Sunday, 24 June 2012

An All-Tube Stereo Level indicator


Add some tube nostalgia to your modern stereo

Every tube loving electronics enthusiast has some tubes in his/her collection that are designed for such specific tasks, that they are very difficult to use for anything else. One of these difficult tubes is the EABC80 (6AK8). They are found in many older tube radios, and contain a triode and three diodes, two of them connected internally to the cathode of the triode. This makes it very hard to use them for anything other than what they were originally designed for.
Another tube that almost everybody has in their collection is the EM84 (6FG6) tuning indicator. Many people agree that these tubes were the eye-catcher of many tube radios. However, today almost no one uses them anymore. Well, things are about to change! With a few hours of work you can bring those nice indicator tubes back into your living room, and as an added bonus the design uses two EABC80 tubes and a tube rectifier.

Schematic diagram


 As you can see in the schematic diagram, the triode section of the EACB80 is used to amplify the incoming audio signal. The internal diodes are then used to create a negative voltage, which is fed to the input of the EM84. The RC combination connected to the grid of the EM84 provides a peak-hold function, and can of course be altered to suit everyone's individual tastes.

Putting it together


Caution! Tube circuits use lethal voltages! Don't say I didn't warn you!

As can be seen in the pictures, i have made a printed circuit board for my level indicator, but the simple nature and low parts count of this design allows many other ways to build it.  Unfortunately I don't have the time to make and send you a copy of this PCB, but for those of you who want to etch the board themselves, here is the layout in PDF format.
The power transformer I used came from an old Philips radio, but almost any transformer can be used, provided it can supply 6.3V @ 2A and about 500VCT @ 10mA. The 10k resistors in the B+ line can be tweaked to get the correct voltage (should be between 250V and 275V DC). The rectifier tube can be an EZ81 (6CA4) but an EZ80 may be even better, because it has the same size as the rest of the tubes.



Usability

Although this stereo level indicator is a great step forward from the usual LED or LCD devices, both in speed and in looks, it is NOT a 100% accurate measuring device. It can be used to compare one recording to another, but lacks the speed and accuracy to provide a reliable clipping detection. But it sure looks cool!
The bandwidth of the device is 20Hz - 100kHz, which is more than enough to ensure an accurate reading for most purposes, and certainly better than most analog meters. However,  as Jukka Tolonen points out in his excellent article on this subject, a 'perfect' peak detector should have an even greater range for adequate peak detection, and should include a full wave rectifier to deal with both negative and positive peaks in the signal. Jukka 's full article can be found here: Peak Reading Level Meter Using Indicator Tubes.
Unfortunately this goal is difficult to achieve with an all-tube design, and would certainly not be a beginners project. Within its limitations however, this level meter gives a fairly accurate view of the incoming audio signal. Adding a full-wave rectifier is of course possible, and could lead to even better results. Solid state diodes are out of the question of course! Besides, that would make things way too easy :-)
A possible idea would be to use EAA91 (6AL5) diodes... if anyone cares to give this a try please let me know!


(Article originally published in VALVE magazine 2/2003)

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