A guide to Communications type Vibrators.

The styling of the box at left implies Motorola vibrators were still being sold well into the 1970's.

In this article we will be looking at the so-called "Communications" type vibrator. While the operation is the same as any other vibrator, and the appearance is the same of any other radio type, there are several important differences.
This type of vibrator was introduced in the mid 1950's in the U.S. and was intended for heavy duty applications, such as radio transceivers. It is a split reed dual-interrupter type. That is, there are two sets of contacts, each with an individual isolated reed. Current rating is somewhat higher than typical radio vibrators such as those found in car and domestic broadcast receivers.
The base is a small UX-7, and the vibrators are series (separate) drive. Coil voltage is 6V, but with an ingenious circuit configuration they can be operated on 12V with no modification, and no series resistor. This change of voltage was required because at the time, U.S. car manufacturers were switching from 6V to 12V electrical systems. Ordinarily, if a transceiver was bought for a 6V car, and later installed in a newer 12V car, modifications would be required to convert it. Typically, this entails replacing the vibrator, vibrator transformer, any dial lamps, and replacing any relays, or connecting resistors in series with their coils. The heater circuit for the valves also has to be rewired to a series parallel circuit, unless there are 12.6V heater types for all the types used - in which case all the valves can be replaced. This additional cost and time required to perform the conversion can be eliminated by using the new type of split reed vibrator circuit. The only change required is in the power plug wiring, depending on whether it is a 6 or 12V supply.

Pin Connection
1 Reed A and coil earth
2 Contact 1A
3 Reed B
4 Coil supply
5 Contact 2B
6 Contact 2A
7 Contact 1B
Most 7 pin communications vibrators will be found to use this pin configuration. A resistor is included internally across the driver contacts in vibrators made by others besides Oak.

Principles and Application.
An article in "G.E. Ham News" for July-August 1957 describes a 100W power supply operating from 6 or 12V, simply by changing the input plug wiring. It is a good example of how these vibrators are intended to be operated.

The transformer has two primary windings, which are essentially operated in parallel for 6V, and in series for 12V. First, examine the circuit when used on 6V. Looking at the plug wiring, it can be seen that the 6V supply feeds the centre-taps of both primaries.
Each primary is connected to its own set of contacts. Each set of contacts has its own reed, which are connected together, and thence to the negative battery terminal. Since the reeds are mechanically, and electrically, connected together, current flows in both of the primary windings in the same direction, and at the same time. The input current is therefore divided in half across the vibrator contacts and primary windings. Effectively, this allows a doubling of power than would otherwise be the case with a single winding and a single set of contacts.
Aside from this, the contacts are larger than a standard radio vibrator, further increasing the power rating. The drive coil is not shown, but is connected to the 6V supply.

Now, consider 12V operation. Note the altered power input plug connections. Starting with the 12V supply, it connects to the centre-tap of the upper secondary. This is switched by the upper contact set, and the return is via the upper reed. In the 6V circuit, this reed would return to the negative in the normal way. But, we want 12V operation! So, instead, the reed returns to the centre-tap of the lower primary winding. This is switched by the lower set of contacts, and current returns to the battery via the lower reed.
It should now be clear that we now have two 6V vibrator power supplies operating in series, but with a common transformer, and with both vibrator reeds mechanically and contacts synchronised. It can be seen that the upper reed and the centre-tap of the lower primary are effectively at the electrical mid point, and therefore at 6V. It is here that the drive coil is connected. As its current draw is minimal compared to the overall primary current, the imbalance it introduces is unimportant. It can be imagined that as soon as power is applied, the drive coil will actually see 12V until the contacts commence vibrating. However, this is for a fraction of a second and is harmless.

From this, we can see that absolutely no changes to the power supply itself are required when switching between 6 and 12V, provided the supply plugs are wired appropriately. Not shown is the heater wiring for the valves. The valves are 6.3V types connected in a series/parallel arrangement. There are two 6V legs of the circuit so to speak, and with suitable arrangement of valve heaters (and if necessary, equalising resistors), each leg is made to draw the same current. This is standard practice for operating 6.3V valve heaters from a 12.6V supply. When the supply is 6V, all that is necessary is to connect the two legs of the circuit in parallel. This is easily done with additional links in the power plug.

Circuit of the G.E. 100W power supply.

