JAN6418 and microphony

[atnaat]

I've read about how hideously microphic these tubes are, If I were to build an Oatley electronics kit (K272) would I be able to get away with the tubes being exposed through the top of the chassis? Obviously I'll have to use some type of damper with them.

My circumstances have changed and for the next couple of months at least all I am going to be able to listen to is my headphones. I need something cheap and super quick to build. I can't go far wrong with this kit can I? Anything other contenders to consider?

[Moksha]

Hi, i have some experience with the Jan6418 tubes - i've designed a few circuits around them now....

Firstly the Oatley kits set the impedance way to low at around 200k - i have tried 2 kits prior to designing my own...

Setting impedance at 1Meg solves the issue of microphony - i currently have my new preamp exposed with tubes just in grommets with none of the high pitched ringing associated with the oatley kits.... with my speakers up quite loud

Also for the tubes to work as per the datasheet impedance needs to be at 1Meg as eg... optimal plate voltage is at around 18-22v (for maximum gain - with 1Meg resistors)... setting impedance at around 200k or lower lowers the maximum gain voltage to around 14 or less - this also produces the unpleasant ringing even at normal listening levels...

I made the mistake of assuming the Oately designs were right and initially desiged a preamp around their circuit - it rang even in a solid metal enclosure...

Detail of a working basic JAN6418 circuit and notes can be found here:

Dystopian Tone - Products (Vacuum Tubes - Valves)

if you navigate around the site you will find complete ready to etch pcb and schematics for various preamps utilising these tubes.

[johnmac0117]

Hi, does anyone know where I can get these Jan6418 tubes?

[Moksha]

The only Australian place i know that sells them is Oatley Electronics in Sydney (just google) - other than that ebay.

[Moksha]

As for the initial question of having the tubes sticking out of the enclosure - change the output impedance resistors to 1Meg... you will have a better chance of not having the ringing..... Also make sure you put on at least 3-4 grommets per tube.... possibly make the hole in your chassis big enough to fit the grommet grooves with your tube firmly fastened to the enclosure... this should minimise the amount of vibrations from your speakers.

[Moksha]

Also flexible rubber feet like those found on turntables on your enclosure should help a little.

[Wavebourn]

This tubes are cheap as dirt and widely available:

1J24B Russian miniature pentode Tubes New Lot 100 - eBay (item 330379930051 end time Apr-16-11 20:54:43 PDT)

Type pentode
Application HF voltage amplification
Cathode type oxide,direct heating
Filament voltage,V 1,2
Filament current,A 0,011-0,015
Anode voltage,V 60
Grid1 voltage,V 0
Grid2 voltage,V 45
Steepness,mA/V 0,48
Reverse grid current,uA 0,1
Microphnic noise,mV 50
Socket type flexible

[Moksha]

I've been buying up a bunch of American and russian subminiatures... they are however higher voltage and i'm working my way up to that slowly... there are a lot of russian (and even american) bulk subminiatures floating around ebay to be had for a pittance... if one is so inclined and has the time to come up with a design no need to spend 12$ a tube (Jan 6418 i have seen between $2-13 in a number of places)..

[Moksha]

here is a link to another thread: Sub-Miniature Tubes - Low Voltage

Lots of info on hundreds of tubes in the link provided,

[johnmac0117]

Thanks guys, lost of useful info in those links.