[Amps] 1 tube or two ??

[Tom W8JI]

> I built a 4-1000A amp in 1969 and I'm still using it with 
> the original
> tube. The thing I like best about it is that I don't have 
> to wait 3
> minutes for it to warm up as with external-anode tubes. 
> Also, the tube
> must be quite rugged or it wouldn't have lasted this long.

A natural progression through amplifiers from the 60's, 
where many of us grew up, would lead us to a 4-1000A. I 
started with an 813, had 810's, and then eventually worked 
up through 400's and 500's to 4-1000A's...going from 
grounded grid to eventually building a pair of them in grid 
driven AB1 when attempts at increasing gain by adding 
regulated screen and bias supplies failed.

I'm certainly not saying it is a bad tube, but it is not a 
big conservative tube for current power rules. We are 
allowed 1500 watts output and even in top form that places 
somewhere around a 1000 watt power dissipation on the 1000 
watt dissipation tube. That isn't a great deal of headroom, 
despite what some might emotionally feel by looking at the 
physical size of the jug.

Additionally it has stability issues when compared to more 
compact tubes with shorter grid leads, and a relatively high 
driving impedance that limits gain in cathode driven service 
when compared to other tubes at the same anode voltage.

On a really good power meter I and everyone else I know or 
knew using amps with that tube needed much more than 100 
watts of drive to get 1500 watts out at 4kV anode voltage. 
To be decent the tube had to run at 5-6kV, and adding a 
screen and bias supply didn't do much in grounded grid 
because the driving impedance didn't change a great deal and 
so nether did the gain.

On a positive note it was a reliable tube, a cheap tube 
readily available used, and it functioned OK in a proper 
circuit.

My only point is it is old technology designed for grid 
driven service and certainly not a "conservative" 1500 watt 
output tube. That thinking is totally wrong unless you run 
the thing at 80% efficiency in class C grid driven, because 
we all know it will only do about 60% efficiency in grounded 
grid AB2 and that means about 1000 watts dissipation at 1500 
watts out...the limit of the anode.

> I *have* considered buying or building a new amp. My shack 
> is small and
> my present amp is huge compared with what is being sold 
> today. But I
> keep coming back to the instant-on advantage of the 
> 4-1000A.

A 3CX1200A7 (or better yet with mods a Z7) will drop in and 
have much more gain at the same voltage and more headroom, 
be cleaner in IMD, last just as long or longer, and is 
instant on. A three by three (3CX3000A7 or F7) will run at 
similar voltages (but more filament current) and be instant 
on, more stable, more gain, much cleaner for IMD, and really 
have headroom.

Not that anyone should change or the 4 by 1 is that bad of a 
bottle, but the point is almost any newer design tube is 
much better technology.

QSK is another nice addition that is easily accomplished 
with vacuum relays or less easily with even faster PIN 
diodes, and they can all be worked into a 4-1000A RF deck.

I certainly don't want to "dis" the old 4 by 1 amps because 
I was also a proud builder of at least a dozen of them, but 
I never looked back when I moved on to more "cathode drive 
friendly" tubes that actually did have significant headroom 
and better performance in every aspect.

We get these memories of how things were and they stick with 
us. We think V beams are spectacular antennas but the very 
best of them can only match the gain of a pitiful 3 element 
Yagi. We think cars from the 60's were fast but cars from 
the 90's and 00's are safer, run longer, run faster, handle 
better, and burn significantly less fuel. The same is true 
for big jugs of all kinds, not just 4-1000's. That hot girl 
from HS probably actually didn't actually look nearly as 
good as we remember either, we just confuse easy to get and 
large with optimum performance.

This doesn't mean any of the old stuff is bad, just that we 
should separate emotion from fact when deciding what to do. 
Everyone was crowing about how conservative a 4-1000 is, but 
the fact is it isn't conservative at all at modern power 
limits. It's actually marginal.

73 Tom