Guide to the PA0FRI "Turbo Mode" Modification for the Heathkit SB-200

 Guide to the PA0FRI "Turbo Mode" Modification for the Heathkit SB-200

The "Turbo Mode" modification, popularized by PA0FRI, increases the Heathkit SB-200's output power from ~600 W to 850–950 W PEP by adding a secondary high-voltage transformer.  This raises the unloaded anode voltage from ~2150 VDC to ~2950 VDC (loaded ~2250–2450 VDC). 

⚠️ Critical Safety Warning

This modification pushes components beyond original design limits.

Voltage Stress: Insulation must be rated for >3000 VDC.

Tube Limits: 572B tubes are stressed at these voltages; ensure they are high-quality (e.g., Taylor/Cetron) and properly reactivated. 

Arcing Risk: Increased risk of internal arcing; a "glitch" resistor and HV fuse are mandatory. 

Lethal Voltage: The amplifier contains >3000 VDC when unloaded.  Discharge capacitors before touching.

1. Required Components & Part Numbers

The core of this mod is a booster transformer wired in series with the original 800 VAC secondary. 

Component Specification Purpose Mouser Part # DigiKey Part #

Booster Transformer 230 VAC Primary / 250–300 VAC Secondary @ 0.6–1.0 A Adds ~350–400 VAC to the HV doubler input. 546-185F250 (Hammond, 250V/0.6A) 237-185F250-ND

HV Capacitors (Upgrade) 8 × 220 µF / 450 V Snap-in Replaces original 6 caps; higher capacity needed for increased voltage/ripple. 598-221A450M2PH (CDE) 493-15715-ND (Nichicon)

HV Rectifier Diodes 10 × BY225 (1300 V / 3 A) or 1N5408 Replaces original string; higher voltage/current rating mandatory. 512-BY225 BY225-ND

Glitch Resistor 15 Ω / 10 W Wirewound Limits current during tube flashover; sacrifices itself to save tubes. 290-15-RC 15F10-ND

HV Fuse 2 A / 10 kV Ceramic Fast-blow protection in anode line. 504-50200 F10200-ND

Toggle Switch DPDT / 250 VAC / 10 A Selects "Normal" (Booster bypassed) vs. "Turbo" (Booster in series). 506-7201K34 CH109-ND

HV Wire 18–20 AWG Silicone/PTFE Rated for >5 kV. 566-300V-18 (Belden) AE1018-ND

Note: The Hammond 185F250 (250V secondary) is ideal. If using a 120V primary unit, wire it for 230V input if possible, or use the 250V secondary directly.

Hammond 185F250 power transformer

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2. Step-by-Step Installation Guide

Step 1: Prepare the High-Voltage Capacitor Bank

The increased voltage and ripple current require expanding the filter bank from 6 to 8 capacitors.

Remove the original 6 electrolytic capacitors from the HV PCB.

Install 8 modern 220 µF / 450 V capacitors.

Since space is limited, mount 4 on the original PCB (if modified) or a new terminal strip, and 4 below the chassis or on a custom bracket near the meter.

Wiring: Ensure the series/parallel arrangement maintains the voltage doubler topology. The total capacitance per leg of the doubler increases, reducing ripple. 

Replace Bleeder Resistors: Install 82 kΩ / 3 W resistors across each of the 8 capacitors (replacing the original 30 kΩ/10 W units).  This reduces heat dissipation significantly.

Step 2: Mount the Booster Transformer

Location: The ideal location is the space vacated by the original large electrolytic capacitors (under the meter/near the front panel). 

Mounting: Secure the Hammond 185F250 transformer using insulated hardware. Ensure it does not short against the chassis or other components.

Primary Wiring: Connect the transformer's primary to the 120 VAC mains (parallel connection of the SB-200's main transformer primaries if running on 120V, or a dedicated tap).

Alternative: Some builders power the booster primary from the SB-200's 6.3V filament winding using a step-up configuration, but direct mains connection is more stable. The PA0FRI design suggests wiring it to the mains or a dedicated winding.

Recommended: Wire the booster primary in parallel with the main transformer primary (after the soft-start circuit) so it powers up simultaneously. 

Step 3: Series Connection (The "Turbo" Circuit)

This step injects the booster voltage into the HV doubler.

Identify the 800 VAC Secondary: Locate the two wires from the main transformer's high-voltage secondary (typically Red and Red/Yellow or similar, feeding the doubler).

Break the Circuit: Cut one of the 800 VAC lines (e.g., the Red wire) going to the doubler input. 

Install the Toggle Switch:

Connect the Cut End A (from transformer) to the Common terminal of the DPDT switch.

Connect Cut End B (to doubler) to Terminal 1 (Normal Position).

Connect the Booster Secondary wires to Terminal 2 (Turbo Position) in series.

Wiring Logic: In "Normal," the switch passes the 800 VAC directly. In "Turbo," the 800 VAC passes through the booster secondary (e.g., 800 V + 250 V = 1050 VAC input to the doubler).

Phasing is Critical: If the voltage drops when switching to Turbo, reverse the two wires connecting the booster secondary to the switch. 

Step 4: Upgrade Rectifiers and Protection

The original diode string is insufficient for ~3000 VDC peaks.

Replace Diodes: Install a string of 10 × BY225 (or 1N5408) diodes in series.

Original design used 16 lower-rated diodes; 10 modern 1300V diodes provide a 13,000 V PIV rating, offering a huge safety margin.

Install Glitch Resistor & Fuse:

Insert the 15 Ω / 10 W resistor and 2 A HV fuse in series with the positive HV output (anode feed) before the plate choke.

Add a 4.7 nF / 3 kV ceramic capacitor from the junction of the fuse/resistor to ground for RF filtering. 

Step 5: Final Adjustments & Testing

Input Network: The higher voltage may require slight retuning of the input network capacitors (especially on 10m/15m) to maintain low SWR.

PA0FRI recommends replacing fixed input capacitors with trimmers or adjusting values (e.g., 10m coil to 6 turns). 

Bias Check: Measure the negative grid bias. It may need slight adjustment to maintain ~90–100 mA idle current due to the higher plate voltage. 

Test Sequence:

No Tubes: Power up in "Normal" mode. Check doubler output (~2100 VDC). Switch to "Turbo." Verify voltage rises to ~2600–2700 VDC unloaded.

With Tubes: Insert tubes. Reactivate if necessary.

Load Test: Connect a dummy load. Key the amplifier.

Normal: ~1900 VDC loaded, ~600 W output.

Turbo: ~2250–2450 VDC loaded, 850–950 W output. 

Monitor: Watch for arcing. If arcing occurs, clean the chassis, check tube quality, or reduce drive power.