The use of the 6550 beam tetrode as a single triode can make a superlative amplifier. My pages on the use of beam tetrodes give much information on the load matching outcomes from using the 6550 as a single amplifying device but many people would prefer to use a multi grid output tube as a triode by connecting the screen grid to the anode.
In fact most people will want to use the 6550 in triode mode for an SE ( single ended ) amplifier rather than use the 6550
in beam tetrode mode, or have a quad of them in parallel to make enough pure class A1 power.

This page has the following content :-

* Fig1. Anode resistance curves for GE6550A in triode from the 1950s.
Explanations of how the curves were obtained and what the curves mean.
* Fig 2. Schematic for testing power triodes.
Triode voltage generator model is included in explanations.

* Fig 3. GE6550 triode curves with 2.5k RL and tangent to calculate Ra, µ and gm for any chosen working point.
Explanation of what load lines are.
How to plot them graphically to read the graphs for gain, calculate power output, and 2H distortion.

* Fig 4. Measured power output vs distortion for 3 load values used with EH6550 in triode.
* Fig 5. Measured power output vs distortion for 3 load values used with GE6550A in triode.
* Fig 6. Measured power output vs distortion for 3 load values used with KT88jj Tesla in triode.
* Fig 7. Measured power output vs distortion for 3 load values used with KT90EH in triode.

* Fig 8. My corrected Ra curves for EH6550 in triode with 3 values of RL plotted.
Comments on loads recommended for EH6550 in triode.
* Fig 9. My anode curves for EH6550 in triode with no load lines so you can download them for use.

* Fig 10. Ra curves for 300B measured after 1990 with 4.5k loadline details.
Comparison of THD with beam tetrodes strapped as triodes.
* Fig 11. Ra curves for trioded GE6550 measured after 1990 with 4.5k load line details.
List of conclusions about beam tetrodes used as SE triodes.

* Fig 12. Intermodulation test rig schematic for measurement.
* Fig 13. Measuring the intermodulation distortion using an oscilliscope wave form.
Comments about THD and IMD significance.

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Fig1.

This set of anode resistance curves is taken from a scan of an ancient GE data sheet and tidied up in MS Paint.
The positions of the curves have been carefully preserved.

To be able to establish these curves in the 1950s a curve plotter machine was used to draw the curves on paper
while someone worked a calibrated analog electronic circuit for testing the tubes.
Another way it may have been done was by photographing an oscilliscope screen and making a black ink on white paper
drawing by tracing over the photo. All these methods were prone to errors, but despite errors there was enough information in the curves to design any circuit using the tube.

The process of viewing the curves shown required that the tube be set up in a test circuit with its cathode grounded,
and its anode taken to a ac signal source of low source impedance less than 50ohms, with a 10 ohm current sensing
resistance. Current wave forms and applied anode voltages are fed to a dual trace oscilliscope.
The oscilliscope is capable of  X-Y function and transfer curve for Ea vs Ia is displayed as a curve as plotted above.
The grid of the tube under test has a range of fixed bias voltages applied while the anode has a large ac voltage swing applied
of up to say 800V peak for many output tubes. There is a separate curve generated for each Eg value.
Each curve represents the dynamic anode resistance known as Ra, for the tube.

Each Ra line is a curve rather than a straight line. This is because where you change the Ea, the Ia changes non linearly
so that Ia = a constant x cube root of Ea squared. To fully understand electrostatic behaviour in tubes one has to study
the old books from the 1920s and 1930s, and read up on Child's Law.

Today hardly anyone can buy a tube electronic curve tracing circuit because of the high voltages involved and lack of demand.
But a few have succeded in using a PC to very easily create a ditital file of the curves which can be printed out,
there are some programs available now which automatically plot loadlines across the tube characteristic curves and calculate
the distortions.

Considering the Ra line in the above set of curves for Eg = 0V, when Ea is raised to 100V, Ia = 100mA, and when Ea is further increased to 200V, Ia rises to 275mA, so the rate of Ia increase isn't linear with applied Ea. Ra varies as explained above but we really only want to know Ra at one very small range of Ea variations where Ra won't vary much.
All resistances can be measured in ohms from Ohm's Law where R = E / I.
The Ra is the dynamic output resistance of the tube and has no strict relationship to the quiescent fixed operating Ea divided by the fixed idle supply current. So because a typical operating point could be with Ia = 100mA, and Ea = 330V, it does not mean Ra = 330 / 0.1A, or 3,300 ohms. The DC operation of the tube should be considered separately to the ac operation.
The equivalant ac model of a triode can be considered a voltage generator which has an extremely low source resistance for its output signal voltage which is the product of the amplification factor, µ, multiplied by input voltage between grid and cathode. There is then a model resistor between the voltage gene output and the anode terminal. This resistor is the Ra, or anode resistance or impedance as it is known. Every amplifying device has its own particular value of "generator resistance"
or "source resistance" or "anode impedance". Even the mains power supply at the wall has source impedance.

