[Patrick J. Jankowiak]

Hello List,

On the 6L6/G/GA/GB, Please help me to better understand these class
AB1 6L6 operating conditions, and compare them to some observed
conditions. I assume the published conditions, having been published
for years by RCA, Sylvania, et al, were once derived from actual test
data.

The official data is shown first, with the tube types according to the
manuals. Last, some measured data is shown. All measurements are done
by driving the amplifier barely to clipping, measuring the peak
voltage on a scope, and figuring the RMS voltage to be 0.707 of this,
and from there using the load resistance to calculate current, then
power, so it is the usual method. To vary the plate to plate load, a
70V/25 watt line matching transformer was used with an 8 ohm load.
70.7V on this amp's 250 ohm output, as called for in the manual, is 20
watts.

At issue is this seemingly incongruous set of data: For the published
conditions of a 9000 ohm load, the output is supposed to be 24.5
watts. The actual amplifier puts out 8 watts. Ok, so I figure I might
have flat tubes, (even though they measure 80% of what a NOS JAN
6L6WGB did on the tube tester). Maybe they cannot pass enough current.
So I tried loading it more lightly so that the load was 18000 ohms and
the power did not increase. At 9000 ohms, there is no extra room for
more voltage excursion under lighter loads. So, why not load more
heavily? Ok. I loaded to 4500 ohms, and Behold! 22 watts. So, no
problem with the current-carrying capacity of the tubes.

But why do I not get the correct power output at 9Kct as stated in the
venerable tube manuals?

Note, the 9K CT specs only seem to show up for early 6L6G material. In
later material it is omitted or relegated to the "additional
data/obsolete" sections.

Could either of these possibilities be true:

1. The data was in error with regard to output?

2. The amplifier in question, a military unit made by David Bogen,
came with two speakers having 250/500/1000/2500 ohm inputs. The output
terminals are stated to be 250 ohms. The mess was intended to be
connected all together, in parallel, with instructions to assure a 250
ohm match regardless of using 1, 2, 4, or more speakers. Could the
amplifier have a wiring mistake? I could not find anything!

Although it is not unusual to get more power when overloading an
amplifier, especially one with a high impedance load to begin with,
this is just too strange.

The bottom line is that I have tried varying the load resistance using
a decade box, and with the 'correct' match, 250 ohms, I just don't get
the power.



tube manual specs:
[[[[[[[[[[[[[
6L6G / 6L6GB

plate 360V, 88/132mA
screen 270v, 5/15mA
bias -22.5
load CT 6600
PoutW 26.5W

plate 360V, 88/132mA
screen 270v, 5/18mA
bias 248 OHMS CATHODE RESISTOR
load CT 9000
PoutW 24.5W

-------------
6L6G / 6L6GB / 6L6 / 6L6GC

plate 360V, 88/132mA
screen 270v, 5/15mA
bias -22.5
load CT 6600
PoutW 26.5W

-------------
6L6 / 6L6GC

plate 360V, 88/132mA
screen 270v, 5/11mA
bias -22.5
load CT 3800
PoutW 18W

[[[[[[[[[[[[[
6L6G in the AM-20 amplifier, has 250 ohm (70V) output terminals.
Tubes check at 80% of a NOS JAN 6L6WGB, 1 KHz sine wave.

plate 380V
screen 265V
bias 250 OHMS CATHODE RESISTOR @23.5V
load CT 9000
PoutW 8W (when load = rated value [250 ohms])
PoutW 22W (when load = 1/2 rated value [125 ohms])




Any comments are welcome.
PJ