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Applications of the RCA-7199 Triode-Pentode in High-Fidelity Audio
Equipment

This Note describes the special features of the RCA-7199 triode-
pentode, and discusses its application in "hi-fi" audio equipment. The
RCA-7199 is a 9-pin miniature type containing a medium-mu triode and a
sharp-cutoff high-transconductance pentode, and having very low hum,
leakage noise, and microphonism. It is specially designed and controlled
for use in high-fidelity audio equipment in stages operating at signal
levels in the order of 100 millivolts. It is particularly useful in
direct-coupled voltage-amplifier phase-splitter circuits, and in
tone-control circuits.

The Note gives two circuits in which the RCA-7199 is used as a direct-
coupled voltage amplifier and phase splitter, and one in which the pentode
unit of the tube is used as a general-purpose voltage amplifier. It also
discusses the advantages to be gained by operation of voltage-amplifier
pentodes at low plate and grid-No 2 voltages, and gives graphs and
calculations showing how these advantages may be achieved for the pentode
unit of the RCA-7199.

Design Features and Characteristics

The RCA-7199 utilizes a specially designed cage assembly and stem which
make it much less susceptible than other triode-pentodes to micro- phonics.
These features also provide a rugged tube structure and minimize leakage
noise. The heaters of the two units are specially designed to produce
very low magnetic hum. Median values of hum and noise, expressed as
equivalent voltages at the control grids, are 35 microvolts for the
pentode unit and 10 microvolts for the triode unit. Maximum values are
100 microvolts for the pentode unit and 150 microvolts for the triode
unit. These equivalent hum-and-noise voltages are for grid-circuit
resistances of 50,000 ohms.

Because of its high transconductance (7000 umhos), the pentode unit
of the 7199 can provide large voltage gains (values of 100 to 350 are
typical). It can also deliver output signals as large as 90 volts peak-
to-peak with low distortion. The triode unit has a mu of 17 and can
handle large signal voltages. This combination of characteristics,
together with its very low hum and noise, makes the 7199 especially suit-
able for use as a voltage amplifier and cathode follower, or a voltage
amplifier and phase splitter (the phase splitter is a form of cathode
follower).

Hum and Noise Performance

The hum-and-noise performance that can be expected from the 7199 when
precautions are taken in circuit layout and wiring to eliminate hum
and noise from other sources is illustrated by the following example:

Assume that the pentode unit of the 7199 is to be used in the input
stage of an amplifier having a sensitivity of 100 millivolts RMS. (The
rated power output or voltage output of the amplifier need not be specified).
The amplifier has an input resistance of 50,000 ohms. Based on the max-
imum value of hum and noise for the pentode unit (100 microvolts), the
minimum signal-to-noise ratio at the amplifier input in decibels will be

20 log 0.1/0.0001 =20 log 1, 000, or 60 db.

Based on the median value of hum and noise for the pentode unit (35
micro-volts), this ratio will be

20 log 0.1/0.000035 = 20 log 2,850 or approximately 69 db.

If the input stage has a gain of at least 20 db (a voltage gain of 10),
hum and noise originating in the grid circuits of succeeding stages can be
neglected, and the signal-to-noise ratio at the amplifier output will be
the same as that at the input. Because the pentode unit of the 7199 can
easily provide voltage gains of more than 100, these conditions are easily
realized. Hum and noise at the output of the amplifier, therefore, will
be at least 60 db below rated output.

It is evident from the foregoing example that if the pentode unit of
the RCA-7199 is used in the input stage of a preamplifier, or in any
other application where the input signal level is only a few millivolts,
the hum and noise at the output of the following amplifier may be sub-
stantially less than 60 db below rated output. For high-fidelity repro-
duction, therefore, the pentode unit of the 7199 should not be used in any
stage where the input signal level is substantially less than 100
millivolts.

Applications

An example of the use of the 7199 as a voltage amplifier and phase
splitter is shown in Fig. 1
(GIF image 19 Kbytes). In this high-fidelity, 15-watt, power ampli- fier,
the pentode unit of the 7199 is direct-coupled to the triode unit, which
drives a pair of RCA-6973 high-fidelity beam power tubes operating in
class AB1 with fixed bias. The amplifier uses 18 db of degenerative
feedback between the voice-coil connection and the voltage-amplifier
cathode. At 15 watts output,total harmonic distortion (measured at 1000 cps)
is less than 0.5 per cent, and intermodulation distortion is lessthan 0.5 per
cent. Hum and noise with input shorted are 84 db below rated output. As
shown in Fig.2, the frequency response of the amplifier at 4 watts output is
flat over the entire audio spectrum and down less than 2 db at 10 cps and
60,000 cps. At 15 watts output,the frequency response is flat from 20 cps to
15,000 cps, and down less than 1 db at 15 cps and 20,000 cps. The amplifier
has a sensitivity of 1.2 volts RMS, and a damping factor of 12.

Figure 2-Frequency-response characteristics of the amplifier shown in
fig.1.

A slightly different voltage-amplifier/phase-splitter circuit which also is
capable of very good performance is shown in Fig.3. In this circuit,grid-No.2
voltage for the pentode unit of 7199 is obtained through

Fig.3 -- Voltage amplifier/phase splitter circuit using the
RCA-7199.

a 220,000-ohm resistor from the cathode of the triode unit. This
arrangement provides regenerative feedback between the two units which can
be used to obtain increased over-all gain at the lower audio
frequencies.The frequency at which this feedback becomes effective depends
on the value of the grid-No.2 bypass capacitor Cl. If C1 has a small
value,it may be necessary to use relatively heavy filtering in the B supply
circuit to prevent motorboating.

