Discussion in 'Amateur Radio Amplifiers' started by WA6MHZ, Jan 11, 2019.
The 4CX250 isn't a power grid tube.
Look at page 3 where the term "power grid tube" is defined:
As far as the tube itself, here is all the math, and instructions for using it, that you will need to design your own amp, or to get a "sanity check" for one you bought.
You're correct Jim, thats it. I just remember thinking I'd like to build one, but never happened.
6 volts max on those filaments, not 6.3 if you change tubes.
Those 6 and 2 meter kilowatt amplifiers were in the ARRL Handbook for several years. All of the info needed to restore one to operation is given in the article.....if someone takes the time to read and understand it.
73 de Jim, N2EY
There's a lot more information about 4CX250B amplifiers in The VHF/UHF DX Book, which is now available as a free download.
Since that book was last published in 1995, further experience suggests that the power and control circuitry is over-engineered for most people's taste; but a tightly regulated screen supply is still very important for low IMD (particularly for tubes of the 4CX250B family).
Also, don't be misled by Eimac's "recommended operating conditions" for SSB. They are far too greedy for power output, promising 470W PEP from a single tube in Class AB1 - but just look at the associated IMD. Beginning at -20dB 3rd order, and spreading out from there with higher-order IMD, such a signal will earn you no friends on the VHF bands. More realistically, a pair of '250Bs will give a very clean 500W PEP output in Class AB1.
Now some good news: it is perfectly possible to operate 4CX250Bs at anode voltages up to 2500V (provided, of course, that the vacuum is still good). There should be no problem at all about the anode voltage rising to 2500V off-load, and falling to 2200-2300V on-load. At the higher anode voltages, the tubes will deliver power more readily, and IMD should decrease even further - provided, once again, that you're not greedy for raw power output.
In the DX Book you will also find details of the classic W1SL 'Plumber's Special' which is a later evolution of the amplifier that you have inherited. You will almost certainly find contact and imbalance problems with the lead-screw anode tuning, so the W1SL arrangement using a floating flapper is a strongly recommended upgrade.
73 from Ian GM3SEK
Remember, America is big and hasn't nearly the level of weak-signal close-together VHF activity the U.K. or all of EU has.
Our major complaint over here is "Where is everybody?" If I can make 200 QSOs on 2m during a major VHF contest, 175 of them will be with stations 250 miles (or farther) away with S1 to S5 signal levels; I wouldn't notice if they had IM7-9 at -25 dB, all that would be way below the noise.
The biggest offenders for IMD here are those using bipolar solid state brick amplifiers, and there are a lot of them; more likely someone within 25 miles is using one of those and creating havoc.
i see measuring Screen current seems critical! so now I need a dual direction ammeter that is going to be mighty tricky to incorporate. I was hoping to use the plate voltage meter as a Multifuncttion meter but it only goes in 1 direction. Negative Screen Current?
I need to tap off the HV for the screen regulator. In the current onfig, they tapped off the 2nd cap from the botton of 6 which means abut 750VDC at that point. The OA2 and OB2s draw 30ma Current. what value of dropping resistor do I need and at what power? They had two 75Ks in the Prior-to-burning amp. They are about 6 inches long and unknown power. They burnt. The newer version used a 25K at maybe 10W for the Zeners. I think two OA2s and a OB2 would give good regulation for the screens.Though it will be higher than 350V (108 +108+150 = 366V) I will put a pot across the regulator to adjust it to 350V and then a 2nd Pot across it to give the 250V What I am worried about is this Mysterious Screen Current being drawn. Though not much, a few mA will stil change the voltage to the screen. Some designs I have seen use a sensor to monitor Screen Current and cause a quick system shutdown if excessive current is ever drawn. that might be worth doing, but the electronics to detect and monitor such will be difficult.
Soldering iron not fired up yet. Just mousing around plans on the design. It needn't be ready for the VHF Contest next weekend (not sure how much I will participate.) but I woud like to get this ready for the June Contest.
OH, and the amount of screen current drawn to BURN UP these big power reisistors must have been MASSIVE!!!