~ Triode Cascode Amplifier Calculator ~

This calculator can be used to calculate the gain
of a triode cascode amplifier with different valves provided µ and
ra for V1 and V2 are known at their
operating condition ~ V1 is a common cathode amplifier with the
input impedance of a common grid stage V2 as its anode load
RLV1

The calculation takes into account additional
loads on V2 due to the input impedance of following stages and
their bias components which can be 'lumped' as RG at the output
~ The default values are for two sections of an E88CC with
Ia ≈ 1mA at Va = 90V ~ Click here
for other calculators

V1

V2

Internal anode resistances —
ra

kΩ

Amplification factors — µ

Anode load resistor — R1

kΩ

Additional anode load — RG

kΩ

Unbypassed cathode resistor — Rk

kΩ

Total anode load for V2 —
RL

kΩ  =  R1||RG

Calculated results for
V1

Anode load due to V2 —
RLV1

Ω  =  (ra2+ RL) / (µ2+1)

Voltage gain at anode — AV1

=  µ1 RLV1 / (RLV1+ ra1+
(µ1+1) Rk)

Voltage gain

dB  =  20log(AV1)

Calculated results for
V2

Voltage gain—
AV2

=  AV1
RL (µ2+1) / (ra2 +
RL)

Voltage gain

dB  =  20log(AV2)

Output resistance at anode — ROut

kΩ  =  ra2 + (µ2+1)
ra1||RL

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 ra' = ra(µ+2) ~
The new values of µ' and ra' give a valve' similar to a pentode
which ~ providing there is sufficient HT voltage ~ can often replace
pentodes giving lower noise and possibly lower distortion ~ See Cascode
a.f. amplifier

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 V2 anode 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 other similar
statements

Almost any dual
or single triodes could 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 and
heater induced hum in low level audio amplifiers are valid and should be
considered