Polski (PL) 
English (United Kingdom) 
Janusz Krzemień (January 8, 2018)
Some time ago, I decided to build my second, large 14-tube stereo amplifier operating in triode mode (previously, I had built a 16-tube amplifier in a push-pull UL Class AB configuration, divided into the main amplifier and the power supply - I posted a report on the TRIODA Forum). In the described amplifier, I used Svetlana KT88 tubes in the output stage.
In the input stage and phase inverter, I used a single 6N8S tube each. The control circuit uses two 6N8S tubes per channel, each controlling one branch of the output stage. The hybrid power supply uses one 5C3S tube (5U4GB) and high-voltage rectifier diodes. The photo below shows the amplifier's appearance. The visual impression is provided by two EM84 tubes. Analog meters indicate the actual input signal, and LCD displays display the current anode voltage for the output stages and the electrolyte discharge status. I used the EM84 tubes out of sentiment, as they remind me of my old equipment from the 1950s and 1960s.
Alpa DIY Push-Pull amplifier in triode mode.
Input and control stage of the amplifier power tube
The schematic diagram is shown in the figure below.
Schematic diagram of the input and control system for one channel.
This section isn't shown in the schematic, but the signal from the input sockets passes through a circuit built on four switching relays. +12V is applied to the relays via a 4-section switch (the input selector is presented later in the description). The signal from the selector is applied to the input of the potentiometer (blue ALPS), then through resistor R-3/2.2KΩ it enters the first grid of the 6H8C (6N8S) tube. As you can see in the schematic, I didn't use a capacitor at the input. A variable resistor (PR) can be used in the tube's cathode to set the tube's operating point. PR resistors R-51 and R-52 at the anodes are used for precise anode voltage adjustment; they can later be replaced with fixed resistors. Resistor R-29/3.3KΩ and capacitor C-25/100pF are used to reduce parasitic interference at supra-acoustic frequencies. The anode of the first triode is connected directly or through the R-53 resistor to the phase inverter grid (the second half of the 6N8S tube). It is very important to obtain the required voltages on resistors R-12 +320V, R-11 +265V to +270V - this part of the power supply has a separate fuse.
Additional notes
In the first stage cathode, I used a wire-wound potentiometer R54/220Ω in series to experimentally determine the tube's operating point. In the anodes, I used potentiometers (PRs on a cement backing) R29 and R52/22KΩ in series with resistors R5 and R7/10kΩ to precisely adjust the anode voltages.
If anyone has any doubts about the purpose of resistor R53 - 100Ω between the anode (1) and grid ( )4, I included it intentionally for experimentation – it can be omitted in the final circuit. The anode voltage at the drive tube anodes is +180V to +182V, at the cathodes 4.6V, with potential correction possible; at 470Ω = 5.6V, the anode voltage is around +200V.
KT88 output tube control system
The circuit is based on two 6H8C (6N8S) tubes per channel. From the anode and cathode of the phase inverter, the signal is applied to each of the driver tube grids via MKP-ESR 0.22uF/1000V capacitors and 2.2kΩ resistors. Each output stage branch utilizes one 6N8S tube, with the tube anodes connected in parallel via resistors R-15 and R-16/100Ω. The anode voltage is applied through resistors R-13, R-14, R-22, and R-23/22kΩ, connected in parallel. The voltages at the tube anodes should be between +180 and +200V, depending on the cathode resistors used. The cathodes of each tube are connected together and share a common resistor.
