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Old 1st March 2011, 10:42 AM   #1
Ardee is offline Ardee  England
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Default 6EM7 Unit 1

Opinion seems divided on whether the high mu section of this tube is worth using. Most critics condemn it after simply looking at the data sheet but there are many circuits out there using it to drive the low mu section. Is it really up to the job? It looks rather like driving a 2A3 with a 12AX7.
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Old 1st March 2011, 12:50 PM   #2
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Quote:
Is it really up to the job? It looks rather like driving a 2A3 with a 12AX7.
I have had no issues using the little side to drive the big side. I have however seen huge tube to tube variations in the gain of the little side. I ran them with LED cathode bias and a CCS load and saw nearly a 2 to 1 gain spread over about 25 different tubes.
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Old 1st March 2011, 12:58 PM   #3
Ardee is offline Ardee  England
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Thanks for the information. I am assuming that the Miller capacitance of the small side is more of a problem than that of the big side?
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Old 1st March 2011, 01:54 PM   #4
6L6 is offline 6L6  United States
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The little side was made to drive the big side -- that is why it's a dissimilar dual triode...

They were made for Vertical and Horizontal hold oscillators in NTSC televisions, the small side would supply the voltage necessary to drive the big side, which could pass enough current to modulate the picture tube yoke.

Anyhoo, as long as you take the Miller into account, the circuit should be fine.
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Old 1st March 2011, 06:19 PM   #5
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I am assuming that the Miller capacitance of the small side is more of a problem than that of the big side?
That would be the case. It has been a few years since I played with that tube, but I don't remember any issues and the amp sounded quite nice. I still have it somewhere. It used the 6EM7 to drive a 6336A cathode follower output stage.

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The little side was made to drive the big side -- that is why it's a dissimilar dual triode...They were made for Vertical and Horizontal hold oscillators in NTSC televisions,
They were intended for vertical deflection use. Unlike the horizontal sweep section of a TV set, the vertical sweep tube operates as a linear amplifier. The little section was used as a gated oscillator generating a 60 Hz (or 50 Hz) sawtooth wave. The big section was used as an SE audio amp except that it had to work at only one frequency. This fact has been exploited for as long as I can remember. I got started with tubes by making guitar amps out of junk TV sets. The vertical section makes a reasonable 2 to 5 watts SE amp. The vertical output transformer will drive a speaker just fine, you just need to use one with 4 wires. This also was the idea behind the Bottlehead SEX amp. It used the 6DN7 and a vertical output transformer.

Miller isn't much of an issue at 60 Hz though. I would keep the driving impedance to a reasonable value. I use a 50K volume pot for these reasons.
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Old 2nd March 2011, 01:38 PM   #6
6L6 is offline 6L6  United States
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Neat! I didn't know that the original SEX amp was also using a TV-surplus transformer as well... Interesting!
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Old 4th March 2011, 12:43 PM   #7
Ardee is offline Ardee  England
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I have now tried a GE manufactured 6EM7/6EA7 and found some problems. The first is that it appears to be more like a 6EA7 than a 6EM7 and I cannot find any curves for the former.

The sound is attractive, with a very lush midrange, but the bass was very woolly. Replacing the small side with a 6N1P cured the bass problem but also lost the attractive midrange. One answer might be to get more current through the small side but I do not fancy plotting curves or just guessing the best operating point. Can anyone suggest the best configuration for the high mu section?
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Old 4th March 2011, 02:17 PM   #8
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6EA7 is the same tube as 6FM7

and 6EM7 is the same tube as 6GF7

http://tubedata.milbert.com/sheets/123/6/6FM7.pdf

http://tubedata.milbert.com/sheets/135/6/6GF7A.pdf

http://tubedata.milbert.com/sheets/049/6/6EM7.pdf

http://tubedata.milbert.com/sheets/093/6/6EA7.pdf
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Last edited by smoking-amp; 4th March 2011 at 02:26 PM.
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Old 4th March 2011, 03:22 PM   #9
Yvesm is offline Yvesm  France
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Just look at the hi Mu section as an half (well know) 6SL7.

As for any triode, they like high B+ and plate load, typically: 300V B+, 220K anode load and bypassed 2K7 cathode resistor.

Yves.
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Old 4th March 2011, 04:58 PM   #10
Ardee is offline Ardee  England
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The 6EM7 and 6EA7 are not identical according to this:

http://www.novotone.be/_site/projets/Projet18/Fig02.pdf

A 220k load is higher than is used in most circuits I have seen, but it looks pretty good at around -2v so I might give it a try. Thanks.
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