5840 mini tube -- overdrive project ideas?

[mordechai]

I picked up a 5840 mini tube, and I'm wondering if it can somehow be used in a project like the Valvecaster?  Actually -- can the Valvecaster be adjusted to accommodate the 5840?

[bluebunny]

Do a search for some of Rick's (frequencycentral) projects.  He's done a simple 5840 booster.  "Le Craquement Thermionique" and "Promiscuous Girlfriend" are two others - both a little more complex.  I've built the girlfriend and it's pretty neat.

[danielzink]

5840 Booster

It's a tube booster that Rick Holt (Frequency Central) designed based around the 5840 sub-mini.

Should be pretty easy to fit into a small enclosure, the layout is 17 holes wide but nothing is stopping you stretching it out if you are using a large heat sink on the LM317T.

If you want to, you could omit the LM317T altogether and just use a 3-5 watt resistor ~ 19ohms to power the heater.

If you do this, remove the following : R2, R3 VR1 (trimmer) and of course the LM317T.

Just connect a 19ohm resistor between 12v and pin 3 on the tube.

If you go with voltage regulator, the trimmer is to dial in the heater voltage (6.3v).

[mordechai]

Thanks Daniel!  I've never done a project that was powered by anything other than a 9V supply.  Is the 5840 unable to run off of 9V?  To get this to run at 12v do I need a charge pump?

If it can run off 9V, it would just have a little less output/drive, correct?  If that is the case, should I still aim for 6.3V going to pin 3 of the tube?

Finally, you mentioned that the layout is 17 holes wide; the image you posted shows 15 -- just a typo, or were you referring to a different layout?

[Gus]

I would save the 5840 for something like a tube condenser microphone.
Tubes like this are not made any more and seems to me to be a waste for a guitar overdrive.
You need to be careful with the leads leave a least 2" and do not bend them at the tube base you can break the vacuum

I would use a currently built tube for guitar overdrive

[amptramp]

The 5840 is not a bad tube for this kind of use if you don't mind its rarity.  It has a 6.3 volt 0.15 amp heater but it has 5000 µmhos transconductance when run at 100 volts on plate and screen with a cathode resistor of 165 ohms (a pair of 330 ohm resistors in parallel would be exact).  It takes 7.5 mA on the plate and 2.4 mA on the screen.  For an overdrive, you can run lower voltages and bias and get the stage to do interesting things.  But it will not run directly from a 9 volt source.  I would get a pair of 6 VAC wall warts - that's right, AC output - and use one to power the unit and another one connected backwards in the unit to turn 6 VAC into 120 VAC which you can rectify and regulate down to a usable (and possibly variable) voltage.

[mordechai]

Thank you all for these insights. For my needs, then, maybe it would be better to build up something like the Matsumin Valvecaster, since it looks like that is more amenable to an overdrive/boost sort of application.  One of the reasons I was interested in the 5840 was because it looks like it doesn't heat up too much when in use...to be honest, I'm a little intimidated by the prospect of something getting really hot inside a pedal enclosure with all those other sensitive parts exposed.  Would the 12AU7 tube in the Valvecaster operating at 9V produce similar heat as the 5840, or does it get hotter than that? 

(I still do want to experiment with the 5840, but maybe not for something like this...)

[amptramp]

The 12AU7 has twice the heater dissipation of a 5840 but it is known to run at low plate voltages, so you may save some power there.  You can connect the heaters either in series (12.6 v 0.15 A) or parallel (6.3 V 0.3A).  If low dissipation is your goal, there are some tubes with directly-heated filaments that offer operation at 1.5 volts and currents ranging from 0.02A to 0.1A and most of these operate at low B+ voltages.  Tubes intended for battery-portable radios or hearing aids have low dissipation because they cannot afford the battery drain.