List of components for the input circuit, inverter and output tubes control circuit
- Tubes 3 X 6N8S
- Potentiometer 100kΩ Alps
- Resistors:
R2 - 470KΩ, R3 - 2.2KΩ, R4 - 680Ω, R5 - 10KΩ, R7 - 10KΩ, R8 - 22KΩ, R9 - 2.2KΩ, R10 - 2.2KΩ, R11 - 4,72KΩ/5W, R12 - 2.76KΩ/10W, R13, R14 - 22KΩ, R15, R16 - 47Ω, R17, R18 - 368Ω, R19 - 2.2KΩ, R20, R21 - 47Ω, R22, R23 - 22KΩ, R24 - 33KΩ*, R29 - 3.3KΩ, R37 - 2.2KΩ, R47, R48, R49, R50 - 330KΩ, R51 - 22KΩ, R52 - 22KΩ, R53 - 47Ω* do 100Ω*, R54 - 220Ω reg. - Capacitors:
C3 - 0.22μF/1000V SCR, C4, C5 - 10μF/470V, C6 - 0.22μF/1000V Wima, C7, C8 - 0,22μF/1000V SCR, C9, C10 - 0.47μF/1000V SCR, C11 - 0.22μF/1000V, C12in the feedback should be adjusted at your discretion, or even omitted.
Amplifier power stagey
The output stage utilizes two KT88 tubes per channel. The signal from the driver tubes is applied to the grids via MKP ESR 0.47μF/100V polypropylene capacitors and resistors R-40 and R-43/2.2KΩ. Polarity is automatic, and the output tube current is set by 470Ω/10W resistors R-41 and R-42. Electrolytic capacitors C-22 and C-23/100μF/100V eliminate local feedback across these capacitors. The tube anodes are connected to the screen grids via resistors R-44 and R-45/100Ω. The output transformers are fused. A WIMA 0.22μF/1000V decoupling capacitor is connected before the +430V transformer input. On the secondary side of the output transformers there are 220Ω/10W resistors that protect the amplifier against damage when the amplifier is turned on without any loudspeaker load.
Schematic diagram of the power stage for one channel.
List of components for the power stage
- Power tubes 4 X KT88 by Svetlana
- Resistors:
R38, R39 - 330KΩ, R40 - 2.2KΩ, R41, R42 - 470Ω/10W ceramic. R43 - 2.2KΩ, R44, R45 - 100Ω, R46 - 220Ω/6W - Capacitors:
C22, C23 - 100μF/250V Rubicon, C24 - 0.22μF/1000V Wima
Hybrid power supply
List of elements
- 500VA toroidal mains transformer
- Chokes 3.2H/0.6A
- Rectifier tube 2 X 5C3S, possible replacement for the Electro-Harmonix 5U4GB tube, many times more expensive than the 5C3S, but working very stably.
- High-voltage diodes D1- D4 BY228,
- Resistors:
R29-R36 - 150KΩ, R25 - 100Ω - Capacitors:
electrolytic: C13 - C14 - 66μF/400V, C16 - C21 - 470μF/400V, C15 - 0.22μF/1000V, - double relays RM 94.
The diagram includes additional circuits in the form of blocks: an anti-interference filter, a soft start with a +12V power supply, a delayed anode voltage switch-on circuit with a +12V power supply and galvanically separated LCD indicator power supplies.
Schematic diagram of a hybrid power supply based on a 5C3S (5U4GB) tube for one channel.
The circuit features a standby switch to disable (block) the anode voltages – a listening break; after switching the amplifier back on, it's ready for listening. The voltages from the relay are applied to the 5C3C or 5U4GB rectifier tubes. The rectified voltage is filtered by the first electrolytic capacitor, 2 x 66µF/400V (two capacitors connected in series), followed by a choke and a block of series-connected electrolytic capacitors. 150K resistors equalize the voltages and discharge the electrolytic capacitors after the anode voltage is turned off. The power supply operates stably at increased output power. The mains transformer is barely warm after an hour of amplifier operation.
Amplifier design - photo report
Amplifier housing
After extensive modifications, I used the chassis from a "factory" receiver. The output transformers, mains transformer, and chokes are separated by 1.5mm thick sheet metal shields. The double top panels are made of 1.5mm sheet steel. The inner panel houses the tube sockets, mounting strips, and connectors. All of this is covered by an outer (decorative) panel. The tube rings are turned from steel and galvanically bonded to the panel. The amplifier's chassis is finished in a metallic "Red Carnival" paint. The front and rear panels are partially painted black.
