Schem check so I don't fry? RE: EM81 tube (and some ?'s)

[mth5044]

Hi, my name is Matthew and I am a tube and power noob 

I've become mesmerized by these 'eye' tubes, so I wanted to start off with a simple little project to get one working, which would be a guitar level indicator. The EM81 (I forget the American code name) is the tube used for the level indicator in the binson echorec and has a twin EM80, which takes the same voltage and has the same pinout, but an almost inverted eye display.

Anyway, here is the datasheets I used:
6AL5: http://pdf1.alldatasheet.com/datasheet-pdf/view/232958/ETC/6AL5.html (embeded in page)
EM81: http://www.datasheetarchive.com/pdf-datasheets/Datasheets-23/DSA-445515.pdf (downloaded)

Also, here is a layout of the pins of the EM81 and connections:



I'm a bit confused about what the 0.3 and the 2 are in the boxes off of pins 5 and 9. Maybe capacitors? I'm also a bit confused about what makes eye display sweep across it's threshold. I think, from the datasheet, it's the variation of -1V to -10.5V? Anyway, here is the schematic I drew up (sans whatever those 0.3 and 2 are in the layout).



So besides comments about that, I have two more questions.

1) Where can I possibly get -1 to -10.5 from a guitar signal? I thought about the LM386 as it was used to drive LED's, but I can't put that into application with my current knowledge.

2) Is there even a transformer that exists to give me those power values? I just made it to the ideal voltages, not to a current production transformer.

I appreciate any comments. I also hope this fits into the 'building your own stompbox' portion of the forum, as it is going to be a subsection of a larger effect.

Thanks again for any info. I really appreciate it.

Cheers,
Matthew

[mth5044]

Woah. Big schematic is big. Let me see what I can do about that 

EDIT: There we go

[idiot savant]

Also, here is a layout of the pins of the EM81 and connections:



I'm a bit confused about what the 0.3 and the 2 are in the boxes off of pins 5 and 9.

1) Where can I possibly get -1 to -10.5 from a guitar signal? I thought about the LM386 as it was used to drive LED's, but I can't put that into application with my current knowledge.

2) Is there even a transformer that exists to give me those power values? I just made it to the ideal voltages, not to a current production transformer.

I appreciate any comments. I also hope this fits into the 'building your own stompbox' portion of the forum, as it is going to be a subsection of a larger effect.

Thanks again for any info. I really appreciate it.

Cheers,
Matthew

the boxes  showing 0.3 and 2 represent the current.

looking at your schematic, you --->need<--- a resistor from pin 9 to V+.

I have used these(not this particular one) "eye" tubes before, they're very fun.

for ideas on how to use these tubes, look @

http://harmonicappliances.com/h86/highway-86-b1.pdf
(bottom right hand corner)

and my own little signal tracer conversion:

http://i4.photobucket.com/albums/y130/idiot5avant/Allied_Radio_HzOG.jpg?t=1251946225

you can dig up schematics for old tube signal tracers in general to find more info.


fun fact: they can be biased into linear operation for use as amplifiers too.

[blackcorvo]

I know the topic is kinda old, but THIS interested me:

fun fact: they can be biased into linear operation for use as amplifiers too.

So, how can that be done?

just asking because it would be cool to make an amp (or pre-amp) out of this type of tubes, since you can find some for pretty cheap!

[idiot savant]

So, how can that be done?

just asking because it would be cool to make an amp (or pre-amp) out of this type of tubes, since you can find some for pretty cheap!

well they do have an internal triode in them. have a look here for an idea:

http://greygum.net/sbench/sbench101/

(in the preamplifier section)

http://greygum.net/sbench/sbench101/Preamps/1629amp.gif

[metallo]

1) Where can I possibly get -1 to -10.5 from a guitar signal?

The 0...-10.5V in the example schematic are referenced to ground.

The tube is ignorant and compares its entire world to its own cathode, so if you reference the cathode to something else than ground, your grid should stay 0...-10.5V with reference to 'something else'. +5V on the cathode with a 5V peak/absolute amplitude would be nice place to start.

Or you can just ignore half the signal and tie the cathode to ground, like in the Hwy86 and Allied Radio examples from idiot savant.

[amptramp]

The 6AL5 is rated for a maximum RMS voltage per plate of 117 volts.  The 250 volt connection may arc and fry the transformer.  You will also need a filter capacitor of at least 10 uF for the 250 VDC or you will get fuzzy edges on the eye tube as a result of it operating from zero to 250 volts every cycle.  I would use a silicon diode bridge for the rectifier to get full-wave rectification.  Also, you want a resistor to ground from the grid of the eye tube to make sure you are able to handle AC-coupled signals and signals with no ground reference.  This can be 1 megohm.

[PRR]

I agree the 6AL5 is dubious for both current and voltage.

And you sure want a filter cap after the rectifier.

Pin 9 does not need a resistor for supply up to 250V. (Pin 7 does, else there's nothing to the control rod.)

You don't need 250VDC. That was commonly available in the radios this tube was designed for, but they will run with less. A bit less bright, somewhat longer life, and a useful reduction in control voltage needed.

Take a 120V secondary through one 1N4007 to a 20uFd 200V cap. This will give you 160VDC. At this low current, it is practical to half-wave rectifiy and over-size the cap. Total current drain is under 2mA, with a good rectifier we "need" 2uFd, a 20uFd is about as small as is worth buying, and one-diode saves a connection.

You can drive control grid pin 1 directly from your loudspeaker output. Yes, it only reads half the wave and is "too dynamic", fuzzy, but so darn simple. However it would be wise to add 10K between input jack and anything else so faults in your wiring don't short your power amp. 1Meg pin 1 to ground will keep it stable when nothing is fed in. And amplifiers over 10W will slam the eye, which does no harm but isn't real interesting to look at. Wire a 1Meg "volume control" between your 10K protection resistor, pin 1, and ground/cathode.

For "nice" display you want a signal rectifier and filter, able to pump -8V. The simple ways would want a >16V supply. Bah, you got the stuff, use a tube. The diode-triode used in ALL AM radios (many different tube-types) can be used this way: