# Selecting a Cathode Resistor Value (to adjust bias)???

#### Lancer X

##### TDPRI Member
Evening, all! I just finished building @robrob's Deluxe Micro Mod, and it sounds freaking fantastic! (No finished pics until the custom logos etc. arrive...)

So, I ended up at 5.7W plate dissipation, which is 103.6% of ideal for this 12AU7 power tube. Not one to do things half way, I want to tweak my cathode resistor to bring her down to ~100%. (This is a cathode biased amp.)

I have been Googling how to calculate

* Plate-to-Cathode 338 V
* Voltage across Cathode Resistor 13.8 V
* Cathode Resistor (actual measured) 810 ohms

Plugging these values into @robrob's "Tube Dissipation Using Cathode Resistor Voltage Drop" calculator, the following values emerged:

* Plate Current 17 mA
* Plate Dissipation 5.7 W
* % of Max Dissipation 103.6%

So, given these values, is there an accurate way to calculate what value of cathode resistor would get me down to ~5.5W of plate dissipation?

I did try to Google for the answer of this question, but didn't gather any useful info. I'm guessing this isn't a simple V=IR situation - seems like the situation is much more interdependent than that??

#### King Fan

##### Poster Extraordinaire
I'm gonna cut clear around the interesting tech part of your question, and suggest that cathode bias is 'self biasing'. The hotter it gets, within reason the more it self-regulates. With standard 6V6 tubes, cathode bias often sounds freaking fantastic (a useful cathode bias term) at something over 100%. Like up to 110 or 115%.

Plus, I'm no tech, but I thought that upping the bias resistor cuts output voltage *by sending more volts to the preamp.*

Now, the only issue: I have no idea if 'self-biasing' is also true when your output tubes are a pair of 12AU7 triodes. But, still, I'm pretty sure 100% and 103% are not very far apart.

#### Lancer X

##### TDPRI Member
I can certainly vouch for the fact that it currently sounds spectacular.

For a class A cathode-biased amp, it is "safe" to bias as hot as 100% of rated plate dissipation. My understanding is that running the amp at 105% wouldn't immediately damage it, but it could decrease the lifespan of my power tube. That's why I was considering cooling it off a bit.

I did just find some info on cathode bias adjustment on Uncle Doug's (most excellent and informative) YouTube channel. Will be checking that out later tonight.

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#### D'tar

##### Friend of Leo's
Just my opinion....

You said it yourself. It sounds fantastic! Leave it be. Lower bias, raise B+, lower B+, raise bias, back n forth, to n fro. My '60 GA5 runs 112% on 6v6, original tubes! My tweed deluxe is zpushing 114%. Wouldnt change a thing!

#### D'tar

##### Friend of Leo's
If you decide to change things, just remeber, its a moving target. Change one thing, cathode resistor, and plate voltage and current change also. Keep refreshing ALL of your nubers each time you edit the circuit.

Have fun, be safe

#### corliss1

##### Poster Extraordinaire
Gold Supporter
Just because they are not a beam power tube doesn't mean it couldn't redplate if it was drawing too much current. If it sounds good, and the tube isn't melting down, leave it. You'd have to get into weird value 1% resistor land to try and get that small of a difference.

#### andrewRneumann

##### Tele-Afflicted
I agree with everyone else--103%? Big deal.

But if you insist...
You have a 56 Ohm resistor of any wattage? Tack that in series with the 810 Ohms you have.

#### wangdaning

##### Tele-Meister
Yeah, using your numbers in Rob's calculator I got that 850ohm would put you are 100%. Not sure if it is really worth it thought.

#### The Ballzz

##### Friend of Leo's
Which is more important to us, having our dual triode powered amp sound great or saving the price of a \$20 tube over the differential course of three years vs four years?
I Thought So!
Gene

#### robrob

##### Poster Extraordinaire
Your bias is perfect. You can't use Ohm's law to figure a new cathode resistor because of what D'tar said.

#### James Knox

##### Tele-Afflicted
Just to note an interesting observation regarding the “everything changes everything” concept. In my recent Champ Dissipation Experiment Build, rather than fooling around with jumpering smaller resistors on the Cathode Bias Resistor, I upped the B+ by using the 325-0-325 taps on the Hammond 290AX PT. In the past, Ive always used the 275-0-275 taps and just “lived” with the cooler bias.

I wanted this Champ HOT - sizzling Hot!

I got my wish; 124% dissiapation, lol. Amp sounded great, no red plating with a JJ, a TungSol, or a NOS GE 6V6.

So, the next day I start experimenting with jumpering some resistors to see if I could bring it down closer to 100%. While taking voltage readings and plugging numbers into RobRobs Bias Calculator, it hits me - the numbers are different today!

Without adding any resistance to the 10W 470R Bias Resistor, I’m now running ~96% dissipation. Just as a matter of habit, I always record what the Wall AC voltage is for every build. So, I check the previous days notes and see that the Wall voltage was 124VAC. Today, the Wall voltage was 117VAC.

Yep, I’m leaving it where it is!

#### printer2

##### Poster Extraordinaire
So once you get it to 100%, will you regularly check it for when the heater emissions reduce and your current drops by 3%?

100% is just an average of a number of tubes. There will be some that are higher and some lower. There is nothing magic to 5.7W How much the plates can actually dissipate also has to do with the temperature of the enclosure the tube was in, the temperature of the room which changes how much heat can get transferred away from the glass. Then they would rate the tube for the length of life wanted. 3% is nothing, forget about it.

#### Lancer X

##### TDPRI Member
Thanks everyone. I really appreciate all the advice!

In the interest of Rock n Roll Science, I did order a few different resistors for my B+ rail. My V1A voltages were pretty high too, so just want to see what I can achieve by tweaking down my B+1. If my calcs are correct, doing so will also bring my B+2 right where I want it, which should also sort out my high preamp voltages.

Not going against all your warnings - this is just for fun and to see what happens. Eventual goal is to design my own, so learning as much by playing as by reading. (Working through Richard Kuehnel's basic amp theory book at the moment.)

So, the next day I start experimenting with jumpering some resistors to see if I could bring it down closer to 100%. While taking voltage readings and plugging numbers into RobRobs Bias Calculator, it hits me - the numbers are different today!

Without adding any resistance to the 10W 470R Bias Resistor, I’m now running ~96% dissipation. Just as a matter of habit, I always record what the Wall AC voltage is for every build. So, I check the previous days notes and see that the Wall voltage was 124VAC. Today, the Wall voltage was 117VAC.

Was this a new build? I definitely had slightly higher numbers the day I first fired up this 5E3M. Seemed like it took a few hours for the components to settle to their steady state values. Does that make sense??

I've seen our wall voltages vary by +/- 2V from day to day. Yours are really lively!