Motorola 330 Watt Base Station Amplifier circa 1959
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This is an informal, narrated tour of a Low-band VHF Motorola 330 Watt Base Station Amplifier with a 1959 manufacture date stamped inside the chassis. The pictures above are of the six-foot tall, floor-mounted cabinet that houses the HV Power Supply deck, the RF Amplifier deck and an accompanying relatively low-power (20 to 30 Watt) exciter. To the right is a close-up of that era's Motorola logo which appears on the front of the cabinet.
"Low-band VHF" within the 2-way land mobile radio industry traditionally refers to the 2-way radio commercial 'spectrum' between 25 and 50 MHz. Commercial 2-way radio 'gear' built in the US (from the fifties well into the nineties) built for VHF Low-band often fell into one of three following frequency ranges: 25 - 36, 36 - 42, 42 - 50 MHz owing to the design component values and the limited adjustment range that the variable components yielded. The amplifier described here was labelled as being in the 40 - 50 MHz range and was therefore suitable for conversion to Six Meters with minimum effort.
This Motorola 330 Watt Base Station amplifier circa 1959 uses two pairs of 'tubes':
The 254 tube has a 4 prong base (picture), is made of blown glass about 6 inches long; a picture of this tube may be seen further down. The plate connection is via a top 'wire', the grid connection is a 'wire' sticking straight out the side of the cylindrical glass envelope, and the directly heated cathode uses just 2 pins out of the 4 pins on the 4-prong base.
The 254 tube (spec sheet) is a power triode with a directly heated cathode, or filament. The filament, as best as I can determine by web research, is made of Thoriated Tungsten and is not carburized (description of the carburization process). The 254 tube specifications state that each tube's 5 Volt filament will draw 7.5 Amperes of current; a nominal 14 Amperes of total filament current will therefore be drawn by the RF Amplifier deck's two 254 tubes.
The pair of 254 tubes are used in a neutralized push-pull amplifier circuit. Push pull operation is accomplished by each tube being connected to opposite ends of the 'plate tank' circuit and the grids likewise being fed 180 degrees out of phase. The 'neutralization circuit' feeds a small amount of opposite 'phase' signal from each tubes plate to the opposite tube's grid via half-dollar sized adjustable open-air capacitors; this effectively cancels any plate-to-grid capacitively coupled signal that is present, resulting in a stable, non-oscillating amplifier.
This amplifier was originally a 40-50 MHz unit but with a rework of the Harmonic Filter (which initially exhibited unacceptable attenuation in the Six Meter band) the amplifier performs well in use on the Six Meter Band.
With a little over 18 Watts of drive from a hybrid (transistorized receiver - tubed transmitter) Motorola Business Dispatcher this amplifier is capable of developing 250 Watts of output power. Additional drive with the added Plate Transformer Variac turned all the way up upwards of 330 Watts of RF output power could be had.
Performance
Conditions/drive source:
Measured performance:
The figures on the far left in blue are the RF output power from the Amplifier, the figures in red several columns to the right are the DC plate current (Ip) and voltage (Ep) respectively.
The far right column is the amplifier DC efficiency irrespective of drive power.
The output power was varied by adjusting the AC (Variac) autotransformer which changed the DC plate voltage.
Documentation
Full size image of the Power Supply deck schematic available here.
Of special note is the 30 second delay timer. This timer provides necessary, critical time for the Mercury vapor 866 tubes to warm up. Failure to allow proper warm up time as authoratatively described in the excerpt below will result in the destruction of the 866 tube's cathode:
The reason the tube [type 82 and 866] was designed was to offer a voltage drop that was more constant with changes in current than was the drop across a vacuum diode. The 12-14 V drop is not particularly low, especially for low currents, but there is some advantage at high currents. This did not seem to appeal greatly to designers, and the tube was rather little used, and eventually was discontinued without the appearance of a later version. The 866, a half-wave phanotron larger than the 82, remained popular for amateur transmitter power supplies. It could handle 250 mA with a peak inverse voltage of 10,000 V, and was generally used in full-wave pairs.When the tube reaches its operating temperature, the upper part of the bulb, which at first condenses a mist, will clear of mercury, which will still collect in the cooler lower regions. At 20°C the vapor pressure of Hg is about .001 mmHg, and at 60°C, about .025 mm Hg. These are roughly the limits of the mercury pressure in the tube. The 82 does not contain argon to start the discharge, since no self-sustaining discharge is initiated. Distinguish carefully between the operation of a phanotron and that of a glow tube, such as the voltage regulators mentioned below. All the current in a phanotron comes from thermionic emission, as aided by the ionic and field effects at the cathode. The maximum current is about 1.8 times the saturation thermionic emission in a vacuum. One should be careful to heat the cathode before applying plate voltage, so that the tube drop does not exceed about 25 V. If it is higher than this, positive-ion bombardment soon destroys the cathode.
