Stereo build- Mikael Abdellah's KT88 SE amp

[alexg]

I have built three of this amp already.

One for me, and two for friends, and it is one of the best sounding amps that I ever built. I find triode strapped is better than UL.

I have plugged an Amperex 6dj8 on the 6n1p position, but I like the 6n1p sound better!

I am thinking of trying the 5670 for a driver, mu is very similar to 6n1p but I think it can have more current.

Any opinions?

 

[Bas Horneman]

I'm actually thinking of breadboarding this amp with parallel KT88's.

If Andrew had problems driving the KT88 with a 6SL7. You'll have twice the problem if you parallel the KT88's. Because you'll have twice the miller capacitance to overcome.

 

[porkchop61]

Okay, doesn't sound too promising then.
Probably not worth the effort.

 

[Sherman]

...
I have plugged an Amperex 6dj8 on the 6n1p position, but I like the 6n1p sound better!

I am thinking of trying the 5670 for a driver, mu is very similar to 6n1p but I think it can have more current.

Any opinions?

I also tried a 6DJ8 and a Sylvania 6922. Both seemed to make the amp produce less bass. Overall the sound, even in the midrange didn't seem as good as with a 6N1P.

I don't have any experience with the 5670 but the current limit on a 6N1P is 25mA. Seems more than enough. I'm only running mine at about 8 or 9 mA.

 

[porkchop61]

I'm running mine at only 4mA, unless I'm calculating things wrong.
4v (at the cathode)/1000R resistor= .004A (4mA)
I have 200V on the plate.
Maybe I should bias a little hotter for more gain?
Now you've got me curious.
Glenn

 

[G]

You could do that. But you'd have less gain. So if you have an active preamplifier with some gain. And / or very efficient speakers you could get away with it.

Gain of the 6SN7 is around 20 and that of the 6n1p is around 30. If you want to go with an octal tube a better tube would be something like the 6SL7 with a gain of 70.

Lot's of people will tell you that the 6SL7 can't drive a KT88 well enough. But a very well reviewed McIntosh amp uses 12AX7's. So you'd have to try it for yourself and see AFAIC.

Or you could try a 396A/5670 with only minor biasing changes.

 

[porkchop61]

Thanks, but I was really looking for an octal based tube.

 

[Bas Horneman]

Glen,

Since you want to PSE KT88 why not parallel 2 sections and 2 tubes of 6SL7 as well. (4 triode sections in parallel in total per channel) that should do the trick. Unconventional but worth trying in my book.

 

[porkchop61]

That is a good idea!
I'll have to start a new breadboard.
Thanks!

 

[mhouston]

I think I'd like to start a new thread on this topic as it might help others when doing this build.

For a little background,
I'm in the process of building a stereo version of this project.
I've built 4 guitar amps, but this is my first hi-fi project. This is also my first non push-pull build, so I have a few new questions.

Sherman has been helping me with a lot of my questions, so I'm very grateful to him for all the advice.

Stay tuned, and I'll post pix as my build goes along.

Has anyone else built this design (other than Sherman), and if so, what to you think of the sound?

Thanks.
Glenn

I have built a second amp based losely on your original schematic. One with Black Sable EL34s and now with Sovtek KT88s. Still an impressive design.

 

[porkchop61]

Nice implementation of this design! I love the cabinet work
I'm also running Sovtek KT88's in mine, when I run them.
I think my favorite in this amp are the Electro-Harmonix EL34's in Pentode mode. Of course this is for Rock music.

Glenn

 

[mhouston]

Nice implementation of this design! I love the cabinet work
I'm also running Sovtek KT88's in mine, when I run them.
I think my favorite in this amp are the Electro-Harmonix EL34's in Pentode mode. Of course this is for Rock music.

Glenn

In my first build I could not use KT88s because of how I mounted the sockets (can be fixed) so I had to run EL34s. I am using matched Black Sables which I find excellent. Also I have listen to Siemens NOS, possibly even better.

I have ordered three matched pairs of JJ KT88 blue glass. I intend to build more of these amps so good tubes will be well used. Other tubes I must try (and even A/B), which I have on hand, are JJ 6L6, NOS 6V6 black glass, Electro-harmonics EL34s, 5881s and some I don't have as yet 6550. Beauty of the voltages etc. I set is that I can plug and play any of these tubes.

I'll let you know of any further developments.

 

[Zekk]

In my first build I could not use KT88s because of how I mounted the sockets (can be fixed) so I had to run EL34s. I am using matched Black Sables which I find excellent. Also I have listen to Siemens NOS, possibly even better.