The full circuit diagram of the 100W power supply shows all the ancillary components required; the buffer circuit, the rectifiers, and RFI suppression components. Also shown is transmit/receive switching by relays. The vibrator drive coil connections can be traced out for both 6 and 12V. Vibrator types suited to this circuit include: Mallory 1701, Oak V6853, Radiart 5722, and G.E. A-7141584.
With a 100W rating, it is obvious that the vibrator current is much higher than that of a broadcast receiver. At 6V, this is an input current of 16.7A, and at 12V, the input is at least 8.3A. Assuming an 80% efficiency, these input currents would be 21A and 10.4A respectively. Note the individual contact current on 6V is not 21A, because of the current being shared between two primaries. Assuming perfectly equal current distribution it will be the same as that for 12V; i.e. 10.4A.


The Mallory 1701 is a common type of communications vibrator.

Included with the Mallory 1701 is information regarding the drive coil. The series resistor required for normal 12V use is 15R. This will be required when the vibrator is used in a conventional circuit, with only one primary winding and a 12V supply. The vibrator pictured appears to have been purchased in 1977 according to the writing on the can. It is new in box. This may indicate that Mallory was still making vibrators at the time.

The last development in vibrator technology was Mallory's 1600 and 1700 series with the new kind of contact.

The 1701 was among the last vibrators developed by Mallory. It was also one of the few series drive vibrators produced by Mallory. Presumably because of the Oak patent concerning spark suppression of the driver contacts, Mallory use a resistor across the driver contacts to prevent sparking. In the Motorola branded example described further down, the resistor is 270R 1/2W.

Radiart / Cornell Dubilier Electronics

Although not clearly visible, the vibrator at right is labelled 5722/6722. It still has a 6V driver coil.

Radiart was taken over by Cornell Dubilier Electronics in 1953, and as such CDE vibrators carry over the Radiart type numbers. A "5" prefix indicates 6 volt, and "6" indicates 12V. Because of the dual voltage operation, there may be either a 5 or 6 prefix for communications types. For example, the 5722 may also be listed as 6722 as the following advertisement shows:

However, it is important to note that the driver coil voltage is still 6V regardless of type number. The "6/12 volt" types listed must not be run directly on 12V.

Type 6722 is the same as 5722. It has a 6V drive coil.


The corrugated can design was used by Oak with some of their later U.S. made vibrators. Presumably, it is intended to be a more effective heat radiator. The type shown here is a 406861.
It is also branded Motorola type 48C830082. Oak vibrators are covered in this article.

The construction is the same as the ordinary radio and inverter vibrators, which have always been series drive. Unique to Oak is the dual winding driver coil, which eliminates the external spark suppression components otherwise required for the driver contact. Driver coil resistance is 22R.
Diameter of the power contacts for the communications type is 5.6mm. An ordinary Oak car radio vibrator has a contact diameter of 4.3mm.

Melvin Instruments.

This 1701A  is branded Melvin Instruments, but appears to be a rebadged Mallory.


With Motorola being a very prominent manufacturer of mobile two way radio equipment at the time, it is no surprise to see a prevalence of Motorola branded vibrators. It appears that they are rebadged Mallory or Oak types.

This shows the inside of a Motorola 48C847730. It is clearly made by Mallory, and presumably is a 1701 type. Coil resistance is 10R, and the spark suppression resistor is 270R (mounted under the stack). An experiment was done in which the reed swing was measured at 6V. It was about 8.5mm. It was found that to get the same reed swing on 12V, the series resistor was 15R, which confirms Mallory's recommendation.

General Electric.

Type 7141584-003. Same as 7141584-P3. It is shown in the list of vibrators suitable for the 100W power supply, and presumably equivalent to Mallory 1701. It is quite likely the unit shown above is actually the latter.

This 7141584-3 is apparently an Oak rebadge, as evident by the corrugated can.


Apparently another Oak rebadge due to the corrugated can. Types 1585-1022P1, 8-21A-16091, 8-21A-12291. While it looks like these are communications types, this has not been confirmed. Coil is 6V.

Vibrator Ratings.

7-Pin comms types:
Motorola 48C830082. Equivalent to 48847730, 48-847730, 48C847730, 6130-00-284-1147, 00-284-1147, 6130002841147, 002841147 (By implication, also Oak 406861 and Mallory 1701).
Frequency 96-105Hz Input current 24A Drive Coil 6V. Dual reed, dual interrupter.

