cores and split windings for 100W for 20Hz I felt it would not be worth the bother, so I did not build any more.

Good luck with your search.

Cheers ............. Graham.
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Old 8th October 2004, 08:56 AM   #4
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just want to share my project
http://www.pbase.com/frankie168/kt88pp
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Old 8th October 2004, 11:18 AM   #5
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Quote:
Originally posted by Graham Maynard
I trust you have a good source for an output transformer, for this single component will determine the quality of the finished amplifier. It needs to be bigger than many I see being purveyed,
The OPT is easy, Lundahl.

KT88 PPP schematic
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Old 9th October 2004, 11:45 PM   #6
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Quote:
Originally posted by Brett
The OPT is easy, Lundahl.

KT88 PPP schematic

You could go crazy and spend nearly 3/4 of a grand on transformer..

I suggest if you're that crazy go for Tamura, the runners up are:

2# Tango
3# Lundahl
4# Plitron
5# Hammond...

Keep in mind though that one brand of transformer may give you that sweet sound but isn't necesserially expensive or popular.

Simular goes for models...

I suggest you get a mid-priced transformer and if you don't like it or think you can do better later down the track, put it up on ebay and use that cash to go and get another one, essentially shop around!

Don't drift from the specifications required by your design though, if in doubt ask us here to verify if you need to do modifications or if a new transformer is suitable for your design, or it can be a drop-in replacement.
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Old 10th October 2004, 05:55 AM   #7
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Quote:
Originally posted by Layberinthius
You could go crazy and spend nearly 3/4 of a grand on transformer.

I suggest if you're that crazy go for Tamura, the runners up are:

2# Tango
3# Lundahl
4# Plitron
5# Hammond...
You could, but the Lundahl LL162x series are $US185 ea ($US325 for the amorphous cores) and $A320ea iirc.

The Tamura and Tango's in PP with similarly large enough capacity are much more expensive, and the Plitrons are hardly cheap either, even in the US.

Having compared the Lundahls and some Hammonds in exactly the same circuits (as well as Lancrofts, Maqnequest, Partridge, Dynaco, Ferguson, A&R and others), the Lundahls are a much better sounding unit.

In Australia, you also have the option of Lancroft, and in the US, there's Electraprint or Magnequest may build you something from their extensive catalogue. There are no doubt others available such as James too.
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Old 12th October 2004, 04:15 PM   #8
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I would have checked with Brian at www.sowter.co.uk first. He makes the most wonderful transformers.
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