The mains supply can be considered to be two elements. The first is a perefct 240Vrms supply with zero output resistance so that any load connected will not affect the voltage. The second is some amount of resistance in series bnetween the perfect
voltage supply and the wall sockets in your house. The voltage at the wall socket will change depending on whether you plug a lamp in which draws little current, or a 2 kilowatt heater because of the copper wire resistance between your appliances and the perfect voltage supply away from your house. The source resistance in ohms can be measured by dividing the voltage change by the current change which occurs for two different known load values. The mains source resistance or a triode's ac anode resistance can simply be measured by recording the voltage change between the unloaded condition and the loaded condition and generator resistance Rg = Vchange / I change.

Suppose a room heater is rated for 2,400 watts. With 240Vrms the current is 10 amps rms.

The resistance of the heater = V / I = 240 / 10 = 24 ohms. Suppose the voltage drop when the heater is turned on is 10Vrms.  The resistance at the connection point and measurement point of the room heater is thus 10V / 10A = 1 ohm.
This resistance includes the wall wiring and wires to the supply pole and back to the generator and all the
other users also connected across the the supply.
Power = I squared x Resistance so where the wire resistance = 1 ohm, there will be a power loss
in that wire = 10 x 10 x 1ohm = 100 watts. This explains why heat is generated in the leads to appliances.

In a triode in a test circuit with a large value choke the load can be made to be such a high value that it is a negligible load.
With a choke of 20H, at 1kHz the choke ac impedance will be F x 6.28 x L = 1,000 x 6.28 x 20 = 125,600ohms,
and well above any value which will seriosly affect the measurement of  most power triodes.
So when such a choke is connected there is little anode signal current and the amplification we see is at its maximum
value which is µ, or the amplification factor, since no voltage drop is occuring across the internal generator resistance
of the triode. 

Here is a schematic of my power tube test rig which includes the triode shown as an "imaginery voltage generator"
with a series resistance to represent the dynamic anode resistance of the tube....
Fig 2.

This actual schematic was used for load testing in Fig 8 below. It includes the dc set up for the triode, the cathode biasing,
and the triode is shown modelled as a voltage generator with a series resistance of the value of Ra also calculated below.
The ac voltage shown at the voltage gene output does not actually appear anywhere really; it is an imaginery signal
created for modelling purposes. But while Vg stays at 29.63Vrms, the voltage at the gene output will always
be 217.57Vrms for the model. if we had 3k for the anode load, there would be a circuit from the voltage gene
to 0V with Ra of 837ohms V in series with 3k, so the signal Ia = 217.57 / 3,837 ohms = 57mA, so the signal at the 3k load, ie, anode output voltage = 0.057 x 3,000 = 170Vrms. The coupling capacitor C2 has an impedance of only 3.2 ohms at 1 kHz.
This test circuit is called a "parafeed" circuit and is actually used for some SET amps where there is a non gapped normal
ac output transformer located in the place of R2 anode load.

Now let us look at some lines plotted on the above curves for the GE6550A....
Fig 3.


These curves are the same as for Fig 1 above.
Let us find out what is the anode resistance, Ra, for this tube.
We have decided to try to use the quiescent idle condition of Ea = 330V, Ia = 100mA which gives
a combined anode and screen dissipation, Pda,  of 33 watts and about 3/4 of the maximum rated Pda = 42 watts.
We would never set up a triode at the rated maximum Pda because the tube may only last a short time. The operating point is called Q, and it is on the Ra curve for Eg = -30V. The Ra we want to know is for a tiny swing voltage on the Ra curve so to find out Ra for point Q a tangent line to the Ra curve is drawn from N to M using a ruler, or mouse with a line draw facility in an image program.
The Ia difference of N to M read off vertically = 370mA.
The Ea difference of N to M read off horizontally = 560V - 250V = 310V.
The resistance value of any sloped line on the graph is determined using Ohm's Law, R = E / I.

Ra = E / I = 310V / 0.37A = 837 ohms.

As you can see by the slope of possible tangent lines, Ra could be 1.8k at Ia = 25mA, and 400 ohms at Ia = 300mA.

