Care and Feeding of Power Grid Tubes in Vintage Collins Radio Equipment

In the Spirit of the Eimac Tradition

In the 1950s through 1970s, the Eimac Division of Varian (later CPI, Eimac Division) published Care and Feeding of Power Grid Tubes, a handbook that became required reading for anyone operating or building equipment with power tetrodes and triodes. This document carries on that tradition, focusing specifically on the power grid tubes found in vintage Collins Radio Company transmitters and amplifiers — the coveted “Gold Dust Twins,” the S-Line, the KWM-2, and the high-power 30S-1 and KWS-1 stations. Whether you are restoring a 30L-1 to full operating condition or nursing a KWS-1 back to life, this guide provides the practical knowledge needed to keep these classic tubes running safely and reliably.

Bill Orr W6SAI (1919–2001) was a genuine giant of amateur radio technical writing, an electrical engineering graduate of UC Berkeley, and an Eimac alumnus of nearly three decades. His direct involvement in Eimac’s applications engineering undoubtedly influenced the Care and Feeding document even if formal authorship belongs to the Eimac laboratory staff. His call sign W6SAI — held from 1938 for the remainder of his life — is commemorated in the ARRL’s annual Bill Orr W6SAI Technical Writing Award.

Power Grid Tubes in the Collins Lineup

1.1 Overview of Collins Transmitters and Amplifiers

Collins Radio Company, founded by Arthur A. Collins in Cedar Rapids, Iowa, produced the finest amateur and commercial radio equipment of the vacuum tube era. Their amateur product line included transmitters, transceivers, and linear amplifiers spanning a range of power grid tubes from modest beam power tetrodes to high-power ceramic tetrodes capable of legal-limit and beyond.

1.2 Tube Families Covered

This guide concentrates on four families of power grid tubes used in Collins amateur equipment:

• Glass Envelope Triodes: The 811A — medium-mu power triode in the 30L-1; the 810 — high-mu transmitting triode in the KW-1 push-pull modulator.
• Glass Envelope Tetrodes: The 4-250A (5D22) — compact power tetrode in the KW-1 RF final; the 4X150A (7034) — glass/metal radial beam tetrode originally in the KWS-1.
• Ceramic/Metal Tetrodes: The 4CX250B (7203) — ceramic upgrade for the KWS-1; the 4CX1000A (8168) — Eimac’s high-power forced-air-cooled tetrode in the 30S-1.
• Glass Beam Power Tubes: The 6146/6146B — PA stages in the 32S series and KWM transceivers.

The 811A Triode — Heart of the 30L-1

2.1 Tube Description and Ratings

The 811A is a medium-mu power triode with a thoriated tungsten filament, a glass envelope, and a plate cap connection. Designed for Class B and Class C RF amplifier service, it was manufactured in quantity by RCA, GE, Amperex, Sylvania, and later by Chinese producers. The UX-4 (4-pin Jumbo) base is identical to that of the 811 and the larger 812 series.

2.2 Operation in the Collins 30L-1

The 30L-1 uses four 811A tubes in a grounded-grid, push-pull parallel configuration operating in Class AB2. The amplifier is rated at 1000 watts PEP input on SSB and 1000 watts DC input on CW across the 80 through 10 meter amateur bands. Typical output power runs 600–800 watts PEP with approximately 65–100 watts of drive from a Collins KWM-2A or 32S-3.

Key operating considerations:

• Filament Management: Apply filament voltage for at least 30 seconds before applying plate voltage. The thoriated tungsten filaments are more tolerant of thermal shock than oxide-coated cathodes, but a proper warm-up period meaningfully extends tube life.
• Filament Voltage: Maintain at 6.3 V ±5% (6.0–6.6 V range). Low line voltage causes reduced emission; high line voltage dramatically shortens tube life. A Variac or bucking transformer at the AC input is a worthwhile investment.
• Plate Dissipation: With four tubes sharing the load, each 811A dissipates well within its 65-watt rating during normal SSB operation. On prolonged key-down CW or RTTY, monitor plate current carefully and reduce duty cycle if plate color becomes visible.
• Parasitic Oscillations: The 811A is prone to VHF parasitic oscillations due to its relatively high grid-to-plate capacitance. The 30L-1 includes parasitic suppressor networks (small inductors with carbon resistors) on each plate lead. Inspect these regularly — carbon composition resistors change value with age and heat cycling.

