1. History, Variants & Specifications

The Alpha 91B was designed and sold by Alpha/ETO and was physically assembled at a manufacturing facility in Bulgaria during the 1990s. This Bulgarian assembly origin has a direct implication for restoration: replacement top covers are no longer available and have not been for approximately 25 years. Alpha RF Systems confirms that the Alpha 99 covers look similar but do not fit the 91B.¹

The 91B is classified as a manual-tune amplifier — unlike the Alpha 87A with its microprocessor-driven autotuning capacitors, the 91B’s Tune and Load capacitors are adjusted by front-panel controls. However, it incorporates several sophisticated protection circuits that go significantly beyond a simple manual-tune design: arc and mistuning detection, plate overcurrent relay, Electronic Bias System (EBS), and a full break-in vacuum relay QSK system rated for continuous high-power QSK operation.

The 91B’s use of the 4CX800A/GU74b tetrode rather than a triode fundamentally changes the circuit architecture relative to amplifiers such as the SB-220, SB-200, or Alpha 87A. A tetrode requires separate control grid bias and a regulated screen grid supply in addition to the plate voltage. It offers significantly higher power gain (~14 dB vs ~6 dB for a typical grounded-grid triode), enabling full rated output with only 50–60 W of drive from the transceiver.²

2. Pre-Restoration Assessment

2.1 Initial Cold Inspection

Before ordering parts or energising the amplifier, perform a cold inspection with the cover off and AC mains unplugged:

• Inspect the 4CX800A/GU74b tube envelopes for cracked ceramic or glass seals, discoloured getter material, and physical damage to the anode fins. The getter in a GU74b tube is a metallic ring visible through the glass envelope — a white or milky colour indicates a vacuum failure.
• Inspect the HV filter capacitor bank (C2–C10) for bulging, leakage, or heat-deformed mounting. There are nine capacitors in this bank; measure voltage across each individually during powered testing (see Section 3.2).
• Inspect MOSFET Q12 (screen regulator, mounted on rear of sub-panel for heatsinking) for burn marks or cracked package. A failed screen regulator is a known service item and will prevent normal operation.
• Check step-start resistors R4 and R9 (wirewound, 10Ω / 15 W) for cracking, carbonisation, or open circuit.
• Examine the plate overcurrent relay K1 and the HV crowbar vacuum relay for evidence of arc damage to their contacts.
• Inspect all HV wiring for cracked or brittle insulation, especially in the transformer area and around the filter capacitor bank.
• Verify the cover interlock S3 operates correctly (actuates when cover is removed, prevents power-up).
• Check the gas-discharge surge suppressor GT (screen grid to ground) for cracks or arc damage.

2.2 Transformer Installation Check

The 91B transformer ships in a separate carton and is installed by the owner. If the unit has changed hands or been stored for years, verify the transformer is correctly installed and that all connector mating is correct. The manual warns that only one orientation of the transformer allows correct mating of all connectors without straining leads. Inspect for any pinched or kinked transformer leads resulting from incorrect installation.³

Verify the primary voltage connection matches your mains supply. The 91B standard version is wired for 200–250 V AC; a 115 V tap is available on the transformer. Incorrect primary voltage connection will cause immediate component failure on power-up.

3. Internal Circuits — Component Restoration

3.1 The Tetrode Advantage — And Its Service Requirements

The 4CX800A/GU74b tetrode’s high gain is achieved through the screen grid (G2) which electrostatically shields the control grid from the plate, eliminating the feedback that limits triode gain. This comes at the cost of a regulated screen grid supply that must be present for correct and safe tube operation. Screen current must also be monitored and limited; MOSFET Q12 provides both regulation and current limiting for the screen supply in the 91B.⁴

Additionally, a gas-discharge surge suppressor (GT) is connected from the screen grids to ground. This device clamps the screen voltage during a tube arc or internal flash-over, protecting Q12 and associated components. The manual states this system has “virtually eliminated RF arc damage in current ETO amplifiers.”

3.2 Power Supply Section

The 91B uses a single main transformer (T1) that provides all raw AC voltages. The high-voltage full-wave bridge rectifier (D12–D31) consists of 600 V / 5 A power diodes; some production units use a moulded block HV rectifier assembly instead. The HV filter bank (C2–C10) provides nine capacitor positions. The screen grid supply uses bridge D1–D4, with MOSFET Q12 providing regulation and current limiting. The bias supply uses D5–D8 with associated components.

