450TH in AB1, AB2. 304TL comparison

[Opcom]

A very interesting question was put to me.

If a 450TH pair in AB2 can do 1400W, and the peak plate current is 800mA, then, if the drive is reduced so that it is AB1, and the peak plate current is only 400mA,

does this also mean that the voltage across the load is cut in half?

does this also mean the power is now 1/4, i.e. 350W, not 1400W?

I note the example of the 304TL datahseet where AB1 and AB2 conditions are shown. nearly the same, except the amount of drive.

for 1500V:
AB1 =  256W
AB2 = 1100W


So, it is obvious the low drive of AB1 does not cause much current flow in the load.
What if the AB1 load impedance were changed to 0.7 * the AB2 figure so more current would flow? Would this just bring on distortion?

A friend has two 450TH and wants to do AB1 modulation, and wants to modulate a pair of 450TL's.
Is he going to be limited severely as to DC input if he wants 100% mod?

[W2PFY]

In one of the ARRL hand books it states that a pair of 304TL's in AB1 will produce about 650 watts of audio with 1500 volts on the plate. I think it was the 1956 handbook. Maybe it was a misprint.

[KM1H]

Yep to all.  AB1 for those tubes is very inefficient as the spec sheets show.

Why does he want AB1? Running in light AB2 requires a bit more driver complexity at a minimal increase in distortion.

He can also run a regulated AB2 bias system that increases bias voltage on voice peaks to a bare tickle of grid current.

Id use 304TL's primarily for the emission reserve.

Carl
KM1H

[Opcom]

He has 450TH's and wants to use them. I only included the 304 sheet for a comparison.

So, then the 450TH are not going to do well no matter what? I guess he is going to get 350W on a good day. (3KV plate)

He wants to run PP 450TL's as the final. I can see running 500W DC input and the modulation being OK with AB1 450TH's, but it would be a shame to have such a beefy rig and not be able to push the throttle to "war emergency power".

I did advise in my private conversation to go ahead and try to use AB2. Could be his reason is lack of grid-driving iron and maybe needing a good driver. His last rig used 304TL in AB1 and was fine.

[Bill, KD0HG]

Random thoughts--A lot has to do with available plate voltage. At 2,000-3,000 volts, a 304 is going to smoke a 450, look at the data sheet, you can drive a 304 to 800-900 mils per tube, that's almost 2 Amps for a pair, maybe 2 KW input if you keep them cool in ICAS or "War Emergency Power".

So my first Q is, how much plate voltage is available? 3 KV or more, 450s will play fine, less than 3 KV, use 304s. Of course, I'm thinking in terms of max smoke, perhaps the fellow isn't interested in that.

Driving a pair or 450s or 304s in Class B shouldn't be a problem. My current audio driver is a 100 WPC solid state stereo amp and a former 5,000 to 8 ohm audio output transformer on the grids.. It's easy. And the solid-state audio amps have a super low output Z and lots of inverse feedback. They just don't care if the class B stage is pulling grid current or not.

Harder to do it all solid-state, but you could still use a pair of audio output trannies back to back if you can't find a big audio driver transformer. I still like triode connected 6550s or 6L6GCs for hollow-state drivers.

[KE6DF]

The 1959 Orr handbook has an article about a 304tl modulator. It gives both AB1 and AB2 operating conditions for a variety of plate voltages.

The max they list for AB1 is 730 Watts output from a pair at 3000 vdc on plates.

The full text of this issue of the radio handbook in on the web in various places as I think they let the copyright expire.

Dave

[W2PFY]

I remember when they were selling 304TL's for 25 cents each. Hams at the time didn't want to use them because they used two much filament current and went out and purchased a new 4-400 for SSB for over $100.00. You could buy a lot of filament run time for that kind of money. What was the cost of power in the 1950-60? Probably 2-3 cents a KWH. I think people in those days operated with a herd mentality.

What does this have to do with the topic? Nothing  

[KM1H]

Also remembering those days and living a subway ride from Radio Row in NYC I have a different take on it. The belief then was that any of the TL tubes were hard to drive dogs. The 250, 304, and 450TH's were grabbed until they almost disappeared and the TL's were left for the po folk. Also since most obeyed the 1000W input rule big bottles werent necessary for modulators since surplus 810 and 813's were also cheap and available. My own amp was 250TH's with 810's audio courtesy of the USAF MARS program.

It wasnt until the mid to late 50's that Orr and others started promoting the 304TL which was mainly used for target practice.

I still use 250TH's but these days 304TL's do the talking.

Carl
KM1H

[w1vtp]

I remember when they were selling 304TL's for 25 cents each. Hams at the time didn't want to use them because they used two much filament current and went out and purchased a new 4-400 for SSB for over $100.00. You could buy a lot of filament run time for that kind of money. What was the cost of power in the 1950-60? Probably 2-3 cents a KWH. I think people in those days operated with a herd mentality.

What does this have to do with the topic? Nothing  

Those days are certainly gone forever.  I tried to get up a collection of 304TLs but ran out of $$ gas very soon.  The audio phools have struck again.  I just did a ePay and they are are listing from  $150 to $250 - unless you want to get sucked into "looks VG" BS.  I got one of those and there was little floaters inside the bottle. So, it's back to my collection of 813s for me.  I still think those 304TLs are a cool toob

Al

[WBear2GCR]

I am unaware that 304TL tubes are of any significant interest to anyone in audio... fwiw.

               _-_-

[n2bc]

Audio foolery here:   http://www.alumrocktech.com/amplifier.shtml

Pair of 304TLs for a mere $85K.  I suspect you need a low oxygen silver line cord and cyrogenicly treated AC recepticals to get the full audio "warmth".  But from the pix, that $85K would make a pretty nifty looking space heater!

[W2PFY]

My own amp was 250TH's with 810's audio courtesy of the USAF MARS program.

Do you still have that transmitter?

[W2PFY]

But from the pix, that $85K would make a pretty nifty looking space heater!

For that kind of money you could heat the average house for 25 years assuming no inflation. Now if it's $170 K for a pair your almost good for a life time supply of fuel.

[KM1H]

Do you still have that transmitter?

Long gone, parted out in the early 60's when I bowed to the call of SSB   Then I got a NCL-2000 prototype for a song that put out 1200W and was smaller than the CE100V exciter. I had that on DSB AM many times until I sold it around 85.

Carl
KM1H