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6N7 - a fun tube!




 
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Patrick J. / KD5OEI
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« on: March 23, 2012, 11:18:54 PM »

Yay! The Audiophools don't like little zero bias triodes! I was doing some experiments with one and I like it and maybe should buy a few.


I'm happy with nit.. It's perfect as Borg.
  • 300V
  • zero bias
  • can make 10W

But as all audio gurus and experts know these are not good for hi-fi absolutely no good at all for that, only for modulators and high voltage amplifiers and that is why they are $5-10 on the internet.

But some folks think otherwise if it says "Mullard"
2Pcs. NOS MULLARD NR73/CV1285=EC​C31/6N7G $269

Not sure what audio amps used them. Just some ham gear and industrial stuff?

The 53 is a 2.5V version.

What other smallish zero bias triodes are there?

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« Reply #1 on: March 24, 2012, 02:12:31 AM »

6A6

1635

6Z7

73DG
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« Reply #2 on: March 24, 2012, 04:32:56 PM »

For dual triodes the 12BH7 is good but its a bit on the audiophool radar.

The 6N7 and 12BH7 were popular for mobile rig modulators.

Ive used the 53 but there is a HD version that was used in elevator controllers but I forget the number.

Carl
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« Reply #3 on: March 24, 2012, 04:47:30 PM »

Quote from: KM1H link=topic=30912.msg241023#msg241023 date=1332621176
Ive used the 53 but there is a HD version that was used in elevator controllers but I forget the number.
Carl
[/quote

That would be the 5608A.  Also widely used in Barber-Coleman boiler controllers.

73DG
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« Reply #4 on: March 24, 2012, 05:23:21 PM »

What got me looking more closely at the 6N7, besides zero bias modulator duty, is this old limiter. It is a little different than most and uses a 5814 which is a 12AU7 in a bridge circuit  (The limiter used a 6N7 for this in early units).

This is a different circuit from the variable-u and variable gain and "screen/supressor/pentagrid" schemes. The compression/distortion specs are interesting @1%<15dB and it claims an attack time as fast as 100us.

The 5814 is a 9-pin miniature dual triode like a 12AU7where the 6N7 is much beefier. I don't know why they changed.

Another interesting "feature" is the way the 6N7 is used in the input circuit. Being a zero bias tube it is still given some cathode bias of 2.6, maybe so the line could drive it meaningfully. The plates feed the bridge directly. I'd like to get my hands on one and run through it with a signal generator and a scope.

Other tubes like the 12BH7, 6FQ7/6CG7, etc. are good but I'm thinking of similar sized tubes that are usually run zero bias with medium or rated plate voltages. The 12BH7 will do that at low voltages like 60-80V.

* collins limiter 26W_Manual.pdf (3458.41 KB - downloaded 455 times.)

* Collins_26W_schematic.gif (223 KB, 2766x1380 - viewed 5329 times.)

* Collins_26W_input.gif (23.7 KB, 499x518 - viewed 3586 times.)
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« Reply #5 on: March 25, 2012, 12:44:39 AM »

The 6N7 was used as the intermediate tube in the speech amplifier section of the Globe champ 175 (between the 6SJ7 and the PP triode connected 6F6's). That rig always got excellent audio reports, and I have subsequently built two speech amplifiers based off the original design. One was used with a T-368 and was identical to the WRL design using triode 6F6's as the driver tubes. The other is currently being used in my homebrew 254W rig. That one uses 6A3's as the drivers. I believe the 6N7 is used as a phase inverter to take the single ended output from the 6SJ7 to the push pull 6A3 grids. The output of the 6N7 is capacitatively coupled to the 6A3's, no transformer. Again, I get excellent audio reports. See the schematic below. The first schematic is the speech amp, the second is the power supply, and the third pic is the actual speech amp. 6SJ7 and 6N7 at top, then ps components. 6A3's at bottom, S-9 driver transformer, 1uf 5000V cap for modified heising, 811A's and S-22 at top left.

Ron


* 254W 002.jpg (268.82 KB, 2338x1700 - viewed 5198 times.)

* 254W 004.jpg (256.93 KB, 2338x1700 - viewed 2604 times.)

* Frankenmitter 004.jpg (2740.62 KB, 3456x2304 - viewed 1694 times.)
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« Reply #6 on: March 25, 2012, 09:28:48 AM »

What got me looking more closely at the 6N7, besides zero bias modulator duty, is this old limiter. It is a little different than most and uses a 5814 which is a 12AU7 in a bridge circuit  (The limiter used a 6N7 for this in early units).