General Electric 7141584-003. Same as 7141584P3. Frequency 115Hz. Drive coil 6V. Shown in the list of vibrators suitable for the 100W power supply.

Military and other types:
Motorola 48B3333. Frequency 109.25-120.75 Hz. Input current 15A. Drive coil 6V. Single reed non-synchronous.

Motorola 48B60392. Equivalent to: 48B60392,  48B61279, 48B61279-24V,  271-1563VB11, SCA12560D, VB11, VB11A, M682
Frequency 115Hz. Drive coil 24V. Single reed non-synchronous. Octal base.

Motorola 48B82036. Equivalent to: 48-880261, V6126, 348
Frequency 106.9-123.0 Hz. Drive Coil 6V Input current 12.5A. Single reed non-synchronous. UX-4 base.

Motorola 48K60393. Equivalent to: 271-1563VB12, 860VIBRAT0R, RA96A, SCA13489, VB12A, VB12
Frequency 108.1- 121.9 Hz. Drive coil 6V. Input current 5A. Single reed non-synchronous.. Octal base.

Motorola 248B1006. Equivalent to: 3356C24V,  M3356C2,  3356C24V,  M3356C2, V6626, 248B1006,  348D107,  SC-C-34091
Frequency 95Hz. Drive coil 24V.  Input current 2.4A. Single reed synchronous self rectifying. UX-6 base.

Motorola 248B1015. Equivalent to: EX2920, V6642.
Frequency 95Hz  Drive coil 12V.  Input current  2.6A. Single reed synchronous self rectifying. UX-6 base.

Motorola 248C3775. Equivalent to: G3356C12-6V,  248C3775,  SMC116293
Frequency ?  Drive Coil  12.6V. Input current  2.4A Single reed synchronous self rectifying. UX-6 base.

Motorola 48B893518. Equivalent to: MV2,  T4003.
Frequency 115Hz.  Drive coil 2V. Input current 6A. Synchronous self rectifying (presumably split-reed). UX-7 base.

This data was obtained from https://www.parttarget.com/Motorola-Inc-055704_nsn-parts_MDA952-4_60-P32530G001.html

What can they be used for?

This box of vibrators was bought for only US$14.00

Vibrators are always available on the U.S. eBay, and the communications types should not be overlooked. In fact, they don't seem to have much demand since they are not a direct plug in replacement for car radios. Because of this, it should be possible to purchase them inexpensively. So what can they be used for? Aside from their original application, they suit any car or domestic radio circuit where a non-synchronous vibrator is used. By connecting the reeds together and paralleling each set of contacts, the vibrator can be used in the normal way. Of course, if replacing the vibrator in an existing set, the socket will have to be changed. As the contact rating is higher than any car radio type, life should be much longer. The series drive coil also helps considerably in this regard.
These vibrators can also of course, be used in 12V circuits, simply by connecting a resistor in series with the drive coil pin. While this resistor is 15R for Mallory and rebadges, it could be different for other types.
Provided the higher frequency is acceptable, they also suit AC inverters, along the lines of this design.
The seemingly high contact rating of 24A is only applicable for 6V operation with two primaries, and that the 24A is the total input current to the vibrator. It is therefore halved across each set of contacts. If the contacts are simply paralleled and used to switch one primary, this rating will be reduced since the current equalisation is less effective. It is hard to say what the current rating would be, but probably 75% would be safe.

Note that the pin connections are the same as many synchronous vibrators, as used in domestic radios, using a self rectifying circuit (such as the Oak V5211). However, the communications type should not be used as a direct replacement. This is because the contacts are timed to open and close at the same time. The synchronous type is adjusted so the second set of contacts (rectifier) open and close slightly later than the first set (primary). This is required for efficient rectification. If a dual-interrupter type is substituted, efficiency will be slightly less, and there could be some contact sparking. Two situations where the substitution is acceptable is where the vibrator is being used a DPDT reversing switch, such as with this type of circuit, or where a separate valve or solid state rectifier is used to replace the second contact set in the synchronous rectifier. On that subject, it is worth noting that the communications vibrator is also an ideal candidate for 6 to 12V conversion of car radios as described here.

Provided the socket is changed from 6 to 7 pin, and the reeds are paralleled, the communications vibrator can also be used as a direct substitute for the Oak V6606 and V6612 dual-interrupter types. Of course, a series resistor is required for the drive coil for 12V.
As always, when substituting vibrators, it is essential to check the suitability of the buffer circuit.