SB-200 High value Capacitor info

 To add an extra capacitor to the high-voltage (HV) line on the Heathkit SB-200 amplifier for additional filtering or bypassing, the recommended specifications based on documented modifications are:

• Capacitance: 4.7 nF (which equals 4700 pF)

• Voltage Rating: 3 kV (3000 volts) or higher 

This modification involves placing the capacitor in parallel with the existing 1 nF capacitor from the bottom of the anode choke to earth ground. It is intended to increase output on the 80-meter band and provide additional filtering. Some builders opt for even higher voltage ratings (4–5 kV) for increased safety margin given the SB-200’s HV supply can exceed 2500 VDC under load. 

Heathkit SB-200 high voltage capacitor mod

SB-200 HV line capacitor upgrade 4.7nF 3kV

If placing the capacitor near the glitch resistor (typically across the HV line to chassis ground immediately after the resistor or at the power supply output), the specifications remain consistent with the primary HV bypass requirements, though the voltage stress can be slightly higher during a fault event before the resistor limits current. 

Recommended specifications for a capacitor in this location:

• Capacitance: 2,200 pF to 4,700 pF (0.0022 µF to 0.0047 µF).

  • Common values used in modifications are 2,200 pF (0.0022 µF) or 3,900 pF (0.0039 µF).

• Voltage Rating: 3 kV minimum, but 5 kV to 10 kV is strongly preferred for this specific location.

  • Since this capacitor is near the raw supply output and the glitch resistor, it may see the full transient voltage before the resistor drops the potential during an arc. Many builders upgrade to 10 kV "doorknob" or ceramic transmitting capacitors here for maximum safety margin. 

Function: In this position, the capacitor acts as the "last line of defense" to shunt RF energy and transient spikes directly to ground before they can travel back into the power supply wiring or cause the glitch resistor to fail catastrophically. 

Heathkit SB-200 glitch resistor installation

SB-200 HV bypass capacitor 10kV doorknob

In the original Heathkit SB-200 schematic, the capacitor connected from the bottom of the anode (plate) choke to earth ground is rated as follows:

• Capacitance: 0.001 µF (which equals 1,000 pF or 1 nF)

• Voltage Rating: 3 kV (3,000 volts) 

This capacitor serves as the primary RF bypass for the high-voltage line. While the original specification is 3 kV, many restorers and modifiers upgrade this component to 4 kV or 5 kV when adding parallel capacitance (such as the common 4.7 nF add-on) to ensure a sufficient safety margin against voltage spikes and arcs, especially given the amplifier's operating voltage of approximately 2,300–2,500 VDC. 

Heathkit SB-200 schematic original capacitor values

SB-200 anode choke capacitor location

Capacitor tolerance has a minimal direct effect on the SB-200's RF output power or stability in the high-voltage (HV) bypass circuit, but it can significantly impact the input tuning circuit and output tank circuit performance. 

HV Bypass Capacitors (Anode Choke to Ground)

For the HV bypass capacitors (such as the original 1,000 pF or the common 4,700 pF upgrade), tolerance is not critical. 

• Function: These capacitors act as a low-impedance path to ground for RF energy and transient spikes. 

• Impact of Tolerance: Whether the value is 3,900 pF, 4,700 pF, or 5,600 pF (standard ±20% tolerance), the reactance at HF frequencies remains sufficiently low to effectively bypass RF. A variance of ±20% does not noticeably alter amplifier performance, stability, or output power in this part of the circuit. 

• Recommendation: Standard ceramic disc or doorknob capacitors with ±20% tolerance are perfectly adequate and commonly used. 

Input Tuning Circuit Capacitors

In contrast, tolerance is critical for the silver mica capacitors in the input tuned circuit (grid circuit).

• Function: These capacitors, along with the coils, form resonant circuits designed to match the amplifier's input impedance to 50 ohms on each band. 

• Impact of Tolerance: The SB-200 is known for having input SWR issues, particularly on 15 and 10 meters. Using capacitors with wide tolerances (e.g., ±20%) can shift the resonant frequency of the input network, causing high SWR and forcing the exciter to reduce drive power. 

• Recommendation: Modifications (such as those by NE7X) specifically specify using silver mica capacitors with ±5% tolerance (or tighter) to ensure the input circuit resonates precisely at the intended frequency, optimizing SWR and power transfer. 

Output Tank Circuit Capacitors

Tolerance is also moderately important for the fixed high-voltage mica capacitors in the output tank circuit (e.g., the 100 pF capacitor switched in for 80 meters). 

• Impact: A capacitor significantly out of tolerance (or drifted due to age/failure) can detune the output circuit, making it difficult to achieve a proper load match or reducing maximum power output on specific bands. 

• Recommendation: While ±10% or ±20% is often acceptable, replacing aged components with known good values (±5% or ±10%) ensures the tank circuit behaves according to the original design calculations.