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Bias Bench
Fixed and cathode bias, push-pull and single-ended, with screen-grid current and PSU sag accounted for. The math from the tech pages, done for you.
High voltage — read this first
The B+ rail in a tube amp sits at 400–500 V DC and it can kill you. Filter capacitors hold that charge for minutes — sometimes hours — after the amp is unplugged. Before your hands go near the chassis: discharge the caps through a bleeder resistor, then verify with your meter that B+ has fallen to a safe level. Keep one hand in your pocket while probing a live amp. If any of this is unfamiliar, this is not the job to learn it on.
Setup
Push-pull · Fixed bias · 6L6GC · ×4
Measure DC voltage between the OT center tap and each plate (outer winding end). You'll also need each half's DC winding resistance and the B+ voltage at the center tap.
Datasheet reference
Pd max 30W · Pg2 max 5W✓ sourced
Datasheet typical (fixed bias, PP): 450V / 58mA per tube → 26.1W
RCA RC-30, pp.353–354 (Design-Maximum rating)
Tube count
Measurement grouping
"Shared per side" is the standard method: measure CT-to-plate drop across each OT half. Individual only applies if this amp has dedicated per-tube test points.
Measurements
Power supply sag — optional
B+ isn't a fixed number — it droops as total current draw rises, because the supply (choke, dropping resistor, transformer/rectifier regulation) has its own source impedance. This only matters for "set a target" measurement points below: it corrects the required voltage for the fact that B+ will be lower once the bias is actually brought up to that target than it is right now at the current (measured) draw.
Estimate it from your supply's choke/resistor DC resistance plus rectifier regulation, or from two real B+ readings at two different current draws: Rsource ≈ ΔB+ / ΔI.
2 tubes
2 tubes
Screen grid (Vg2) — optional
Fill this in if screen voltage runs noticeably below B+ (power-scaled/variable-Vg2 designs) or you want screen dissipation checked independently. Leave blank for a plain pentode/tetrode running screen straight off B+ — it's not needed for the plate-dissipation numbers above.
Results
6L6GC · Push-pull · Fixed bias · Quartet (4) · Shared per side (2 points)
Enter B+ and each measurement point's resistance/voltage above — or set a target — to see results.
Bias target threshold
A common conservative target is 60–70% of rated dissipation for tube longevity. 100% is the datasheet's absolute figure, not a safety margin.
Operating point
Amp profiles & history
Saves winding/resistor values and tube setup — not today's voltage readings, so you're prompted for fresh measurements every time you re-bias.
Reading history
No saved readings yet.
Backup
Tube ratings sourced from datasheets where noted; unverified entries are flagged. This tool estimates dissipation from your measurements — it does not replace datasheet judgment or amp-specific service literature. All data stays in this browser; nothing is uploaded.
Where this comes from
The method behind the numbers
Bias Bench uses the output-transformer resistance method — safer than putting a meter in series with the plate current, and second only to a scope on a dummy load. The full bench procedure, with the math worked through step by step, is written up here.
Also on the bench
The other half of the amp
Biasing sets how the output stage behaves. What reaches it was already shaped by the tone stack — and that circuit is where most of an amp's character actually lives. Tone Stack Lab plots it live, with the real component values from 55 amps and pedals.
Free, and staying that way
If this saved you a trip to the bench, chip in
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