~ Triode Cascode Amplifier Calculator ~

This calculator can be used to calculate the gain of a triode cascode amplifier using different valves provided µ and r_a for each valve V₁ and V₂ are known at the required operating current

V₁ is a common cathode amplifier with the low input impedance of the common grid stage V₂ as its anode load R_LV1 which reduces the voltage swing at V₁ improving the linearity and reducing noise

If V₁ and V₂ are in the same glass envelope a 'grid stopper’ resistor R_stop may be required at V2 plus ferrite beads where shown to prevent high frequency oscillation when not using a ‘cascode’ specific valve with inter-anode screening [see comments at bottom of page]

The calculation takes into account additional load on V₂due to the input impedance and bias components of the following stage which can be 'lumped' as R_G ~ R_k may be partially bypassed to give a specific gain by inserting a resistor in series with C_bypass and using its value for R_G

The default values are for two sections of an E88CC with Ia ≈ 1mA at V_a = 90V per valve and make a valve similar to the EF86 as used in my QUAD II power amplifier modification ~ Click here for other calculators

Circuit Variables for —	V₁	V₂
Internal anode resistances — r_a		kΩ
Amplification factors — µ
Anode load resistor — R_L		kΩ
Additional anode load — R_G		kΩ Load of next stage
Unbypassed cathode resistor — R_k		kΩ Output Significant Figures
Calculated results for V₁
Total anode load for V₂— R_L		kΩ = R_L ∥ R_G
Anode load on V₁ due to V₂ — R_LV1		Ω = (r_a2+ R_L) / (µ₂+1)
Voltage gain at anode — A_V1		V/V = µ₁R_LV1/ (R_LV1+ r_a1+ (µ₁+1) R_k)
Voltage gain — A_V1		dB = 20log (A_V1)
Calculated results for V₂
Voltage gain— A_V2		V/V = A_V1R_L(µ₂+1) / (r_a2 + R_L)
Voltage gain — A_V2		dB = 20log (A_V2)
Modified anode resistance — r_a2'		kΩ = r_a2 + (µ₂+1) r_a1
Output resistance at anode — R_Out		kΩ = r_a2' ∥ R_L

Keith Snook Triode Cascode Amplifier Calculator In practice the valve used for a cascode is often a double triode and assuming the 2 sections are good and thus identical the cascode can be considered as a single valve with µ' = µ(µ+1) and r_a' = r_a(µ+2) ~ The new values of µ' and r_a' give a valve similar to a pentode which ~ providing there is sufficient HT voltage ~ can often replace pentodes with lower noise and possibly lower distortion ~ See Cascode a.f. amplifier

No Silicon Heaven ~ Then where do all the Calculators go ? The common grid or grounded grid section of the cascode V₂ has a high non–inverting gain which can extend to UHF ~ Grounded grid triode amplifiers are used to amplify VHF and UHF signals because the screening action of the grid both prevents Miller capacitance and stops positive feedback from anode to cathode

In the 1950s commercial dual triodes specially made for cascode amplifiers like the ECC84 ECC85 and ECC88 were produced ~ These have a screen between the 2 anodes to prevent signal coupling between V₂anode and cathode when V₁ anode is connected to V₂ cathode ~ Some like the ECC84 have a triode grid connected internally to the inter–anode shield

There is a book about 'valve amplifiers' which states the only general purpose valve that was designed to work well in a cascode is the ECC88 / 6DJ8 or E88CC /6922 . . . . Try other valves, by all means, but do not expect the performance to be as good maybe the 5th or 6th edition will now correct this along with many other misleading statements

Almost any dual or single triodes can be used to make an audio cascode amplifier if screening and/or grid stoppers etc. are used to prevent positive feedback at RF ~ Other statements relating to cascode heater/cathode voltage rating and heater induced hum in low level audio amplifiers are valid and should be considered

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" Two against nature they got that stuff ~ Good things happening when you see about us "