Mercury has an ionization potential of 10.43 V. When electrons have been accelerated to this energy in the cathode-plate field, they can knock electrons off the neutral atoms and produce positive ions. These positive ions neutralize the space charge, producing a plasma that is very conductive. This is the effect of the gas; no glow discharge with its characteristic cathode and anode phenomena is initiated. The anode-cathode voltage must only remain high enough to replenish the stock of ions. Electrons of lower energy can excite mercury atoms to upper levels. It takes only 4.9 eV to excite the atom so that it emits its strong ultraviolet line at 253.7 nm. Most of the glow is produced by such excitation by inelastic electron collisions, as well as by recombination of the ions. With a hand spectroscope, you should see the familiar lines 454 nm (blue), 546 nm (green) and 578 nm (yellow) of the mercury spectrum in the glow.
If the voltage across the tube should rise above 22 V, the disintegration voltage, the positive ions acquire such energy that they sputter and destroy the oxide cathode. This can happen if the current is raised too high, or if anode voltage is applied without sufficient gas pressure. These tubes work with an efficient oxide cathode only because the discharge is maintained in mercury vapor at a low enough voltage.
On the amplifier I bought this timer didn't work, the amplifier therefore wouldn't go into transmit mode without bypassing this timer.
Meters aligned along the top of the 330 Watt Amplifier - five meters in all - monitoring the health and status of the base station exciter and RF Power Amplifier.
The most important two meters on the meter panel, from left to right, Amplifier Plate Current and Amplifier Plate Voltage respectively.
The Amplifier Plate Voltage meter reads from 0 to 3,000 Volts. The Plate DC milliampere meter displays currents ranging from 0 to 500 milliAmperes.
Looking up into the RF Amplifier deck.
A cast aluminum frame 20 Watt EG&G Rotron model MX2A3 Muffin-XL series fan was added to keep the two 254W RF amplifier tubes cool. The original fan, mounted low in the cabinent and well below the RF Deck, had failed. The original fan simply 'pressurized' the cabinet with air that eventually made its way out the top of the cabinet; past the Power Supply deck, past the exciter and then finally past the RF Amplifier deck and through screened grill-work on the top of the cabinet as well as over the top of the RF deck and out the back.
You can see that the fan was mounted on two brackets that 'tilt out' from the RF deck chassis to allow the fan to be mounted at an angle to keep both tubes bathed with a steady flow of cooling air. The 20 Watt EG&G Rotron fan runs at low speed during standby (via several series power resistors) and is brought up to full speed during transmit.
One of the two 254W tubes can be seen to the left of the fan.
Same shot as picture above, except the flash was not used and the brilliance of the 254 tube's directly heated filament can be seen.
One of the 254W tubes provides its own illumination for a picture. These filaments 'burn' awfully bright, not the usual soft, warm 'glow' that is associated with most tube gear.
Same picture as above - but with flash to better illuminate the tube's internal structure and other components on the RF deck.
Note the 'wings' on the plate, there is a wing on each side (180 degrees apart) of the plate jutting straight out. This seems to be more an artifact of the way the cylinder comprising the 254's plate is constructed as opposed to some technique to aid in cooling the plate (although additional unheated surface area in contact with the actual plate would serve to aid in cooling the actual plate).
View of the two 254 tubes installed in the amplifier and viewed from the left side of the amp.
For this view the filaments only are energized. The small half-moon shape near the top of the plate lets a little bit of the light from the filament 'escape'. During operation, a little blue 'light' can be seen in this area as a result of the tubes containing trace amounts of Oxygen and that gas being impinged by electrons escaping from those small half-moon slivers.
View of the two 254 tubes viewed from the right side of the amplifier.
During key-down at high output powers the plates in these tubes can be made to glow cherry red - REALLY punishing these tubes and that glow gets brighter until the outline of the grid can be seen on the plate in a slightly darker shade of red.
This gives one an idea the size of these tubes - not really that big.
On the right side of the tube sticking right straight-out of the tube's glass envelope is the grid connection.
