High output screen driven linear amplifer

[kb3ouk]

http://www.813am.qsl.br/artigos/moduladores/hileman/high_output_linear_amplifier_cq_set_61.pdf

Is this too good to be true or actually possible? If I'm reading this right, then using this circuit it should be possible to get about 145 watts of carrier from a single 813. A pair of them should do almost 300 watts, where a pair in conventional class B grid driven serivce will only do about half that.

[Ralph W3GL]

After a quick  scan of that CQ piece from 1961, a pair or 3,  4CX250's with
2KV applied to the anodes should really talk in SSB mode and with enough
air moving produce plenty CW or AM carrier.  But remember, you have to
bias the #1 grid to make it work...

Lots of those tubes as well as the sockets out there at reasonable prices.

I think I even have several, still in the sealed plastic wrap, in my stash ...
 
Edit:  More thinking (dangerous condition at times!) the carrier generated
         should be good for CW and FM use but might fall short in AM due
         to lack of "head-room issues" at full bore ...

[kb3ouk]

That article is saying you can get a peak efficiency of 77% in screen driven service. So figuring 2 kv at about 900ma for 3 4CX250s (probably a conservative guess, looking at how hard they were pushing a single 813), that's almost 1400 watts pep, which roughly translates into about 350 watts of carrier.

[KM1H]

And on SSB at least they are massive IMD/splatter generators. Similar to the G2DAF abortion.

There is no magic bullet but increasing the bias more into Class B in a standard GG amp raises efficiency with only a very small IMD hit. Ive run Dentron L's on AM at 450W carrier and no plate color. Plus excellent reports and scope pictures and the transformer was not overheating after 2 hours of ragchews.

Carl

[kb3ouk]

So it really doesn't have any advantage over a normal class B amp? You could probably make it cleaner by backing off the voltage and plate current, but then you loose output power, which probably means no more power than a conventional amp.

[W3RSW]

Pretty much right.

Pure class B, you get crossover distortion.

Class AB, advantages of class A in the crossover, low power region.  As you raise the quiescent, ( more "A",less "B") no signal current, you get less efficiency.

Pure class A, much less distortion,  close to perfectly linear transfer characteristics or as good as a real world tube, built properly, can deliver.

All this assumes constant, no sag, voltages at all the tube elements for best results.
And therein is a common failing of less expensive audio and RF amplifiers.

[Opcom]

4CX250B and 4X150 control grid is extremely delicate. G1 is rated 2W.

The all time best datasheet on the 4*150 and 4*250 is probably the STC application report. http://www.bunkerofdoom.com/tube/man/STCx.pdf and elsewhere.

Both tubes are rated 250W plate. They are not always interchangeable.

[kb3ouk]

The screen dissipation of a 4CX250B is 12 watts. In that circuit the screen gets the RF drive and the grid is only drawing a small amount of current at a low voltage.

[Opcom]

One similar to the 6BQ5 diagram was seen in a Wawasee amplifier on 27MHz. One 6BQ5, 600VDC, 75W peaks. It wasn't very linear but it belted the power out.

[ke7trp]

I had an amplifier called a Delcon T210.  It was made by HP for the airforce and used in mars stations.  It ran 3 x 4cx250R tubes.  The PS was seperate. It was extreme quality with dual vac caps and silver tank.  The PS had hypersil iron all around.  This amp ran 1500 watts out easy.

Current was about an amp and I think the HV was 2600.  I just sold it to a collins collector on Ebay.  They where made to run with the S line.  One thing I did notice is that this amp produced an incredible amount of heat when in use.  The idle current was 350!! MA.   I backed that down to 200 and never used it over 1000 watts out. But it sure ran HOT HOT HOT.  The SB 220 that is in normal use here runs cold in comparison at the same 1000 watts out.  Those where some HOT little tubes!

That article is saying you can get a peak efficiency of 77% in screen driven service. So figuring 2 kv at about 900ma for 3 4CX250s (probably a conservative guess, looking at how hard they were pushing a single 813), that's almost 1400 watts pep, which roughly translates into about 350 watts of carrier.

[W2VW]

I had an amplifier called a Delcon T210.  It was made by HP for the airforce and used in mars stations.  It ran 3 x 4cx250R tubes.  The PS was seperate. It was extreme quality with dual vac caps and silver tank.  The PS had hypersil iron all around.  This amp ran 1500 watts out easy.

Current was about an amp and I think the HV was 2600.  I just sold it to a collins collector on Ebay.  They where made to run with the S line.  One thing I did notice is that this amp produced an incredible amount of heat when in use.  The idle current was 350!! MA.   I backed that down to 200 and never used it over 1000 watts out. But it sure ran HOT HOT HOT.  The SB 220 that is in normal use here runs cold in comparison at the same 1000 watts out.  Those where some HOT little tubes!

That article is saying you can get a peak efficiency of 77% in screen driven service. So figuring 2 kv at about 900ma for 3 4CX250s (probably a conservative guess, looking at how hard they were pushing a single 813), that's almost 1400 watts pep, which roughly translates into about 350 watts of carrier.

An amplifier  with 1500 watts out of 750 watts worth of dissipation is pounding the crap out of the tubes or making a nasty signal. Do not confuse ease of drive verses sustained power capability. Heat is a clue.

[ke7trp]

I measured the signal with the Spec AN. It was pretty clean by todays standards up to 1000 watts where it was designed to run.

[W2VW]

I measured the signal with the Spec AN. It was pretty clean by todays standards up to 1000 watts where it was designed to run.

What figure did you get for transient intermodulation distortion?

[Steve - K4HX]

Here is the spec sheet. It was never supposed to be run anywhere near 1500 watts.

[ke7trp]

Yes. I had the manual from w7FG   Mine tested a bit cleaner then the specs.  Probably because my transciever is very very clean. Cleaner than most of the radios of time period.

Each tube has a 250 watt plate rating, I biased the amp down a bit and found that even at only 175 to 200 MA of current for three tubes, the signal was linear and clean.  So 1500 is not out of the realm of three R's with that beefy power supply.

When used with the S line for continuous use, you where supposed to set the ALC control so that you had 1000 watts ouput which is where I ran it, Mainly on 15 and 17 meters,  Even then, That amp made serious heat.  This amp had original tubes as the date matched the date of the amp with in a month and the rings around the tarnished tubes all matched up to the brackets. They still made full power and ran clean.  Wrong AGAIN w2vw.  Its seems that happens alot to you.


C

[W2VW]

What r u smoking besides radio equipment?

[Ralph W3GL]

Boy, this thread got hi-jacked and really deteriorated but,

        YOU GO DAVID!
 

[Steve - K4HX]

Now Ralph, you well know that Dave knows little or nothing about amps. He's never worked on one, built one or maintained one for a living.

[Ralph W3GL]

Yeah, and I'm a reformed bootlegger...       

[KM1H]

Unless the HV was run well above spec the 4CX250R is only good for a 300W maximum on SSB and the IMD is in the high -20's by then.
Run as in the spec sheet which is loafing them the stated "over -30dB" is a reality but not by much.

You dont get prime rib from a pig either and the 4CX250R is a true porker of late 50's design. The R was for "rugged" and not for electrical performance.

Carl

[Steve - K4HX]

Please stop bringing facts and reason to the discussion.  Let Clark have his fantasy.