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4CX250 tubes in London Power amps

6/1/1999 6:15 PM
4CX250 tubes in London Power amps
This company uses these unusual tubes in the power section of certain amps and seem to get quite a good amount of wattage from them when compared to the usual tubes we see in amps (6L6, EL-34 and company). Is London Power the only amp maker to use these tubes for guitar amps and does anyone have playing experience with these tubes and/or amps? How do they hold up tone wise. The canadian amps are very pricey so I was wondering if the money is worth it. Also, has anyone homebrewed with these tubes??
6/1/1999 7:04 PM
Dave Chun

If I remember correctly, the 4CX250s are triodes more suited for RF applications (they can dissipate over 250W each.) There's lots more info at the Svetlana website,
6/2/1999 12:57 AM

A pair of 4CX250s in push pull class AB1 at audio frequencies will put out way over 600 watts with the right power supply!  
Try and find an OT for them though! $$$$  
6/2/1999 5:16 AM
Dave Chun

The London Power Bass Amp does just that - a pair of 4CX250s for 700w. I emailed them about the OT and it's a custom Hammond type that weighs *35 lbs.* (!).
6/2/1999 5:46 PM
Reid Kneeland

There are also some data sheets at  
6/5/1999 2:55 AM
Ned Carlson

4CX250B isn't a triode, it's a radial beam tetrode. 4CX250B is like the 6L6 of the RF world, it and its predecessor 4X150A (basically, same item with a glass seal instead of ceramic) have been around since the 1940's.They're still made in the USA, Russia & China. If you want deals on these tubes, look for the ones with weirdo basing and filament voltages, like 4X150G.  
The problems with these tubes are:  
(a) Require forced air cooling, via an Eimac type air system socket & chimney.  
(b) The voltages used are literally killer. 2,000 volts is NOT for neophytes, to be really prepared to deal with kind of stuff, you ought to have some background in high power transmitters & modulators. I think the American Radio Relay League " target="_blank">"> has a lot of info on this. AAMOF, studying to get a ham radio license (which I could kick myself in the butt for not having) is an excellent primer on electronics in general and high power transmitter/modulators in particular.  
Any rate, anything with the voltage and current that the London Power amps have, have got to have outboard voltage & current meters and interlocks so there's absolutely no reason for or possibility of humans interacting with 2,000 volts. You do not get second chances when dealing with this kind of juice, one mistake and the only way you'll ever post on Ampage again is if you are reincarnated.  
Any rate, 4CX250B certainly can be used as audio amplifier tubes, and Eimac issued specifications to use them that way.
6/5/1999 9:49 AM

Obviously, with those high voltages and custom trannys I can understand why these tubes are not being used that often in amps. I was asking these questions because I was curious. Altough a push pull pair of these tubes can push out 650+ watts, some of London Power's amps max out at 60 watts per side... I suppose these amps use much lower voltage settings and more common output transformers. I was also curious how these tubes sound, sure, mega wattage from a 20 year lifespan tube is neat, but what about tone??
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