12ax7 bias problem or plate voltage prob ?

[Ice-9]

I'm trying to get a good circuit using a 12ax7 to work as a distortion/overdrive, ive just done a basic circuit to get it woking but the problem i have at the moment is that it is sounding like a really badly biased jfet. i've done a bit searching and have found that 12ax7 valves don't really work too well at lower plate voltages and suggestions of using 12au7 is a better bet. But what is to low a plate voltage for the 12ax7, i have 60v from my psu. I've tok some voltage readings and put them on the schematic. I've also tried different resistor values on the plate and catode but the results were the same. the question is am i wasting my time trying to get a 12ax7 to work at these voltages or have i just done something wrong ?





Sorry i double posted this earlier

[markusw]

I think you forgot to add a plate cap at the first stage. This should increase plate voltage at the first stage.

[Jered]

  This is DC volts, yes?

[frequencycentral]

I think you forgot to add a plate cap at the first stage. This should increase plate voltage at the first stage.

+1 on that. Try a 47n between pin 1 and the 250K pot. I'm also not sure if you need the 100n at pin 7. You should get a bit more drive if you increase the value of the 100K plate resistor at pin 1, maybe 220K or 470K.

I've never used a 12AX7, but there is some onfo on the Valvecaster thread about the 12AU7 sounding better at low voltage. But yours is not exactly a starved plate circuit.

[gnognofasciani]

These 51 volts on pin6 are probably your problem! you should increase that 100k resistor until you reach something near 25-30v
If you're not using a 1M resistor from pin 7 to ground, you may want to use a 1M pot for the gain instead of a 250k
Hope i helped!

[Johan]

I think you forgot to add a plate cap at the first stage. This should increase plate voltage at the first stage.

+1 on that. Try a 47n between pin 1 and the 250K pot. I'm also not sure if you need the 100n at pin 7.

..actually, that 100nF is blocking the bias to the second stage and shouldnt be there.( this is why there is 51volt on the second plate..the tube is basicly "off" ) but something needs to block the DC-path from the first plate to ground...so, just as you suggest, a cap there would be a good thing

j

[frequencycentral]

I think you forgot to add a plate cap at the first stage. This should increase plate voltage at the first stage.

+1 on that. Try a 47n between pin 1 and the 250K pot. I'm also not sure if you need the 100n at pin 7.

..actually, that 100nF is blocking the bias to the second stage and shouldnt be there.( this is why there is 51volt on the second plate..the tube is basicly "off" ) but something needs to block the DC-path from the first plate to ground...so, just as you suggest, a cap there would be a good thing

j

Nice explanation - I often know where a components should be, but not why it should be there!


EDIT: This is worth reading: http://www.diystompboxes.com/pedals/tubedummy.html

[Ice-9]

thanks foor all this info, i wil work my way through it today, to answer some of the questions in these posts, Yes the voltage is DC i also previously had a 22n cap on the plate to pot junction of v1a, but i still had the 100n cap in place on the grid of v1b so i will rework this bit.  Nice link on the tubes for dummies.

[Ice-9]

Thats helped quite a lot, I have removed the 100n cap on the grid of the second part of the valve and put a 1m resistor to gnd on the same grid (pin 7). I have also put a 22n cap on the plate of the 1st part (pin 1 to 250k pot)

To test the sound of each section i removed the cap on pin 1 to pot and put my guitar input to this point running the signal through the second stage only. I am getting a really nice tone which is crystal clear with a perfect signal decay over the full range of the pot, also at full pot setting i get quite a big volume increase.
Second test I put the input back to the first stage and took the output from the centre of the pot adding a 47n cap to that. This stage is giving me some nice distortion and tone but slightly breaking up.

Using the full circuit together results in an awful, badly biased sound at higher gain setting so i'm going to do some reading up today and try and get this cracked over the weekend.

I will try and set up my gear to get a recording done and get some clips up soon.

[Gus]

Not sure what your design is about.   Some things to think about.  The tube curves at the operation point you can achieve with the B+ and tube used you are operating them at low plate currents and voltages.  first stage 51.8VDC - 32.6VDC = 19.2VDC   19.2VDC/100K= .192ma  2nd stage there is leakage current in tube between pins that can act like a very high value resistor this is one reason you have a bias on the tube -.6VDC at the grid and .01VDC at the cathode note the plate current is very small .5VDC drop across the 100K .5VDC / 100K =.005ma

   read this    http://www.w8ji.com/fusing_and_floating_grids.htm

The schematic post looks like you made a contact point bias mixed a bit with cathode bias setup in the stages.  The grid measuring more -VDC than the top of the cathode resistor.

