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In Reply to: RE: Using 6sn7 + 6bl7 for SE driver? posted by DAK on September 09, 2014 at 14:16:25
You will need a separate CCS ( and bypass cap(s)) for each output tube, so 4 CCS per channel. For parallel output tubes you'll need separate coupling caps, so that the DC bias for each tube is independent of the other. They come together at the plate connection to the output transformer. Of course, it's best to have reasonably matched output tubes in each channel.
Kevin Carter
K&K Audio
www.kandkaudio.com
Follow Ups:
So that would be 2 for the 6sn7 and 6bl7 + 4 for the kt88. thank you.
Hi,
The SET 'front end' of 6SN7 and 6BX7/6BL7 with CCS plate loads is the same as my modified Sun 2A3 amp. The sound is excellent. The stock Sun used a 6SN7 as driver and had a kind of "polite" and whimpy sound. The 6BX7 corrected that and the CCS plate loads added more clarity and detail. It's very musically involving now. Not polite, but still more refined. Good luck with your project. I know that you will enjoy listening to it.
Hi JKT, nice work on the Sun amp. I noticed that the 6sn7/6bl7 , each triode half, is used per channel, do you therefor use a CCS per each triode half? I am also inexperienced with the "sound" of using CCS for current sourcing. How would you describe the difference from stock to the of CCS? thank you again for chiming in! best regards, Dak
Think of the plate load CCS as replacing the plate load resistor. You would not use the same plate load resistor for each channel. That would result in a mono output like Kevin said, not a stereo output. So yes, each triode half has it's own CCS plate load. I do have to tweak the current output of these in my Sun amp to keep the plate voltages in line so the direct coupling works properly and the driver stage has enough headroom. Such is the problem of living with-in the Sun's limit of 330V B+. Kevin's circuit was designed for the 6SN7/6BN7 and this should NOT be a problem. In my case adding the CCS plate loads is worth the extra hassle that I have to put up with. Less distortion and a more direct connection to the music, is what I hear with the CCSs. While the impedance of a CCS is not infinite, it is very high. As a result the tube can swing quite a bit of voltage with little change in current. As you know, any curve begins to look linear if you look at a small enough section of it. This is true of the plate curves of a triode. Also, the load line with a CCS plate load is practically horizontal. Draw a horizontal line through the plate curves of a triode and you can see that this is a linear load line. Thus the lower distortion and clearer sound. Hope this helps.
"I do have to tweak the current output of these in my Sun amp to keep the plate voltages in line so the direct coupling works properly and the driver stage has enough headroom."Am I reading this right.
Leaving the current the same and just changing the cathode resistor value (thus changing the plate voltage which of course IS the bias voltage for the output tube) can leave you with too little headroom in the driver stage?
I always liked the idea of an adjustable cathode resistor on the driver tube as the bias adjustment for the output tube with a CCS loaded driver tube and direct coupling.
And being a SE guy, if I ever build a direct coupled SE amp this is how I would plan to fine tune the bias of the output tube.
BTW I haven't seen your schematic and I may not understand.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 10/27/14 10/28/14
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