[Amps] 1 tube or two ??

[Tom W8JI]

> Peter Voelpel wrote:
>> I imagine the two holer was built to run the ham limit.
>> If you inspect the Pi values and calculate back, you will 
>> see for what power
>> level they are constructed.

Why would that be? The range of Q is one of the least 
critical parameters so long as it is a minium of  something 
significantly greater than the square root of the loadline 
impedance over the load impedance. I can use the same tank 
component values at 10,000 watts as 1,000 watts with very 
little change in performance so long as they are large enogh 
in size and meet the minimum Q requirement.

>> The drive requirement will be lower with two tubes for 
>> the same output

It won't be significantly lower in grounded grid.

In a grounded grid PA that is not saturated the gain is 
largely determined by  the ratio of driving to load line 
impedance. Placing two tubes in parallel changes both a 
nearly equal amount. As a matter of fact, this is a huge 
worry in amplifiers using multiple tubes. Say you have three 
8874's in parallel.  If a single tube fails the drive power 
to the other two tubes simply increases, and the net output 
is very close to the same amount. This can quickly lead to 
loss of another tube from excessive drive.

There is some gain increase, but it certainly is not 
anywhere near a 1:1 improvement.

A 4-1000A is a good 1000-1200 watt output tube when in 
grounded grid, but it is generally gain starved in a good 
stable circuit. The reason is the driving impedance of a 
4-1000A is high, around 70-90 ohms. The high driving 
impedance reduces the gain compared to other tubes like 
3-1000Z's or 3-500Z's operating at the same anode voltage. 
It generally takes 140-150 watts to make 1500 watts on a 
4-1000A in cathode driven service even when the anode is at 
4 or 5 kV.

Of course power meters are all over the place and 
regeneration from poor layout can increase gain, so we can 
expect to see higher claimed gains but all in all the 
4-1000A is not the "big tube" we fondly recall. Like the 
Rhombic or V Beam or a 66 Mustang GT Hi-po that only runs 
15's in the 1/4 mile, they gained that reputation when we 
just didn't know what a good thing really was.

73 Tom