2.3 Substitute and Replacement Tubes

The 4X150A Tetrode — Power Behind the KWS-1

3.1 Tube Description and Ratings

The Eimac 4X150A (JEDEC designation 7034) is a forced-air-cooled, external-anode, radial-beam tetrode with a glass-and-metal envelope. It was one of Eimac’s early compact tetrode designs, offering a maximum plate dissipation of 250 watts. The tube operates at full ratings up to 150 MHz, with reduced ratings applicable to 500 MHz. An oxide-coated cathode — rather than the thoriated tungsten filament found in the larger 4-250A — is a defining characteristic that imposes stricter voltage management requirements.

3.2 Operation in the Collins KWS-1

The Collins KWS-1 transmitter uses two 4X150A tubes in push-pull Class AB1 configuration, delivering approximately 100 W carrier on AM and 600 W PEP on SSB across the 80–10 meter bands. The 428A-1 external power supply provides plate and screen voltage to the PA stage and includes a dedicated blower for forced-air cooling.

Key operating considerations:

• Forced-Air Cooling is Mandatory: The 4X150A requires forced-air cooling whenever filament voltage is applied. The built-in blower in the 428A-1 power supply must be operational at all times. Loss of airflow with filaments energised will destroy the tubes within minutes.
• Screen Voltage Protection: Screen dissipation is limited to only 12 watts. Never apply screen voltage without plate voltage present. If plate voltage is lost for any reason, screen current will spike and can destroy the tube in seconds.
• Heater Voltage: The rated heater voltage is 6.0 V. Maintain within ±10% short-term; sustained over- or under-voltage degrades the oxide-coated cathode.
• Neutralization: As a grid-driven amplifier, the 4X150A stage requires proper neutralisation. Poor neutralisation causes instability, oscillation, and potential tube damage.
• Control Grid Limits: Maximum grid dissipation is only 2 watts. Never exceed recommended drive levels. The grid must always have a DC return path.

3.3 Operating Tips for the 4X150A

The 4X150A demands more operator discipline than glass-envelope tubes with thoriated tungsten filaments. These tips will maximise tube life and performance:

1. Always verify blower operation before applying any voltage. Place your hand over the exhaust chimney to confirm positive airflow. A stalled or sluggish blower will not provide adequate cooling.
2. Monitor ambient temperature. The 4X150A’s cooling requirements assume 25°C ambient. In hot environments, reduce power output by 10–15% or increase airflow.
3. Never hot-switch bands. Reduce drive to zero, remove plate/screen voltage, retune, then reapply voltage. Changing bands under power with the plate detuned subjects the screen to extreme dissipation.
4. Watch for cathode poisoning. After long storage, apply filament voltage only for 30 minutes, then gradually increase to operating voltages over 15–20 minutes. This reforming process can recover marginal tubes.
5. Minimise tune-up time. The 12 W screen limit means every second spent with the plate off resonance is damaging. Pre-set tuning and loading controls to known band positions before transmitting.
6. Log tube hours. The 4X150A has a typical service life of 2,000–5,000 hours. A logbook helps predict replacement timing.
7. Inspect the socket regularly. SK-600 series socket contacts develop oxidation in humid environments. Clean with DeoxIT or isopropyl alcohol every six months.
8. Check the glass-to-metal seals. Unlike the ceramic 4CX250B, the 4X150A has glass-to-metal seals vulnerable to thermal and mechanical stress. Inspect for hairline cracks or darkening.

3.4 The 4CX250B Upgrade for the KWS-1

The 4CX250B (JEDEC 7203) is the ceramic-envelope successor to the 4X150A and represents the single most valuable upgrade available for the KWS-1. Many KWS-1 units in current service have already had this modification performed.

Why Upgrade?

The 4X150A suffered from glass-to-metal seal reliability problems. Eimac developed the 4CX250B specifically to address this issue by replacing the glass envelope with an all-ceramic/metal construction. The electrical ratings are essentially identical, while the mechanical construction is substantially superior.

