Stereo build- Mikael Abdellah's KT88 SE amp

[Bas Horneman]

My first cap is a 100uF, 10H choke, 47uF.

Yes, I think so. Most likely the psu will be less resonant.

Probably the 47uF->10H->100uF will be better theoretically...although I doubt one can hear any difference.

 

[tigrine]

My firts tube amp project

This is my firts tube amp project. A Miakel Abdellah'S KT88 SE amp. Firts impression: better sound than my Yaqin MC-10L (a push pull 42W). The sound are very musical, more coherency than the MC-10L. My B+ is to high 450 volts, I have to raise it before a long listening session.

Thanks Miakel for the schematic and Francois for your help.


Riken for All resistors
Power transformer and output transformer James
Capacitor: Solen, Audio Note, Black Gate.

 

[tigrine]

My firts tube amp project

the inside....

 

[castlesteve]

KT88SE amp

Here`s my attempt:

 

[porkchop61]

Re: My firts tube amp project

the inside....

Nice!
Are those chokes on the underside of the chassis? I was thinking of adding a choke cap decoupling filter for my build, for the 6N1P driver.

I'm going to be in Boucherville on Tuesday to visit a customer. It's close to St-hyacinthe, right?
Glenn

 

[porkchop61]

Now that I have this project running, I had a few questions.

I like the sound of the EL34's, but I'm running close to the rated 25W plate dissipation. Can I lower this by increasing the value of the cathode resistor? I'm currently using the 470R called for in the original schematic. Maybe something over 500R?

Is it advisable to use a decoupling scheme in the B+ supply to the 6N1P driver? Just a resistor/electrolytic capacitor combo?

Has anyone tried running this amp with a KT66 in Triode mode? Seems like it should be able to handle it if the B+ was lowered a bit.

Thanks
Glenn

 

[Sherman]

...a few questions.

... Can I lower this by increasing the value of the cathode resistor? I'm currently using the 470R called for in the original schematic. Maybe something over 500R?

Is it advisable to use a decoupling scheme in the B+ supply to the 6N1P driver? Just a resistor/electrolytic capacitor combo?

Has anyone tried running this amp with a KT66 in Triode mode? Seems like it should be able to handle it if the B+ was lowered a bit.

Thanks
Glenn

Increasing the cathode resistor value would do it.

I had oscillation in mine when driven with a preamp that had some bass boost. Decoupling the driver stage solved it. I don't use that preamp as a source anymore but have left the decoupling in and I think it is probably a good idea, especially if you are running the output tubes close to their limit.

Haven't tried KT66s, or even EL34s in mine. I've been happy with the KT88s.

 

[porkchop61]

Thanks Sherman.
I'll pay around with the cathode value a little. I don't like running right at the max value for plate current.

I had some oscillation when overdriving the amp with a portable CD player. Backing down on the volume fixed that. I may still add a decoupling network for the preamp tube.
What did you wind up using for resistor/cap values for your amp?

Glenn

 

[Sherman]

IIRC I used about a 15K resistor and a 22uF cap. I then adjusted the plate resistor on the 6N1P to around 33K.

 

[porkchop61]

Thanks!
Time to start tweaking the design a little for my own tastes
Glenn

 

[tigrine]

Like I said, I'm a newbie.
I used the shematic of this link,

http://diyaudioprojects.com/Tubes/KT88/index.htm

Yes, Glenn. It's is very close approximately 20 minutes.

 

[porkchop61]

Be careful with that schematic, it has an error. The output from the 6N1P tube should be taken from the plate, not the screen as shown. Alex already knows about this.

What did you use for chokes? They look like Hammonds.

 

[tigrine]

Yes, these chokes are Hammonds.
I know for the error, I used the original to correct it.

Thank you for the warning!

 

[castlesteve]

Re: KT88SE amp

Here`s my attempt:

Lets try again:

http://img.villagephotos.com/p/2007-4/1255358/0-Audio003.jpg

 

[porkchop61]

Looks good!
Seems like everyone uses a similar layout
How do you like the sound?
Glenn

 

[porkchop61]

Anyone out there that has built this amp tried driving the KT88's with a 6SN7? Looks like it doesn't require too many changes. I'm still somewhat of a noob, so I don't know what to look at to even see if a 6SN7 should be considered.
I think I'm suffering from "Tube envy" with that 6N1P sitting next to those KT88's
Glenn

 

[Bas Horneman]

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.

 

[Sherman]

...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.

...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.

I'm with Bas on this. I think as a grounded cathode voltage amp about the best one could hope for with a 6SN7 would be a voltage gain of around 14.

I breadboarded a SE KT88 amp using a 6SL7 into a 6SN7 as a cathode follower which drove the KT88. I used a B+ of 425V which is about what I have on my monoblocks built to Mikael's schematic. It worked fine and sounded fine but didn't sound better than using a 6N1P and it added unneeded complexity and required a beefier PS.

I didn't try driving the KT88 directly from the 6SL7. Probably should have given it a shot when I had the setup on the bench!

 

[porkchop61]

Thanks Bas and Sherman.
That's the kind of things I wasn't sure about. I might just give the 6SL7 a try on another build. I'm actually thinking of breadboarding this amp with parallel KT88's. I might try the 6SL7 on that amp.
Thanks again for the information.
Glenn

 

[Drewaudio]

I spend a lot of effort trying to drive a KT88 with a 6SL7. I couldn't pass enough current through the 6SL7 to overcome the KT88 millar capacitance so changed to a 12AT7 (ECC81). I also tried a ECC35 which has an extra 50% plate dissipation over a 6SL7 but still reverted to the 12AT7.

With the 6SL7, the response was 2 dB down at 20KHz on the output transformer (a Hammond 1672SE transformer) with no feedback.

I think the 6SL7 or ECC35 sound better than the 12AT7. Maybe the millar effect would be more acceptable if the KT88 was connected in triode...

Andrew