3.3 Electronic Bias System (EBS)

The EBS is a distinctive Alpha/ETO circuit that automatically reduces the idle plate current from 350–400 mA (during active RF) to 30–50 mA during key-up pauses in speech or between CW characters. A detector on the Control Board senses the presence of RF drive; when RF drive is removed, the EBS shifts the control grid bias more negative, cutting plate current to the low-idle level. When RF drive resumes, the bias returns to normal and full-power operation continues without operator intervention.

The EBS substantially reduces average power supply loading, heat generation, and wasted filament energy during typical SSB operation. It also provides a mild degree of RF drive-level detection that contributes to the mistuning protection circuit.

3.4 RF Deck — Tank Circuit & Tetrode Input

The 91B’s output uses a switched pi-L network with plate RF choke L4 switched by band switch section S1C to optimise performance across all nine amateur bands from 1.8 to 29.7 MHz. The two-section Tune (C20) and Load (C26) capacitors are switched to provide an “electrical vernier” for smooth, accurately-resettable tuning by-the-numbers on each band.⁵

Unlike grounded-grid triode amplifiers, the 91B uses an untuned input: the RF input is terminated in a non-inductive 50Ω resistor (R1) on the Input Board. This resistive termination provides consistent 50Ω input impedance across all bands without band-switched input networks, at the cost of power dissipated in R1 during operation. R1 is a service item; inspect for discolouration or cracking.

3.5 Vacuum Relay T/R System (QSK)

Unlike the Alpha 87A with its PIN diode T/R switching, the 91B uses vacuum relay switching for T/R and QSK functions. Vacuum relays offer very low contact resistance, high contact voltage and current ratings, and long life in HF switching service. They are, however, mechanical devices that can wear, become contaminated, or fail in ways that PIN diodes cannot.

3.6 Control Board & Metering

The Control Board is located on the front of the 91B sub-panel. It houses monitoring and protective circuits on the left half of the schematic, warm-up timing and status indicators at the lower right, and keying/switching functions at the upper right. The Display Board on the rear of the front panel carries all indicator LEDs and their driver chips.

4. Troubleshooting Reference

The following table summarises the official troubleshooting symptoms from the Alpha 91B user manual, with additional context from the restoration community.⁶

5. Safety: High Voltage, Multiple Supply Rails & Interlocks

5.1 Cover Interlock (S3)

The 91B includes a cover safety interlock switch S3 that prevents the amplifier from powering up when the cover is removed. This is a power-prevention interlock, not an active discharge circuit — it does not short the HV to ground when the cover is removed. If the amplifier is powered up normally and the cover is then removed during operation, S3 will open and cut AC power. However, the filter capacitors will retain their full charge and must be discharged through the bleeder resistors over several minutes, or manually discharged as described in Section 5.2.

5.2 Safe Discharge Procedure

  STEP 1 ─── Switch amplifier to STANDBY; press power OFF
                │
  STEP 2 ─── Unplug AC mains cord from wall outlet
                │
  STEP 3 ─── Wait 5 minutes (bleeder resistors discharge HV filter bank)
                │
  STEP 4 ─── MAIN HV CHECK:
             Multimeter set to 4000V DC range.
             Probe (+) to HV positive on filter cap bank.
             Probe (-) to chassis. Confirm < 50V.
                │
  STEP 5 ─── SCREEN SUPPLY CHECK:
             Separately measure screen supply output node.
             Confirm screen supply < 50V before touching screen circuit.
                │
  STEP 6 ─── MANUAL DISCHARGE (belt & suspenders):
             Insulated discharge stick (10kΩ / 25W + insulated handle).
             Touch probe to each filter capacitor positive terminal,
             and to the screen supply output node, holding 10 seconds each.
                │
  STEP 7 ─── Re-measure ALL supply nodes. Confirm < 10V on each.
                │
  STEP 8 ─── Safe to work internally.
             ─────────────────────────────────────────────────────
             IMPORTANT: Dielectric absorption in large electrolytics
             can cause charge recovery. Re-verify before each session.
             IMPORTANT: The screen supply (400V) requires its own
             verification — it does not discharge through the
             main HV bleeder path.