Key Advantages of the 4CX250B

• Higher plate voltage rating (2500 V vs. 1250 V) — substantially more operating headroom
• Ceramic construction eliminates glass seal failures — the most common 4X150A failure mode
• Higher seal temperature tolerance (250°C vs. ~200°C) — more forgiving of minor cooling lapses
• Higher transconductance (11,000 vs. ~9,000 µmhos) — slightly higher gain
• Better availability — the 4CX250B remains in production and is far easier to source than the discontinued 4X150A

Performing the Upgrade

1. Remove the original 4X150A tubes
2. Inspect and clean the SK-600 sockets thoroughly with high-voltage-rated contact cleaner
3. Insert the 4CX250B tubes, ensuring proper seating
4. Check bias voltage and idle current — the higher transconductance may require slight bias adjustment
5. Reneutralise the PA stage per the KWS-1 manual procedure
6. Verify screen current remains within the 12 W maximum under all operating conditions

Recommended 4CX250B Variants

3.5 Substitute and Replacement Tubes (Original 4X150A)

3.6 Investigating the 4CX350 Family as KWS-1 Substitutes

A frequently asked question in Collins restoration circles is whether the 4CX350B — and by extension other members of the 4CX350 family — can serve as a higher-power upgrade in the KWS-1 PA stage, following the same logic that made the 4CX250B such a successful replacement for the original 4X150A. The answer requires careful examination of both tube specifications and the KWS-1’s circuit requirements.

4CX350A (8321) — Full Specifications

The Eimac 4CX350A is a ceramic/metal, forced-air-cooled radial beam tetrode described by Eimac as “externally identical to the 4CX250B” but with a larger cathode area and higher amplification factor. It was designed primarily for Class AB1 audio and RF amplifier service in broadcast and communications equipment.

Detailed Analysis — Why the 4CX350A Is Not Recommended for the KWS-1

1. Screen dissipation is actually worse. The single most important number in this comparison — and the one most likely to surprise — is that the 4CX350A’s maximum screen dissipation is 8 W, compared to 12 W for the 4CX250B. The KWS-1’s screen protection circuitry was designed around a 12 W screen limit. Substituting a tube with a 33% tighter screen limit into an unchanged circuit does not improve the situation — it makes the tube more vulnerable to the screen burnout failure mode that the upgrade is intended to address. This alone is sufficient reason to reject the 4CX350A as a KWS-1 upgrade candidate.

2. Transconductance is approximately double. The 4CX350A’s transconductance of approximately 22,000 µmhos is roughly twice that of the 4CX250B at 11,000 µmhos. In the KWS-1 PA circuit, this means that for a given grid voltage swing, the plate current excursion will be approximately double what the circuit was designed to produce. The consequences include: grossly incorrect operating point at existing bias settings; substantially different input and output impedances affecting the tank circuit tuning range; and potential instability as the much higher gain interacts with the neutralisation network. Complete re-engineering of the bias supply, drive level, tank circuit, and neutralisation would be required — this is no longer a “tube swap,” it is a circuit redesign.

3. Heater current demands verification. Two 4CX350A tubes draw a combined 7.2 A from the 6 V heater winding, compared to 5.2 A for two 4CX250B tubes. The KWS-1’s heater transformer winding and associated wiring must be capable of sustaining this 38% increase in current without excessive voltage drop or thermal stress. The heater supply voltage at the socket under the higher load must be measured and confirmed to remain within the ±10% tolerance (5.4–6.6 V) required by an oxide-coated cathode. A voltage drop that takes the 4CX350A heater below 5.4 V will degrade cathode emission and shorten tube life.

4. The plate dissipation advantage is largely notional. The 4CX350A’s 350 W plate dissipation rating does provide 40% more headroom than the 4CX250B’s 250 W. However, the KWS-1 operating at its design power level with correctly adjusted bias and loading remains well within the 4CX250B’s plate dissipation rating. The plate dissipation advantage of the 4CX350A confers no practical benefit in a correctly operating KWS-1, while its liabilities — particularly the tighter screen limit and doubled transconductance — are very real.

The 4CX350AC — Current-Production Option

For completeness, the 4CX350AC (8321A) from Penta Laboratories is the current-production ruggedised version of the 4CX350A. It is electrically identical to the 4CX350A in all respects relevant to this analysis, with the sole difference being a slightly extended warm-up time (40 seconds vs 30 seconds). It shares all the same liabilities discussed above and is equally unsuitable for a direct KWS-1 substitution without circuit modifications.

