Tube preamp - RIAA corrector - headphone amplifier.
If you want to add tubes to your system to take advantage of their enhanced level of musicality the preamplifier is the place to start. Preamplifiers, more accurately called control amplifiers, come in two basic types. A passive preamp acts only to control the level of a source signal and pass it along, with no additional gain, to the power amplifier. An active preamp has a separate gain stage that allows it to amplify a signal that might otherwise have insufficient voltage to efficiently drive a power amplifier (or high sensitive speakers directly). This preamp uses tubes for voltage gain only, not for rectification or regulation.
Signal from input jacks X1-X6 follows to input selector S1 and discrete resistor volume control R1 with 24 2dB steps. The signal afterwards drive to the stage with tube 6Z43P (6Ж43П), (PDF) russian high frequency pentode in triode connection. You can also use tube 6Z11P (6Ж11П). Close resemblance of this tube is E280F (7722). Siemens tube D3A is applicable too (in this case R13 is 180 Ohms).
Preamplifier works with transformer output stage. Transformers UU type core is made of high-grade electrical sheet steel. Cores from transformers OCM-0.063 is applied. The sizes of section are 25x25 mm. Primary winding partitioned in three sections, every section consists 1000 winds of 0.18 mm diameter wire. The secondary winding is wound between sections of the primary winding as in diagram shown. Secondary winding consists two sections 1400 winds of 0.18 mm diameter wire and two sections 70 winds 0.56 mm diameter wire. All sections of winding connected in series. One final point of interest with a SE transformer is the air gap. This is necessary in order to bring the operating point of the core to the correct region on its B-H curve. Paper spacer 0.05 mm thickness is applied. The transformer is designed on 8 ohm high sensitive speakers (output SPK, 0.6W) and drives power amplifier (output X7) in a separate box. At input amplitude 1V (X1-X5) output amplitude is 45V (X7). It's possible drive output stage tube (EL500, EL509, 807, KT88, EL34 etc) directly.
Frequency band is displayed in charts. Topmost chart displays work of device with zero load, when transformers are on idle run. Lower chart displays work of device when its transformer are loaded with 36 kOhm resistors. You can see, that curve is a bit straighter, - these peaks were never too high though (never exceeded 2,5dB). Conditionally high „mountains“ are displayed because of logarythmic scale of the chart.
Measurements were taken with these devices: oscillator ГЗ-102, osciloscope C1-49, tube voltmeter ВК7-9. Amplification of degree with a 6Ж43П tube is about 45. With a signal imput of 1V we get pretty, non-disorted sine curve of 45V on X7 output, which is enough to swing powerful enough tube, such as 6П36С (EL500) or 6П45С (EL509).
RIAA corrector uses two russian super miniature pencil tubes 6N16B (6Н16Б) (PDF) and one 6N1P (6Н1П). Pencil tubes were the last thermionic devices to be developed, together with nuvistors, and were built to rigid, military specifications. They’re typically very small, and instead of being socketed they have flexible leads like passive components. Even in the breadboard stage without any shielding, the pencil tube preamp is VERY quiet. I used DC heating and the minus pole of the heater power supply is connected to the +50V (R26, R27). Standard signal level 1V adjusted by trim. potentiometer R9. Preamplifiers power supply can supply 300V at 100mA and under 2mV peak ripple under load. Two transformers for halogen lamps 12V is used with connection in series. Secondary windings (12V) connected in tandem. The power supply isn't dual-mono throughout, but it does have separate regulators for the two channels' filament 6.3V and +Ua voltages. Device is mounted into body of an used CD player. Tubes are mounted into case, and are layed on the side, none of them are sticking out, so you can place all audio devices on top of each other. The height is only 8cm. All input (X1-X6) and output (X7 and PP) sockets are placed on the back of device. The discrete sound regulator (R1), ON/OFF switch (S3), input switch and indicators are placed on the front panel of device. Even since device is developed for controlling high-power amplifier, it is possible to listen background music, - all you have to do is connect speakers of high sensitivity (around 93dB or more) to device's low-ohm output (SPK). Output power is about 0,6-0,7W. It is amazing, that tubes of so small size are capable of producing such a sound. For example, when listenning to CD music, signal is traveling a short path – just one tube and a transformer. Quality of a sound is very high.
Final layout of preamp plus power amplifier is displayed HERE. Cable connections are also displayed in the layout. When high-power amplifier is not powered-up and only the primary amplifier is turned on, speakers through relay K1 are connected to primary amplifier, so you can listen to sound at low-power, about 0.6 W. When high-power amplifier is powered-up, relay K1 switches speakers to the output of more powerful amplifier. This is basically universal device. There is DESCRIPTION and PHOTO of power amplifier.