ECF80 Voltages KEL84

[John Caswell]

There have been a lot of comments on the Forum recently regarding the voltages measured on the ECF80 valve especially when in the KEL84.

The nett conclusion reached by the member of the team involved is that, in this case, it does not seem to matter one iota that the voltages are not as the designer specified on the circuit diagram the amplifier will work perfectly satisfactorily, and to specification.

To whit:
A random sample consisting ECF80s and E80CFs, 12 in total, were taken and fitted into a KEL 84 and the resulting voltages measured and tabulated as shown, with row 1 being the designer bogey voltages, rows 2 & 3 being the existing valves as supplied in the KEL84, and rows 5 to 16 being the samples.
The gain, as can be seen, varies by up to ±0.08dB. This is totally insignificant and can be discounted. Similarly total harmonic distortion, "A weighted�" varied from 0.03% up to a whopping! 0.08% once again totally insignificant.
It must be noted that if the Pentode Anode (Ap) volts are high the corresponding Screen grid (G2) are low, and vice versa, thereby maintaining the average current flow through the Cathode (Kp). These two voltages appear to equalise at around 90 V so it is suggested that this is the norm� with a corresponding Cathode voltage of approx 1.9V.
If these figures are taken as the norm then the corresponding Triode voltages will be Anode (At) 187V and Cathode (Kt) 98V, and because the Triode grid is directly coupled to the preceding Anode (Ap) this must necessarily have the same voltage as Ap.
It would therefore seem that the voltages should to be re-specified as follows:
Anode Pentode = 90V ± 20% e.g. 72V - 108V
G2 Pentode = 90V ± 20% e.g. 72V - 108V
Cathode Pentode = 1.9V ± 10% e.g. 1.71V - 2.09.

The member is of the opinion that there will still be valves that will fall outside these ranges but is certain that as long as the pentode cathode voltage is as stated above, ie 1.9v ±10% then the amplifier will function perfectly.

Valve type Make Ap G2 Kp At Gt =Ap Kt Gain in dB

Specified voltages 73 110 2 205 80 0

ECF80 - L Mullard 100 99 1.8 181 104 0
ECF80 - R Mullard 94 98 1.9 186 99 0

ECF80 - 1 Mullard 99 89 1.87 184 104 0
ECF80 - 2 Mullard 113 86 1.76 168 116 -0.05
ECF80 - 3 Mullard 81 102 1.98 199 88 -0.05
ECF80 - 4 Mullard 90 97 1.9 190 94 0
ECF80 - 5 Mullard 92 83 1.92 187 96 0.03
ECF80 - 6 Mullard 72 102 2.06 207 79 0.08
ECF80 - 7 Mullard 93 91 1.9 187 98 -0.02
ECF80 - 8 Mullard 86 87 1.97 194 91 -0.03
E80CF - 9 Mullard 113 99 1.71 169 116 -0.1
E80CF -10 Mullard 92 105 1.88 190 97 -0.08
E80CF -11 Mullard 100 105 1.82 182 103 -0.08
E80CF -12 Mullard 80 112 1.96 200 87 -0.08


The FAQ Team