Cathode bias R

[dude]

In a SE Fender champ, using a 6L6GC is a 5 watt brick K resistor ok, or do you think l need a 10 watt. A 5y3 gives me 360 on the plate, plays fine with a 5 watt and 6L6 but was thinking about increasing the voltage with a 5AR4, much less V drop, or perhaps a 5V4, in between V drop.
I know it can’t hurt to use a 10 watt but just experimenting first, then if l like the results open the amp and change to 10 watts.

[sluckey]

P = E²/R

Measure the voltage across the resistor. Square that voltage. Divide by the value of the resistor. That's the actual power being dissipated by the resistor. Now choose a resistor with a wattage that is at least twice the actual power for a safety margin.

This works for any resistor in any circuit.

[pdf64]

It’s worth noting that the voltage should be checked under worst case conditions, eg at high power output, fully cranked.
Maybe not much of a difference with a Champ, but for AB amps it can increase a lot.

[dude]

Thanks, l know a pair of cathode bias 6L6s AB PP, a 10 watt resistor should be used, that would cover a lot voltage spikes. The 5 watt probably would be close in a 6L6 SE Class A amp like the Champ running at higher voltages.
But doing the math is the key, thanks Sluckey, “Give a man a fish, feed him for a day, teach a man to fish, feed him for life”

[mresistor]

[mresistor]

Of course there is more to practical application and knowing which forumula to use..   

[shooter]

Maybe not much of a difference with a Champ

pretty much all my SE builds were max current at idle, no signal.
once you add signal and crank, the voltage would change < 10%, usually up, indicating current went down

[sluckey]

Give a man a fish, feed him for a day, and tomorrow he will want a boat!   

[HotBluePlates]

... all my SE builds were max current at idle, no signal.
once you add signal and crank, the voltage would change ... usually up, indicating current went down

Look again at Ohm's Law and think about how it is possible for Volts to increase if Current decreases.

What you saw was Screen Current increasing when the tube is driven.  You might also be seeing the effect of distortion (if the tube was distorting) if single-ended, or one side cutting off for a push-pull pair transitioning into class AB.

[pdf64]

Maybe not much of a difference with a Champ

pretty much all my SE builds were max current at idle, no signal.
once you add signal and crank, the voltage would change < 10%, usually up, indicating current went down

The detail of the asymmetry at high signal levels of the output of the preceding stages will tend to affect how that plays out.

[shooter]

I = E/R ?
what I observed was the tube changing the flow of electrons based on drive signal applied.


I typically test to Max clean, you're right, when you drive class A into AB linear math gets replaced by circle math 
Most of my builds allowed the guitarist to make square waves into the inductor, if so inclined.

[tubeswell]

But doing the math is the key,...

So what answer did you get?

[dude]

But doing the math is the key,...

So what answer did you get?

Haven’t opened the amp yet, a Fender combo is a pain in the butt to get the chassis out  but will post results.

[HotBluePlates]

I = E/R ?

Volts (across cathode resistor) = Current / Resistance

If current increases, voltage across the cathode resistor increases.  If current decreases, voltage decreases.  This is true unless you have the "negative resistance" of certain active circuits (where current increases even though volts pushing that current decreases).

Overall, a single-ended stage operated cleanly has similar current-change in both the positive & negative directions.  The result is the average current through the resistor is unchanged, giving a steady d.c. bias voltage.

You can get something else when the single-ended stage is not center-biased & is driven to distortion:  one side of the wave flat-tops while the other side increases.  That unequal-current in each direction causes the average current through the cathode resistor to change, and the bias voltage follows.

However, when you drive the output tube towards max power, screen current rises.  This is shown in the top graph of Page 4 here by the way the dashed screen current lines increase at low plate voltage, which is where the late is pulled when it is moving towards peak plate current.

Or look at the Class A conditions and see that from idle to max-power the plate current only increases 15-20% (due to distortion) while screen current rises 50-280%.

[mresistor]

a Fender combo is a pain in the butt to get the chassis out  but will post results.

Not if you've done it a few times already,  its fairly easy for a medium to small amp..  Now the supers and twins can be a pain especially when the chassis mount bolts are bent and cross threaded.

[dude]

a Fender combo is a pain in the butt to get the chassis out  but will post results.

Not if you've done it a few times already,  its fairly easy for a medium to small amp..  Now the supers and twins can be a pain especially when the chassis mount bolts are bent and cross threaded.

yeah, l know, only the one nut on the PT side is a little tricky, I usually use a small piece of double sided tape to hold the nut. But that little champ is much easier than a twin, baseman or even a Deluxe.
I’m interested in the current draw when idling and on full distortion, l’ll get to very soon  Will post.

[Colas LeGrippa]

Hi my friends,


In my builds, se. Pp, 12 or 60 watters, I put 10 w cathode resistors for peace of mind.
I put 2 W in the rest of the circuit for the same reason though it is overkill


Some amp builders recommend 2 or 5 w maxas k  bias resistors, as they act as fuses in case of a shortout output tube.


Up to this point it is better install a B+ fuse as it protects the entire b+ circuit, and strong k resistors


Colas

[bmccowan]

“Give a man a fish, feed him for a day, teach a man to fish, feed him for life”

Give a man a fish, feed him for a day, teach a man to fish, and give him another reason to drink beer.

[mresistor]

Getting that PT side front chaiss mount nut off is easily done with a socket and short extension to clear that pt,  I use a Chapman ratchet sometimes but mostly just a 1/4" ratchet with the extension. Oh and a small flashlight is helpful.

[Willabe]

I use a nut driver.