Has anyone tried a 6S4A as a 300B driver tube? - Page 2 - diyAudio
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Old 25th January 2004, 12:15 PM   #11
MIKET is offline MIKET  United States
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Default Here is the circuit after some minor tweeks

All filiments are DC.
1 minute timer for B+
Constant Current Source found at
http://home.zonnet.nl/horneman/mosfet.htm
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File Type: gif 300bv2.gif (6.7 KB, 632 views)
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Old 18th July 2011, 03:18 PM   #12
alexg is offline alexg  Philippines
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Hi MikeT, did you ever build this amp?

I like the 6s4a (I have an amp using 6s4a as power tube and it sounds great). I also built a 6s4a WOT preamp and I like it.

Am thinking of building a 300 SE with 5687 input tube and 6s4a as driver.

What do you think guys?
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Old 18th July 2011, 10:12 PM   #13
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6S4 is a good looking triode, but is designed for 150V operation, and a limit of 160V.

Meanwhile the 300B is normally biassed at -60 to -80V, and to allow some margin, the driver must be able to swing +/- 100V. With anode voltage = 150V idle, 6S4 can manage the negative swing, but going +ve, the curves become obviously nonlinear above 200V or so, even if CCS loading is used.

Only high quality option is to try a series cascode, which is what the tube was designed for. IME, an NPN transistor in the upstairs position is best here.
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Old 18th July 2011, 10:24 PM   #14
SY is offline SY  United States
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Quote:
Originally Posted by Rod Coleman View Post
6S4 is a good looking triode, but is designed for 150V operation, and a limit of 160V.
Tung Sol datasheet for 6S4 says 500V max. GE datasheet for 6S4A says 550V max. The 6S4 is a terrific driver tube and can even make a pretty decent output tube. You'll be interested to see its use in the upcoming 4th edition of "Valve Amplifiers."
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Old 18th July 2011, 10:54 PM   #15
kevinkr is offline kevinkr  United States
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I was hoping it was an octal, should have known better..

Linearity looks really good, and sufficient power handling for the likes of a GM70 driving it somewhat into class A2.. A 300B should be a walk in the park..
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Old 19th July 2011, 03:42 AM   #16
alexg is offline alexg  Philippines
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Thanks Kevin and SY, I will breadboard the 300B amp, either 6sn7 or 5687 for input, 6s4a driver, 300B output.

I need to gather parts, this will be an exciting project!
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Old 19th July 2011, 05:55 PM   #17
Pano is offline Pano  United States
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A nave question:
Does the 300B really need a driver? Isn't there a nice pentode that can do duty as a voltage gain stage and have enough current, too?
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Old 19th July 2011, 06:15 PM   #18
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"Linearity looks really good"

Umm, did you look at the 3rd page of the datasheet? The Mu curves are horrendous. Not even constant Mu at constant current. Should work OK for P-P, but SE looks like a disaster. As suggested earlier, it looks just like a bigger version of a 6C4.

http://frank.pocnet.net/sheets/093/6/6S4A.pdf
http://frank.pocnet.net/sheets/093/6/6C4.pdf
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Last edited by smoking-amp; 19th July 2011 at 06:40 PM.
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Old 19th July 2011, 06:17 PM   #19
SY is offline SY  United States
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The test bench says otherwise.
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Old 19th July 2011, 06:58 PM   #20
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"The test bench says otherwise."

Well, if the 6S4A datasheet is to be believed, it might work for 2nd Harmonic driver cancellation with the 300B. Has very high 2nd harmonic like a 12AT7.
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