electronics | simplepleasuretubeamps

This is a truly one-of-a-kind preamplifier built around the legendary Type 26 direct-heated triode (DHT), renowned for its exceptionally pure, natural, and transparent sound signature.

The design utilizes a fully separated dual-chassis architecture, with the power supply housed independently from the main preamplifier section. The custom screened umbilical cable carries DC only, minimizing noise and ensuring optimal isolation from AC radiation — a critical consideration when working with sensitive DHT tubes such as the Type 26.

Special attention was given to achieving ultra-low noise and minimal microphonics. Since Type 26 tubes are highly susceptible to hum and interference from nearby AC fields, the isolated power supply configuration significantly improves performance.

The power supply chassis contains:
• Independent filament supplies for each Type 26 tube
• Regulated DC supply for the remote-control circuitry
• Dedicated high-voltage supply section

Each filament supply employs 40,000uF of filtering capacitance alongside highly regarded Coleman filament regulators operating as constant current sources (CCS). High voltage rectification is handled by a 6BY5GA tube rectifier feeding a CLCLC filter network, followed by a MOSFET shunt regulator capable of reducing ripple to microvolt levels.

Protection circuitry includes:
• One rear-panel 1.5A slow-blow main fuse
• Two independent 0.75A slow-blow filament supply fuses

The main power transformer is a premium Hashimoto unit equipped with an internal thermal fuse for enhanced safety. Because this fuse is non-replaceable, the use of a properly calibrated, high-quality 1.5–1.6A slow-blow fuse is essential to prevent irreversible transformer damage.

The power supply connects to the preamp chassis via a custom-built screened umbilical cable terminated with military-grade Amphenol connectors. Each connector is uniquely matched to its enclosure to prevent incorrect connection and ensure operational safety.

The main preamp chassis incorporates:
• High-voltage shunt regulator
• Single-stage amplification circuit
• Remote-controlled volume control

The signal path is intentionally minimalist and completely free of coupling or decoupling capacitors. This was achieved through:
• Filament bias topology using a low-value 10-ohm resistor without bypass capacitors
• Lundahl LL2763NC nanocrystalline output transformers

A Hoyt USA analog panel meter monitors filament voltage for each channel via a left-center-right toggle switch:
• Left: left channel measurement
• Center: meter disengaged
• Right: right channel measurement

Precision potentiometers on each side of the meter allow fine adjustment of filament voltage between 1.4V and 1.5V. Lower filament voltage reduces microphonics and may be adjusted down to approximately 1.2–1.3V depending on tube characteristics, while maintaining visible filament glow. A 1.4V setting generally provides the best balance between low microphonics and optimal output impedance.

The Lundahl output transformers feature four secondary windings with selectable gain configurations accessible via rear-panel switches:
• High gain: 12–13dB, 2:1 output ratio, 2400-ohm output impedance
• Low gain: 6–7dB, 4:1 output ratio, 600-ohm output impedance

To further reduce vibration sensitivity, the tube sockets are mounted on a floating sub-plate suspended by four neoprene damping standoffs. Tubes should be inserted and removed carefully to minimize mechanical stress.

Volume control is handled by a motorized TKD CP-2511 high-end potentiometer paired with a Bent Audio remote-control system known for exceptionally quiet operation.

Additional features:
• 1 input / 2 outputs
• Heavy-duty 1/8” aluminum chassis by Landfall Systems
• Custom CNC-machined and anodized top plate by Front Panel Express
• Hand-finished ¾” Peruvian walnut exterior with General Finishes Antique Oil urethane

Performance Specifications:
• THD: 0.02% (predominantly second harmonic) at 2Vrms output
• Frequency response: 30Hz–30kHz ±0.5dB (-1dB at 20Hz)
• Output impedance: 600 ohms (low gain), 2400 ohms (high gain)
• Gain: 6–7dB (low), 12–13dB (high)
• Power consumption: 70W
• Main fuse: 1.5A slow-blow

Dimensions & Weight:
• Main chassis: 19 lbs — 12.5” x 15” x 9”
• Power supply chassis: 22 lbs — 11.5” x 15” x 9”

The result is a preamplifier with a smooth, clean, highly resolving, and remarkably natural presentation.

This project started when I found a pair of LL2765/75mA amorphous core waiting to be used. I’ve built a 300B HP amp using these types of transformers. I needed something different. I needed an HP amp that was more versatile and could be used as a high gain preamp and low cost tubes, the power being preferable DHTs. The DHP wired triode 4P1L was the proper tube for this implementation. I’ve been successful building several PSE4P1L amplifiers and tried them on headphones. The result was a very pleasant (2nd harmonic predominant), quick and clean sound. 4P1L in triode mode has quite a bit of input capacitance and 2 in parallel will give the driver about 400pF to deal with. Therefore, the driver must be a powerful one and considering the gain I needed, I chose and tested the 6H30PI and the 6H6P. These are inexpensive tubes and a very good match to the 4P1L. I initially considered a dual chassis HP amp. However, I was confident I can make it quiet in one enclosure, and I did. Despite my efforts to make it as compact as possible, the amp/preamp enclosure is 19.25×15.5×11” ( WXDXH) and weights 45 lbs.

The power supply section takes about 2/3 of the real estate. It has an audio grade 200VA Antek toroidal that powers the high voltage and the DC filament regulator for the driver. There are 2 separate supplies for each of the 2x4P1L filaments and it is regulated by the highly praised Coleman filament regulators. The high voltage is tube rectified, using TV dampers 6CJ3, and CLCLC filtered using Lundahl LL2742 chokes in CMMR configuration for the highest common mode ripple rejection. This power supply provides DC all around and makes this HP amp/preamp very quiet.

The signal section of the amp is very simple. The driver is loaded on a CCS and it is capacitor coupled to the power stage. The power stage is loaded on the Lundahl LL2765AM/75mA and it is self-biased as well as the driver stage. I implemented it with high quality and appropriate type of parts and materials.

The chassis is made of 5/4” American cherry hardwood stained with General finishes Antique walnut and coated with 5 coats of General finishes oil based topcoat. The top plate is a sturdy 1/8” brushed aluminum that holds all the equipment besides the IEC which is installed on the back of the wood chassis.

The exquisite sound headphone amp is provided with a high Quality Neutrik phone jack. The volume pot is a TKD CP2511 and a 1.5” NOS bakelite knob. There are few features on the top plate:

On the left side is the mains On/Off switch
In the middle there is a 500mA NOS Hoyt panel meter. This measures the high voltage supply current which should be around 170mA
On the right side there are 2 sets of RCAs. One in the back is the input and the other the output when used as a preamplifier.
On each side of the 4P1L tubes there is a set of tip jack insulated test points. Measuring the voltage (either side) you will basically read the total plate current of the corresponding channel. The voltage in mV corresponds to the current in mA.

The spec measurements of this device showed the following:

Maximum output power is 3W on 32 ohms headphones, 0.56W on 120 ohms and 0.22W on 300 ohms
The input sensitivity is about 0.9Vrms (6H30PI driver) and 0.6Vrms (6H6P driver) for full power output.
Output impedance of this HP amp/preamp is around 7 ohms
The frequency response is 20Hz-20kHz +/- 0.5dB
The THD is 0.55% at 1W on 32 ohms, 0.81% at 2W on 32 ohms
The overall gain is 19dB when using the 6H30PI driver, and 22dB when using the 6H6P driver
The max output voltage is 8Vrms
Power consumption 130W