Reading tube pin datasheets

This is a beginners question.   

I understand the concept behind the simple testing of tubes for connectivity with a multimeter, but I don't understand how to check my results to a datasheet.  For example, with the EL84 I understand that pins 4 and 5, the filament pins, should have connectivity and can see that they are shown as connected in the pin diagram.  I know from experience that pins 1 and 2 also have connectivity.  How is that connectivity shown in a tube pin diagram?  I feel like I'm missing something obvious, but I haven't been able to find an answer to this basic question.  

In the event I still don't understand, what pins, if any,  should have connectivity, other than pins 4 and 5 the filament pins, on an EF86 tube?  I ask as that is in an amp I am working on at the moment and don't have any EF86 tubes to compare it to.

Thanks for any assistance.

Mark

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Hello Mark.

Let's first answer your question concerning the EL84 pinout in general terms. Besides the intentional electrode connections, sometimes the pinout diagrams will assign a pin as either IC or NC, the first meaning Internal Connection and the second meaning No Connection. If a pin is marked as IC, then it should NOT be used for external connection at the tube/valve base. Conversely, if it is marked as NC, that pin assignment is often used as an under chassis connection point at the tube base. Think of it just as an extra tag to use there. Now to your question specifically. You say, "I know from experience that pins 1 and 2 also have connectivity. How is that connectivity shown in a tube pin diagram?" My answer would be that Pin 1 is assigned the code IC, so it is not intended to be used even though it is connected internally to Grid 1 (Pin2), and so it is not shown as connected. Whenever you can, my advice is to use a reputable manufacturer's data sheet rather than just trust any internet pinout diagram. This is especially true for tubes manufactured in recent years, like those intended for audio aficionados, where I have seen one data sheet which shows Pin 1 as NC, which may indeed be the case.

Now to the EF86, which was designed as a high gain low noise AF pentode. In addition to pins 4 and 5 showing heater connectivity, pins 2 and 7 will also show connectivity because they are connected to an internal screen. These pins are typically connected to chassis ground at the input of the amplifier. If you search for the Mullard 3-3 Amplifier on the web, you will see how these pins are connected. Having used an EF86 in a record player amplifier many years ago, I found out to my cost that it required an acoustically coupled tube base because microphony was a noticeable problem with such a high gain stage when the loudspeaker was in the same enclosure.

I'm sure there are some tube experts here at RMorg who can give you even better information than the above.

Kind regards,

Stan Roberts

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Thanks, Stan.  I appreciate the explanation for the IC and NC.   

My lack of experience is showing and maybe I'm trying to simplify it too much.  I pulled this image from the RMorg site on the EL84 and this is the type of datasheet that I have been referencing.  Is there anything on this sheet that would show me that pin 1 and pin 2 are connected?  If the information isn't on this sheet, what document would I look for to get that information?  Again, I am simply connecting my multimeter leads to pins and checking for connectivity as a first step in determining if a tube has an obvious flaw.

I appreciate your guidance.  

Mark

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Mark,

There is nothing on that sheet to indicate that Pins 1 and 2 are connected internally. That’s normal and that’s just how the convention has been interpreted over the years when it comes to internal connections. I don’t think you’ll find any additional information that discusses internal connections.

I grew up at the very end of the vacuum tube era, so I yield to the experts here when it comes to the vagaries of tube design and pinouts. Perhaps unused pins were used to aid the mechanical and internal construction, support and rigidity as dimensional tolerances were very important in the control of electrical parameters. This arose because many of the B9A tubes were merely ‘shrunken’ versions of earlier octal and side contact types and so required improved physical and dimensional tolerances for improved efficiency and frequency response. In this respect, the EL84 seems to be an overgrown version of the older Rimlock EL41, which is itself a derivative of the earlier EL3. For years, we’ve tended to think of die-shrinking as a normal technique in the advancement of semiconductor design, but perhaps we overlook that vacuum tube technology went through similar iterations in design a couple of generations ago that eventually resulted in Nuvistors (6CW4/7586) and sub-miniature tubes such as the 1AD4/DF62.

Enjoy the vicissitudes of an earlier technology!

Stan Roberts

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Thank you, Stan, that is a big help and answers my primary question.  I was driving myself crazy looking at datasheets to figure out pin connectivity for my basic tube test.

On the EF86, you stated that pins 2 and 7 are connected by an internal screen.  The tube that I have shows pins 2, 3 and 7 are connected.  Does that make sense to you?  I don't have another tube to compare it to.

Thanks,

Mark

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Mark,

That doesn't sound right to me that the cathode (Pin 3) is internally connected to the screen (Pins 2 and 7). I checked a couple of tubes that I have here, both are Mullard, one an EF86 and the other the higher grade version M8195. Pin 3 does not show a connection on either sample, so I can only conclude that yours has an internal short. It still may be usable as long as you let the screen float at the tube socket, but that may sacrifice some of its low noise/hum quality. Fortunately, you can still find these tubes at low cost on eBay.

Kind regards,

Stan

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Thanks, Stan.  That's a huge help and I appreciate all your other information.  All very helpful and much appreciated.

Mark

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Hello Stan:

I'm working my way through this and thought of your comment about the acoustically coupled tube base.  I am not familar with those.  Do you have any information on those or can you direct me to a source of information?

Thanks,

Mark

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Hello again Mark.

These tube bases are commonly known as anti-vibration or vibration-resistant bases and are still available on the web. I also found a different type, which was a thick rubber sleeve on a 6267 tube (a close equivalent to an EF86) in an old Akai M7 tape recorder. I assume it was intended to dampen out any motor vibration. Examples are shown below.

You may also be able to fashion a homebrew version by using rubber grommets as shock mounts in the chassis in a similar way to how tuning capacitors were frequently chassis mounted in tube radios.

Does that help?

Kind regards.

• Anti-vibration base (98 KB)
• Rubber_Sleeve (20 KB)

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Yes, thanks, that does help.  I'm in the process of troubleshooting this little amp and it's exhibiting problems that I haven't encountered before (I'm new to the hobby).  I'm not sure the EF86 is the issue, but your suggestions may be part of the answer.  

Thank you,

Mark

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