This is a different circuit from the variable-u and variable gain and "screen/supressor/pentagrid" schemes. The compression/distortion specs are interesting @1%<15dB and it claims an attack time as fast as 100us.

I know where there's a Collins 26W, no longer in service, just holding a place in a rack. Will check whether early or late for tube complement.  If you want us to power it up and check behavior or other parameters let me know.
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« Reply #7 on: March 25, 2012, 10:12:43 PM »

Thanks! I guess I wanted to vary the input voltage and observe the AC and DC levels on the input amp electrodes and the limiter bridge tube electrodes as well as across the driver transformer primary to better understand it. Its the only unit I know of that uses a bridge. It would take all day fiddling about. I would not ask you go to to that much trouble.
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« Reply #8 on: March 25, 2012, 10:59:12 PM »

The 6N7 was used as the intermediate tube in the speech amplifier section of the Globe champ 175 (between the 6SJ7 and the PP triode connected 6F6's). That rig always got excellent audio reports, and I have subsequently built two speech amplifiers based off the original design. One was used with a T-368 and was identical to the WRL design using triode 6F6's as the driver tubes. The other is currently being used in my homebrew 254W rig. That one uses 6A3's as the drivers. I believe the 6N7 is used as a phase inverter to take the single ended output from the 6SJ7 to the push pull 6A3 grids. The output of the 6N7 is capacitatively coupled to the 6A3's, no transformer. Again, I get excellent audio reports. See the schematic below. The first schematic is the speech amp, the second is the power supply, and the third pic is the actual speech amp. 6SJ7 and 6N7 at top, then ps components. 6A3's at bottom, S-9 driver transformer, 1uf 5000V cap for modified heising, 811A's and S-22 at top left.

Ron

That's a very good looking modulator there. 6A3's are kinda scarce, I would probably go the 6F6 (or more likely other more common pentode or tetrode) route and use then as triodes. How did you like the 6F6 in triode mode? Did it have good regulation driving the 811's? Can you recall about B+ for them and the bias method or value? Some limiters and compressors have a small power stage and some don't, but I tend to want one. One question, since the 6N7 can make 8-10 watts, was why didn't you use it for a driver to the 811's? Not enough overhead?
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« Reply #9 on: March 26, 2012, 12:20:00 AM »

Running a class 'B' tube into another 'B' audio-wise is a bad idea.  A good driver for 6N7's was another 6N7 with the elements tied together, forcing it into more of an AB operation.

If you want to use a real triode in driver service without having to resort to neg feedback, try PP 6B4G's.  They are sorta on the audiofool list, but can be had for not a lot of money.  Same as a 6A3 with a octal base.

Russian surplus examples are available NOS as well.

53-6A6-6N7's also makes a good balanced crystal oscillator with the crystal feeding into the two grids.

73DG

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« Reply #10 on: March 26, 2012, 12:42:17 AM »

Here's a little table of triode connected 2 and 3 grid tubes. Maybe this can be kept up with and added to as more applications are found here and there.

so far: 6L6, KT88, EL34, 6F6, 807

It's interesting that the 6F6 data found was class AB2. I did not find AB1 or A data for the 6F6, but I've only looked on Bunker and on Pete Millett's site.

The 807 data in the Sylvania '59 manual had a column for ICAS but the data was exactly the same as for CCS ?? huh.

"ultralinear" and other "tapped transformer" data was omitted because it is not true Triode operation and we don't generally see that in ham radio modulator drivers and such driver transformers as would be required for it are not off the shelf items.

The "special triode connection" where G1 and G2 are driven together was also omitted because that is not the same thing. What is the general opinion about whether that is appropriate? It would do no harm but the focus of the whole 6N7 topic and the triode connection subtopic seems to be triode connected driver amplifiers that do not themselves require meaningful drive power.

Other beam tubes and pentodes that would be interesting would be the 6AQ5, 6BQ5, 6K6, and certain low voltage ones like the 6Y6

* triode connected .xls (16.5 KB - downloaded 297 times.)
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« Reply #11 on: March 26, 2012, 08:46:27 AM »

The 6N7 is a strapping little tube. Both sections in parallel transformer coupled to another one with both sections in P/P can be pushed to close to 10w outpoot. One as a P/P class-B modder would be great for a small PW rig.

