Tube fixed bias Ra and Rk?

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simonsez

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Nov 14, 2008
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Anyone can tell me how to calculate Ra and Rk for fixed bias circuit? Is it different from common cathode using load line calculation? What if using transformer coupled? I'm playing with varimu stage and try different tube. Any explanation or may be pointing me to the article about this? Thank you ...
 
Sorry Simon, I'm at work.

Is this for the actual gain-controlling stage in a vari-mu? You should be able to set it up in an idle state as per usual, and then calculate the effects of shifting the operating conditions on gain using valve characteristic data.

I'd read PRR's posts and description of his own vari-mu circuit which describes things nicely.
 
Thank you Rod, sorry for disturb your work. I'm poor in this kind of theory, when i'm build my varimu, i just look at so many schematic, but i can't see the general rules for that component value (Ra). i see many use high Ra, and many use only a few K ra, Now one of my bottle generate some kind of hum...i thing it's defect now, i swap the bottle and the hum stays in that bottle. I suspect there is something wrong with my build. Yes, i read many post about varimu here, but only little stays in my brain (not my fault...my brain is.. :() below is the schematic of my built..may be you can see something not right...many thanks Rod..
 

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Thanks Magneto, How you can get that 10k?
MagnetoSound said:
What is the purpose of the 10k across the primary?

You tell me, i saw in few schematic that use interstage tx, if i take it out, the low freq is gone..
 
It could be a serious imbalance between both sides. Check that the cathode currents are identical, by measuring voltage across cathode resistors. For some reason, one of the tubes seem to be off.
 
abbey road d enfer said:
It could be a serious imbalance between both sides. Check that the cathode currents are identical, by measuring voltage across cathode resistors. For some reason, one of the tubes seem to be off.

Thanks Abbey, I did trim the cathode balance until the wave is symmetry (minimum thump)at the output and minimum voltage between both side cathode (near zero volt). For a few day i try feed it with 200vdc... :(. Is this can cause damage for the tube?
 
>How you can get that 10k?


Check this example - 6BA6 and 6386 are fairly comparable, your cathode values look small as well compared to this, how much plate current is flowing?

cbs.jpg



What is the primary impedance of the interstage?

Is it tapped and gapped? If so, maybe you can apply B+ at the CT. If not, any imbalance between plate voltages will pass DC through the primary which will affect freq. response. Your resistor might be mitigating this. A better approach would be to use blocking caps, as in the image above.

 
Thanks magneto sound, i read the data sheet of both tubes....6386 had 4250 ohms plate resistance, so 10k plate resistor is fine i think, But 6BA6 had 0.25 Mohm (as pentode)...i don't know how to define the Ra, Disa limiter use 6k8 Ra and 300ohm cathode trim. But i'll try with 10k and 240 ohm, thanks for the pic.  I plan to get LL1660 for the interstage (the only iron i can get locally and i'ts very expensive), but for now i used Ptx with 4:1 turns ratio ( :-[). I can't get rid of thump when using coupling cap (no matching tubes), and i really like the sound of using "interstage Tx". Everything is ok except hum in one bottle ( only heard when i crank the monitor level up). I swap the bottle and it's stay in that particular bottle . I'll get another one this week.

Many thanks MagnetoSound...
 
Well ... if you like the sound you're good, no?

simonsez said:
i read the data sheet of both tubes ... 6386 had 4250 ohms plate resistance ... But 6BA6 had 0.25 Mohm (as pentode)...

Strapped as triode though, the plate resistance comes down considerably. (Anyone know how to calculate this if screen resistance is not given?) Transconductance is not so dissimilar. In a vari-mu application, this should have greater impact on the compression curve, I would have thought.

You want the tubes in a good operating range. Further than that, make sure both tubes are healthy and the sound will be whatever it is for those tubes.

 
Hey magneto sound,  suddenly the hum is gone....really weird....now it's very quite, i hope it's ok now....Thanks everyone...
 

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