jfets and tubes...help

[rx5]

hi mates,

Im planning to build a 12AX7 tube audio output buffer...tube only, in leu of a Jfet...hope you could help me out on this one...

1.) I wish to make it by powering it by a single 9V supply(regulated)... lets just use one troide only for simplicity...can I power it by 9volts alone?? ill connect the HEATER filaments(pin 4 &5) to 9VDC....then power the triode by 9VDC...can I ?? or should I use a voltage multiplier (say, about +45VDC) to power the triode???

-planning to get the output of the tube by pin "1".... (in njfet terms, the Drain)

2.) I think Ive read someone saying that Tubes are like Jfets... since I know that using the N-Jfet that when you get the output from the DRAIN, you should adjust the resistor connected from V+ to the DRAIN leg to get about 1/2 supply...should this be the same biasing with triode tubes pin "1"??

3.) how should I bias the "gate" (pin 2) of the triode?? or where should I get my bias voltage?? should it be between {1/2 of +45VDC}??  or  {1/2 of 9VDC}??? or what ?? please note that Im using single +9VDC -and- a voltage multiplier of +45VDC@15mA(i  think??) gonna use CD4049 for voltage multiplier circuit....  

4.) what is the input impedance of 12AX7?? the output impedance??

5.) can I connect a simple 1 transistor amp(like input buffer of BOSS) to the input(pin 2) of the triode??

6.) should I buffer the output of the Triode ?? can a simple 1 transistor buffer be enough?? or should I use an op-amp output buffer??

here is my planned setup:
==========================
obsidian-T (w/out tone controls) + tube circuit (as buffer/"squasher") + passive tone control( from the obsisdian's) + transistor buffer(or should I ommitt this transistor buffer)...

hope you could help me.... OR any links about info how to use 12AX7 tubes..... pls....

sorry for many questions.. hope you all could help me out..I know a bit bout bipolar transistors and N-Jfet's but totally ignorant bout tubes.....

thanks...


-RAlph

[puretube]

http://www.tubecad.com/

although that above site is more an advanced tube-builder resort,
you can learn some basic tube-tech there:

I hope some other "forumites" will supply you with more additional
useful links....





//www.puretube.com

[Peter Snowberg]

I'll add some to that...

For #1: 12AX7s will take either 6 or 12 volts for the fillament dpending on how you wire it. If you use 9V on a 6V fillament you will burn it out very quickly. If you use 9V on a 12V fillament, it will not get hot enough, life may suffer, and tone will suffer badly. You must use the propper voltage and there is no way around that.

For #2 & #3: Tube bias is a little different. You want to have ground potential on the grid (with a resistor to ground), and you want some voltage on the cathode. The cathode resistor takes care of that, raising the cathode to have a higher voltage than the grid. The cathode bypass capacitor provides the cathode with an AC ground.

For #4: I think output impedance of a 12AX7 is roughly 62K when running at 250 volts. That goes UP when running at lower voltage. I don't know about the input impedance when wired up as a basic triode.

For #5: Sure, you could buffer the input to get some more gain. Just make sure you use a coupling capacitor between the transistor and the tube.

For #6: You don't need to buffer the output, but if you want to lower the output impedance, I like the NE5532.


I would suggest looking at Doug Hammond's Firefly as an example. Just leave out the self-split output stage and you have a nice little tube preamp. Use only the first stage with a volume control after the coupling capacitor and you'll have a nice buffer.

I don't think you'll like the results at 45 volts running as a buffer. If you want tube fuzz it may be great, but for adding some tube character to the signal should be done with the tube running at 2 to 6 times as much voltage.

Good luck!

-Peter

[puretube]

watch this thread, too:

http://diystompboxes.com/sboxforum/viewtopic.php?t=11887

[Mike Burgundy]

If you're somewhat experienced with calculations on circuits, this will help:
http://www.aikenamps.com/Equations.htm
Data on impedance and such can be found in datasheets:
http://tdsl.duncanamps.com/tubesearch.php

Note that if you want to buid an *audio* buffer, running the tube at low voltage(including 45VDC) will put it in a very non-linear range, and totally muck up dynamics to boot.  I'd aim for a minimum Va of 100V.
If it's a distortion box, hey, the non-liear thing is what we wnat right? Unfortunately low-voltage tube distorters tend to sound mushy to my ears. (The famous tubedriver uses the tube to *smooth out* the distorted output of a 4558 opamp - the main distortion is not generated by the tube. Same goes for the Mesa VTwin)

Note:
High voltage is LETHAL. Don't fool around with this unless you feel comfortable that you know what you're doing.

[rx5]

thanks for the links guys.... STUDY STUDY STUDY

wish I could build one..... just gonna read and understand first....

links looks cool..... lot of info..


cya....


-Ralph