applications for 6418 pentode

[nisios]

I have  5 or 6 6418 pentodes laying arround.....does anyone know any application for those tubes?

they are low voltage....would work nicely on 16 volts i think.

http://datasheets.electron-tube.net/sheets/127/6/6418.pdf

This is the datasheet.....i really dont even undestand this datasheet.....so im unable to do anything with this by myself.

Where is the cathode for instance?

[Threefish]

I built a LM1875 practice amp a while back, and I tried to build a preamp around a
kit that had a 6418 valve in it. Seemed like a good idea at the time  , but I nearly went insane trying to get it to work how I wanted it to. Heres a link to the kit:

http://secure.oatleyelectronics.com//product_info.php?products_id=256&osCsid=fd0999d9c2808d901bad17679798be03

Maybe I was trying to use the wrong ingredients (and it was very early in my pedal/amp building education), but I pretty much gave up on it. It worked, but had nowhere near enough gain for a preamp (it was only around 8x – your average 12AX7 is about 100x I think). It has a power supply I never really worked out as well. I have considered going back to it recently, as I recall that a boost in front of it gave you a pretty good distortion, though lacking in treble. Probably usable for a pedal if you didn't expect a volume boost.

I have the kit schem here somewhere. I'll post it up on my putfile page when I get home. It would be interesting to get some opinions from the valve gurus on the forum about the way it's been used in this kit. I suspect that it could work a lot better than it has for me.

[Ronsonic]

For heater efficiency it is a direct heated tube. Meaning that the heater IS the cathode. Usually you'll use a pair of resistors to create a center point - or use a center tap on the heater transformer - or just use one leg of the heater and declare it the cathode all depending on the application.

Got anything in mind you want to build? Walkie Talkie is what these came from. I've got a number of these type tubes myself, but haven't felt the need to build anything interesting with them yet.

I'd be happy to try and work out a design with them if you'd like. 

Ron

[Arfman]

Interesting...I've never used this tube. However, the filament and the cathode are the same. It says 1.25 volts AC or DC, but I've got to think that running AC would not be a particularly good idea...note the similarity between that 1.25 filament voltage and the typical bias voltage on most triodes (12AX7, for example)...So I guess instead of establishing a bias point with a resistor to ground you apply a positive voltage. I also have to think that pin 3 is tied to ground since the suppressor grid is also tied to that pin...

I would have guessed that the grid would have been at ground potential but the datasheet lists typical as -.8 to -1.2volts.

Yes, 16 volts would work, 22 would be better. 30 better yet (more on this below). A 16 volt RadioShack transformer into a bridge rectifier, filtered (might even think about using a voltage regulator which would really give you clean B+) would work nicely.

Biggest problem I see is dealing with a load resistance of 100k...I've just never looked into it, perhaps it's not that difficult...but it would be real tempting to use 4 or more in an AB setup, or just multiple tubes in a class A SE, just to get that down. This is also a good reason to go with the higher plate voltage...the load  resistance goes down as well...note that te 15 volts is 150K!! Doesn't RadioShack carry a 1K:8 small audio transformer? That could work, just hook the primary of one to the secondary of the other...then you'd get 8->1K->125K...I have no idea what it would sound like, but now you could use one tube, or two in AB if your adventurous, you'd just have to decouple the plate voltage from the transformer...no big deal..

The only schematics I found were for a tube mic...makes sense to use this tube for that.

If you decide to take this on keep posting...i'd be willing to help.

[Threefish]

I discovered the gallery.... (duh)

Here's that schematic I mentioned above, if it's any interest to you. Apologies for it being upside-down.
http://www.aronnelson.com/DIYFiles/up/6418_preamp.pdf
(this used to be available on the website link I posted above, free to anyone, so I don't see any copywrite or intellectual property probs Aron)

Now that you've turned it right side up:
The upper part of the schematic, 4/5 of the way down the page on the left, is the preamp valve + components.
(The bottom bit is the power supply schem, and probably worth a thread all it's own, somewhere other than here. It's way too complex for me.)

My valve knowledge is a bit shakey, but here goes:
Note the 5v going to the cathode and filament via the 270r resistor.
36v to the anode via a 33k resistor. Would upping that 33k get you more gain?
Load resistor? Is that the function the 100k pot on the output is serving?
Grid 2 tied to the anode.
22n at the input. Could probably be a larger value.

Anyway, I'm interested to hear what everyone thinks about the component values etc. in reference to what you think would work. Just after the 22n input cap are two links - for higher or lower sensitivity (the top and bottom respectively). I used the A link, finding the lower B link got you almost no signal at all. 

Nisios, if I'm hijacking your thread please say so and I'll shut up. I thought this schem could be a good (or bad) starting point, since while it's not great in the form as presented here, at least it passes a signal....

[Threefish]

Oops -
from the  PDF - "filament voltage 1.3v" after that 270r resistor...

[widdly]

Not a pedal but you did say you had 4 or 5 of them.  You can make a tube condenser microphone very cheaply.

This schematic uses a Panasonic WM-61A capsule which are meant to sound ok..

http://i13.photobucket.com/albums/a300/mr_sevs/MSH-4.gif

It's a copy of this mic...

http://www.naiant.com/msh4specification.html

[nisios]

Hi everyone.


Threefish:
Hijack at will!!!  Thanks for that schematic seems interesting.


Ronsonic:
What i was thinking was just to make a simple booster....didnt need to boost too much. Instead of a npn simple booster i think a tube one whould be a great thing.
And what about overdriving one of this tubes with a transistor circuit before it? just to see if it clips good.
Im a complete newbie in what comes to tube design. (and other designs too.......only learned how to design properly a transistor stage a couple of weeks ago)
I am starting to read radiotron bible but i can tell you that things are getting pretty ugly at this point.