These tubes have a 4 pin base - only two of these pins are used, and they are used as connections for the directly heated cathode or filament.
The top pin of this tube in this picture has a problem; the filament connection had always been intermittant on this particular tube requiring occasional jiggling to get the 254 (located on the right side of the RF Deck) tube's filament to 'fire up'. In October of 2004 it finally became completely open, no jiggling would help and so the amp was torn down to salvage this tube. The glass envelope had also become de-glued from the base requiring minor disassembly of the amplifier rather than simple removal of the tube by rotating it counter-clockwise in its socket.
K9STH (Glenn) recommended the use of hi-temperature Silicone RTV for re-glueing the glass envelope back to the base; something I had considered and so this was done along with re-soldering the bad pin.
I used the high-temperature black Silicone RTV that Autozone sells for use as gasket sealer. It worked well; I used a wooden cue-tip as a small chisel tool and worked the RTV in between the glass envelope of the tube and it's base. Masking tape was used to 'hold' the base in the proper orientation in relation to the glass envelope. I had just changed the water pump and the Optispark module (installed a rebuild kit from AutoZone actually) on a 90's Chevrolet B-body L99/LT1 series engine so I had this on hand as it was!
Beautiful view of the grid connection and the plate in the 254 tube.
Notice the little 'wing' running vertically along the plate.
Really close-up view of the grid 'cage'. There is also a wire (shown winding up the grid wires to the left) that winds spirally around the entire length of the grid to hold the vertical wires that make up the grid in place; this 'spiral' wire can be seen on the plate, as a shadow, when these tubes are really 'pushed hard' and the plates are glowing red. It is apparent that not as many electrons strike the plate where this 'shadow' shows up on the plate.
The two 254/254W tubes removed from the Amplifier RF deck and parked on an anti-static mat.
An anti-static mat is NOT required for working on this technology ...
Closer view of the two 254 tubes.
A .7 mm Pentel mechanical pencil in the foreground gives some size perspective.
Amplifier RF Deck with the 254 tubes removed. Looks empty without 'the glass' installed.
The plates in the two 254 tubes connects to the ends of this large, silver-plated plate tank coil.
'Output coupling' is accomplished using that flat, movable coil that is seen there near the center of the plate tank coil.
This is the plate tank variable tuning capacitor.
One of the small plate-connection heatsinks can be seen dangling from a bit of braided cable can also be seen in this picture.
This amplifier design required that certain 'stray' signals, namely, the 'feedback' within the tube from output (plate) to input (grid), basically an S12 quantity, be effectively 'cancelled'.
These caps are part of that cancelling network called a "Neutralizing circuit". I have used an HP 8405 Vector Voltmeter (low frequency network analyzer) to adjust this parameter with the amplifier 'cold' (turned off).
Failure to properly neutralize an amplifier can result in oscillation; in this case, that could result in 300 Watts worth of 'spurious' signal emitted from this amplifier!
From EIMAC's 1967 booklet titled Care and Feeding of Power Grid Tubes SECTION 5 - NEUTRALIZATION we get this:
5.4.1 Symmetrical Grid-Driven AmplifiersA symmetrical or push-pull grid excited amplifier with grounded cathode is shown in Figure 62. If the inductance of the leads is considered to be negligible at the operating frequency, independence between the input and output circuits is generally obtained by cross-connecting the grids and plates through capacitors Cn having values equal to the internal grid-plate capacitance, Cgp, of the vacuum tubes. The requirements of stability and neutralization are fulfilled simultaneously because the input circuit is connected between the grids (in the case of a symmetrical stage) or between the cathode and grid (in a single-ended amplifier).
RF Deck sans the 254W tubes and sans the muffin fan.
View of bottom of RF Amplifier Deck.
The Matching network for input circuit matching is located on the underside of the RF Amplifier Deck.
The ceramic tube sockets can also be seen. The tube pin contacts are silverplated.
View of rear of RF Amplifier Deck.
The output of the amplifier is sent through a Harmonic Filter, then on to the T/R relay.
View of the front of Power Supply Deck.
The two Mercury vapor rectifier tubes can be seen on the left with the white plate caps.
This power supply uses multiple chokes and several transformers to provide the required filament voltages for the 254W tubes and the Mercury Vapor recrifier tubes as well.
A "Variac" (variable autotransformer) can be seen at the lower left; this is wired in to control the plate voltage and allows the output power to be varied from zero to 330 plus Watts.