Mechanical work involves a lot of manual work. Hand-drilling the holes for tube sockets in the inner panel and making larger holes for tube bases in the outer panel requires considerable effort. Holes between the two panels must be centered. Numerous ventilation holes are drilled in the lower and upper panels.
Amplifier housing element (color "red carnival")
Panel ready for mounting tube sockets.
External decorative panel after painting.
Mounting the lower part of the amplifier
"Fitting" the basic components of the amplifier onto the chassis.
In the foreground two low-frequency chokes, in the second an "empty" base for the electrolytic capacitor block.
Visible large chokes from "Kaka" (TRIODA Forum User), and capacitors on the base shown in the photo above.
The lower part of the power supply in all its glory.
As you can see in the photos, the power supply takes up a significant amount of space in the amplifier's chassis. Compared to some inexpensive commercial amplifier designs, this is a good, uncompromising result. It ensures solid bass reproduction even for moderate evening listening.
Mains filter and mains socket.
Mains filter mounted next to the electrolytic capacitor block.
Electrolyte block supplemented with equalizing resistors.
Combined electrolytic capacitor block - top view.
The toroidal output transformers were specially designed by Piotr (from the TRIODA Forum) for triode operation. The mains transformer was also made by Piotr, and the powerful low-frequency 3.2H/0.6A chokes were custom-made by KaKa (from the TRIODA Forum).
The mains transformer was made by Piotr (User from the TRIODA Forum)
The mounted mains transformer is screwed to the base which also serves as a screen.
Chokes and centrally located mains transformer.
The interior of the amplifier during assembly.
Screens separating chokes and mains transformer.
The "top" screen of the mains transformer.
The "upper" screen is mounted on the mains transformer.
Miniature sockets and plugs ready for connection.
One more photo before screwing the front and rear panels on.
Connection sockets and support systems
Four RCA signal inputs are available. The input selector circuit is implemented using relays.
4-input selector board.
Rear panel and XLR sockets ready for installation.
"Gentle" switching on of the amplifier is ensured by soft start.
Schematic diagram of the "soft start" system - diagram provided by Amplifon.
Board with soft start circuit.
The delay circuit assembled on a printed circuit board has its own +12V power supply. It is built on BC516, BC337 transistors, a 3V9 diode, and an RM-94P-12V relay. The voltage from the 2 x 400V secondary anode windings for the left and right power supplies passes through a circuit delaying the turn-on of the anode voltages by 90 seconds. The +12V power supply circuit is assembled on a printed circuit board in which I used a 4 x 1N4004 bridge rectifier, a 7812 regulator, BC516, BC211 transistors, a 3V9 diode, a Rempol RM-94P-12V relay, and other components.
Schematic diagram of the anode voltage switch-on delay circuit - diagram provided by Amplifon.
Board with anode voltage switch-on delay circuit.
The speaker cable switching system for the additional amplifier uses a double relay. This eliminates the need to laboriously reconnect speaker cables.
Board for switching speaker cables to a second amplifier.
The LCD metering for anode voltages caused some confusion. The amplifier is supposed to be user-friendly. My previous amplifier with analog meters also stirred up excitement. I'm particularly interested in the functionality, features, and reliability of the device. Everyone has their own thoughts on amplifier design and the use of necessary or unnecessary components. Why did I use LCD metering for anode voltages and not another solution? They take up little space, which was no longer available for another option. The LCD metering displays the discharge status of the electrolytic capacitors and the anode voltage drop to zero. You get used to everything after a while, even with LCD metering.
Front panel before painting.
The front panel after painting before installing the indicators and displays.
The front panel after applying paint and installing indicators and LCD displays.
