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JAN6418 and microphony

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atnaat

Member
Joined 2005
#1
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?
 
#2
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.
 
#5
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.
 
#7
#8
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)..
 
#16
I'd try get my hands on even basic greencaps instead of electrolytics as neither side is being fed to ground... regarding the basic circuit... you may want to go as high as 1uf on the initial input cap and 0.47uf on each subsequent cap in the circuit....

I found with my RIAA preamp that 0.1uf simply cut off too much bass response - those cap values 1uf and 0.47uf seem to work really well... this should also be the case for microphones etc...

Sorry for not replying earlier... i didnt get an email showing any responses - i just clicked a referrer link back to this website from my website stat program.
 
#17
Also make sure that the tube is connected properly - where i have the two pins connected together in basic triode mode should be where your tube is marked with a RED dot (Pin 1 & 2 - pin one is marked with red dot) - another thing i will be adding to the circuit description is that its ok to change the initial input impedance to match your device (R1) eg for a MM turntable cartridge R1 can be 47k or line level 200k whatever the output impedance is of the device you are connecting to the circuit - output impedance of the tube part really needs to be above 500k (ideally 1Meg or higher - i have tested with 500k on one of my designs and works well too) and if using multiple tubes - the second/third etc stage input and output impedance should be at about 1MEG
 
#19
Hi Moshka,

Thanks for the great help! I hope to get to it soon and post back.. I made a trip to the shop the other day and got some more ceramic caps (0.1uf), I'll try them out first.
I got the red dot in the right place, I was just wondering a few things:
- is a plate resistor necessary for the circuit to work, I will most likely put a trimpot like you mentioned to play with the voltage/volume.
- what are the benefits to having more than one stage, aside of more volume?

I am planning to make a pre-amp / line buffer.
 
#20
You can supply voltage directly to the plate.... this tube functions differently from other tubes? as there is no bias voltage etc (i'm still trying to get my head around it for the next tube project)... in the basic circuit from my website you can supply voltage directly to the plate... be careful of the wattage rating on you trimpot... i found that in tride mode i needed a 1/2 watt pot to control the voltage & volume on one set of tubes (both channels)... next project i design i will use an LM317 adjustable voltage regulator instead of doing it just with a pot... if you dont know how to do this i have a schematic here for an adjustable regulator you could implement in your design: Dystopian Tone - Simple LM317 Adjustable Voltage Regulator

there are formulas when you look on google to customise the voltage drop more effectively for your circuit (as opposed to a fixed rate of 28v to 1.5v regulation). this page has a formula i found works well Adjustable power supply you can use this to fine tune the LM317 circuit for your required voltages (if you use the LM317 method instead of a pot - you can use any potentiometer - or if you are just building a buffer you can attenuate the input voltage with a dual gang pot)

I would suggest you invest in audio grade capacitors once you get your prototype working they really do make a big difference in sound with these tubes... don't have to go overboard but something like sprague orange drops won't break the bank but should give a nice pleasant sound.

More tubes = more gain - sound also changes the more capacitors and tubes it goes through - maybe a little warmer (perhaps slight distortion on some frequencies gives the ipression of warmth)... might be worth experimenting to see if you prefer two tubes running at 12v or so as opposed to one tube at 18v... i use multiple tube stages as i require that much gain to offset passive filters, balance controls etc

hope that helps... if you have any more questions my email is on my website (if i'm not responding here)... i'll try to answer any questions here first
 
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