Now the real way to build this is to curve trace the tube at the operation points you can archive with the circuit B+ voltage and current.  The problem is this is a more curved area were parameters change more than at the more straight parts of the curves.

http://www.mif.pg.gda.pl/homepages/frank/sheets/093/1/12AX7.pdf

note rp u and gm are not even drawn at that low a current page 2.  Also look at page 3 and how the curve bend the other way the closer to 0Volts plate and 0ma plate at lower - grid voltage

EDIT how much current can you get from the B+ supply before it drops a lot and gets a lot of ripple?  You can test this with a load resistor 60VDC with a 60K is 1ma.  60VDC witha 33K is 60VDC/33K = the ma etc.  I would try 100k and 47K a first to see what happens.   P=I^2 x R or P=E X I for the power rating of the resistor.  Knowing the advailbe current will help to adjust the circuit.

[Ice-9]

Thanks Gus for the info on using the tube graphs and maths for selecting the resistors.

here is the revised schematic and voltages, which operate much better, but now with the info,  i will do some maths and try and select a linear part of the graph and voltages to improve it.

[frequencycentral]

That 680K grid stopper resistor looks massive. Tube Dummies says: "Another modification you may see is a Grid Stopper Resistor, this can also control gain between stages and also interacts with the tube to roll off highs. Values can be 1.5K->100K. Larger values roll off more highs and reduce gain between stages."

If you're not using a 1M resistor from pin 7 to ground, you may want to use a 1M pot for the gain instead of a 250k

+1, I think you will get more gain with a 1M pot.

[Ice-9]

Even with the 680k resistor on the grid of section 2 i still get massive gain. with the pot set at minimum its about 2 times as load as bypassed.

I've took some voltage measurements from the psu under load.

no load    = 56.3v
56k load  = 45v
39k load  = 42.1v
10k load  = 26.5v

[Lurco]

decrease the 33k to 3k3 or less
use a logarithmic pot (audio taper)
use 1M like mentioned above

[markusw]

For the 250k gain pot (edit: better log as already mentioned) you might add a 250k or even larger between the 22n plate cap of stage 1 and the gain pot.
A 250k should reduce gain by approximately 50%, 500k by about 75%.
Might work better than increasing the grid stopper.

I think something like 200-250k should be fine for the grid stopper.
680k might already reduce highs too much.

2nd edit: maybe also add a master volume pot (e.g. 1M log) to the output. This way you could adjust distortion with the gain pot and overall output by the master.

[Ice-9]

2nd edit: maybe also add a master volume pot (e.g. 1M log) to the output. This way you could adjust distortion with the gain pot and overall output by the master.

I will be adding a master volume at some stage. My goal here is to build a full pedal with tone stack vol etc. What i am trying to achive at this stage is build each block of the effect and tailor it to work the way i like, i'm not sure if it may seem a bit backwards to build in blocks like this, but my thinking on this is to get each stage correct and working properly. Once the valve section is working i can move onto the tone stack. 

From this thread alone i have learned quite a bit about valve circuitry already and for me this build is more about learning how and why each component works or doesn't work, so i f i ask any questions which seem stupid or obvious please bear with me

[MohiZ]

I'm doing something similar, although at first I'm just getting insight by playing around with a single triode stage. I've put pots in place of the grid resistor, plate resistor and the cathode resistor to learn how changing them will affect the sound. Can anyone link some good pages to read about the biasing and tube circuits in general?

[frequencycentral]

I'm doing something similar, although at first I'm just getting insight by playing around with a single triode stage. I've put pots in place of the grid resistor, plate resistor and the cathode resistor to learn how changing them will affect the sound. Can anyone link some good pages to read about the biasing and tube circuits in general?

http://www.diystompboxes.com/pedals/tubedummy.html

http://www.tubecad.com/

http://www.thermionic.info/

The best is probably http://www.freewebs.com/valvewizard/ They have a great PDF called 'Common Gain Stages'.

[MohiZ]

Thanks, this was what I was looking for. Sometimes the best resources are not the easiest to find.

[biggy boy]

Hi Rick
Do you use the Tube Cad?
If so, would you recommend it as being a useful tool worth paying for?
I came across the tube cad website last week and it caught my eye very intriguing software!!!!

Glen

I'm doing something similar, although at first I'm just getting insight by playing around with a single triode stage. I've put pots in place of the grid resistor, plate resistor and the cathode resistor to learn how changing them will affect the sound. Can anyone link some good pages to read about the biasing and tube circuits in general?

http://www.diystompboxes.com/pedals/tubedummy.html

http://www.tubecad.com/

http://www.thermionic.info/

The best is probably http://www.freewebs.com/valvewizard/ They have a great PDF called 'Common Gain Stages'.