Summary: 4CX250 Variants That Are Recommended for the KWS-1

Rather than exploring the 4CX350 family, restorers seeking the best available tube for the KWS-1 should focus on the proven 4CX250 series variants below, all of which are socket-compatible and require only minor bias and neutralisation adjustment:

3.7 The Svetlana 4CX400A (GS-36B) — A Further Candidate

The Svetlana 4CX400A — also known by its Russian industrial designation GS-36B — is a compact, high-perveance, forced-air-cooled radial beam tetrode with a plate dissipation of 400 watts. Manufactured by Svetlana Electron Devices in Russia and used widely in military and commercial amplifiers, it appeared in surplus quantities during the 1990s and 2000s and attracted interest from the amateur community as a potential upgrade for equipment using the 4CX250B. The SK-600 series socket is explicitly listed as compatible in the Svetlana datasheet.

The pattern established by the 4CX350A analysis repeats with the 4CX400A: a larger cathode area produces higher transconductance and more plate dissipation capability, but the screen grid — a physically small and delicate structure — cannot scale proportionally, so the screen dissipation limit actually decreases relative to the 4CX250B it might nominally replace. The 4CX400A’s 8 W screen limit is identical to the 4CX350A’s and carries all the same operational risks in the KWS-1 circuit.

The 4CX400A’s transconductance of 26,000 µmhos is 2.4 times that of the 4CX250B. Combined with the GS-36B’s “high-perveance” cathode design (optimised for low-voltage, high-current operation), the tube would operate at a substantially different and incorrect operating point in the KWS-1’s existing PA circuit without significant redesign of the bias supply, tank circuit impedance, and neutralisation network. The KWS-1 was not designed to accommodate this level of gain, and stability cannot be assumed.

The 4CX400A’s heater current of 3.15 A nominal per tube (6.3 A total for a pair) is a smaller departure from the 4CX250B’s 2.6 A than the 4CX350A’s 3.6 A, making the heater transformer impact more manageable — but the screen dissipation and transconductance problems remain disqualifying for a straightforward swap.

3.8 Definitive KWS-1 PA Tube Substitute Master Reference

The following table consolidates all known SK-600-compatible PA tube candidates for the KWS-1, rated by practical suitability. The recurring theme is unambiguous: no tube in the 4CX family larger than the 4CX250B improves the KWS-1’s most vulnerable characteristic — screen protection — while all of them introduce higher transconductance that requires significant circuit work to accommodate.

Neutralisation Note Applicable to All Substitutions

Whenever any tube is substituted in the KWS-1 PA — even a 4CX250B replacing the original 4X150A — the PA stage must be reneutralised. The change from the 4X150A’s glass-envelope construction to the 4CX250B’s ceramic construction reduces interelectrode capacitances, which alters the neutralisation requirement. Detailed notes from W8MAQ document that the KWS-1’s butterfly neutralisation capacitor often needs to have its wings carefully and evenly bent back to near-parallel to reduce the neutralisation capacitance range when changing from 4X150A to 4CX250B. Any restorer performing a tube substitution should obtain and follow this procedure. The neutralisation must be re-verified on all bands after any tube change.

The 4-250A Tetrode and 810 Triode — The KW-1 Transmitter

4.1 The Eimac 4-250A (5D22)

The 4-250A is a compact, ruggedly constructed glass-envelope power tetrode with a maximum plate dissipation of 250 watts and a thoriated tungsten filament. It requires forced-air cooling through the base and chimney assembly. Its enormous filament current (14.5 A at 5.0 V — 72.5 W per tube) means the KW-1’s filament transformer must supply nearly 30 A for the pair alone. The 5D22 is simply the military JEDEC designation for the same tube.

4.2 The 810 Transmitting Triode

The 810 is a high-mu, thoriated-tungsten filament power triode in a glass envelope rated for 125 watts plate dissipation. Used in the KW-1’s push-pull Class B modulator, it provides the brute-force audio power needed for high-level AM plate modulation of the kilowatt-class RF final. The VT-127A is the military designation and is fully interchangeable.

