Fixed Bias vs Cathode Bias: The Formulas, and Cool, Warm or Hot Settings for Popular Amps
Author: Kevin M Date Posted:20 October 2026
Our post To Bias or Not to Bias explains which amps need biasing. This one covers the maths: the handful of formulas a tech uses to work out the right idle current, how fixed bias and cathode bias differ on paper, and what cool, warm and hot settings look like for the amps we're asked about most.
You don't need to be an engineer to follow it. Every formula comes down to Ohm's law and one number printed on the valve's data sheet: its maximum plate dissipation.
The one number that matters: plate dissipation
When an amp is idling, each power valve turns some of its supply into heat at the plate. That heat is the plate dissipation, measured in watts. Every valve type has a maximum it's designed to handle, and biasing is really just choosing what percentage of that maximum the valve sits at while you're not playing.
Rearrange the first formula and you get the one most people actually use, the target idle current:
Maximum plate dissipation of common power valves
| Valve | Pmax (W) | Found in |
|---|---|---|
| EL84 / 6BQ5 | 12 | Vox AC15 and AC30, Marshall 18 Watt, Fender Blues Junior |
| 6V6GT | 12 | Fender Champ, Deluxe, Deluxe Reverb, Princeton Reverb |
| JJ 6V6S | 14 | A heavier-duty 6V6, popular in hot-running tweeds |
| 5881 / 6L6WGB | 23 | Tweed Bassman, early Fender and Marshall JTM45 |
| EL34 / 6CA7, KT77, KT66 | 25 | Marshall, Hiwatt, Orange; KT66 in the JTM45 |
| 6L6GC | 30 | Fender Twin Reverb, Super Reverb, Bassman, Mesa/Boogie |
| 6550, KT88 | 35 | Bass amps such as the Ampeg SVT, and hi-fi |
These are the classic design-centre ratings. Some modern versions are rated higher (for example some 6550 and KT88 types at 42 W), but working from the classic figure gives you a safety margin.
Cool, warm or hot: what the percentages mean
For a fixed-bias, push-pull amp (two or four power valves running class AB), the usual working range is about 50% to 70% of maximum plate dissipation. Where you set it within that range is a matter of taste.
Cool
More clean headroom and a tighter, glassier sound, and the longest valve life. Go much below this and you'll hear crossover distortion: a thin, fizzy edge on notes as they decay.
Warm
The middle of the road and a good default. Fuller tone, smooth breakup and sensible valve life. Most techs start here.
Hot
Earlier, creamier breakup and more harmonic richness, but shorter valve life. Treat 70% as the ceiling for a fixed-bias class AB amp.
Fixed bias: the formulas
In a fixed-bias amp, a negative voltage on the control grid sets the idle current, and a bias pot (or a fixed resistor) sets that voltage. You turn the pot until the current matches your target. The grid voltage itself isn't the target: two valves of the same type can need quite different grid voltages to draw the same current, which is why you set bias by measuring current.
1. Work out the target
This formula works out how much current each power valve should draw at idle so it runs at the percentage of its heat rating you've chosen. Watts are volts × amps, so once you know the plate voltage and how many watts you want, you can work out the current. Here's each part, using a Twin Reverb as the example:
| Part | What it means | Twin Reverb example |
|---|---|---|
| Pmax | The valve's maximum plate dissipation in watts, from the data sheet | 6L6GC = 30 W |
| % | How hard you want it to run at idle: cool 50%, warm 60%, hot 70% | Warm, 60% = 0.60 |
| Pmax × % | The watts you want the valve to idle at | 30 × 0.60 = 18 W |
| ÷ Vp | Divides by the plate voltage to turn watts into amps (amps = watts ÷ volts) | 18 ÷ 460 = 0.039 A |
| × 1000 | Converts amps to milliamps, the unit your bias meter shows | 0.039 × 1000 = 39 mA |
Why the plate voltage matters: the same valve needs a different current in a different amp. A 6L6GC at 60% (18 W) wants 18 ÷ 430 = 41.9 mA in a tweed Bassman at 430 V, but 18 ÷ 460 = 39.1 mA in a Twin Reverb at 460 V. Higher plate voltage means less current for the same heat, which is why no single "set it to 40 mA" rule suits every amp, and why you should measure your own plate voltage.