I have ordered three matched pairs of JJ KT88 blue glass. I intend to build more of these amps so good tubes will be well used. Other tubes I must try (and even A/B), which I have on hand, are JJ 6L6, NOS 6V6 black glass, Electro-harmonics EL34s, 5881s and some I don't have as yet 6550. Beauty of the voltages etc. I set is that I can plug and play any of these tubes.

I'll let you know of any further developments.

I've tried NOS Genalex KT88, GE KT66 and Tung Sol 6550 in it, but, to my ears, Amperex Holland EL34 beats them all!

Enjoy!

 

[mhouston]

I've tried NOS Genalex KT88, GE KT66 and Tung Sol 6550 in it, but, to my ears, Amperex Holland EL34 beats them all!

Enjoy!

I don't know and have never heard of the tube you mention.

Just got 6 X matched pairs of JJ KT88 Blue Glass last night. They produce a little more bass weight and mids a little more froward but I think the tubes need at least another 20 hours. But who cares how they sound they just look so cooool

http://diyaudioprojects.com/Gallery/displayimage.php?pos=-502

 

[88man]

Before I cut the metal, I was wondering if the layout of the SE KT88 amp is OK?... To create symmetry with the tubes on the back row, there's a longer run of AC to supply the 5U4-G. Will this be a source of potential hum?...

Scale 1:2. Dimensions 12x16in, 0.125in T6061 aluminum top plate.
PTX and Choke: Hammond
OPT: James 6123HS
PSU caps: ASC 440VAC

Thanks!

 

[Curly Woods]

Before I cut the metal, I was wondering if the layout of the SE KT88 amp is OK?... To create symmetry with the tubes on the back row, there's a longer run of AC to supply the 5U4-G. Will this be a source of potential hum?...

Scale 1:2. Dimensions 12x16in, 0.125in T6061 aluminum top plate.
PTX and Choke: Hammond
OPT: James 6123HS
PSU caps: ASC 440VAC

Thanks!

I would try to get that rectifier tube farther away from your output/input tubes. Could you mount one or more of those big caps under the hood? It looks like you have a 12"x16" top plate. Is that right? The output tubes are fairly close together at 1" spacing between them.

What about moving your choke closer to the front of the chassis and shifting the caps between the PTX and the choke. Then move your rectifier tube closer to the choke. I like to get rectifier tubes as far away from the signal tubes as physically possible. I may be all wet here too though so hopefully some more experienced builders will chime in on your layout

 

[mhouston]

Before I cut the metal, I was wondering if the layout of the SE KT88 amp is OK?... To create symmetry with the tubes on the back row, there's a longer run of AC to supply the 5U4-G. Will this be a source of potential hum?...

Scale 1:2. Dimensions 12x16in, 0.125in T6061 aluminum top plate.
PTX and Choke: Hammond
OPT: James 6123HS
PSU caps: ASC 440VAC

Thanks!

My top plate is only 300 X 300mm but I use SS rectification and mount the choke and OPTs on thier side in the base.
Vacuum Tube (Valve) Audio Projects/KT88 SE UL tube power amp-underside - DIY Audio Projects Photo Gallery

 

[alexg]

Before I cut the metal, I was wondering if the layout of the SE KT88 amp is OK?... To create symmetry with the tubes on the back row, there's a longer run of AC to supply the 5U4-G. Will this be a source of potential hum?...

Scale 1:2. Dimensions 12x16in, 0.125in T6061 aluminum top plate.
PTX and Choke: Hammond
OPT: James 6123HS
PSU caps: ASC 440VAC

Thanks!

I would space the KT88 sockets around 4" from each other, most KT88 data states that you need to space them arounmd 4", they generate a fair amount of heat.

 

[Curly Woods]

I would space the KT88 sockets around 4" from each other, most KT88 data states that you need to space them arounmd 4", they generate a fair amount of heat.

Does he really need 4" space between the KT88's? I would think 2 1/2" would be pretty good spacing, but again I will defer to the more seasoned builders suggestions on the spacing issues.

 

[Curly Woods]

Thank you Alexg, Curly, and mhouston! I thought, I could get by with the larger tubes in a nice straight line. I guess not. Here is another sketch, I placed the rectifier near the power supply corridor. I'll center the KT88s in front of the OPT, but space them about 2.5in apart. Is this better?...

BTW, what is the power output of this amp with B+ @ 410-420V?...

Oops. You forgot the sketch On another note, I meant to say 2.5" between the envelopes pf the tubes, which is a little closer to 4" between the sockets, as Alex stated in his post. Sorry for the confusion!