4.3 Operation in the Collins KW-1

The KW-1 is an earlier Collins transmitter using two 4-250A tetrodes in the RF final amplifier and two 810 triodes in the push-pull Class B modulator, with an 807W driver stage. The combination provides full kilowatt-level operation on AM and CW. The 872A mercury-vapour rectifiers in the power supply require their own 30-second warm-up before plate voltage can safely be applied to the PA stage.

4-250A Key Operating Considerations

• Filament Warm-Up: Apply 5.0 V filament voltage for a minimum of 60 seconds before plate and screen voltages. Maintain within ±5%.
• Cooling: The 4-250A requires forced-air cooling through the SK-410 socket and SK-406 chimney. Monitor plate seal temperature — maximum 200°C.
• Screen Protection: Screen dissipation is limited to 35 W. Always tune for plate current dip (resonance) before increasing loading.

810 Modulator Key Operating Considerations

• Filament Voltage: The 810 requires 10.0 V at 4.5 A from a dedicated filament winding. Maintain within ±5%.
• Plate Dissipation: Limited to 125 W. Barely perceptible red plate colour indicates operation near maximum. Reduce loading or duty cycle if a brighter glow appears.
• Mercury Vapour Rectifiers: The 872A rectifiers in the KW-1 power supply must be warmed up for a full 30 seconds before plate voltage is applied, without exception. Failure to do so risks catastrophic arc-over inside the rectifier tube.

4.4 Maintenance Checklist for the 4-250A

Perform this checklist every 500 operating hours or every 6 months, whichever comes first.

Electrical Checks:

• Measure filament voltage at socket pins under full load — must be 5.0 V ±5% (4.75–5.25 V)
• Record idle plate current and compare to previous readings — rising trend indicates cathode deterioration
• Verify screen voltage under load — must match manual specification
• Check bias supply voltage — drift causes incorrect idle current
• Test for interelectrode leakage with tube tester if available
• Measure plate current balance between the two 4-250A tubes — imbalance exceeding 15% indicates a weak tube

Mechanical and Thermal Checks:

• Inspect glass envelope for cracks, darkening, or discoloration near seals
• Check plate cap connector for arcing, pitting, or discoloration — clean and retension as needed
• Verify SK-410 socket contacts are clean and have firm spring tension
• Inspect SK-406 air chimney for cracks, proper seating, and unobstructed airflow
• Confirm blower motor operates at correct speed — lubricate bearings if applicable
• Check airflow through chimney with tissue paper test — air must flow freely from base through chimney
• Inspect plate and base seal areas for any sign of thermal stress (discoloration, milky appearance)

Associated Circuit Checks:

• Inspect parasitic suppressor resistors — check value with ohmmeter (carbon comp types drift with age)
• Check plate RF choke for continuity and signs of overheating
• Verify bleeder resistors across filter capacitors are within 20% of rated value
• Inspect plate tuning and loading capacitors for signs of arcing or flash-over
• Test all interlock switches for proper operation

4.5 Biasing Procedure for the KW-1 Tube Complement

4-250A RF Final Amplifier Bias Adjustment

1. Install a matched pair of 4-250A tubes. Allow filaments to warm up for a minimum of 2 minutes.
2. Set the KW-1 to CW mode. Connect a dummy load — do not use a live antenna.
3. Apply plate and screen voltages with no RF drive applied.
4. Measure the total resting plate current of both 4-250A tubes. Refer to the Collins KW-1 manual for the correct idle current specification for your revision (typically 50–70 mA combined).
5. Adjust the bias potentiometer until the specified idle plate current is achieved.
6. Verify screen current at idle is near zero — significant screen current with no drive indicates a bias problem.
7. Apply a small amount of RF drive and confirm plate current increases smoothly. Retune plate and loading per normal procedure.
8. Record bias voltage and idle current values for future reference.

810 Modulator Bias Adjustment

1. Install a matched pair of 810 triodes. Allow filaments (10.0 V) to warm up for a minimum of 2 minutes.
2. With no audio drive applied, apply plate voltage to the modulator stage.
3. Measure the total idle plate current of the push-pull 810 pair (typically 25–40 mA combined for Class B idle).
4. Adjust the modulator bias control until the specified idle current is reached.
5. Verify balance between the two 810 tubes by measuring individual cathode currents — should match within 10%.
6. Apply audio drive and verify clean modulation on an oscilloscope or modulation monitor. Crossover distortion (notch at the zero-crossing) indicates insufficient idle current — increase slightly.
7. Excessive idle current wastes plate dissipation. Find the minimum idle current that eliminates crossover distortion.
8. Record all bias settings.