The circuit that Ron posted is a pretty classic 6N7 fuzz inverter for a twin triode with a common cathode. The common cathode seemed to be a stumbling block for designers in audio service, but is very handy for other applications like mixers, product detectors, etc.

I have always been curious how 2 of them would work in P/P parallel. But I have never seen anyone do that. The only parallel applications I have seen are usually class-A low level stuff.
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« Reply #12 on: March 26, 2012, 12:49:16 PM »

The 2A5 and 42 should be listed with the 6F6;  the 46 and 59 also has triode ratings.

One tube Ive not remembered seeing listed for audio service is the 837.
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« Reply #13 on: March 26, 2012, 08:03:25 PM »

I started receiving 6N7's from eBay purchases. It is apparently a VT-96. One of the tubes is an Arcturus brand. I didn't know they made those.

added the 2A5 and 42.
I'll work on the chart some more later.

Maybe I should include single ended triode connections, special class B connections (g1 to g2) and small zero bias triodes. Each implementation should have a separate tab.


* the_triode_connection_2012_0326_1927cst .xls (18 KB - downloaded 267 times.)
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« Reply #14 on: March 26, 2012, 08:53:37 PM »

Here are a whole bunch of distortion specs for the 2A5/42/6F6

http://tubedata.tigahost.com/tubedata/sheets/127/2/2A5.pdf

The only thing I could find on the 807 is 2% in AB1 under Philips
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« Reply #15 on: March 26, 2012, 09:36:20 PM »

The 6N7 noise limiter in the post war SP-400 Superpro puts both sections in parallel and is basically a fast low impedance shunt audio switch. Cleverness in the form of emission starvation is used by means of a series resistor off the 6VAC in series with the fils to set the tube to a point where it responds nicely to impulse noise. But this almost always goes out of whack due to capacitor, tube and resistor aging basically guaranteeing that the circuit does not work very well in any living example!
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« Reply #16 on: March 26, 2012, 11:45:39 PM »

Patrick,

I don't really know how to analyze an audio chain properly. Audio circuits have always been more difficult than RF circuits for me to understand. I find a schematic that I like, I build it, and hopefully it works. If it works well, that's icing on the cake. In this particular case, I noticed that everyone raved about the audio from my Globe Champ 175, so when I needed to build a speech amp, I just duplicated the GC175 circuit and again got a good result. Interestingly, the modulator in the Globe champ 175 also uses triode connected tetrodes. Specifically, it uses push pull parallel 6L6s in triode configuration to modulate a single 812. That was a great little (big) rig. There was one at the W0ZUS auction in like new condition that sold for $160.00. That was a good deal.

In the first speech amp that I built, the GC175 clone, I believe I had about 300 volts on the plates of the 6F6's. Screen and plate tied directly together. Cathode bias with 700 ohm/15 watt resistor (one resistor for both tubes). These were metal envelope tubes and I remember they ran very hot. Perhaps they would have been happier with 250 volts on the plates. On the other hand, the tube data sheet that I have shows that you should run the plates at 350 volts for PP class AB2 service. Using -38V grid bias and 350V on the plates, power output is 13 watts, probably somewhat less with cathode bias I would assume. Plate to plate load resistance is 6000 ohms at those values. I would not ever hesitate to use triode connected 6F6's as driver tubes, the experience I had with them was very good. They will probably always be easier and cheaper to get compared to the 2A3/6A3/6B4 family of tubes.

I had forgotten this until today, but the speech amplifier/driver that I built for my homebrew rig is actually out of the Radio Handbook 15th edition (1959). The circuit is shown on page 652 and is practically identical to the GC175 circuit except that it employs 6B4 triodes instead of triode connected 6F6's. I substituted 6A3's in my unit since that is what I had on hand, but it is the same tube except for the base. Again, I believe that I have about 300 volts on the plates of the 6A3's. I am using cathode bias again, this time with a variable WW 1000 ohm resistor. The 6A3 plate circuit is metered, and the bias resistor is set for 80 mA of static current. Power output should be about 10 watts, and plate to plate load resistance is about 5000 ohms.

