12ax7 load lines

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12ax7 load lines

Postby ThisLifeILead on Tue Feb 03, 2015 5:33 pm

Howdy gents, i picked up an old eico signal generator back in the day and well, it finally bit the dust. It has 2 12ax7 preamp tubes. I am working on a design to convert it into a relay Bypassable jcm800 preamp. My concern is after rebuilding the power supply im only getting 195v of b+.
Thats rather low in my experiences. so i need to learn how to properly use Load lines to determine my plate/cathode.
Suggestive reads?
"Great spirits often encounter violent opposition from mediocre minds" - A.Einstein
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Re: 12ax7 load lines

Postby PeterS on Tue Feb 03, 2015 10:38 pm

Well, my favorite is the classic Radiotron Designer's Handbook. But if you just want the answer, the RCA tube manual, which you can probably find online, has tables of resistor values for biasing various preamp tubes (including 12ax7) under various conditions. I'd give you some numbers but my book is at work and I'm at home...
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Re: 12ax7 load lines

Postby ThisLifeILead on Wed Feb 04, 2015 7:35 pm

Ok i think i found The chart You were refering to Peter.wow this is a super handy chart.
Pulled from the chart, a somewhat standard gain stage.
B+ of 300v (ebb)
Rp= 100k
Rk= 900ohms.
gain = 50


the chart indicates with a b+ (ebb) of 180v (my case exactly)
Rk= 1.1k
Rp= 100k
gain = 46

the trend im seeing is a slight increase in Rk (cathode) to compensate for the lower b+ (ebb).
:jam:
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Re: 12ax7 load lines

Postby PeterS on Wed Feb 04, 2015 10:42 pm

the trend im seeing is a slight increase in Rk (cathode) to compensate for the lower b+ (ebb).


That makes sense (after the fact, I'm not sure I would have predicted it!). The current is only weakly sensitive to plate voltage. For lower B+ you want less current, in order to have less voltage across the plate resistor. To lower the current you increase the cathode resistor to get the same cathode voltage at a smaller current...
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Re: 12ax7 load lines

Postby ThisLifeILead on Mon Mar 23, 2015 7:32 pm

very cool!
got a nice wave on my output but im getting some heavy motor boating that seems to be cancelling out any output signal (all i hear is motorboating when power up). im using two 47uf @350v caps one for each tube with a 1K dropping resistor between them. I am going to rebuild the power supply to try and decouple some more.
I would have thought 2x47uf would have been enough to smooth the power supply out but i guess its more of a problem with my design then the cap values themselves. :jam:
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Re: 12ax7 load lines

Postby PeterS on Tue Mar 24, 2015 7:14 pm

That's weird, 47uF should be plenty. Try increasing the dropper from 1k to 10k...
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Re: 12ax7 load lines

Postby ThisLifeILead on Tue Mar 24, 2015 7:40 pm

the way i have it wired is:

bridge rect out ----> 47uf (node A feeding v2) ---> 1k dropping resistor --->47uf (node B feeding v1).

i will try 10k :)
thnx Peter
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Re: 12ax7 load lines

Postby ThisLifeILead on Mon Mar 30, 2015 1:23 am

did 10K and bumped it up to 20K. still motorboating but not quite as bad.
Can I scope the power supply for ripple since the voltage is 200v? If there is very little ripple what else can cause motorboating? :jam:
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Re: 12ax7 load lines

Postby PeterS on Mon Mar 30, 2015 11:47 am

Yes you can put the scope on it, on AC-coupled. There's usually a 400 volt coupling cap.

Are you sure it's motorboating and not a huge hum? If it could be a hum you might have a heater-to-whatever short somewhere.
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Re: 12ax7 load lines

Postby ThisLifeILead on Wed Apr 08, 2015 2:51 am

im getting a 20mv saw tooth ripple that pulses on the PS nodes. I clipped in a 100uf cap parallel to the input 47uf cap. Ripple didnt change.
So looking back to the original schematic : http://s1259.photobucket.com/user/relichtr/media/Eico324_zps99379d26.jpg.html
It employs a have wave rectifier. Im using a bridge.
Heaters are also wired different.
Will try returning the PS to stock.
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