• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

KT88 SE Abdellah (modified by Alex Gendrano) + AK..

emartine said:
Great. How much power do you get with 372Vdc at the plates?
I have 360VDC at 6CA7 plates, and only get 3.5Wrms per channel.
Will try to use SS rectification and change 10uF filter cap to 22uF so I can increase B+ a little bit.
Click to expand...

This design requires a preamp to reach full power. It's one of the reasons I ended up going with a different driver tube, with a higher amplification factor, on my next build. Mark talks about this amp needing the additional drive to reach full power in the videos. And by "360 at the plate", are you talking about the voltage across the tube or just measuring the V at the plate pin referenced to ground?
 
How many, who have built this amp, use a preamp with it?

I haven't, and even running 6L6GCs it seems to go quite loud with 87 dB speakers. With different bias setting and 6550 tubes it will go noticeably louder. I don't have the equipment to measure what the output is, though.

Didn't Mark get 8W out of his KT88s without a preamp?
 
These measurements are done with a signal gen and a scope. For mine to reach full power/clipping took more input voltage swing than you would normally see out of a line signal. Mark very specifically said it would need a preamp to get -full power-, doesn't mean it wouldn't be loud enough without one.

Here are his comments about that starting at 8:50:

 
Last edited:
I'm just curious, with so many people building this one, how many run a preamp with it.

Also, I'd hate to see someone pass on a great design like this because they think it has to have a preamp. And for that matter, I'd think something like a JDS Labs Atom would make a good option for a 'preamp' for it to get more voltage in, or maybe even build an objective2 into the case.
 
Last edited:
StepheK said:
These measurements are done with a signal gen and a scope. For mine to reach full power/clipping took more input voltage swing than you would normally see out of a line signal. Mark very specifically said it would need a preamp to get -full power-, doesn't mean it wouldn't be loud enough without one.

Here are his comments about that starting at 8:50:

Click to expand...

He finishes it by saying a volt and a half 'really gets this thing singing'; don't most DACs put out 2V?
 
My speakers are 102 dB sensitivity, so half volume on Spotify via my phone (Chromecast => DAC) is as loud as I would ever care to listen.

I've recently wired it Ultra linear, replaced the driver Rk with LED's and switched to SS diode rectification. No regrets and it's going to stay this way for now.

411VDC at the EL34 plates. 270VDC at the 6SL7 plates.
 
StepheK said:
This design requires a preamp to reach full power. It's one of the reasons I ended up going with a different driver tube, with a higher amplification factor, on my next build. Mark talks about this amp needing the additional drive to reach full power in the videos. And by "360 at the plate", are you talking about the voltage across the tube or just measuring the V at the plate pin referenced to ground?
Click to expand...

Yes. 360 Vdc referenced to ground.
 
emartine said:
Yes. 360 Vdc referenced to ground.
Click to expand...

So you need to subtract the cathode voltage to get what the real plate voltage is.

an EL34/6CA7 would be much happier with the B+ around 410, 405 at the plate measured to ground and you should end up with 30V at the cathode with a 500 ohm resistor load wired as a ultra linear. That's 375 across the tube, 21.5W plate dissipation, which is well within the limits of that tube. My EL34 SE UL amp sings at that operating point.
 
StepheK said:
So you need to subtract the cathode voltage to get what the real plate voltage is.

an EL34/6CA7 would be much happier with the B+ around 410, 405 at the plate measured to ground and you should end up with 30V at the cathode with a 500 ohm resistor load wired as a ultra linear. That's 375 across the tube, 21.5W plate dissipation, which is well within the limits of that tube. My EL34 SE UL amp sings at that operating point.
Click to expand...
Yes. Mine is a little low. I have 360vdc at the plate. 29.5 vdc at the cathode.
 
I would also work to get the cathode voltage on the driver tubes under 2V. Are you using a red LED + small resistor? If so reduce the size of the resistor in series with the LED to get it between 1.8 and 2V. You are running the output tube at 75% so I wouldn't expect more than 4W at best.
 
I know this is an old thread but I put this info out for folks like me that don't sling solder.
I just bought this version of the SEUL KT-88.
Nice and plenty loud on my Tekton Double impact speakers.
You probably want 90+ db speakers for this and my room is kinda small.
sites.google.com

naturecoastelectronics - Stereo Vacuum Tube Amplifiers

Live recording of EL34-KT88 SEUL Vacuum Tube Amplifier Speakers: Altec 604E Super Duplex Loudspeaker Audio Recorded with Sony ECM-DS70P Stereo Microphone Song clip: "Make Me A Memory" from the EP "Tribute To Grover Washington, Jr." - performed by Rocco Ventrella - used by permission
sites.google.com sites.google.com
 
Scott was very helpful with this NCE amp.
Answered all my dumb questions and everything.
The Bass is great with the JJ KT-77's
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom