Free Bench Resource
Estimate target current and compare a tube's operating point against its plate-dissipation rating.
Warning: tube amplifiers contain lethal live and stored voltage, sometimes hundreds of volts after power is removed. If you are not trained to work safely inside a live chassis, do not use this tool to guide measurements. Use the one-hand rule when probing a powered amp and stop immediately if you are unsure about any step.
Calculator
Fixed-bias mode compares your measured per-tube current to live plate voltage. Cathode-bias mode uses cathode current per tube as a conservative operating point.
Step 1 - Setup
Choose this for adjustable fixed-bias amps when your probe, meter, or test point reports current for one tube.
Sets the plate-dissipation target for the calculator.
Step 2 - Enter Measurements
Measured at the plate pin while the amp is operating normally.
Use the current your meter, probe, or sense resistor reports for one output tube.
Selected Tube
Rating
30W
Calculated Operating Point
Inputs Used
46.7 mA per tube at 450 VDC
Result
21.02W against a 30W rating
Measured current per tube
46.7mA
Plate voltage
450VDC
Plate Watts
21.02W
Some probe and cathode-sense readings include a little screen current, so read this as a practical bench reference.
Target current table
450 VDC plate
| Dissipation | Target mA | Plate Watts |
|---|---|---|
| 50% | 33.3mA | 15.0W |
| 60% | 40.0mA | 18.0W |
| 70% | 46.7mA | 21.0W |
| 80% | 53.3mA | 24.0W |
| 90% | 60.0mA | 27.0W |
| 100% | 66.7mA | 30.0W |
Technical Guide
The cleanest bias math in the world is useless if the measurement itself is sloppy or dangerous. These are bench habits, not bravado. If your setup is not stable, stop before the live measurement and regroup.
Clip your meter ground first with the amp powered down, then keep one hand away from the chassis when taking the live reading.
Use insulated probes and a meter rated for the voltage you expect to see. Do not improvise with worn leads.
Stabilize the chassis before measuring. A slipping probe can short high-voltage nodes in an instant.
After power-down, verify that filter capacitors are discharged before touching the circuit or moving probes around.
If bias current is drifting hard, the amp is red-plating, the mains fuse keeps opening, or plate voltage is nowhere near what the circuit should produce, you are no longer doing a routine bias check. You are troubleshooting a fault, and guessing gets expensive fast.
At that point, the right move is any qualified tech with a good bench setup, whether that is your local repair shop or someone you already trust.
Reference Library
The dropdown covers more tube types than the guide below. These are the families most players and techs run into on the bench, with quick context for tone, common applications, and typical bias habits. Tube ratings here are common reference values; verify the manufacturer datasheet for the exact tube in hand when the margin matters.
Plate Dissipation Rating
25W rating
Tonal Character
The classic British-rock bottle: forward mids, quick attack, and aggressive breakup when pushed.
Common Amps
Marshall 50/100 watt heads, Orange, Laney, some Hiwatt variants
Typical Bias Window
Usually 60-70% in fixed-bias Class AB amps
Modern 6CA7 variants can behave differently from slim-bottle EL34s, but the same dissipation math still applies.
Plate Dissipation Rating
30W rating
Tonal Character
Big clean headroom with tighter lows and glassier highs than an EL34.
Common Amps
Fender Twin Reverb, Bassman, Bandmaster, many Mesa circuits
Typical Bias Window
Usually 60-70% in fixed-bias Class AB amps
Plate Dissipation Rating
14W rating
Tonal Character
Lower-watt American favorite with earlier compression and sweeter breakup.
Common Amps
Princeton Reverb, Deluxe Reverb, Champ-derived builds
Typical Bias Window
Often 60-70% fixed bias, sometimes hotter in smaller cathode-biased amps
Plate Dissipation Rating
12W rating
Tonal Character
Chimey upper mids, quick compression, and a lively feel at lower wattage.
Common Amps
Vox AC15/AC30, Matchless Lightning, many 18-watt Marshall circuits
Typical Bias Window
Frequently run hot, especially in cathode-biased designs
Many EL84 amps are cathode biased, so treat the cathode-current result as a conservative operating point.
Plate Dissipation Rating
42W rating
Tonal Character
Massive low end, high headroom, and a more hi-fi top end in powerful amps.
Common Amps
Ampeg SVT, Sunn, Hiwatt DR and bass platforms
Typical Bias Window
Often 50-65% in high-voltage fixed-bias applications
Plate Dissipation Rating
25W rating
Tonal Character
KT66 leans warm and broad; KT77 sits closer to an EL34 with firmer lows.
Common Amps
JTM45-inspired builds, boutique British-voiced amps
Typical Bias Window
Typically 60-70% in Class AB push-pull circuits
Plate Dissipation Rating
19W rating
Tonal Character
Compact bottle with strong output, used heavily in vintage Ampeg and hi-fi gear.
Common Amps
Ampeg Reverberocket, Gemini, Scott and Fisher hi-fi amps
Typical Bias Window
Generally 60-70%, but verify circuit-specific plate voltage first
Safety Protocol
A tube amplifier chassis can contain lethal AC and DC voltage, including stored voltage after the amp is unplugged. A calculator cannot make live chassis work safe or diagnose a faulted amp. Use proper tools, stable probe placement, and training. If you are second-guessing the measurement, probe placement, or condition of the amp, stop there.
If you are not experienced working around high voltage, have a qualified amp technician service the amplifier. A competent local tech is a much better next step than pushing forward on a live amp when you are unsure.
FAQ
For many Class AB guitar amps, techs often start around 60-70% of the tube's plate-dissipation rating and then verify stability, plate voltage, and tone from there. The right target still depends on the specific circuit and the tube family in use.
Cathode-biased amps lift the cathode above ground, so the tube does not see the full B+ value across plate-to-cathode. Subtracting cathode voltage from plate voltage gives the effective voltage used in the dissipation estimate.
The calculator uses the current you type in. Depending on how you measure, that number can include a small amount of screen current. Treat it as a practical bench reference and verify that the amp is stable.
Use it only if you already know how to work safely around live tube amps. If the amp is unstable or any measurement feels uncertain, hand it to a qualified tech.
If you happen to be in western Massachusetts and would rather hand the job off, I do offer repair and restoration work through Moddjobbs. The main goal here, though, is to make a solid free reference available to the wider amp community.
Contact Moddjobbs