The TW500A is a 500 Watt output 110V/220V mains powered commerical Solid State amplifier which occasionally is found on the second hand market.

Documentation for this amplifier seems to be very scarce so I have scanned the circuit and manual to assist owners of this amp.

I receive a large number of Emails asking for the part number of the PA transistors used in this amplifier. Virtually all of the Emails report that the PA Transistors in their amplifiers only show in-house part numbers.

In my amp there are 8 x Motorola MRF454 devices.

It is, of course, possible that my amp does not have the original PA Transistors and that the MRF454's are substitute replacement devices. However the PCB does not show any signs of the re-soldering associated with device removal and replacement so there is a high probability that these are the original devices.

A word of warning.

From the Emails I receive it would seem that there may be more than one version of this amplifier which bears the TW500A model number (!) and some would appear to use 24V or even 50V devices whereas the MRF454 is a 12 volt device. The model to which this page relates is a desk top unit housed in a black anodised aluminium case approximately 440mm x 200mm x 330mm with a large 100A panel meter to the top right of the front panel. The power switch is on the left and there is a band switch centre left. Below the meter is a circuit breaker. (See pictures in manual)

Please verify that your amplifier is of the same type before substituting a failed device with an MRF454. It may also be advisable to replace the other device in the push-pull pair to maintain balance.

Update 18 June 2007

I have received an Email reporting that Trans World have also used the MRF458 in this amplifier. However the MRF458 appears to be obsolete and may therefore be difficult to obtain. Of the two devices, the MRF454 would appear to be the better of the two devices and is readily available.

Page last updated 18 June 2007