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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

SE KT88 Amp

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Mikael Abdellah said:
I'm afraid I have no good advice on that. I use my amp directly connected to a cd-player.

I'll figure that one out, one of the ECC82 schematics I was looking at had a recommendation for replacing the pot. I can use that as a starting point.

What output transformers are you using? I'm considering Hammond 125 ESE (because I have a couple) with an eye to future replacement with 1627SE when budget permits.
 
Mine are custom from sowter. But a hammond should work. The tubes are biased at 75mA so any 5k (or maybe 3,5k) transformer that can handle the bias-current should work. I think 2,5k might be a little low but you get double the output power. The 125ESE should work really well for testing, but personally I would get the 1628SE.
 
I'm sold on Plitron, but they are costly.

For my next SE project, I'm looking at One Electron. They are about the same price as Hammond.

There's another thread on James transformers, which look very interesting...

However, my final SE amp will use Plitron, because I think they are the best.
 
Mikael Abdellah said:
I get somewhere between 4-5W from my amp.


Excellent output. My "main" speakers are 93-94db/W/1M so they should be a decent match. This is looking better and better!

I've started to put together my parts list (and comparing it to what is in the parts bin). I am looking forward to this project.

I was checking out various KT88 tubes and saw the one with blue glass at Antique Electrical Supply. Hmmm... very pretty, high WAF for putting the amps on display!
;)
 
What about a 6Sn7 driving a KT88?

What is your opinion about this idea?

It based on the "JE-Labs 300B" with the 6SN7 driving a KT88.

Cheers,

Manuel
 

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pingfloid said:
...so I am also going to try Mr. Abdellah's schematic. I suppose the 1627SE will also work for it.

Thanks,

Manuel


Manuel,

I have built a pair of monoblocks using Mikael's schematic. They sound great! I was struck by the simplicity of the design and felt it would be a good amp to experiment with. Changes if necessary would be very easy.

I used tube rectification with 5U4-G rectifiers. They feed into a 40uF ASC cap, an 8 Henry choke and then a 100uF ASC cap. The result is a very clean power supply. SS rectification would be smaller and less expensive but for this project I wanted to keep things "all tube".

Mikael's schematic calls for an OPT with a 5K primary. I believe the 1627SE is 2.5K. I think it would work and you would have somewhat more power and also higher distortion.

The Hammond 1628SE has a 5K primary but it reportedly has linearity issues. For my monoblocks I used James transformers (6123HS). Here in the US the price was the same as the Hammond 1628SE and most users reported them to be excellent OPTs. After listening for a few months now I have to agree! A real bargain.

Good luck with your project. I'm sure you will be happy with the result, Mikael did a great job with this schematic.
 
Re: KT88 SE PSU

DocLorren said:
Mikael, Sherman,

Since I got quite curious about the KT88 you designed/built is it possible to share the schematics for the PSU so we can see how you got your amps powered? Furthermore, can you elaborate about the input sensitivity? Thx in advance.


Here is my PS schematic. Very simple and effective.

As for input sensitivity I haven't measured it but I would guess an input of about a volt would drive it to full power so sources like CD or DVD players or tuners would be fine. For my monoblocks I didn't change anything other than elminating the volume pot and substituting a fixed resistor. So technically they have more sensitivity than a power amp probably should.

A unity gain preamp would be fine as an input. I'm using a preamp with a gain of 5.6 and seldom move the volume control past 9 o'clock but everything works and they sound great.
 

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Where to buy James transformers now?

It seems that Euphonia is no long selling anything at all as of October 17th. Does anyone have a source in the US? Also what are the voltages of the power transformer for Mikaels' KT88SE amp? Does anyone have a suggestion on what James power transformer would be proper for Mikael's KT88SE amp? I like the look of the potted transformers :)
 
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