The voltage-amplifier/phase-splitter circuits shown in Figs. 1 and 2
produce extremely low harmonic distortion, have frequency-response char-
acteristics extending well beyond the audio range, and permit the use of
relatively large amounts of over-all feedback. The phase-splitter stages
are easily balanced at low frequencies by adjustment of the plate and
cathode resistors, and have input impedances of several megohms, thus
making it possible to achieve very high gains in the pentode stages.

Some of the special considerations involved in the use of such circuits
are:

(1) As the plate-load resistance for the pentode unit is increased,the
distortion produced by this unit increases more rapidly than its gain;

(2) If the plate-supply voltage is low, the use of direct coupl- ing
between the pentode and triode units may result in undesirably low plate
voltage for the pentode unit;

(3) To obtain equal response at the higher audio frequencies in the
two output circuits of the phase splitter,it may be necessary to equalize
the capacitances to ground of the plate and cathode of the triode unit.

Operation of Voltage-Amplifier Pentodes at Low plate and Grid-No 2
Voltages

There are two methods by which a pentode may be operated so as to
produce low harmonic distortion. One is to operate the pentode over the
most nearly linear region of its characteristic. The other is to select an
operating point just above the "knee" of the characteristic, so that
second-harmonic distortion is partially cancelled by third-harmonic dis-
tortion. Although the latter method provides the highest gain, and has
recently been used by some circuit designers for that reason, it is not
recommended for production equipment because of the initial adjustment
required. Normal variations among pentodes of the same type are such
that a quiescent point which is just above the "knee" for one tube may be
on or under the "knee" for another, and thus result in low gain and very
high distortion.

Although operation over the most nearly linear region of the character-
istic provides less gain, it provides much more latitude for normal varia-
tions in tube characteristics, and, therefore, is the recommended method of
operation for pentodes. To achieve maximum latitude in this method of
operation,it is necessary to take into consideration the effects on
harmonic distortion of normal variations in transconductance, plate
resistance, amplification factor, cathode current, and plate-to-grid-No. 2
current ratio. Fig. 4 shows the total harmonic distortion and voltage gain of
the pentode unit of the RCA-7199 as a function of plate-load resistance. The
three distortion curves show the spread of this characteristic for tubes

Fig.4 -- Total harmonic distortion and voltage gain of the pentode unit of
an RCA-7199 as a function of plate-load resistance. Distortion curves are for
typical tubes having high, average, and low plate currents; gain curve is for
typical tube having average plate current.

having high, low,and average values of plate current. This figure also
shows the voltage gain of a tube having average plate current as a
function of plate-load resistance.

Fig.5 -- Amplifier circuit used to obtain data shown in Fig. 4. Cathode
resistor was adjusted to provide grid-No.1 bias of -1 volt.

These curves were obtained with the tubes operated in the circuit
shown in Fig.5, using a constant signal-input voltage of 0.1 volt (100
millivolts) RMS, and with the cathode resistor adjusted to provide a grid-
No.1 bias of -1 volt. The average maximum output voltage obtained in
this circuit was 32 volts RMS (representing a gain of 320), or 90 volts
peak-to-peak, which is sufficient to drive the largest audio-output tubes.

Fig.6 - General-purpose voltage-amplifier circuit using the pentode unit of
an RCA-7199.

The first minimum point on each distortion curve is that obtained by
operation over the most nearly linear region of the tube characteristic.
The second minimum point is for operation in the knee region, which is
not recommended. It is evident from Fig. 4 that the optimum value of
plate-load resistance under the conditions employed is between 180,000 and
240,000 ohms. (The average value of cathode resistance required to provide -1
volt of bias for this range of plate-load resistance values was 750 ohms).

Fig. 7 -- Total harmonic distortion of a typical RCA-7199 pentode unit as a
function of plate-load resistance and grid-No.2 resistance.

The circuit of a general-purpose amplifier stage using the pentode unit of
the 7199 with the recommended component values is shown in Fig. 6. In this
circuit, the average grid-No.2 voltage of the pentode is 35 volts. The
spread of a typical curve of distortion vs plate-load resistance,
such as the curve for an average pentode shown in Fig. 4, can be broadened so
as to make circuit performance less critical with respect to individual
tube characteristics and component values by' an increase in the value of the
grid-No.2 resistor. Fig. 7 shows the distortion of a typical 7199 pentode
unit as a function of grid-No.2 resistance and plate-load
resistance. These data were obtained in the circuit shown in Fig. 8.

Fig.8 -- Amplifier circuit used to obtain data shown in Fig. 7.

It is evident that as the grid-No.2 resistance is made larger--i.e.,
as the grid-No 2 voltage is reduced--the plate-load resistance for
minimum distortion increases and becomes much less critical. This effect
can be used to advantage in the design of direct-coupled pentode-triode
circuits such as those shown in Figs.1 and 3. In such circuits, the
pentode unit must be operated at low plate voltage to provide proper bias for
the triode unit, and, therefore, must also be operated with low grid-
No.2 voltage. The use of high-value plate-load and grid-No.2 resistors of
relatively wide tolerance satisfies these requirements, minimizes
the sensitivity of the circuit to variations in tube characteristics and
component values, and reduces manufacturing costs.

The curves in Fig. 7 also indicate that in any pentode amplifier
application it is possible to achieve minimum distortion for any value
of plate-load resistance by adjustment of the grid-No.2 resistance.
This feature makes it possible to optimize gain and distortion for any
desired high-frequency "roll-off" characteristic.

Information furnished by RCA is believed to be accurate and reliable.
However, no responsibility is assumed by RCA for its use; nor for any
infringements of patents or other rights of third parties which may result
from its use. No license is granted by implication or otherwise under
any patent or patent rights of RCA.

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