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4X150A and 4CX250B

U

Unger

This may have been discussed before but I can't find any information on it. I know a few folks who have Harris FM 2.5K/H transmitters which use the 4X150A for a driver. Given the declining stock of replacement 4X150A's can the 4CX250B be used as a replacement? They seem to be a lot more common. I have looked at the data sheets and can't see any major differences other than the 4CX250B is designed for more power.

Has anyone ever done this swap?
 
Check specs for different inter-electrode capacitances, which might throw things off.
 
Looks like the 4X150A has an output intereclectrode capacitance of 15.5pF and the 4CX250B has 4.5pF. That's sounds like a lot to me. I'll have to look at the schematic to see what kind of range I have in tuning.

Tom Wells said:
Check specs for different inter-electrode capacitances, which might throw things off.
Click to expand...
 
I have interchanged them with no problem, other than a slight retune. Unless your tuning caps are at or near max or min should be OK. YMMV
 
Why bother?

1. What exciter are you using?
2. What TPO do you need?

Both the H and the K series allow you to feed the final directly from an exciter. The MX/MS series exciter will do about 15 watts--enough for around 1200 watts.
Need more? Find a used FX 50 or 802A or B. About 20~25 watts is enough to drive the final to full 2500 watts output.

Caveats:

You will, of course, need to carefully match the final's input for minimum vswr to the exciter. Easily done, and once matched, should stay this way. As a safety measure to protect the exciter, "T" a shorted quarter wave section of coax into your feed to the final. Prevents a short in the final circuit from taking out the exciter.

I ran a 2.5H this way for several years using an FX-50 exciter.
 
Using a Continental 802A. I bypassed the IPA for quite a while using that exciter but found there wasn't much headroom once the final started to age.

Whats a good deal on a 50 or 60 watt exciter these days?

TomT said:
Why bother?

1. What exciter are you using?
2. What TPO do you need?

Both the H and the K series allow you to feed the final directly from an exciter. The MX/MS series exciter will do about 15 watts--enough for around 1200 watts.
Need more? Find a used FX 50 or 802A or B. About 20~25 watts is enough to drive the final to full 2500 watts output.

Caveats:

You will, of course, need to carefully match the final's input for minimum vswr to the exciter. Easily done, and once matched, should stay this way. As a safety measure to protect the exciter, "T" a shorted quarter wave section of coax into your feed to the final. Prevents a short in the final circuit from taking out the exciter.

I ran a 2.5H this way for several years using an FX-50 exciter.
Click to expand...
 
If you watch E-Bay and similar sources, you can find 802A's for around a grand. I've seen FX-50's and 802B's for $2500 used.

Haven't tried it, but believe a new exciter like an Armstrong would have enough output to drive the final to full power.

BTW--buy the 4CX1500B's rather than the A series, you will get better life. I never saw the headroom problem with my "H" though I sold it before the tube really began to die--about 3 years as I remember.
 
The primary difference between the two tubes is that the 4CX250B is a ceramic tube (hence the "C") which allows higher seal temperatures and greater plate dissipation. It will work just fine in the 2.5H/K. You can often find NOS 250s on EBay. However, as others have mentioned, using a 25 watt (or higher) exciter and bypassing the IPA works quite well.
 
I've fed several Harris 2.5's with an exciter, bypassing the IPA stage...talk about a move that pays for itself. A 10 watt exciter will produce 600 watts or so, a 25 watt exciter will produce 1.2KW, a 50 watt exciter easily produces full power. Ditch the IPA tubes...you'll never regret it.
 
TomT said:
I ran a 2.5H this way for several years using an FX-50 exciter.
Click to expand...
The new owner of that 2.5H is running it the same way :)
 
Good to hear--they almost tipped over their van getting it out of my site. It's in Monroe County, Ohio, aptly named the "Switzerland of Ohio."

With an MX-15 & 15 watts I was getting about 1200 watts (the licensed TPO was 1050 at the time).
Later, when we were granted a power increase, the FX-50 only needed 25 watts to obtain the licensed 2450 TPO.
 
The only gripe they have is common to all of them...the slightest power bump and off she goes. I like to Bias them as hard as I can and punch through with more drive...improves the efficiency. That old transmitter is never gonna die :). Still using one of the tubes you sent along with it.
 
No surprise--the "logic" consists of a couple 110 volt relays. I just programmed my Sine to scan and kick it back on when the plat voltage sample went away.
 
TomT said:
No surprise--the "logic" consists of a couple 110 volt relays. I just programmed my Sine to scan and kick it back on when the plat voltage sample went away.
Click to expand...
Thought about that but what safeguard do you have if it keeps licking off each time? I was afraid it would beat itself to death by the time a live person caught it.
 
You can program the Sine to try a certain number of times, then quit. Sort of like the three times overload reset on the bigger Harris tube transmitters.
 
In my experience with a transmitter on 102.7 Mhz, using a 4CX250 worked, but the IPA tuning was MUCH more critical. Literally breathing on the controls would make a huge change in output power. The 4X150A tuned much broader.
 
Just did another bypass operation today. A 2.5K with various problems, including a failure in the IPA circuit. Licensed TPO 1350 watts, connecting the MX-15 directly to the final got it back on with 1100 watts. Suggested to management that they look for a good used 30 watt (or better) exciter; or get an inexpensive new 30 watt exciter; rather than fuss with the IPA..

Has the MX sitting in the transmitter--and rewired for 220. Audio processing Optimod 8100--not good--too long a run for the composite signal. Hardly any stereo separation. Need something that can be mounted nearer to the 8100.
 
Tom,
Just a thought on the long mpx cable, i assume you have the 8100 mpx out as per manual.
Would it be worth changing the mpx out for higher current to drive the cable capacitance better?
As std the mpx out is 470 ohm, if you change 1c704 on card 7 from the ad518 to a ne5534 and replace r739 from 470 ohm to 10 ohm, this will vastly increase the current o/p.

Paul.
 
Using RG-62 (93 ohm) for composite cable helps reduce the strain on the stereo generator output circuit and improve stereo performance.
 
I've suggested they look at replacing the MX with another inexpensive exciter with more output (they probably need 20 watts)--and mounting it directly above the Optimod. The MX-15 is in the transmitter, and, from the markings on the connector, running on 240 volts. (In this transmitter the original design had an external 110 feed through the filament contactor, this has been modified for unknown reasons). Going from a 5 foot to 18 inch cable should dramatically improve separation.
 
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