PP002 Push-Pull 300B mono-block amplifier.
PPOPT 300
5Ka-a:0-4-8
Push-pull output
transformer
300B
PPIT100
1:1+1 inter-stage
transformer
PURPLE
PURPLE
15K
BLUE
RED 8
10,000uF
10V
560pF
f2
f1
GREEN
47R, 2W Cermet Pot
YELLOW
GREEN 4
GREEN
15K
6C45pi
6
RED
BLUE 0
560pF
1K5
YELLOW
7
1.3V
Bias test
points
BLACK
10R
8
420V
2.5V
190V
4
A
B
5
100k
470uF
10V
220R
47K
2W
47uF
250V
300B
47uF
100V
10K
5K
10 turn
2W
-100
C
470uF/250V +
100k, 2W
x2
GZ37/GZ34/GZ32
5AR4/5R4/5U4G
8
CH100
3H,200mA
A
Yellow
2
1N4007
2n2
2kV
Ceramic
disc
1N4007
470uF/250V +
100k, 2W
x2
6
Red
Purple
4
Black
0.1uF
250V AC
X2
Purple
B
CH107
20H, 25mA
2K2
47uF
450V
Red
10k
Carbon
comp
Red
Yellow
2 x1N4007*
+ 10n 1kV
47K
2W
SPST
Red
Purple
47uF
250V
47uF
250V
2 x1N4007*
+ 10n 1kV
4.7K
Purple
C
FLTX030
Filament
transformer
300B heater supply
0.82R 5W ***
Green
f1
5V
7.5-0
4A
4 x 1N5401**
+10nF
10,000uF
10V
f2
Green
L
4
Input valve
heater supply.
Blue
3.15
0.1uF
250V AC
X2
N
DPST
E
Black
100R
5
Blue
3.15
SPST
0.1uF250V AC X
* 1000V soft recovery types can be substituted
** hexfreds or soft recovery diodes can be substituted. Schottky types can also be substituted but an increased resistor value will be
required.
*** If an external PSU is used, this resistor value will need to be adjusted to achieve 5V on the 300B filament
TENT labs filament supplies can also be used directly with this filament transformer. If an external supply is used, place the TENT supply
in the audio chassis.
PP002 Design Notes.
PP002 is a push-pull 300B design and is transformer coupled. Despite the fashion for all things single-ended, pushpull 300B amps are also very musical and are often more versatile than SE’s. Push-pulls are more load tolerant,
have better bass dynamics, go louder and are capable of playing a wider range of musical styles.
Audio Circuit description:
6C45pi
A Russian 6C45pi combines input and driver duties and drives a pair of 300B’s via an inter-stage transformer.
One of the so called ‘super triodes’ the 6C45pi has very high transconductance which makes it an ideal transformer
driver and its high gain makes it possible to construct a 2 stage amplifier with reasonable sensitivity. The high
transconductance means a grid stopper is essential to prevent oscillation and a quick look at the data sheet shows
there are multiple grid and cathode connections. Some designers advocate connecting all the grid connections and
cathode connections together whilst others advocate separate grid stoppers and cathode resistors (plus by-pass
capacitors) to each connection. The simple scheme shown in the diagram gives excellent sonics but the keen
experimenter might want to try out different grid and cathode wiring schemes. Whichever scheme is eventually
chosen the lead length of the resistor-valve base connections should be kept as short as possible and direct
mounting of the resistor to the valve base pins is advised.
Inter-stage transformer
Using the driver transformer allows excursions into Class A2 positive grid which will increase power output on peaks.
This amp should provide 20W/channel in Class A but there is a lot more available if a blind eye is turned to
distortion. A Zobel network is required across the secondary of the driver transformer to tame resonances and
improve the frequency response.
Gain and Feedback
This is a low gain design needing approx 1V RMS to generate full power output and should ideally be partnered with
an active pre-amplifier. It has no negative feedback and the non-ideal behaviour of the inter-stage transformer
precludes its application.
Biasing
The 300B’s are operated in fixed bias which is more efficient than cathode bias and also gives tighter bass. There is
only one bias supply for each pair of 300B’s meaning only a single adjustment is required but this does mean that
matched pairs of 300B’s must be used.
Heaters
A separate filament transformer is provided and each pair of 300B’s are DC heated from a single resistor-capacitor
supply. There is no need for individual heater supplies in a fixed bias design as both cathodes are at the same
potential. Each diode in the DC heater circuit is ‘snubbed’ with a small value capacitor. Higher quality diodes can be
substituted such as hexfreds or soft/fast recovery types can be substituted.
The output from the filament transformer is specified to allow the use of a TENT labs filament supply, something we
endorse highly.
The input/driver valve is AC heated.
Power Supply
Driver supply H.T. and bias supply.
The HT and bias supplies use solid state rectification for reasons of ruggedness and longevity. The diodes in the
heater, bias and input/driver HT supply are by-passed with snubbing capacitors to kill switching noise. It is
remarkable how smooth solid state diodes can sound when these measures are taken. To avoid the large voltage
drop required in the power supply from the 400V output stage to the 200V input/driver stage, and the attendant large
hot resistor, a separate supply is provided to the input stage. This supply uses choke input filtering for good
regulation and noise suppression. A 10 turn pot is recommended for the bias supply to make accurate adjustment
possible.
Output stage H.T.
The output stage supply is unusual as it uses a valve rectifier in parallel with solid state diodes to combine the low
impedance and high current capability of solid state with the good sounds of valves. This allows a relatively large
input capacitor to be used (not possible with a valve rectifier) which is good for regulation and therefore bass
performance. If the arrangement looks unusual it is, and there are no doubt sceptics that will doubt it’s efficacy, but it
does sound like a valve rectifier with good bass. It is possible to run without the valve rectifier to save on costs in
which case some more snubbing caps will be a good idea. A low DCR choke is used for good regulation and noise
filtering.
The separate filament transformer allows the filaments to be warmed before the HT is applied as using a valve
rectifier in this configuration does not give a slow start. 10-15 seconds is recommended before turning on the HT.