4.6 Substitute and Replacement Tubes

4-250A Substitutes:

810 Substitutes:

The 4CX1000A Ceramic Tetrode — The 30S-1 Flagship

5.1 Tube Description and Ratings

The Eimac 4CX1000A (also designated 8168) is a ceramic/metal, forced-air-cooled, radial-beam tetrode with a rated maximum plate dissipation of 1000 watts and an extraordinarily high transconductance of 37,000 µmhos. A single 4CX1000A in the Collins 30S-1 provides full legal power output with approximately 70 watts of drive — a testament to its exceptional efficiency. The tube was, at the time of the 30S-1’s introduction, among the most powerful single tubes available for amateur service.

5.2 Operation in the Collins 30S-1

The 30S-1 is a single-tube grounded-grid linear amplifier covering 80 through 10 meters, producing approximately 1000 watts PEP output. It was marketed as Collins’s companion to the KWM-2A and 32S-3 and represented the pinnacle of Collins amateur amplifier design.

• Warm-Up Sequence: The 30S-1 includes a timed warm-up interlock sequence. Do not bypass or defeat this circuit — it protects both tube and operator.
• Forced-Air Cooling: Verify blower at full speed before and during operation. The blower must continue running after shutdown via the time-delay relay until the tube has cooled.
• Screen Grid Vulnerability: Despite being a 1000-watt tube, the 4CX1000A has a maximum screen dissipation of only 12 W and grid dissipation of only 1 W. Never defeat protective circuits.
• Plate Voltage: Approximately 2500–3000 V — immediately lethal. Always follow high-voltage safety procedures.
• Tune-Up: The 37,000 µmhos transconductance means small grid voltage changes produce large plate current swings. Always tune at reduced drive power.

5.3 Tube Replacement and Seating (Breechblock Socket)

The 4CX1000A uses a breechblock socket (SK-800B) — a design where the tube is inserted straight and then twisted to lock into a positive-contact position. This is a more complex operation than a standard socket and requires care:

• Insert straight and rotate to lock into breechblock mechanism
• All contact fingers must make firm, clean contact
• Inspect for discoloration, pitting, or loss of spring tension before inserting a new tube
• Air chimney (SK-806) must be properly seated to maintain correct airflow
• Clean socket contacts with high-voltage-rated contact cleaner before inserting a replacement tube

5.4 Substitute and Replacement Tubes

The 6146/6146B Beam Power Tube — S-Line and KWM Series

6.1 Tube Description

6.2 Application in Collins Equipment

Used in pairs in the 32S-1, 32S-3/3A, KWM-1, and KWM-2/2A as the final PA stage. While modest compared to the power tubes in the larger Collins station equipment, the 6146B punches well above its weight class — a matched pair can deliver 100 W PEP SSB output from the transmitter, sufficient to fully drive a 30L-1 or 30S-1 to rated power.

• Use matched pairs for push-pull operation — current imbalance causes distortion and unequal tube wear
• 6146B is preferred — higher plate voltage and dissipation ratings provide a meaningful safety margin
• Inspect parasitic suppressors — a common failure point in Collins transmitters; carbon resistors change value with age
• Screen voltage must be well-regulated; the screen protection circuits are especially important during tune-up
• Adjust bias for correct idle current after installing new tubes

6.3 Substitute and Replacement Tubes

Performance Comparison — 4X150A vs. 4-250A

The 4X150A (KWS-1) and 4-250A (KW-1) are both 250-watt plate dissipation tetrodes, but they are fundamentally different designs intended for different applications. This comparison illuminates the engineering philosophy behind each Collins station.

7.1 Specifications Compared

7.2 Analysis

Power Supply Requirements: The 4X150A operates at lower plate voltage (1250 V max) with higher plate current, while the 4-250A operates at up to 4000 V with lower current. This has direct implications for transformer design and safety — lower voltage is relatively less immediately lethal, though both are clearly dangerous.

