Another DHT switchable preamp

Built for my audiophile pal Paul, this is a one enclosure preamp  using 101D or 26 vacuum tubes.


This is an all DHT( direct heated tubes) preamplifier/ line stage.  It has one stage and is built within the same enclosure. This a very good option for anyone who want to try and compare the sound of the 101D  DHT triode and the 26 DHT triode.

The power supply uses Lundahl LL1683 for main HV in a choke input 0.47uF(Obbligato gold film)-10H Lundahl LL1638-68uF(Obbligato Film-Oil)- 10H Lundahl LL1638-68uF(Obbligato Film-Oil). The rectifier used is the 274B and can be replaced by any rectifier in the 5U4G family. HV is supplied with a low ripple which is perfectly adequate considering that the shunt regulator SSHV1 will add at least 80dB PSSR bringing the ripple at extremely low levels ( I could not measure it with my equipment).  The raw power supply for the filament biases are using Hammond 167 series transformers at 12.6Vac, followed by a rectifier bridge with 1N5822 and 10mF-2×0.33ohms-10mF-0.1uF-220pF. The two raw filament supplies are delivering up to 1 Amp to the Coleman regulator.  The Coleman regulator is adjusted at 1A which is perfect for the 101D tube and slightly starved for the 26.


The preamplifier  uses either the 101D or the 26 tube in a filament bias configuration. In order to switch between 101D and 26 tubes, there are 2 sets of DTDP high quality Carling switches. The anode load of the DHTs is Lundahl  LL1660/10mA connected step down Alt. Q 4.5:1 ratio. This allows to deliver a 20 times lower output impedance compared to the plate resistance of the DHT . The input of the preamplifier is DC connected, therefore care must be taken when connect to other audio equipment to make sure that there is no DC to feed this preamplifier. The preamp is provided with a stereo remote volume control by Khozmo in 64 steps, shunt stepped attenuator having only two resistors in the signal path at a time. All components and wires are high end audio such as Vishay nude, silver gold wire, NEOTECH – Solid Core Copper, *PTFE Jacket
(99.9999% UP-OCC Mono-Crystal design).


The following measurements are only to give you a sense  of the approximate performance. I used a computer software and interface and this comes with some artifacts and noise that are not present when the amp is connected in a audio path. One, if owns high quality audio analyzer, can take more precise measurements

  • Frequency response: better than 30Hz-30kHz @ -1dB
  • Distortions better than 0.2% at 3Vrms output, for most tubes if carefully selected for low distorsion
  • Gain using the 26: about 5.5dB
  • Gain using the 101D: about 4.8dB
  • Input impedance: 50 kohms, unbalanced
  • Output impedance under 400 ohms for the 26 and under 300 ohms for the 101D

More pictures: Top plate from Front Panel Express with a very nice powder coated finish…

26-101d-main-schematics.jpg26-101D_PSU schematics

5 thoughts on “Another DHT switchable preamp

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