10.4 phase reversal level

10-67

10.4.3 differential amplifier (long tail)

This circuit type combines two different tubes basic circuits: the first tube

works in cathodes basic circuit with current feedback, the second tube works in

basic grid-connection; it is used by the first pipes on the cathode is activated. In the

fender history is the difference amplifier at the end of a development series: Paraphase

(1946 - 1951), Paraphase with feedback (1951 - 1954), Kathodyn (1955 - 1957), diffe-

Renz amplifier (from 1956). Other manufacturers such as VOX (1958) or Marshall (1962), the

only a good decade after fender with the reinforced construction began to used the

differential Amplifier right from the beginning.

Geko

fig. 10.4.8:

Differential Amplifier with cathode coupling.

The picture on the left of

fig. 10.4.8

Shows the basic circuit of the differential amplifier. Ac-

Voltage level of the left tube changes its anode and cathode current, and leads

in an AC voltage drop at the anode and cathode resistance. As with the

cathode voltage, the control voltage of the right-hand tube changes, your anodes-

Or cathode current is changed (mesh base-circuit). As

example

: If the (against earth

defined) Grid voltage of the left tube to 2 mV increases, the cathode voltage

by 1 mV. Your grid/cathode voltage has thus increased by 1 mV, while the

grid/cathode voltage the right tubes by 1 mV has decreased. In the event of the same raw materials

rens part of this amount were the same, but against phase anode voltages result.

This example used in books like has however a small error:

The sum of the Anode Power

changes

would be zero and so were the cathode potential

constant - the right tube was not activated. With a small change but it's:

The left gate potential rises by 3 mV, the cathode potential increase 1 mV, the anodes-

Potential changes are opposite phase - but not more equal! In normal dimen-

Sion picking would be the ac voltage gain the right tube only about half as

as big as the left, and it would also be very heavily on the individual tubes-

Data-dependent. This is the reason why the cathode resistance is increased, causing the

voltage gain of both tubes is reduced, but rohrenun

dependent (current-

Against coupling). The middle image in Figure 10.4.8 shows such a circuit (VOX

AC-30), the image on the right contains an additional input for the output transformer

inclusive great feedback loop (Marshall, Fender from 1956).

The typical differential amplifier tubes is first of all the fender

12AX7

(7025, ECC83),

the Black Face era is then the change to the output rather low impedance

12AT7

(ECC81). VOX uses the ECC83 (12AX7), Marshall as well.

Data Sheet Specifications

: Internal resistance = 30 k

Ω

(ECC81) or 63 k

Ω

(ECC83).