Screen Grid Tolerance: The 4-250A’s 35 W screen dissipation limit is nearly three times the 4X150A’s 12 W, making the 4-250A far more forgiving during tune-up and fault conditions. The 4X150A demands meticulous screen protection circuitry.

Filament Power: The 4-250A draws an enormous 14.5 A at 5.0 V (72.5 W per tube) compared to 2.6 A at 6.0 V (15.6 W) for the 4X150A. The KW-1’s filament transformer must supply nearly 30 A for the pair alone.

Gain and Drive: The 4X150A’s much higher transconductance (~9,000 vs. 4,000 µmhos) and amplification factor (75 vs. 5.1) mean it requires far less drive power. The KWS-1 in Class AB1 requires only RF voltage swing from the exciter. The KW-1’s 4-250A stage requires a substantial driver (the 807W) to develop the required grid swing.

Frequency Performance: The 4X150A is rated to 150 MHz; the 4-250A to only 30 MHz. Both are adequate for HF amateur service, but the 4X150A has substantially lower stray capacitances and shorter internal lead lengths, giving it superior high-frequency behaviour.

Durability: The 4-250A’s thoriated tungsten filament is inherently more rugged and longer-lived than the 4X150A’s oxide cathode. However, the 4-250A’s glass envelope and higher operating temperatures make it more vulnerable to seal failures over time.

Failure Comparison — KWS-1 vs. KW-1

8.1 Common Failures by Equipment

8.2 Reliability Summary

The KW-1 is generally considered the more robust design from a tube survival standpoint. The 4-250A’s generous screen dissipation rating (35 W vs. 12 W), thoriated tungsten filament, and higher tolerance for less-than-perfect operating conditions make it more forgiving of operator error and component ageing. However, the KW-1’s more complex tube complement (4-250A + 810 + 807W + 872A rectifiers) means more tubes to maintain and source — and the 872A mercury vapour rectifiers are particularly unforgiving of warm-up shortcuts.

The KWS-1 offers superior performance and more elegant circuit design, but the 4X150A’s fragile glass seals, razor-thin screen dissipation margin, and absolute dependence on forced-air cooling make it less tolerant of neglect. Upgrading to the 4CX250B substantially mitigates the KWS-1’s reliability concerns and is strongly recommended.

General Principles of Tube Care

9.1 Filament and Heater Management

The single most important factor in power grid tube longevity is proper filament/heater voltage management:

• Measure at the socket: Line voltage variations, transformer regulation, and wiring losses mean the voltage at the socket may differ significantly from nominal. Always measure under load.
• Thoriated tungsten filaments (811A, 4-250A, 810) are robust but sensitive to overvoltage — running 10% over nominal can halve tube life through accelerated thorium evaporation.
• Oxide-coated cathodes (4X150A, 4CX250B, 4CX1000A, 6146B) are damaged by both overvoltage and undervoltage — maintain within ±5% of nominal.
• Minimise on/off cycles: Each filament start-up thermally shocks the cathode. If operating again within a few hours, consider leaving filaments on with plate/screen off.

9.2 High-Voltage Safety

All Collins transmitters and amplifiers contain lethal voltages (1500–3500 V).

• Always discharge filter capacitors before working inside — use a proper resistive discharge tool, not a bare screwdriver
• Use the buddy system: Never work on high-voltage equipment alone
• Keep one hand in your pocket or behind your back when measuring live voltages — prevents a bilateral path across the heart
• Never defeat interlock switches
• Verify bleeder resistors across filter capacitors are intact and within tolerance
• Allow adequate discharge time — large filter capacitors in the KW-1 and KWS-1 can retain dangerous charge for many minutes even with bleeder resistors fitted

9.3 Forced-Air Cooling Requirements

9.4 Tune-Up Best Practices

1. Start with minimum drive power
2. Tune for plate current dip first (resonance)
3. Increase loading gradually while monitoring plate current
4. Watch screen current — rises sharply when plate is detuned
5. Keep tune-up transmissions brief — five seconds on, five seconds off
6. Check the antenna system first — a mismatched antenna makes proper loading impossible

