A 811A(24W) SE Amp with regulator tube drive (CF + CCS)

Photo of 811A SE Amp

1) Introduction
The purpose of assembling 811A SE Amp is for perfect drive of new SP system B& W Nautilus 802 (91dB) replacing Altec horn systems(100dB) done in year 2000.
Since I have been concentrated on single-ended Amps based on direct heating triode
transmitting tubes designed for B & C class operation.
In the mean time I have developed a breakthrough solution comprising twin triode regulator tube (5998A) being directory connected between cathode follower output
and 811A grid, while another unit pulling as a series constant source load with minus
power supply.

Regulator tube drive (CF + CCS) Benefits:
1) Can handle higher grid current of power tube.
2) Can handle wide range of bias voltage (minus region to + 40V).
3) High Gm (5998A), makes cathode follower output impedance lower.
4) Wide range of application.
5) Strong against every transients and performing stable operation.

2) 811A:
Plate loss of 65W (ICAS), Filament voltage 6.3V (4A), Amplification factor 160 and
generic UX socket.
It is a compact size transmitting tube, easy to designing and constructing for Audio Amp.
811A is originally used in B class P.P. operation at more than 1,000V or Grounded-grid
linear Amplifire service.
This RCA 811A SE Amp operates in an A2 class at 600V, 80mA (no signal) with applied
+18.5V grid bias, and 85mA (maximum signal of 47Vrms) to achieve 22W output.
Output transformer is Ex-TANGO XE-60-5SNF possessing 5K ohm load impedance and
36 ohm cathode NFB coil. When cathode NFB circuit is functioned, actual load impedance shall increase to approx. 5.9K ohm.
When connected 811A cathode to KNFB coil, just 8dB of negative feedback is applied
and as resulting, the dumping/factor becomes 2.0 from 0.2.
A significant improvement of D/F is attributed due to its high amplification factor of 811A. Tried 811A ex-CETRON, developed 24W output under the conditions of 21V
grid bias and grid current downed to 50%.

3) 5998A
5998A is the best tube for this application, as plate loss of 15W, plate load resistance of
350 ohm, and a gm of apporox.15.5mS, which corresponding to an output impedance of 64 ohm.
Working conditions:
Plate current of the cathode follower unit is 52mA (no signal) ~ 57mA (maximum 47V rms
applied) and current of the constant current source is 39mA (R ch).
The grid current of 811A is a balance of the above, 13mA ~ 18mA.
To get 80mA of plate current for 811A, the grid bias of 5998A (R ch) is adjusted resulting minus 19V. Although deviation of grid bias of 5998A is rather big, even in the same tube
but, it is not necessary to select the tubes, as the intent of the adjustment is to get 80mA of plate current for 811A and not about grid bias voltage of 5998A.

4) Power supply and others
MX-165, Ex-TANGO is used for power-transformer.
When 5R4 is used as rectifier together with capacitor input circuit, maximum current
can be taken to 170mA which is sufficient for this Amp.
Film condenser manufactured by Toitsu denki is used for high voltage filter capacitors
to save space (10+50+50 microfarads, 700V).
Regarding low voltage (+,-220V) are used for choke input circuit to obtain better voltage
regulation with TANGO TC-10-130W, double-wound choke-coil + 470 microfarads capacitors.
For Filament of 811A is used ripple filter with transistor circuit and it can be omitted
10 ohm VR and replaced pair of 25 ohm fixed resisters.
A 150mA full scale meter is used to read plate current of 811A.
To cope with trangents response for start-up time, Ham, Buzz and increasing of
plate-current of 811A occured due to unable to control of grid bias instantaneously,
30 second muting circuit is provided.
Exception of summer season, cooling fan is not really necessary and install
2P Terminal strips to removable cooling fan easily.

5) Measurements and the sound
Fig 2 , 3 and 4 are Performance graphs of distortion, input/output voltages and
frequency response characteristics of 811A SE Amp.
When cut-off local feed back signal from cathode of 5998A, the distortion of 0 ~ 8W
region is lowered, because of cancellation of distortion between output tube and
voltage amplifier tube is functioned.
I have 4 sets of SE Amps based on transmitting tubes.
A 801A x 3 SE Amp is conventional designed minus bias with no-feedback SE Amp
which I used it my reference Amp.
Compared to the reference Amp, the sounds difference is very little and no one can
tell which one is which by hearing test by instantaneous chengeover in two Amps.
I can say 811A SE Amp has well balanced quite normal sound with typical thoriated-tungsten-filament type twinkling clear sonic in high frequency.

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