What your meter reads: this formula gives plate current. A bias probe or 1 Ω cathode resistor reads cathode current, which is about 5% higher (see step 2), so for the Twin Reverb example aim for about 41 mA on the meter.
2. Measure the current
There are three common ways to read idle current. The first two measure cathode current, which is plate current plus a little screen-grid current.
- 1 Ω cathode resistors: many modern amps (and amps fitted with our bias test points) have a 1 Ω resistor from each cathode to ground. Because I = V ÷ R, every 1 mV across it equals 1 mA. A reading of 41 mV means 41 mA.
- Bias probe: an adaptor that plugs in between the valve and its socket, with the same 1 mV = 1 mA reading.
- Output transformer resistance method: measure the DC resistance of each half of the output transformer primary with the amp off and drained, then measure the voltage drop across that half with the amp running. Ip = voltage drop ÷ resistance. This reads plate current only, but it means measuring across high-voltage points on a live amp, so it's a job for an experienced tech.
3. Check the result
This is the target formula worked backwards. Once you've set the bias and taken a reading, it tells you what percentage of its heat rating the valve is actually running at, so you can see whether you've landed on cool, warm or hot. Here's each part, using the same Twin Reverb 6L6GC example as step 1:
| Part | What it means | Twin Reverb example |
|---|---|---|
| Ip | The idle current you measured, converted from mA to amps (divide by 1000) | 39 mA = 0.039 A |
| Vp × Ip | Plate voltage times current gives the watts the valve is turning into heat at idle (watts = volts × amps) | 460 × 0.039 = 17.9 W |
| ÷ Pmax | Compares those watts with the valve's maximum rating, giving a fraction of the maximum | 17.9 ÷ 30 = 0.60 |
| × 100 | Turns the fraction into a percentage | 0.60 × 100 = 60% |
Reading the answer: about 50% is cool, 60% is warm and 70% is hot. In a fixed-bias class AB amp, anything over 70% is too hot, so turn the bias pot down and measure again. Well under 50% risks crossover distortion.
If you measured cathode current: a bias probe or 1 Ω resistor reading includes about 5% screen current, so divide it by 1.05 first. In step 1 we aimed for about 41 mA on the meter. That's 41 ÷ 1.05 = 39.0 mA of plate current, which works out to 460 × 0.039 = 17.9 W, or 60%: right on warm. Without the correction, 41 mA would look like 63%, a little hotter than the valve is really running.
Cathode bias: the formulas
In a cathode-biased amp there's no bias pot. A resistor between the cathode and ground (Rk) sets the bias: current through it lifts the cathode above ground, which has the same effect as putting a negative voltage on the grid. You can't "set" a cathode-biased amp, but you can check it with a few voltage readings.
This is Ohm's law (current = voltage ÷ resistance). All the current the power valves draw flows out through their cathodes and down through the cathode resistor to ground. That current lifts the top of the resistor above ground, so measuring the voltage across a resistor of known value tells you how much current is flowing, without breaking into the circuit. Here's each part, using the Fender 5E3 Tweed Deluxe from the worked example below:
| Part | What it means | 5E3 example |
|---|---|---|
| Vk | The DC voltage across the cathode resistor, measured from the cathode end of the resistor to ground with the amp idling | 22 V |
| Rk | The cathode resistor's value in ohms. Read it from the schematic or the resistor itself | 250 Ω |
| Vk ÷ Rk | Voltage divided by resistance gives the current in amps | 22 ÷ 250 = 0.088 A |
| × 1000 | Converts amps to milliamps | 0.088 × 1000 = 88 mA total |
| ÷ number of valves | Shares the total between the valves on that resistor | 88 ÷ 2 = 44 mA per valve |
Shared or separate resistors: the 5E3's two 6V6s share one 250 Ω resistor, so you divide by 2. A Vox AC30's four EL84s share one 50 Ω resistor, so you divide by 4. A 5F1 Champ has one valve on its own 470 Ω resistor, so there's nothing to divide. Dividing assumes the valves draw equal current, which is one reason matched pairs and quads matter in cathode-biased amps: one valve can be working much harder than the other and the shared reading won't show it.
What this current includes: it's cathode current, which includes the screen current as well as the plate current. That's why the next formula multiplies by 0.95 when working out plate dissipation.