9.5 Storage and Handling

• Store in original packaging or equivalent, in a cool dry location
• Ceramic tubes resist mechanical shock better than glass, but ceramic seals can crack if dropped
• Avoid touching glass envelopes with bare hands — skin oils create localised hot spots during operation
• Reform cathodes on long-stored tubes before full-power operation (see Section 3.3, Item 4)

9.6 Recognising End of Life

• Reduced emission: Cannot achieve rated output at normal drive levels
• Increased grid current: Indicates cathode deterioration in tetrodes
• Gas: Bluish glow between electrodes (distinct from the normal blue glass fluorescence)
• Intermittent arcing: Sudden plate current spikes, audible clicks
• Hot spots on plate: Localised red/orange spots indicate overheating from emission non-uniformity
• Mechanical noise: Loose electrode structures cause microphonic behaviour

Sourcing Tubes Today

10.1 Availability Summary (as of 2026)

• 811A/572B: Currently manufactured (Chinese production). NOS American-made (RCA, GE, Amperex) command a premium and are the preferred choice for Collins restoration work.
• 4X150A: No longer manufactured. NOS Eimac stock from specialty dealers. Upgrade to 4CX250B is strongly recommended.
• 4CX250B: Currently manufactured (Penta Laboratories and others). The most practical tube for KWS-1 service today.
• 4-250A: No longer manufactured. Rebuilds from Richardson Electronics and Penta are the best available source.
• 810: Available as Chinese manufacture. NOS US-made (RCA, GE) are significantly more reliable for KW-1 modulator service.
• 4CX1000A: Limited new manufacture. Rebuilds from Penta are practical and cost-effective.
• 6146B/6146W: Readily available NOS. GE, RCA, and Sylvania brands are preferred. Current Chinese production is adequate for most applications.

10.2 Recommended Vendors

10.3 Testing on Arrival

• Emission test: Below 70% of nominal — retire the tube from final amplifier service
• Mutual conductance (Gm): Should be within 20% of published specs
• Gas test: Any tube showing gas should be discarded immediately
• Interelectrode leakage: Check for shorts between all elements
• Ceramic tubes: Visually inspect seals for cracks, discoloration, or evidence of overheating

Complete Substitution Quick Reference

* 4X150D requires heater supply adjustment — not a straightforward drop-in.
† 4CX1500A compatibility with SK-800B socket and SK-806 chimney must be verified before installation.

Troubleshooting Common Problems

12.1 Collins 30L-1 Issues

12.2 Collins 30S-1 Issues

12.3 Collins KWS-1 Issues

12.4 Collins KW-1 Issues

Specifications Quick Reference

13.1 Comparative Tube Data

* 4CX250B is the recommended upgrade replacement for the original 4X150A in the KWS-1.

13.2 Recommended Filament/Heater Voltages

Appendix A: Glossary of Terms

Appendix B: Recommended Reading and Resources

• Eimac Division of Varian / CPI Eimac Division, Care and Feeding of Power Grid Tubes (multiple editions 1951–2003) — authored by the Eimac Laboratory Staff, with Robert I. Sutherland credited on the 1967 edition
• Collins Radio Company, individual equipment instruction manuals for the 30L-1, 30S-1, KWS-1, KW-1, 32S-3, and KWM-2A
• ARRL, The ARRL Handbook for Radio Communications — transmitter and amplifier chapters
• Orr, William I. (W6SAI), Radio Handbook (Howard W. Sams / Editors and Engineers) — the classic amateur radio engineering reference; 23rd edition and earlier are particularly relevant to this era of equipment
• WA3KEY Collins Information Center (wa3key.com) — tube and crystal complements for all Collins equipment
• r-390a.net — VK6ADA’s Collins and vintage receiver community resource
• vk6ada.com.au — Collins Radio restoration, SDR integration, and Boatanchor Legends

This document is provided for educational and reference purposes in support of the vintage amateur radio restoration community. It is not affiliated with or endorsed by Collins Radio Company, Eimac, CPI, or their successors. Always follow the original equipment manufacturer’s instructions and applicable electrical safety regulations when working with high-voltage equipment.

Fact-check corrections and technical enrichments applied April 2026. Where corrections are made, sources are cited. Original erroneous claims are clearly identified.

73 — Good DX and keep those filaments glowing.