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.
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.
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.
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.
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.
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?
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?
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
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
Last edited:
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.
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.
Oh, see what you mean. The GE handbook has the data identical for them.
The 6EA7 datasheet is evaluated at different operating currents than the handbook 6EA7 data or the 6FM7 datasheet & handbook data.
Only way to tell if they are the same would be to put them on a curve tracer,
or see if the 6FM7 curves evaluate to the same data as the 6EA7 data at those different rating currents.
Could also be that later issue tubes were made identical.
The 6EA7 datasheet is evaluated at different operating currents than the handbook 6EA7 data or the 6FM7 datasheet & handbook data.
Only way to tell if they are the same would be to put them on a curve tracer,
or see if the 6FM7 curves evaluate to the same data as the 6EA7 data at those different rating currents.
Could also be that later issue tubes were made identical.
The 6EA7 datasheet data looks very similar to the RCA 6GL7 datasheet data. Interestingly, for triode #1, the 6GL7 datasheet also lists a second set of data for triode #1 equal to the 6FM7 data. Triode #2 6GL7 data is at 46 mA versus 48 mA for the 6EA7 datasheet and is very similar too. Kind of odd for two tubes on the same base to be identical though.
Then there are 6DR7, 6FY7
Then there are 6DR7, 6FY7
Last edited:
As I stated earlier I have seen a large tube to tube variation with these tubes. Looking at my collection some are marked 6EA7 some are 6EM7 and most are 6EA7/6EM7. Maybe it didn't make much difference in a TV set, but there seems to be two or more different tubes being sold as 6EA7/6EM7. There are three different constructions of the tubes marked with both numbers. The ones with the wide flat plates for the big section seem to have more bass, but they red plate easier than the other flavors when pushed hard.
I tried a couple of Sylvania 13GF7 and 13FM7 in the curve tracer, and they are definately the same tube. They both look the same, all black plates, the big flat plate on section #2. Only difference is that the 13GF7 has a little cooler fin atop g1 of section #1.
Then I looked thru some other brands, and after only 12 tubes out, I see 3 different constructions of each type too. All the others had no cooler fin on section #1 g1 (but small or large cooler fins on section #2 g1 on all). The GE and RCA usually have grey plates, but one RCA has black plates (Sylvania black plated). Section #2 is sometimes a big flat plate (Sylvania, RCA, Zenith) and sometimes a rectangle at 90 degrees in the middle (GE). Section #1 is usually a rectangular box, but some have flat fins out the narrow ends.
The most significant difference I see is that section #1 sometimes has a round cathode (Zenith, GE) and others have a flat cathode (Sylvania, RCA). Usually this means different curves, but I put one of each on the curve tracer and the same curves still, but a DC bias difference. With only 12 tubes out so far, looks like construction was a free for all with these tubes. But the curves were remarkably similar on the few I have tried (only tried 4 on the tracer).
I bought a box of 156 of the 13FM7 type when VT.net had the box sale, something like $0.35 each. Probably got every brand possible here. Lets see, Dumont, Tung-Sol, Philco, Raytheon, International, PhilipsECG, Zenith, RCA, GE, Sylvania, Lindal, Westinghouse, Amperex, El Menco, Canadian Marconi, Admiral, Sears, Silvertone. Am I missing any? And that was just the 13FM7 box.
Then I looked thru some other brands, and after only 12 tubes out, I see 3 different constructions of each type too. All the others had no cooler fin on section #1 g1 (but small or large cooler fins on section #2 g1 on all). The GE and RCA usually have grey plates, but one RCA has black plates (Sylvania black plated). Section #2 is sometimes a big flat plate (Sylvania, RCA, Zenith) and sometimes a rectangle at 90 degrees in the middle (GE). Section #1 is usually a rectangular box, but some have flat fins out the narrow ends.
The most significant difference I see is that section #1 sometimes has a round cathode (Zenith, GE) and others have a flat cathode (Sylvania, RCA). Usually this means different curves, but I put one of each on the curve tracer and the same curves still, but a DC bias difference. With only 12 tubes out so far, looks like construction was a free for all with these tubes. But the curves were remarkably similar on the few I have tried (only tried 4 on the tracer).
I bought a box of 156 of the 13FM7 type when VT.net had the box sale, something like $0.35 each. Probably got every brand possible here. Lets see, Dumont, Tung-Sol, Philco, Raytheon, International, PhilipsECG, Zenith, RCA, GE, Sylvania, Lindal, Westinghouse, Amperex, El Menco, Canadian Marconi, Admiral, Sears, Silvertone. Am I missing any? And that was just the 13FM7 box.
Last edited:
I'm part way through a 6EM7 build with a pair of Salvania tubes (NOS I'm sure). But "tracer" not heard of this before.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.