Any one knows a simple to understand, hands on design tube literature?


Arfman:
As i sayd earlyer im not good on this but heres my thought anyway: wouldnt a cathode follower do the trick of lowering the output impedance instead of a transformer? But.....is it even possible to implement a cathode follower with this direct heated thing?
The thing is that i dont have access to audio transformers here.

Widdly:
I have done that mic and i dont like it.
First of all i dont think phantom power is intended for this kind of power consumption.
In one mixer the mic was very low ( the plate voltage got really low ) and in other it worked nice.....so its not a stable design.....
And the sound is just as good as not having any tube at all IMO. Because the mic as an internal fet thats the one doing all the amplifing work i think.

[nisios]

oh and just a final thought.

I was aiming at a 18 volt thing because it can be powered from 2 9v's wich would make it fit nicely in the stompbox realm. Anything beyond would imply the use of transformers wich would IMO make this design less apelyng than any other great designs out there that use better tubes.

[Arfman]

oh and just a final thought.

I was aiming at a 18 volt thing because it can be powered from 2 9v's wich would make it fit nicely in the stompbox realm. Anything beyond would imply the use of transformers wich would IMO make this design less apelyng than any other great designs out there that use better tubes.

Ah...3 9 volt batteries = 27 volts if your box will accomodate it...

Are you doing a stompbox or an amp? Big diff on the impedance...

You are correct re: the cathode follower if you don't mind using one tube for no gain. The purpose of the cathode follower is not to impact the resistance of the next circuit, usually a tone stack. That said, you can factor in the impedence of the output of a gain stage and adjust the slope resistor accordingly.

This would probably easily make a headphone amp for guitar...but again, I really don't know much about this tube so don't know how good it would sound...being that it's used for condensor mics clearly it's good for audio purposes...

If this is a stompbox you shouldn't need a transformer. If you use the higher voltage and are going to use an volume control on the output just slap in a 250k. If you are going with the 15 volts I'd use 100k...

[Minion]

I am designing a project at the moment that uses a 6418 tube....i uses a single OPA134 opamp in non inverting config and then it goes into the 6418 tube and then to the output...
it also has a couple clipping diodes after the OPA134 that can be enabled or disabled....It uses a LM317 Regulator to get the 1.5v fillament Voltage and the Plate is run off of the same +15v supply the OPA134 runs off of.....

I have allready designed the curcuit and the PCB and just have to etch and test and debug the curcuit....

i"ll let you know the results I get......

Cheers

[nisios]

I remembered.....
I think i could fit a charge pump ic and make it 30 volts.
Ill try that and see what i get.

[chris2002rocklin]

Has anyone worked on these 6418 tubes for audio electric guitar applications?

[iccaros]

here is another to get ideals from
http://diyaudioprojects.com/Tubes/6418-Tube-Phono-Preamp-Kit/

[amptramp]

I couldn't open the link from the first post, but another version of the tube data is here:

http://www.mif.pg.gda.pl/homepages/frank/sheets/127/6/6418.pdf

The file:

http://www.aronnelson.com/DIYFiles/up/6418_preamp.pdf

shows the tube triode-connected, presumably to get a lower output impedance.  All the tube specs are for the pentode connection, so you are getting into unknown territory here.  The tube gain is low, so even in the pentode connection, a high plate resistor is necessary to get reasonable gain.  Minion has mentioned using it in the middle of a design with op amps buffering the input and output and that may be the best way to use it - the output impedance is going to be fairly high, not good unless you are feeding a wah or a fuzz face.  It should work as a microphone amplifier and may work as a buffer, but this tube was optimized for low filament and plate current drain for portable applications - not necessarily important for a stompbox design unless you are using battery power.

[PRR]

> another version of the tube data is here:
> All the tube specs are for the pentode connection, so you are getting into unknown territory here.

Triode curves are on page 3 of the sheet you linked. At likely op-points Rp is around 50K and Mu is near 8.

Assuming filament-bias at -0.625V grid-cathode, 24V supply, 100K load resistor, plate will sit near +13V, gain is about 1.2Vpp in for 7Vpp out or 6.

(The anonymous hifi preamp plan with +36V supply takes 1V (or 10V) input for 7V output with fairly low THD (~~2%).)

1.2Vpp or 0.4Vrms input overload means it will not overload direct from guitar. You could try lowering B+ to 2V-3V... that would get input overload in the dozens-mV range, and also get the output clipping down from an amp-burning 7V down to "normal" zone.

B+ power consumption is so very low that a 9V batt will run for a week straight, so don't fool with elaborate power schemes, use a batt until you find it is (or isn't) your cuppa tea.

The filament battery should be a small "1.5V" cell. It will drop to 1.4V quickly, then fade to 0.9V as the hearing-aid craps-out. One D-cell will run 2 months straight, an AA or AAA cell is appropriate. Switch the filament power off, there will be no drain on the B+ power.

FWIW, that sheet has a typo. Filament leads have polarity; not to get hot but because the "natural" grid-cathode bias is also half of the filament voltage. You want most of the cathode (heater) +positive+ of grid. So "Fil-" goes to ground and "Fil+" goes to +1.25V. Tungsol's sheet-writer screwed-up and wrote "F+" on both F leads; Vademecum says the _center_ lead is "-F". OK, except many similar tubes show "+F" in the center.

I don't think it would be a terrific mike-amp, except that it would be possible (with heavy thinking) to buzz-down Phantom Power to run the tube. Still, 300uMho promises such a high hiss-level that only the largest condenser-heads could overwhelm it with their own self-hiss, and 12% THD at +3dBM does not lead to "low" distortion at the -30dbm to -10dbm peak levels of huge condenser heads. I believe there _is_ a commercial mike using this tube, FWTW.