pdf version - Menno van der Veen

How to make Super-Triode without a separate cathode winding?:
As said before, the output transformer inside the UL40-S2 contains no separate cathode winding. I
do need extra local feedback to change the Ultra-Linear tube into Triode behavior without loosing
output power. Fortunately there exists another technique of local feedback. This technique applies
local feedback at the control grid of the power tube instead of at the cathode. See for details figure
3.
R1
R2
C
1
input
100 pF
2
100 pF
3
Vht
4
inverse
5
input
R1
R2
C
Fig. 3 : New Super-Triode circuit with control grid feedback. Copyright 2004 Vanderveen
A certain part of the alternating voltage at the anode is sent to the control grid through a voltage
divider (created by R1 and R2. Forget C for this moment. Its function only is to stop the DC voltage
at the anode to enter the control grid). When I make the voltage division through R1 and R2 equal
to the turns ratio of the cathode winding to the primary winding, then the control grid will receive the
same amount of local feedback as in the Super-Triode circuit. So, only with two resistors (and a
capacitor as DC stopper) I can reach the same goal as with a separate cathode winding.
Is this way of thinking correct? Yes, else I would not have published. Also, there is nothing new
under the sun. See my book “Modern High-End Valve Amplifiers based on toroidal output
transformers”, page 240, figure 20.1. There the resistors R5 and R6 perform the same trick.
What resistance should the resistors R1 and R2 have in the UL40-S2? Look into the schematics of
this amplifier, to see that R1 in figure 3 equals R12 and R22 in the UL40-S2, while R2 and C in
figure 3 have to be added as new components in the UL40-S2.
R12 (in UL40-S2) = 82k (or series circuit of 47k resistor + 50k trim pot, see later)
R22 (in UL40-S2) = 100k
In parallel to R12 and R22 a Styroflex capacitor of 100 pF (see remarks 28-11-04)
Rnew (in UL40-S2) = 2M2 (This Rnew equals R2 in figure 3)
Cnew (In UL40-S2) = 10nF/1000V (This Cnew equals C in figure 3).
Why do R12 and R22 not have the same resistance? The output impedance of the upper halve of
the phase splitter (6N1P) has an effective plate resistance of about 18k. This resistance is in series
with R12, meaning that R12 + 18k = R22. The output impedance of the lower halve of the phase
splitter is negligible due to the large amount of local feedback around this lower halve circuit.
Therefore R22 does not need extra compensation. The resistors R12 and R22 should be ¼ Watt
power resistors. The capacitor C can be ordered at Farnell, order number 106-364 (order per 5
pieces).