I have always wondered what prompted the engineers at WRL to use the triode 6F6 as a driver. I would guess that it had something to do with costs and mass production, but in 1948, I would think that 6A3's, 6B4's and 2A3's were all plentiful and cheap. Not true today of course. By the way, 6A3's are still commonly seen on Ebay. New ones typically go for $40.00-50.00. If you watch close, you can sometimes get a better deal. I got a group of four used 6A3's for $30.00 about a year ago. They are definitely out there if you are willing to pay the price. If you know how to do hi fi tube audio, they are probably worth it. For the rest of us who don't know how to use oxygen free copper wire properly, it's just for braggin' rights. Wink

Ron
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« Reply #17 on: March 27, 2012, 01:04:07 AM »

Thanks Ron for that info. You did right to not re-invent the wheel, and that info's great and I'll look it up and keep it. For my part RF has always been the challenge, and even some audio stuff is tricky. I don't design tube audio as a profession, just have repaired many tube amplifiers over the years and built a lot of them.

The reason for all the interest in triode connecting high-grid-count tubes was initially for possible replacements for tubes like the 6N7 that I thought were scarce and costly (but it seems they are not, today). Nonetheless it's worth collecting as many examples as possible of triode connection operating conditions for reference. I guess the specs for triode connecting larger tubes like the 6CA7 or 6L6 have always been around but until I got started in RF I didn't realize the value of triodes for drivers.

]]]]

About those distortion figures, those are without feedback, so a great deal of improvement is possible. 2% isn't bad but vintage "storebought" ham gear seems to fall short of the mark.
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« Reply #18 on: March 27, 2012, 08:54:25 AM »

Patrick,
            Reading your post the other day stuck with me as I attended an AWA swapmeet on Saturday. Just before leaving, I thought I'd browse through the tailgate section and ran across a fellow selling tubes for a buck apiece. Remembering the tube number, I picked up half a dozen NIB GE 6N7's. A PW project might be in order now.

Phil 
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« Reply #19 on: March 27, 2012, 09:24:06 AM »

Just FWIW, The older Super Pros (SP-200, 210, Bc-779, and 1004) that everyone loved the audio from used a triode connected 6F6 as the driver and triode connected 6F6s for the outpoots.

I think the true pentodes lend themself much better to Class-A triode cnnected applications then the beam tetrodes. I'm sure there is someone out there that knows more than me that can explain why. There are also triode connected specs published for all or most of the "beam power amp" tubes like 6V6, 6L6, etc
But the triode connected 6F6, 42, and a few other pentodes were more popular.

Just another interesting sub note. Class-A and other non grid current classes of operation had the screen and supressor tied to the plate. Class-B and other classes that sucked up a little grid current always had all of the grids tied together as one. I guess having the grids tied to the plate gives a higher Mu which is more suitable for class-A operation.
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« Reply #20 on: March 27, 2012, 04:04:54 PM »

The Super Pro triode phase started with PP 42's in the SP-10, 6F6's from the SP-100 to early SP-400 and 6V6's in the later 400's.

Carl
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« Reply #21 on: March 27, 2012, 07:40:30 PM »

I made a 6N7 Jones oscillator transmitter a couple of years ago and it is a slick little unit.

Mike WU2D


* 6N7JonesPIC.jpg (391.83 KB, 2055x1524 - viewed 1315 times.)

* 6N7JonesType.jpg (68.04 KB, 1154x608 - viewed 4204 times.)
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« Reply #22 on: March 27, 2012, 08:21:50 PM »

I've done all I really want to for now. I did't fill out the sheet too well for the special class B triode connection. There is little info for it except in the modulator sections of the ham mags. Nothing a ruler and some plate curves wouldn't fix though.

* the_triode_connection_2012_0327_1909cst.xls (55 KB - downloaded 301 times.)
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« Reply #23 on: March 27, 2012, 08:28:55 PM »

Patrick,
            Reading your post the other day stuck with me as I attended an AWA swapmeet on Saturday. Just before leaving, I thought I'd browse through the tailgate section and ran across a fellow selling tubes for a buck apiece. Remembering the tube number, I picked up half a dozen NIB GE 6N7's. A PW project might be in order now.

Phil 

Good! $1 is cheap for that much tube.


I made a 6N7 Jones oscillator transmitter a couple of years ago and it is a slick little unit.

Mike WU2D

Are the gimmicks under the socket or are they the capacitances of the tube and wiring?
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« Reply #24 on: March 27, 2012, 08:39:35 PM »

I made a 6N7 Jones oscillator transmitter a couple of years ago and it is a slick little unit.

Mike WU2D

That is the very circuit I was referring to in a previous comment.

Great minds having one thought..... Wink

73DG
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