4CX250B heavy duty replacement

[n8fvj]

I there a tube such as a 4CX350 that replaces the 2CX250B with same operating parameters and uses the 4CX250B chimneys? Does the replacement tube require more air flow?

[Opcom]

The eimac datasheets are here:
https://frank.pocnet.net/

The input capacitance is significantly different as is the G2 to plate amp factor, but the rest is close. You have to make your own decision based on the application.

[KD1SH]

There are also the Svetlana 4CX400A (hen's teeth, these days) and Russian GS36B tubes (still somewhat plentiful), but neither will accept the 4CX250 series chimneys, where the 4CX350 will, and like the 4CX350 their screens are a tad more more delicate than those of the 4CX250B.

[W1ITT]

I looked into the 4CX350B a few years back.  Their appeal is not so much their plate dissipation, but that they were designed for low IMD in linear operation.  The 4CX250 series were best at Class C and had a reputation for poor IMD figures.  I've read that they don't do much better than minus 20 db near full ratings.  When Tom K1JJ embarked on his very low IMD amplifier campaign a few years ago, I seem to recall that the 350Bs were among his bag of tricks.  Availability of copious spares is something to watch for.
73 de Norm W1ITT

[K1JJ]

I looked into the 4CX350B a few years back.  Their appeal is not so much their plate dissipation, but that they were designed for low IMD in linear operation.  The 4CX250 series were best at Class C and had a reputation for poor IMD figures.  I've read that they don't do much better than minus 20 db near full ratings.  When Tom K1JJ embarked on his very low IMD amplifier campaign a few years ago, I seem to recall that the 350Bs were among his bag of tricks.  Availability of copious spares is something to watch for.
73 de Norm W1ITT

Hi Norm,

I used the 4CX-350J, an ultra-linear tube for the job as IPA.  Fair Radio had a huge supply of them at a stiff price about 7 years ago... They are probably hard to find these days.

I was able to get upwards of -55 3rd IMD when running a class A 4CX-350J at a few watts output to drive a single class A 8877 > into a pair of class A/B 8877s.

Yes, the 4CX-250s needs some RF negative feedback to get them out of the "abominable" sidebands rating.... :-)  I used to have a TMC linear using 6146 > pair of 250Bs with NFB. It did about -40 3rd IIRC.
Reasonably clean.
T

[KD6VXI]

There is no drop in upgrade tube.

The closest is the 4CX250R.   Bias is slightly different on the grid, but man, it's a hot tube!

Then there is the 8930.   Military tube, the hottest drop in, but it's not.  Slightly different g1 bias as well as a different chimney that's pretty rare now.

4cx800 (really a 600 Watt tube) almost drops in, but voltages are different enough to matter.

All of the 350 and 300 tubes are different enough in parameter that it matters.  I've dropped them in a pride dx300 and with a zener diode change they worked, but where easily destroyed in am operation as linear.  Grids are just too fragile for any use exceeding parameters.  They ain't a 4-1000!

8930 is the way to go.

Finding one isn't easy.

--Shane
WP2ASS / ex KD6VXI

[n8fvj]

It appears the 4CX350 is not a good match in my KWS-1. The 8930 and 4CX250R are scare and very expensive new. Seeing the KWS-1 shipped with the 4X150 I stay with the 4CX250B. Thanks all.