Homebrew Linear Amplifier Plate Current Anomaly
You are testing a homebrew 3-500Z grounded-grid linear amplifier. At 1,800 V plate voltage and 400 mA plate current, the output is 400 W into a dummy load. The plate current meter shows a sudden 50 mA increase when you touch the tuning capacitor shaft, and the grid current meter reads higher than expected. The amplifier is keyed by a relay in the transceiver, and the RF drive is 100 W. The plate parasitic choke is a 1 µH inductor with a 100 ohm resistor in parallel. You notice the plate cap connector is slightly loose.
1. What does the sudden plate current change when touching the tuning capacitor indicate, and what is the risk?
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A change in plate current when touching the tuning capacitor shaft indicates RF current flowing on the capacitor shaft or chassis, likely due to poor RF grounding or parasitic oscillation. The human body adds capacitance to ground, changing the circuit's stray capacitance and detuning or altering the parasitic path.
Risks:
- Parasitic VHF oscillation can destroy the tube or cause arcing.
- High RF voltage on the shaft presents a shock and RF burn hazard.
- Unstable tuning can lead to excessive plate dissipation.
Immediate action: shut down and discharge the plate supply, then inspect the chassis bonding, the parasitic choke, and the capacitor mounting. Do not touch the capacitor while transmitting.
2. How do you evaluate whether the parasitic choke is adequate?
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The 1 µH inductor with a 100 ohm resistor in parallel forms a parasitic suppressor intended to damp VHF parasitic oscillations. Evaluate:
- Resonance: The choke should present a high impedance at the parasitic frequency (often 100-200 MHz for a 3-500Z). A 1 µH inductor resonates with stray capacitance; the 100 ohm resistor damps the resonance.
- Power rating: The resistor must handle the RF current; a 100 ohm, 2 W carbon composition is common.
- Placement: The suppressor should be close to the tube plate and connected with short leads.
- Test: Look for stable plate current with no drive, and check for VHF output with a receiver or spectrum analyzer. If oscillation persists, add a second suppressor or change the inductor value.
A loose plate cap connector can also cause arcing and parasitic excitation; tighten and inspect it first.
3. The amplifier produces 400 W output at 400 mA plate current and 1,800 V plate voltage. Estimate the plate dissipation and efficiency, and comment on whether the tube is being overstressed.
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Input power: P_in = V_plate × I_plate = 1,800 V × 0.400 A = 720 W.
Output power: 400 W.
Plate dissipation: P_diss = P_in − P_out = 720 − 400 = 320 W.
Efficiency: η = 400 / 720 ≈ 56%, which is reasonable for a grounded-grid linear amplifier in class AB.
The 3-500Z has a plate dissipation rating of 500 W. At 320 W, the tube is at about 64% of its rating, which is acceptable but leaves limited margin. If the loose plate connector or parasitic oscillation causes increased current, the tube could exceed 500 W dissipation. Fix the connector and verify stable operation before sustained use. Also check that the grid current is within the tube's rating (typically 150-200 mA).