Heathkit SB-200 CTR Engineering Arc Protection Kit Revised Installation Guide

Heathkit SB-200 CTR Engineering Arc Protection Kit Installation Guide

This guide details the installation of the CTR Engineering SB-200KP Arc Protection and Relay Buffer Kit specifically for the Heathkit SB-200 amplifier using 572B tubes. All references to the Collins 30L-1 have been omitted.

⚠️ Safety Warning

High Voltage Lethal Hazard: The SB-200 contains voltages exceeding 2,400V DC. Capacitors can hold a lethal charge long after unplugging.

Unplug the amplifier.

Wait 15 minutes.

Discharge all high-voltage capacitors and the plate tank circuit using a grounded insulated screwdriver.

Verify zero voltage with a multimeter before touching any internal components.

Phase 1: High Voltage (HV) Glitch Resistor Installation

Purpose: Limits current during an internal tube arc, protecting the power supply rectifiers, filter capacitors, and transformer.

Location: Under the chassis, connecting the Power Supply PCB to the RF Deck feed-through.

Installation Steps

Access: Turn the chassis upside down. Locate the Power Supply PCB (large board with capacitors and rectifiers).

Identify Wire: Find the heavy gauge blue wire soldered to the lower-left corner of the PCB (near the HV filter capacitors). This wire carries +2,400V to the RF deck.

Disconnect: Unsolder this blue wire from the PCB pad.

Mount Standoffs: Remove the two #6 nuts securing the left side of the PCB to the chassis. Install the provided insulated standoffs in these holes.

Mount Resistor: Secure the 10Ω 9kV Glitch Resistor (sandwiched between the two Lexan shields) onto these standoffs.

Reconnect Input: Solder the original blue wire (from the PCB) to the input side of the glitch resistor assembly.

Connect Output: Solder a new piece of heavy-duty HV wire (provided or salvaged) from the output side of the glitch resistor to the original feed-through grommet leading into the RF deck.

Note: Ensure the resistor assembly does not touch the chassis or other components. The Lexan shields prevent arcing to ground.

ASCII Schematic: HV Glitch Resistor

FROM HV RECTIFIER/FILTER CAPS (+2400V)

| (Original Heavy Blue Wire)

.-----+-----.

| PCB Pad | (Original Solder Point)

'-----+-----'

(Unsoldered Original Wire)

.-----+-----.

| 10Ω 9kV | <--- GLITCH RESISTOR (Kit)

| (Lexan | Mounted on Insulated Standoffs

| Shields) |

'-----+-----'

| (New HV Wire Extension)

.-----+-----.

| Feed-Through|

| Grommet | --> TO RF DECK / PLATE CHOKE / TUBE PLATES

'-------------'

Phase 2: Grid Circuit Arc Protection

Purpose: Prevents grid-to-cathode arcs from destroying the bias supply, meter, and driving transceiver.

Location: At the 572B Tube Sockets (Under the chassis or inside RF cage depending on socket type). Note: The SB-200 uses 4-pin UX sockets for the 572B.

Pin 1 & 2: Filament

Pin 3: No Connection

Pin 4: Control Grid

Installation Steps

Access: Remove the RF cage cover to access the tube sockets.

Identify Grid: Locate Pin 4 on both tube sockets. This is the grid connection.

Cut Grid Lead: Cut the existing wire connecting Pin 4 to the ground/bias network (usually leading to the 33Ω resistors R21/R22).

Install Surge Resistor: Solder a 33Ω 1W Metal Composition Resistor (provided) directly to Pin 4. Keep leads extremely short.

Reconnect Circuit: Solder the original grid circuit wire to the other end of this new 33Ω resistor.

Install Grid GDT: Solder a 230V Gas Discharge Tube (GDT) from the Pin 4 side of the new 33Ω resistor (directly at the pin) to Chassis Ground.

Function: If an arc occurs, the GDT fires instantly, shorting the grid to ground and killing the arc before it damages the bias supply.

Install Bypass Capacitor: Solder a 0.001µF (1000pF) HV Disc Capacitor in parallel with the GDT (from Pin 4 side to Ground) to ensure RF grounding.