Figure 1. Alpha 91B multi-supply safe discharge procedure before internal access.

5.3 HV Crowbar Vacuum Relay

In addition to S3, the 91B includes an HV crowbar vacuum relay visible in the top-view diagram of the manual. This crowbar activates in response to certain fault conditions and shorts the HV positive rail to chassis ground. After a crowbar event, verify that the relay contacts have not been welded closed by the arc energy — a welded-closed crowbar relay will prevent HV from building up on the next power-on and will show as a short from HV positive to chassis.

5.4 AC Line Safety Capacitors

Any capacitors installed across the AC mains lines or from mains to chassis must be Class X2 (line-to-line) or Class Y2 (line-to-chassis) safety-rated components. Standard ceramic or film capacitors are not rated for mains voltage and can fail short, putting mains potential on the chassis and creating an immediate fire and electrocution hazard. Inspect all AC input filter components; replace any non-line-rated types from Mouser, DigiKey, or a qualified electrical parts supplier.

5.5 Cooling Clearance Requirements

The 91B manual specifies a minimum of 12 inches (31 cm) of unobstructed clearance around both the air intake and exhaust areas. The blower runs at a reduced voltage (~100 VAC from a transformer tap) to minimise acoustic noise; this means airflow is lower than many other amplifiers of similar power. Blockage of the air path causes rapid overheating of the 4CX800A/GU74b tube anodes and can result in tube failure in minutes at full power. For continuous key-down operation exceeding five minutes (RTTY, FM, SSTV), the optional auxiliary cooling fan must be installed.

6. Cabinet Restoration

6.1 Bulgarian Assembly — Cabinet Parts Availability

The 91B was assembled in Bulgaria, and the cabinet was manufactured locally at that facility. As of the current date, replacement top covers are no longer available and have not been for approximately 25 years. Alpha RF Systems has confirmed this, noting that the Alpha 99 covers look similar but are not interchangeable. If the top cover of a 91B is damaged or missing, fabrication from sheet aluminium using the original as a template is the most realistic path; skilled metalwork shops can replicate the perforated ventilation pattern and form.⁷

6.2 Cabinet Finish

The 91B uses an aluminium chassis with a dark grey powder-coat or anodised finish similar to other Alpha amplifiers of the same era. Minor scratches can be addressed with a matching grey powder-coat aerosol from an automotive or industrial supplier. The front panel uses silk-screened or printed legends over anodised aluminium. Cosmetic restoration parts including reproduction front panels are available from Island Amplifier USA which specialises in legacy Alpha/ETO amplifier restoration.

7. Parts Sources & Reference Documents

• Alpha RF Systems — 91B Parts — alpharfsystems.com/91b-parts — factory source for 10Ω / 15 W step-start resistors and other 91B-specific parts. Online store intermittently closed during facility move to Ohio; email or call for availability. 91B documentation (manual, parts list, bias adjustments) archived at tentec.com/alpha-91b.
• Alpha RF Systems — 91B Blower Assembly — alpharfsystems.com — 115 VAC blower assembly; exchange basis (return original required).
• Alpha Power ETO 91B User Manual (ManualsLib) — manualslib.com — complete user manual including specifications, transformer installation, circuit description, troubleshooting, and schematic diagrams.
• Island Amplifier USA — islandamplifier.com — specialised Alpha/ETO amplifier repair and cosmetic restoration parts; replacement front panels, meters, and indicators for 91B and other legacy Alpha models. Contact: KF6I, (714) 412-7399.
• KØHM Electronics Repair (Brad Focken, KØHM) — k0hm.com — former Alpha factory technician with 17+ years experience; one of the designers of the 9500 and 8410. Offers professional service for 91B, 99, 8100, 87A, 89, 9500, and 8410.
• 4CX800A/GU74b Tube Sources — rfparts.com; pentalabs.com; dxengineering.com (NOS, while stock lasts). DX Engineering warrants tubes one year from purchase date. Penta Laboratories offers current-production 4CX800A/GU74b as direct replacements.
• AK6R Alpha 91B page — qsl.net/ak6r — practical operating and maintenance notes from an active 91B user.