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Cathode bias resistor calculation


 
7/31/2000 9:28 PM
Johnny
Cathode bias resistor calculation
I have built a pp el34 triode amp which is working fine. I got the value of the Cathode resistor from other circuits (220 ohm) however my circuit has a lower B+ than the circuits I copied from (370v vs 400-430v). Hence each tube is drawing only 45mA each. With such a low B+ I was hoping to run the tubes near class A with high current.  
 
To increase the tube current I would expect to lower the value of the Cathode resistor but I am afraid that this will lower the cathode voltage and reduce the max voltage swing onto the el34 grids and hence reduce the output of the amp.  
 
Can anyone fill me in on the calculations for the optimum cathode resistor.  
 
As mentioned before, the B+ is 370v, the tubes are wired as triodes, the OPT is 6.6K and the present cathode resistor is 220ohms.  
 
Any help is appreciated  
 
John
 
7/31/2000 9:42 PM
Ken Gilbert

if you increase the standing current of the amp, i.e. push the amp more towards class A, you WILL get lower power output. that's all there is to it.  
 
the voltage developed across the cathode r will decrease a bit, but that is offset by the action of the tube pulling more current. actually for a given Vg2-k, the bias voltage will be the same.  
 
according to the amperex data sheets, typical operating points or triode connection for a SINGLE tube would be:  
 
B+ = 375V  
Vg3 = 0V  
Rk = 370R  
Rl = 3k  
Vin = 19Vrms  
Ip = 70mA  
Po = 6W  
 
soooooo if you've got a shared Rk for two tubes (push pull pair) then you've got to halve that value of 370R, and use 185R. of course, if you've got another 370R resistor, just wire it in parallel with the existing one and call it done.  
 
note that your power output will be about 12W (6W * 2).  
 
kg
 
7/31/2000 9:46 PM
Ken Gilbert

quote:
"of course, if you've got another 370R resistor, just wire it in parallel with the existing one and call it done"
 
 
oops... you don't have a 370R resistor in place... it's 220R.  
 
that means you could parallel it with a 1k2 resistor and be right on 185R.  
 
kg
 
7/31/2000 11:11 PM
dutch

quote:
"of course, if you've got another 370R resistor, just wire it in parallel with the existing one and call it done  
 
oops... you don't have a 370R resistor in place... it's 220R.  
 
that means you could parallel it with a 1k2 resistor and be right on 185R.  
 
kg"
 
 
Another way you can do it is to use the old Fisher hi-fi bias adjustment trick:  
 
1: Wire up a 22k resistor in series with a 5k-L pot, with the pot at the ground end of the series string.  
 
2: Then tie the EL34s' bias feed resistors to the pot's wiper instead of ground.  
 
Easy adjustable bias for cathode-biased amps!  
 
What this does is provide a source of slight *positive* bias on the output tubes' grids, so I'd recommend changing the 370R resistor to a 390R for a bit better adjustability, i.e., to be able to go a bit in either direction.... Why *everyone* building cathode-biased amps doesn't do this easy trick is beyond me. It works great, and lets you adjust the amp easily to suit each new set of tubes you put in.
 
7/31/2000 11:31 PM
Johnny

Many thanks guys  
 
John
 
8/1/2000 4:30 PM
Ken Gilbert

dutch,  
 
you always seem to have these little tricks up your sleeve.. i'm building a cathode biased amp atm and came up with the same scheme in my head--after pondering it for about 2 days!  
 
the only difference was that i "used" a higher wattage pot and passed the whole cathode current through it. of course, that limits your choices of pots, so there's benefits to your higher impedance method.  
 
like you, i wonder why more cathode biased amps don't use this...  
 
kg
 
8/1/2000 8:53 PM
dutch

quote:
"you always seem to have these little tricks up your sleeve.. "
 
 
I just happened to see it in an old Fisher amp that I did some work on, and logged it in the ol' "utility cobweb storage facility" for future use. I've used the circuit to good effect on a little cathode-biased EL84 amp I built.  
 
The main requirement of the circuit is a slightly higher-than-normal Rk (maybe 10-15% high) to give adjustment range up *and* down....  
 
BTW, the same amp used the preamp's 12AX7 filaments as part of the cathode resistor--paralleled with a resistor to trim the current on the filaments at idle to where they wanted it. As long as you bypass the whole mess with a decent-sized Ck (50uF? the exact value's lost in the cobwebs...), it's *cheap* DC filaments for your pre. You can apparantly go as much as 20% low on the filament voltage with no harm or detrimental operation, and about 10% over.....  
 
C ya,  
Dutch
 
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