Front panel from the back - LED displays, control indicators, mains switch board on the relay.
Ready panel, LCD displays for anode voltages.
And now some photos of the rear panel.
Rear panel assembly phase - XLR output and input sockets, cable switching system for the second amplifier.
Output transformers visible, rear panel assembly completed.
Completed assembly on the rear panel.
Rear panel ready, additional 8 ohm output jacks.
The lower section of the amplifier has been assembled. Here are photos of it.
The lower part of the amplifier is ready.
The lower part of the amplifier is ready.
After completing the work on the lower part of the enclosure, I could begin the electronics assembly on the amplifier's top panel. My thoughts revolved around organizing the spatial assembly so that it wouldn't be a tangle of running wires. I also appreciated the possibility of using mounting strips on which to place some of the components. Everyone has their own mounting method. As an amateur, and within my limited capabilities, I decided to use a mixed assembly method. I ordered printed circuit board strips from a company a few years ago and used them in my amplifier. Components mounted this way can be quickly desoldered for servicing.
Mounting strips and two 10A voltage sockets.
On the power strips, I mounted the power supply components for the input stages and the drive stage based on 6N8S tubes. To drive the KT-88 output tubes, I decided to use two of these tubes per channel. The tubes are connected in parallel, one 6N8S per branch, allowing for proper drive of the KT-88 output tubes without the use of cathode followers.
The second mounting strip houses the power stage components and dual 2x4 BY228/1200V rectifier diodes powering the 5C3S rectifier tubes, one per channel, in a hybrid configuration. Two smaller boards house 2x66μF/400V electrolytic capacitors in series, along with 150KΩ voltage equalization resistors. The 2x66μF electrolytic capacitors are the first stage of filtering the outgoing rectified voltage from the 5C3S tubes. Voltage is supplied via 10A multi-pin connectors through 3.2H/0.6A chokes to the electrolyte banks located at the bottom of the housing.
Power strip.
Power strip and other PCB-based strips screwed to the mounting base.
Power strip and other PCB-based strips screwed to the mounting base.
Part of the hybrid power supply.
10A sockets are mounted on the sides.
Power strip and other PCB strips screwed to the mounting base.
Power strip and other PCB strips screwed to the base.
A little explanation about the mysterious mounting strips at the top of the vacuum tube board.
After connecting the motherboard to the connectors on the bottom of the power supply, the upper chassis cannot be inverted to perform measurements due to the short wire leads to the connectors. I used short wires to avoid introducing any interference. To ensure that measuring different operating points was possible and convenient only after removing the case cover, without inverting the upper chassis, I used special measuring strips. These can be seen in the photo below.
Tube sockets and measuring strips mounted on the panel.
Measuring strips mounted on the left and right sides of the panel are intended for starting the amplifier.
Connected measuring strips.
Ready-made amplifier module with electron tubes.
Starting the amplifier
I loaded the power supply for one channel with three 40W lamps in series. After powering up, the soft start and delay circuits worked properly. The power supply worked flawlessly; the lamps illuminated softly. Then, after inserting all the vacuum tubes for one channel into their sockets, I loaded the amplifier output with a 6.8R/50W resistor. I turned on the power supply, and the tubes were filament-free, and the soft start and anode voltage delay circuits worked correctly. While observing the tubes, I decided to measure the voltages on the measuring strips. The anode voltage stabilized at +430V.
Activation
Now some photos of the signals at the amplifier output.
Square wave with a frequency of 1 kHz. The reference and output waveforms are identical.
A sinusoidal waveform with a frequency of 1 kHz.
10 kHz square wave.
15 kHz square wave.
20 kHz square wave.
Finally, a few more photos of the amplifier ready for operation.
I invite all interested parties to read the thread dedicated to this amplifier on the TRIODA Forum at: [Wzmacniacz lampowy stereo w konfiguracji triody].
Prepared by: Janusz Krzemień