View of the rear of the Power Supply Deck.
Numerous lethal voltages are accessable here including a) AC Line potential, b) 3,000 VAC and c) 3,000 VDC.
Close-up of the plate (top left in picture) and the grid (with the grid connection coming in from the right) on one of the 254W tubes.
Illumination is provided by the tube itself with power applied to the filaments.
The little heatsink can be seen attached to the end of the grid connection at the left. This little heatsink insures that the grid wire will remain cool to the point where it will not melt the glass through which it passes.
Picture of one of the filaments inside a 254W tube. This picture was taken by turning the filament voltage down low enough to allow a picture to be taken of it. With full filament voltage applied these tube filaments don't just glow brightly, they are brilliant!
Link Summary/References:
- Vacuum Tubes - History, the underlying physics and suggested experiments as well as many scientific observations on vacuum tubes. A highly informative web page worth visiting and bookmarking.
- Spec sheet 866A tube
- Spec sheet 254 tube
- Discussion of PL-254W made by Penta Labs
- Discussion of HK-254 in relation to other era 'power tubes'.
-
Description of the carburization process as applied to a Thoriated Tungsten filament in a high power vacuum tube.
- Extending Tube Life - Reprint from Eimac Bulletin #18
Excerpt portion dealing with filaments, tungsten and thoria:
For a better understanding of this sensitive aging mechanism, the filament itself must be understood. Most filaments in high-power, "gridded tubes are a mixture of tungsten and thoria with a chemical composition of W + THO2. A filament made of this wire is not a suitable electron emitter for extended life applications until it is processed. Once the filament is formed into the desired shaped and mounted, it is heated to approximately 2100° C, in tbe presence of a hydrocarbon. The resulting thermochemical reaction forms di-tungsten carbide on the filament's surface. Life is proportional to the degree of carburization. If the filament is over-carburized, however, it will be brittle and easily broken during handling and transporting. Therefore, only approximately 25% of the cross-sectional area of the wire is converted to di-tungsten carbide. Di-tungsten carbide has a higher resistance than tungsten; thus, the reaction can be carefully monitored by observing the reduction in filament current as the carburizing process proceeds.
As the tube is used, the filament slowly de-carburizes. At some point in life, all of the di-tungsten carbide layer is depleted, and the reduction of thoria to free thorium stops. The filament is now de-carburized and is no longer an effective electron emitter.
The key to extending the life of a thoriated tungsten filament emitter is to control operating temperature. Emitter temperature is a function of the total RMS power applied to the filament. Thus, filament voltage control is temperature control; temperature varies directly with voltage. As the emitter temperature rises, the de-carburizing process is accelerated and tube life shortened. Figure 2 shows that the useful tube life can vary significantly with only a 5% change in filament voltage.
Of great importance to long tube life is the temperature of the elements, and the ceramic-to-metal seals. Element temperature can be held within proper limits by observing the maximum dissipation ratings listed in the data sheet. Seal temperatures should be limited to 200° C, at the lower anode seal, under worst case conditions. As element temperature rises beyond 200° C, the release of contaminants, locked in the materials used in tube manufacturing, increases rapidly. These contaminants cause a rapid depletion of the all-tungsten carbide layer of the filament.
- EIMAC's booklet titled
THE CARE AND FEEDING OF POWER GRID TUBES originally published 1n 1967.
- Section on filaments (such as the type found in the 254 tube):
POWER GRID TUBES, Thoriated Tungsten Filaments by EIMAC
Excerpt:
2.6.2 Thoriated Tungsten Filaments
A thoriated tungsten filament is one form of an atomic-film emitter. Thorium is added to the tungsten in the process of making tungsten wire. Typically, about 1.5 per cent of thorium in the form of thoria (thorium oxide, ThO2) is added. By proper processing during vacuum pumping of the tube envelope, the metallic thorium is brought to the surface of the filament wire, and emission increases approximately 1000 times. The thoriated tungsten filament is also 26 carburized. The small amount of tungsten carbide formed in the carburizing process reduces the evaporation rate of the thorium and thus increases the life of the filament. At a typical operating temperature of approximately 1900° K, a thoriated tungsten filament will produce a specific peak emission of about 70 to 100 mA per watt of filament heating power. This is normally 5 to 10 mA of CW anode current per watt of filament power. A thoriated tungsten filament is more tolerant of ion bombardment than an oxide cathode, and, therefore, higher voltages can be applied to the tube.
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