The current transmitters are standard Marconi units installed in 1985 and re-tuned from 200KHz to 198kHz in 1988

Actually the Pulsam modulators were changed for something even more modern and solid state by continental in the late '90s, and the efficiency (mains power to RF conversion ) is in the 80s of %.

The valves last upwards of a decade in their current config, and they had quite a few spares when I last asked.

some history lifted from

https://www.bbceng.info/Operations/transmitter_ops/Reminiscences/Droitwich/droitwich_calling.htm

"

By late 1985 the new long-wave installation was complete and testing was taking place.  The two transmitters, type B6042 and again manufactured by the Marconi Company, were each rated at 250 kilowatts output and used a new method of modulation called the "Pulsam" system.  This was a new application of Pulse Width Modulation (P.W.M.).  So efficient was this new system that a figure of 70% overall was claimed, 10% better than even the B6034s.  The two large valves in each transmitter used a form of cooling known as vapour cooling.  Water flowing into the valve anode jacket (known as the boiler) is vaporised by the heat dissipated, fed to a cooler and condenser and  re-circulated,  the waste heat being removed by an air blast radiator. Only a few gallons of water (purified) are needed compared with the 7500 gallons required by 5XX!   Consequently a water leak, although undesirable, is not as catastrophic as in the old transmitter. The two transmitters were arranged to work in parallel with an output of 500 kilowatts using a diplexer combining circuit and automatic switching to reduce breakdown time in the event of a failure of one unit.  By the end of 1986 the new long-wave transmitters were in use for Radio 4, still providing a frequency standard using the rubidium drive, and continuing the RDS service.  On February 1st 1988, the carrier frequency of Radio 4 was changed from 200 khz to 198 khz.  This was the final  requirement of the Geneva Plan its deferment until this date was part of the plan.  The change was so small (1%) that it was unnoticeable on most receivers but it brought Radio 4 into its new allocated channel.

The last part of the re-engineering of Droitwich Transmitting Station was nearing completion at the end of the 1980s.  This was the replacement of the original Diesel Alternators by new, more efficient machines.  The four English Electric sets, admired by so many visitors in the past were scrapped and their place has been taken by two high-speed turbo-charged machines which take up half the space of the originals and provide more power!  These new engines are fully automated, running themselves up in a few seconds when required in a power failure.  In comparison the old machines, being manually controlled, took about twenty minutes to prepare for service with a consequent break in programmes of this duration if a failure of incoming power occurred."

Mike

The current transmitters are standard Marconi units installed in 1985 and re-tuned from 200KHz to 198kHz in 1988

Actually the Pulsam modulators were changed for something even more modern and solid state by continental in the late '90s, and the efficiency (mains power to RF conversion ) is in the 80s of %.

The valves last upwards of a decade in their current config, and they had quite a few spares when I last asked.

some history lifted from

https://www.bbceng.info/Operations/transmitter_ops/Reminiscences/Droitwich/droitwich_calling.htm

"

By late 1985 the new long-wave installation was complete and testing was taking place.  The two transmitters, type B6042 and again manufactured by the Marconi Company, were each rated at 250 kilowatts output and used a new method of modulation called the "Pulsam" system.  This was a new application of Pulse Width Modulation (P.W.M.).  So efficient was this new system that a figure of 70% overall was claimed, 10% better than even the B6034s.  The two large valves in each transmitter used a form of cooling known as vapour cooling.  Water flowing into the valve anode jacket (known as the boiler) is vaporised by the heat dissipated, fed to a cooler and condenser and  re-circulated,  the waste heat being removed by an air blast radiator. Only a few gallons of water (purified) are needed compared with the 7500 gallons required by 5XX!   Consequently a water leak, although undesirable, is not as catastrophic as in the old transmitter. The two transmitters were arranged to work in parallel with an output of 500 kilowatts using a diplexer combining circuit and automatic switching to reduce breakdown time in the event of a failure of one unit.  By the end of 1986 the new long-wave transmitters were in use for Radio 4, still providing a frequency standard using the rubidium drive, and continuing the RDS service.  On February 1st 1988, the carrier frequency of Radio 4 was changed from 200 khz to 198 khz.  This was the final  requirement of the Geneva Plan its deferment until this date was part of the plan.  The change was so small (1%) that it was unnoticeable on most receivers but it brought Radio 4 into its new allocated channel.

The last part of the re-engineering of Droitwich Transmitting Station was nearing completion at the end of the 1980s.  This was the replacement of the original Diesel Alternators by new, more efficient machines.  The four English Electric sets, admired by so many visitors in the past were scrapped and their place has been taken by two high-speed turbo-charged machines which take up half the space of the originals and provide more power!  These new engines are fully automated, running themselves up in a few seconds when required in a power failure.  In comparison the old machines, being manually controlled, took about twenty minutes to prepare for service with a consequent break in programmes of this duration if a failure of incoming power occurred."

Mike