ASCII Schematic: Grid Protection

TUBE SOCKET PIN 4 (GRID)

.----+----.

| 33Ω Res | <--- Surge Resistor (Kit)

'----+----'

+-------||------+

| | |

[GDT] [C] [D1]

(230V) (1000pF) (1N5408)

| | |

GND GND GND

| (To Bias Network / Meter / ALC)

.----+----.

| Zener | <--- 3.9V Zener (Bias Stability - if included in specific kit rev)

'----+----'

GND

LEGEND:

[GDT] = Gas Discharge Tube (Arc Short)

[C] = 0.001uF HV Capacitor (RF Bypass)

[D1] = 1N5408 Diode (Positive Clamp: Anode to Grid, Cathode to GND)

Phase 3: Filament Arc Protection

Purpose: Protects the filament transformer from cathode-to-filament arcs.

Location: At the 572B Tube Sockets (Pins 1 & 2).

Installation Steps

Identify Filament Pins: Locate Pin 1 and Pin 2 on the tube sockets.

Install GDTs:

Solder a 150V GDT from Pin 1 to Chassis Ground.

Solder a 150V GDT from Pin 2 to Chassis Ground.

Lead Length: Keep leads as short as possible to minimize inductance.

Phase 4: Meter & Bias Protection

Purpose: Prevents high-voltage spikes from pegging or burning out the meter movement and stabilizes bias.

Location: Under the chassis at the Meter terminals and Bias Resistors.

Installation Steps

Meter Protection:

Locate the terminals on the rear of the front-panel meter.

Solder two 1N4007 Diodes across the meter terminals in inverse parallel (Anode of D1 to Cathode of D2, and vice versa).

Function: Clamps reverse voltage spikes to ~0.7V, protecting the delicate meter coil.

Grid Positive Clamp:

Locate the common grid bias line (after the 33Ω surge resistors).

Solder a 1N5408 Diode from this line to Chassis Ground.

Orientation: Anode to the Grid Line, Cathode (Stripe) to Ground.

Function: Prevents the grid voltage from ever going positive during a severe arc.

Bias Stabilization (If applicable to kit version):

Install the 3.9V Zener Diodes in the bias supply divider network as per the specific schematic included in your kit envelope to stabilize idle current against line voltage variations.

Phase 5: Relay Buffer (Soft Keying)

Purpose: Isolates modern solid-state transceivers from the SB-200's harsh -120V DC keying voltage.

Location: Under the chassis, near the original T/R Relay.

Installation Steps

Identify Keying Line: Locate the wire carrying the -120V DC keying voltage to the T/R relay coil.

Disconnect Original Key: Disconnect the original keying input (usually from the front panel "Key" jack or rear accessory jack).

Install Buffer Input: Connect your transceiver's PTT/Key Line to the Input of the CTR Relay Buffer board.

Install Buffer Output: Connect the Output of the buffer board to the T/R relay coil.

Power: Connect the buffer board's power input to the SB-200's -120V DC source (so the buffer switches the high voltage internally).

Ground: Connect the buffer board ground to the chassis.

Phase 6: Final Verification

Visual Check: Verify all solder joints are solid. Ensure no component leads are touching high-voltage points or the chassis improperly.

Resistance Check: With power OFF, measure resistance from the HV input (after the glitch resistor) to ground. It should read the DC resistance of the plate choke (typically a few hundred ohms).

Power Up (No Drive):

Install 572B tubes.

Turn on Filament power. Wait 2 minutes.

Switch to Operate. Verify the relay clicks.

Check Plate Current: Should be stable (approx. 90-110mA total for the pair).

ALC Check: Connect a transceiver. Key the amp. Verify the ALC voltage does not exceed -10V at the radio (use a multimeter). If it exceeds -10V, the Zener clamp (if installed) or external clamping is required to protect the radio.

Summary of Modifications

Component Stock SB-200 With CTR Kit Benefit

HV Line Direct Connection 10Ω 9kV Glitch Resistor Limits arc current; saves caps/rectifiers.

Grid (Pin 4) Direct Ground/Bias 33Ω Res + 230V GDT Instantly shorts arc; saves bias supply.

Filament Unprotected 150V GDTs Protects filament transformer from arcs.

Meter Unprotected 1N4007 Diodes Prevents meter burnout during faults.

Keying -120V Direct Relay Buffer Safe for modern solid-state radios.