How to bias directly heated filament -- 6418 subminiature tube

[spacelabstudio]

So today I'm playing with some 6418 tubes that I scored on Ebay.  I'm intrigued by the idea of a battery operated tube pedal of some sort.  Here's the datasheet:

http://www.datasheetarchive.com/6418%20tube-datasheet.html

One new thing I haven't dealt with before is the directly heated cathode.  I know that the current through the tube is a function of the potential difference between the grid and the cathode.  But the cathode is a filament with a voltage across it.  In this, case about 1.25V.  So for determining bias, what figure do I use for the voltage of the cathode?  Does the tube basically behave as though the cathode is at the average of whatever it is that's across it?  .6125 V in this case?  Or what?  Does it matter which direction I run the filament current in?  What's the standard procedure for directly heated filaments?

Thanks!
Chris

[sluckey]

Just heat the filaments with a floating 1.25V source. IOW, don't reference the filament supply to anything that the tube circuit will be referenced to. Then just use the tube as a standard tube.

It's kinda like the 5 volt rectifier tubes. The 5 volt filament floats and even though the cathode is tied to the heater AND has B+ on it, it doesn't matter.

[spacelabstudio]

Cool, that works.  I've got the filament running off of a C battery just strapped across pins 3 and 5.  Otherwise I have it hooked up like your typical common cathode triode gain stage, with cathode bias.  Using two 9V batteries stacked for an 18V plate supply. 

The guitar signal actually sounds pretty good.  Not a ton of gain, but it sounds nice.  Only problem is this thing wants to oscillate up somewhere around 10k or so (imprecise "by ear" estimate).  I've got five of these tubes and they all do it.  I've found that by tapping on the tube I can cause it to get louder and even go up and down in pitch a little bit.  The only tube that doesn't do it reliably is the one that's audibly microphonic.  If I tap it, it will oscillate for a second or two and then die down.  My only guess is there's an actual electro-mechanical resonance setting up between two of the elements.  I don't know.  Just start sticking capacitors across terminal pairs and see what happens?

 

[PRR]

> a floating 1.25V source. IOW, don't reference the filament supply to anything that the tube circuit will be referenced to.

No; the filament IS the cathode and must return to B-. Otherwise no plate-cathode current.

> Does the tube basically behave as though the cathode is at the average

Yes; but for the battery series the published bias is referenced to a specified end of the filament.

Problem: the best datasheet for 6418 (from Frank's) has a typo.... BOTH ends of the filament are marked "F+,G3".

Look at 6419. Lead 3 (center) is F-, lead 5 is F+.

Ground lead 3. Apply +1.2V to pin 5. Return G1 to ground through a large resistor. The average bias is -0.6V. Don't worry that this is not the data-sheet bias, fiddle G2 and plate resistors to get into a linear realm.

[spacelabstudio]

> a floating 1.25V source. IOW, don't reference the filament supply to anything that the tube circuit will be referenced to.

No; the filament IS the cathode and must return to B-. Otherwise no plate-cathode current.

> Does the tube basically behave as though the cathode is at the average

Yes; but for the battery series the published bias is referenced to a specified end of the filament.

Problem: the best datasheet for 6418 (from Frank's has a typo.... BOTH ends of the filament are marked "F+,G3".

Look at 6419. Lead 3 (center) is F-, lead 5 is F+.

Ground lead 3. Apply +1.2V to pin 5. Return G1 to ground through a large resistor. The average bias is -0.6V. Don't worry that this is not the data-sheet bias, fiddle G2 and plate resistors to get into a linear realm.

Done, thanks.  Actually applying 1.5ish to filament, but seems to like that just fine.  With 18V supply and 100k load resistor I'm getting ~14.25 idle at the plate.  Sounds fine, albeit, again, not a lot of gain.  Still running as triode at this point.  The annoying thing, really, is the ringing.  I've finally figured it's really some form microphonic feedback.  I might need bury this thing in a plastic box with a bunch of cotton balls or something.  Or pot it in wax like they do with guitar pickups.  Ideas welcome.  If the ringing could get got under control, these might be nice for a buffer pedal or something.

Chris

[DummyLoad]

1AD4 may be of more use to you in this application - gm is around 2000umohs vs. 300umohs of the 6418 - 1AD4 has higher B+ rating, is relatively inexpensive & available in quantity. i bought a few a while back to fiddle around with.

fiddling with the 6418 triode load lines: i like 91K ra; 12K rk with 25V B+. vk should be parked around 1.25V under those conditions with about 100uA bias.

http://scottbecker.net/tube/sheets/138/1/1AD4.pdf

--DL