The amplification factor, µ, is the voltage gain of the tube achieved with a load that does not cause any current change.
This over simplifies the idea of µ, but its all I have room for now. µ is determined by the designer by dimensioning the inter-electrode distances.
A load where no current change occurs is called a constant current source. Such a load is easy to provide to the tube and can be a high value choke to supply the dc idle current which may have 200,000 ohms of ac impedance at 1 kHz. A 32 Henry
choke has 200,000 ohms of ac impedance at 1 kHz, and such a load with say 250vrms applied across it results in only
1.25mA of ac anode current. If you draw  the load line for 200k, it is virtually a horizontal line on the graph.
The constant current source, CCS, could be another tube or transistor connected in such a way to act as an
"active load" and equivalent to many megohms of resistance. We would need to then apply an input signal and measure
the output signal to calculate the voltage gain which in this case be equal to µ.
But we don't have to do that if the curves are accurately drawn for the tube. To plot a loadline on the tube curves where no current change occurs, the line will be a horizontal one at the dc current level.
So let us look at the horizontal line at Ia = 100mA. This line is intersected by all the Ra curves and the Ea change caused by an Eg change can easily be read off.
So for Eg = 0V, Ea = 100V, and for Eg = -60V, Ea = 540V.
Therefore a grid voltage change of -60V gives an anode change of +440V.
The gain = µ = Ea change / Eg change = 440V / -60V = - 7.33. The sign for µ is accurate, but for general purposes
nobody bothers to worry about it and just refers to µ as a positive number. But in other gain equations it may be important to
consider µ with the correct -ve sign.

For all tubes, transconductance, gm,  in amps per volt, is the ability of the grid voltage to cause anode current change and

gm = µ / Ra.

In this case gm = 7.33 / 837 = 0.00876A/V = 8.8mA/V

So by working with curves we know the 3 basic parameters for the tube for the working Q point. For all tubes,

Signal voltage gain, A, = µ x RL / ( RL + Ra ) This is the very useful universally applicable equation for tube voltage gain.

Now we want to see what outcome we may get with a load resistance. Once we have an operating point, we can calculate a reasonable "centre value" for RL for any power triode.

183 - 142 ) / ( 183 + 142 ) =  6.3%.

The smart people amoung you will notice that the Ra lines for the GE6550 are not spaced along any horizontal
line at even spaces; the Ra curves tend to crowd closer as you move to the right side of the graph.
If you measured the +ve and -ve anode swings for -/+ 30V grid change, you find that with a CCS load at Ia = 100mA
that there is about 2.5% 2H distortion which isn't very good because many power triodes are capable of
2H less than 0.5% even with a large anode V swing.

So were the curves drawn in 1955 correct? I decided to measure a couple of slightly used samples of EH6550 to see if there was much difference between today's Russian made EH6550 and the tubes made all those years ago in the US.

My test rig circuit has a high value choke to supply DC to the tube, switched load resistances of  9.7k, 7.2k, 4.5k, 3k, 2k, and 1k across the choke. I have switched cathode resistors and a massive cathode bypass cap to set the bias and a regulated
B+ supply.  The basic schematic is in Fig 2 above .There is a low distortion triode signal amp for grid signals producing less than 0.5% 2H at 50Vrms.
The test signal from the ultra low THD oscillator is at 1 kHz. The conditions for the test were Ea = 420V, Ia = 73mA,
Eg = -47V.

Here are three following graph sheets each with the same 3 loads showing the THD for each load vs output voltage.
I measured samples EH6550, GE6550A, KT88jj, and KT90 :-
Fig 4.
 
The 2H is lower when no load is connected at < 1.6%. at 235vrms output. The old GE6550A triode curves
are not correct for the EH6550.
Fig 5.

The sample of GE6550A tested has 2,000 hrs of use and is middle aged, yet it managed to measure better than EH6550
for a 3k RL, but only slightly worse for 4.5k and 9.7k. Ra and µ were about the same.
The results indicate that the original GE anode curves are slightly fictitious.
Fig 6.

This KT88 measures slightly better.....

Fig 7.

The KT90 THD is less than each of the 3 previous tests. Part of the reason is the slightly higher Ia = 78mA, placing the operating region further "up the curves" and thus further away from the bottoms of the curves which swing round to the left
and thus contribute to the distortion. If the Ra lines were straight lines and all exactly parallel, there would be no distortion,
but amoung the laws of nature there is seldom any linearity.

Here are my corrected Ra curves for EH6550 with load lines plotted which must be correct because the above
measurements must co-relate with the data curves :-
Fig 8.

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