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Using 6sn7 + 6bl7 for SE driver?

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Posted on September 7, 2014 at 11:58:06
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Hello, I was wondering if anyone has used the 6sn7 + 6bl7 front end of the PP KT88 schematic as a driver for a PSE KT88 amp. I would like to try this since I have the tubes and the output transformers. I already built a power supply which has a no load output of 430vdc and a -63volts for fixed bias. Any suggestions will be much appreciated, best regards, Dak

 

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RE: Using 6sn7 + 6bl7 for SE driver?, posted on September 9, 2014 at 13:48:16
KevinC
Manufacturer

Posts: 2984
Location: NC
Joined: April 19, 2001



Try this similar circuit, which has been built and sounds quite good. If you want an interstage transformer loaded driver to drive the output tube you can delete the plate load CCS on the 6BL7GT/6BX7GT and increase the PS dropping resistor and PS bypass capacitor values to taste.

Kevin Carter
K&K Audio
www.kandkaudio.com

 

RE: Using 6sn7 + 6bl7 for SE driver?, posted on September 9, 2014 at 14:16:25
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Thank you Kevin! I am going to parallel the kt88 do I need to make any other changes? Also the CCS I can use one per tube right? So, for my project I would need 3 or 4 CCS? Please let me know so I can order them before you go on holiday. best regards, Dak

 

RE: Using 6sn7 + 6bl7 for SE driver?, posted on September 9, 2014 at 14:26:10
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Hi again, I wonder if you have a larger version of this schematic? the one here is too small to read the component values. thanx again. Dak

 

RE: Using 6sn7 + 6bl7 for SE driver?, posted on September 9, 2014 at 18:45:17
KevinC
Manufacturer

Posts: 2984
Location: NC
Joined: April 19, 2001
 Send me an e-mail and I will return a better quality pdf.

Kevin Carter
K&K Audio
www.kandkaudio.com

 

RE: Using 6sn7 + 6bl7 for SE driver?, posted on September 9, 2014 at 18:51:21
KevinC
Manufacturer

Posts: 2984
Location: NC
Joined: April 19, 2001
 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

 

RE: Using 6sn7 + 6bl7 for SE driver?, posted on September 9, 2014 at 19:50:09
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 So that would be 2 for the 6sn7 and 6bl7 + 4 for the kt88. thank you.

 

RE: Using 6sn7 + 6bl7 for SE driver?, posted on September 10, 2014 at 04:38:54
KevinC
Manufacturer

Posts: 2984
Location: NC
Joined: April 19, 2001
 You will need a total of 4 CCSs per channel. One for the 6SN7GT plate load, one for the 6BL7 plate load, and one each for the two KT-88 cathodes.

Kevin Carter
K&K Audio
www.kandkaudio.com

 

RE: Using 6sn7 + 6bl7 for SE driver?, posted on September 10, 2014 at 08:55:38
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Hi KevC, is it not possible for 1 CCS to drive both plates of the single 6sn7 and another CCS to drive both plates of the 6bl7? Dak

 

RE: Using 6sn7 + 6bl7 for SE driver?, posted on September 10, 2014 at 09:24:37
KevinC
Manufacturer

Posts: 2984
Location: NC
Joined: April 19, 2001
 Well, I can't say that I am really sure what you are asking. It is possible to use one CCS, if you are using both triodes of the 6SN7 for the one channel in parallel.

Kevin Carter
K&K Audio
www.kandkaudio.com

 

RE: Using 6sn7 + 6bl7 for SE driver?, posted on September 10, 2014 at 11:07:13
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Hi Kev, In the schematic, 1/2 of a 6sn7 and 1/2 of the 6bl7 is used per channel, with a CCS on each tube's plate. I was wondering if the CCS could not be shared for both plates of the 6sn7 and shared for both plates of the 6bl7 instead?

 

RE: Using 6sn7 + 6bl7 for SE driver?, posted on September 10, 2014 at 11:17:28
KevinC
Manufacturer

Posts: 2984
Location: NC
Joined: April 19, 2001
 Not unless you listen only to mono.

Kevin Carter
K&K Audio
www.kandkaudio.com

 

RE: Using 6sn7 + 6bl7 for SE driver?, posted on September 22, 2014 at 14:45:37
JKT
Audiophile

Posts: 612
Location: Midwest
Joined: November 26, 2002
 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.



"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

RE: Using 6sn7 + 6bl7 for SE driver?, posted on September 22, 2014 at 16:10:08
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 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

 

RE: Using 6sn7 + 6bl7 for SE driver?, posted on September 23, 2014 at 16:03:29
JKT
Audiophile

Posts: 612
Location: Midwest
Joined: November 26, 2002
 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.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

RE: Using 6sn7 + 6bl7 for SE driver?, posted on October 6, 2014 at 22:21:55
Tre'
Industry Professional

Posts: 17296
Location: So. Cal.
Joined: February 9, 2002
 "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
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