This is the same plate dissipation formula as before (watts = volts × amps), with two corrections for the way a cathode-biased amp is measured. It tells you how many watts each valve is turning into heat at its plate while the amp idles. Here's each part, carrying on with the same 5E3 numbers:
| Part | What it means | 5E3 example |
|---|---|---|
| Vp | The plate voltage, measured from the plate (pin 3 on a 6V6) to ground with the amp idling | 360 V |
| Vp − Vk | Subtracts the cathode voltage to give the voltage actually across the valve, from plate to cathode | 360 − 22 = 338 V |
| Ik per valve | The current per valve from the formula above, in amps (mA ÷ 1000) | 44 mA = 0.044 A |
| × Ik per valve | Voltage times current gives watts | 338 × 0.044 = 14.9 W |
| × 0.95 | Takes out the screen current, leaving just the plate's share | 14.9 × 0.95 = 14.1 W per valve |
Why subtract Vk: your meter measures the plate voltage to ground, but the cathode isn't at ground. It sits on top of the cathode resistor, 22 V up in this example. The valve only "sees" the difference between its plate and its cathode, so that's the voltage that heats the plate. Skip this step and you'd overstate the dissipation, by about 6% here. In a fixed-bias amp the cathode is at or very close to ground, which is why the fixed-bias formulas don't need it.
Why × 0.95: the current through the cathode resistor is cathode current, which includes about 5% screen-grid current. The screen current heats the screen grid, not the plate, so taking it out leaves just the plate current. Multiplying by 0.95 is near enough the same as the ÷ 1.05 used in the fixed-bias section.
What to do with the answer: divide it by the valve's Pmax to get a percentage, just as in the fixed-bias section. For the 5E3, 14.1 ÷ 12 = 118% of a 6V6GT's rating. The worked example below explains why that's normal for this amp.
The cathode resistor carries all the current of the valves above it, so it gets hot too. This formula tells you how many watts it turns into heat, so you can choose a resistor that won't burn out. It comes from two versions of Ohm's law put together: power = voltage × current, and current = voltage ÷ resistance, so power = voltage × voltage ÷ resistance. Here's each part, with the same 5E3 numbers:
| Part | What it means | 5E3 example |
|---|---|---|
| Vk | The voltage across the cathode resistor, the same reading as before | 22 V |
| Vk² | That voltage multiplied by itself | 22 × 22 = 484 |
| ÷ Rk | Divided by the resistor's value gives the watts it dissipates | 484 ÷ 250 = 1.9 W |
| × 2 | The minimum rating to fit, for a safety margin | 1.9 × 2 = 3.9 W, so fit a 5 W resistor |
A quick cross-check: you get the same answer from Vk × Ik total, using the current from the first cathode formula: 22 V × 0.088 A = 1.9 W.
Why at least twice the rating: a resistor running at its full rating gets very hot, and it usually sits in a hot chassis close to other parts. The current also rises when the amp is played hard or fitted with hotter valves, and a failing valve can draw far more. Doubling the rating keeps the resistor cooler and lets it ride out those peaks. Resistors come in standard ratings (1, 2, 3, 5 and 10 W are common), so round up to the next one.
Don't forget the bypass capacitor: most cathode resistors have an electrolytic capacitor across them, and it sees the same Vk. Choose one rated comfortably above it. With 22 V on a 5E3's cathode, a 25 µF 50 V capacitor gives plenty of margin.
Worked example: Fender 5E3 Tweed Deluxe
Two 6V6s share a 250 Ω cathode resistor. Say you measure 22 V across it and 360 V on the plates.
- Total current: 22 ÷ 250 = 88 mA, so 44 mA per valve
- Plate dissipation: (360 − 22) × 0.044 × 0.95 = 14.1 W
- Percentage: 14.1 ÷ 12 = 118% of a 6V6GT's rating
- Resistor power: 22² ÷ 250 = 1.9 W, so the original 5 W resistor has plenty of margin
Over 100% looks alarming, and it is hard on valves, but it's normal for a 5E3. Cathode-biased amps run hotter at idle than fixed-bias amps because they hold back the current as they're played harder: more current lifts the cathode voltage, which pulls the bias colder. That self-correction is why many cathode-biased amps idle anywhere from about 70% to over 100%. It's also why rugged valves like the JJ 6V6S (rated 14 W) are a popular choice in tweeds. See the full circuit in our 5E3 parts list.
