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Special 26 DHT Preamplifier — Engineered with a Separate Power Supply for Superior Performance

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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.

Shiny Eyes 26 DHT Preamplifier

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This is a “one of a kind” preamplifier. It uses the 26, an early DHT, capable of a pure sound . Who ever is familiar with the 26 sound can say that it excels in musicality and emotion.

At the demand of the client, the preamplifier was carefully designed in one enclosure. Despite the challenges of a single enclosure DHT preamp, the 26 device is as quiet and as little microphonic as possible. 26 tube is known for picking up noise/hum from proximity to the AC radiating area. I highly recommend a dual enclosure scenario, with a separate power supply. However, if used with low power tube amps that have low to medium overall gain, this preamp will perform great.

The preamp has 2 distinct zones (looking from above): on the right hand side is the power supply zone and on the left hand side is the main signal path zone.

The power supply area is packed with separate supplies for each filament of the 26 tube, the regulated DC for the remote control unit and the high voltage unit. Each 26 tube filament has a raw DC supply using 40,000uF of filtering capacitance per channel, and a much praised Coleman filament regulator. The high voltage supply is using a type 80 tube as rectifier and a CLCLC filter, followed by a special Mosfet shunt regulator that brings the ripple down to uV levels.

The signal path of this preamp is as short as it can be, and free of coupling or decoupling capacitors. That was possible by:

  •  Using filament bias circuit, where the filament is in series with the low value (10 ohms) bias resistor. This resistor doesn’t need to be decoupled. The filament current is adjusted by the Coleman regulator (which is a CCS-constant current source).
  • Using a line output transformer by Lundahl LL2763/7mA

The line output transformers are made by Lundahl in Sweden, and they have 4 balanced secondary windings. There are 3 4PDT switches on the back panel, aligned with the left and right RCAs. If they are all toggled upwards, the gain of the preamp is 12-13dB, and if they are downwards the gain of the preamp is 6-7dB. The high gain position implies a 2:1 output ratio and the output impedance of the preamp is 2200 ohms. The low gain position implies a 4:1 output ratio and the output impedance of the preamp is 600 ohms.

The input of the preamp is DC coupled using a motorized Alps Blue Velvet remote control by Bent Audio. This is a high quality, super quiet remote.

The preamp is provided with 3 inputs switched by a high quality Seiden selector switch made in Japan.

The enclosure is a very sturdy 1/8” thick aluminum chassis by Landfallsystems.

The top plate is custom CNCed and anodized at Front Panel Express. The sides and front are wrapped with ¾” American cheery hardwood that was stained with General Finishes Antique walnut gel stain and coated with 4 layers of oil based urethane.

The preamp will sound smooth, clean and clear sound with 0.05% THD second harmonic predominant at 2Vrms output.