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Old 19th July 2011, 06:15 PM   #21
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Quote:
Originally Posted by Pano View Post
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?
You are absolutely right!

The 6BW7 in pentode, or even cheap IF pentodes like EF80 sound much better than 2-stage medium-mu stuff. In almost every case, set VG2 = 150V, Ia= 10mA or so, Va as high as you can get, consistent with desired gain; but 200V min.

There's endless fun to be had with this architecture, since ultra cheap tubes can be tried, and often sound better than expensive NOS relics. And when you want to take a step further, the same valve in the same socket can be triode connected & run shunt-cascode for even higher performance. In this circuit the gain starts at 250, and can be set with little regard to the operating current.
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Old 19th July 2011, 07:00 PM   #22
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Quote:
Originally Posted by smoking-amp View Post
"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.
I've posted my test data on 12AT7/ECC81. The "high second harmonic" turns out not to be the case, with the JJ versions being particularly good.

edit: Copy'n'paste from my earlier posting:

ECC81 (JJ, average of ten sections) 1.7V bias, red LED (CCS plate load)
15VRMS output, 1kHz
-66dB 2nd
-91dB 3rd
-99dB 4th
<-100dB 5th
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Old 19th July 2011, 07:29 PM   #23
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by smoking-amp View Post
"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
I only looked quickly, but they didn't look too bad on first inspection. Guess I should have looked a little more closely, might evidently have missed something.. (Or maybe I just have lower expectations.. )

In any event I am using D3A triode or pentode connected for a number of chores and those do exhibit good linearity on the bench and can drive a 300B.
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Old 20th July 2011, 02:30 AM   #24
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Maybe it's just GE versions (ie the datasheets were by GE) that have bad curves? I've noticed big differences between GE and Sylvania for 6CM7, GE had round cathodes and Sylvania had flat cathodes.
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Old 20th July 2011, 04:17 PM   #25
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by smoking-amp View Post
Maybe it's just GE versions (ie the datasheets were by GE) that have bad curves? I've noticed big differences between GE and Sylvania for 6CM7, GE had round cathodes and Sylvania had flat cathodes.
I was looking at the Sylvania curves, but only cursorily so I might still have missed something..

I'm curious enough that I may eventually get some to experiment with..
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Old 10th May 2012, 06:18 AM   #26
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Quote:
Originally Posted by smoking-amp View Post
"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
Given I cannae afford a matched quad of 45s I'm looking at either 6SN7s or 6S4A as PP drivers to PP 2A3. I only "need" 45V swing but 100V will allow the circuit to drive into AB (if I add Brooks style sliding bias)

I did build a sub one watt SE 6S4A amp and was very happy with the sound
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Old 10th May 2012, 10:14 AM   #27
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For a PP 2a3 amp I recommend you try 4P1L running at 15ma-20mA and about 150v anode as differential pair. Starve the filaments a little. Details on other threads here - mogliaa is an expert. Cheap enough and works very well. I strongly recommend a PP plate choke. I use a Hammond 124B, just the 90K side with the centre tap. works like a charm. I've used the 124B with 6SN7 drivers as well - it's half nickel and very good value.

6S4A is OK - I use it as PPP outputs in a Leak Stereo 20. But there are better choices of drivers in the DHTs.

andy
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Old 10th May 2012, 10:48 AM   #28
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Quote:
Originally Posted by smoking-amp View Post
Maybe it's just GE versions (ie the datasheets were by GE) that have bad curves? I've noticed big differences between GE and Sylvania for 6CM7, GE had round cathodes and Sylvania had flat cathodes.
Morgan Jones notes in VA4 that the GE 6S4 tubes gave him better and much more consistent performance than the Sylvanias, fwiw.
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Old 10th May 2012, 01:05 PM   #29
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Quote:
Originally Posted by andyjevans View Post
I strongly recommend a PP plate choke.
I've got some PP:PP interstage transformers (parmeko and trimax/ericson) end to end. It'll end up loosely based on Lynn Olsen's Karna with 5687, 6S4A* and 2A3 pairs

* subject to changes!
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Old 11th May 2012, 04:58 PM   #30
Zoran is offline Zoran  Serbia
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Spice model for 6S4-A from GE datas.
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