Cool, warm or hot in a cathode-biased amp
You change cathode bias by changing the resistor:
- Cooler: a larger Rk. A step of about 10% to 20% (for example 250 Ω to 300 Ω in a 5E3) noticeably lowers the idle current and lengthens valve life.
- Hotter: a smaller Rk. Only do this if the valves are well under their rating to begin with.
- Starting value: Rk ≈ Vk ÷ target Ik total. This is only a starting point, because Vk changes as soon as you change the resistor, so fit it and measure again.
Bias chart for popular amps
Targets below are plate current per valve, in mA, worked out from the typical idle plate voltage of each amp. Your amp's voltage will differ, especially on Australian 230 to 240 V mains, which often runs a few percent above the voltages on American schematics. Measure your own plate voltage and use the calculator below for figures that suit your amp. If you read cathode current (1 Ω resistors or a bias probe), add about 5%.
Fixed-bias amps
| Amp | Power valves | Typical Vp | Cool 50% | Warm 60% | Hot 70% |
|---|---|---|---|---|---|
| Fender Deluxe Reverb (AB763 and '65 reissue) | 2 × 6V6 | 430 V | 14.0 | 16.7 | 19.5 |
| Fender Bassman 5F6-A (tweed) | 2 × 6L6GC | 430 V | 34.9 | 41.9 | 48.8 |
| Fender Twin Reverb (blackface and silverface) | 4 × 6L6GC | 460 V | 32.6 | 39.1 | 45.7 |
| Fender Super Reverb | 4 × 6L6GC | 460 V | 32.6 | 39.1 | 45.7 |
| Marshall JTM45 | 2 × KT66 | 440 V | 28.4 | 34.1 | 39.8 |
| Marshall JCM800 2204 (50 W) | 2 × EL34 | 460 V | 27.2 | 32.6 | 38.0 |
| Marshall 1959 Super Lead and JCM800 2203 (100 W) | 4 × EL34 | 480 V | 26.0 | 31.2 | 36.5 |
| Hiwatt DR103 Custom 100 | 4 × EL34 | 500 V | 25.0 | 30.0 | 35.0 |
| Mesa/Boogie Dual Rectifier * | 4 × 6L6GC | 470 V | 31.9 | 38.3 | 44.7 |
* Most Mesa/Boogie amps are fixed bias with no adjustment. Use Mesa-spec valves, and use these figures only to check that a new set is running within a safe range.
Cathode-biased amps
| Amp | Power valves | Cathode resistor | What to expect |
|---|---|---|---|
| Fender 5F1 Champ | 1 × 6V6, single-ended | 470 Ω for the one valve | Class A, so it idles near or over 100%. See our 5F1 parts list. |
| Fender 5E3 Tweed Deluxe | 2 × 6V6 | 250 Ω shared, 5 W | About 110% to 125% is common. Rugged 6V6s recommended. |
| Vox AC30 | 4 × EL84 | 50 Ω shared | Runs its EL84s very hot by design. Use good-quality matched quads and check them regularly. |
| Vox AC15, Marshall 18 Watt and similar | 2 × EL84 | One shared resistor (value varies by model) | Check with the cathode formulas above. About 80% to 100% is typical. |
Bias calculator
Pick an amp to load its typical figures, or enter your own measurements. The calculator does the arithmetic only. It can't tell you whether a reading is safe to take on your amp.
Work out your bias
Enter what you measured. Results update as you type.
⚠️ Safety warning
Every measurement on this page is taken on a live amp, with plate voltages of 300 to 500 V DC or more, and filter capacitors that hold a lethal charge even after the amp is switched off and unplugged. Use one hand, insulated probes and a meter rated for the voltage. If you're not trained to work inside a live chassis, have a qualified amp technician bias your amp. Please read our valve amp safety notes.
Shop power valves
We stock matched pairs and quads of 6V6, 6L6, 5881, EL34 and EL84, plus bias test points and bias pots. Browse all power tubes, or contact us if you're not sure what your amp needs. Changing to EL34-family valves? Read E34L vs EL34 first, because the E34L needs rebiasing.
