Here is a great post by Rui Lourenço who finished a sublime incarnation of the 26 DHT preamp Gen2.  I hope you find this post as inspiring as delightful it is for me to see someone taking my 26 DHT preamp version to the next level of perfection. I think Rui’s pictures will speak for themselves showing the great craftsman skills and amount of effort and dedication put into this art work.

My Type 26 Tube Preamplifier adventure – by Rui Lourenço (Portugal)

I’ve started my interest in diy audio about 2 decades ago, basically at that time because I had no means of buying some of the fantastic equipment’s I saw in several Audio Shows, also because the bug was there, and some of my friends were doing it. Today, fortunately I have the means, but the bug was kept and personally I believe that good implemented diy projects can most of the times be several notches above commercial products, or otherwise you would have to spend some important thousands of Euros to acquire them at a similar performance level. You all agree that the personal filling of achievement vs your friends drop jaws when looking to some of the wonderful projects we find everywhere, is also very good.

The project

After several preamplifiers done in the past, I decided to try to build a direct heated triode preamplifier myself; I had established some basic principles:

• It had to a 2 chassis design
• Retro looking
• With valve rectification and non-electrolytic filtering
• I wanted to use Salas fantastic HV regulators
• Dual mono (or almost)
• With output transformers

I studied several possibilities, but having seen in the past Ale’s first #26 implementation, after looking at the wonderful job Thomas Mayer develops, I knew I had to go this way. I then came across Ale’s version 2 of the preamplifier and finally decided to go this way, of course using also some of my predetermined ideas and principles.

The Schematics

Ale’s version 2 schematics:

• First implementation done for testing with a big number of alligator clips, but it worked.
• HV supply is built based upon a Telefunken AZ11 (mesh plate) rectifier in graetz mode using to U4007 diodes. First cap is ASC X386 top blue line with 8uF; this feeds a Lundahl 1685 10H choke, feeding an ASC X386 normal line 50uF capacitor. The HV line then splits in two; feeding one Hashimoto 30H choke by channel, finally feeding one more ASC X386 normal line 50uF capacitor by channel, these last two capacitors have a 220K /5W Kiwame resistor across their terminals. Ground is common and arranged in star connection through a copper bar where all grounds connect together.
•  Supply filament uses exactly Ale’s schematic with Panasonic high ripple capacitors, three times 15.000uF by channel, plus 2 coil chokes by channel in a PI arrangement. The transformers are 15V/50VA Weiss manufactured, again one by channel. Rectification is done through the very good Per Anders audio rectification boards, built around YXYS Hexfred devices, all snubbered with 100R Kiwame resistors and 0,1uF capacitors.
• Mains transformer (incredibly good built to Thomas specs), wood chassis (high quality real wood, no veneers), tube damping and output transformers were bought from Thomas Mayer in Germany.
• Main schematic, is exactly Ale’s design, with some details:
  • The HV lines coming from the supply chassis through an eight pin Neutrik connector, are fed in a custom made double 2,2uF/400V Kelvin-connected Morgan Jones capacitor
  • The two separate HV lines are feeding one Salas HV regulator by channel
  • These feed, one KK audio CCS by channel
  • These then feed two VR75 type tubes in series, by channel, both bypassed by a Russian PIO capacitor, plus a 510K/5W Kiwame resistor on the top VR tube.
  • I have Salas regulators working with 175V/60mA
  • KK CCS is regulated to have 4,5mA on the 1R (Mills) resistor on the plate of the #26 tube
  • I use one Rod Colleman filament supply by channel, fed separately from the above mentioned supplies
  • The bias is set through five 50R/20W Dale resistors in parallel by channel to get 10R, and around 10V on their terminals
  • The HV line is reaching around 145V on the 1R already mentioned resistor
  • Again ground is very important, and the same implementation used on the supply chassis, is used on the main unit, using a chassis insulated copper bar, where all ground connections are linked. Chassis is connected to this ground in one point, and mains earth is connected through a separate line (also to the chassis via a 1M resistors and a by-pass capacitor), earth ground can be lifted through a switch.
  • Both HV and filament supplies are completely clean on the scope.
  • The all unit is DEAD QUIET, and you can’t hear any noise from the speakers, even with your head deep into the speaker units.

 

 

 

 

 

Some general information

All used components are top notch:

• All internal power supply cabling is done with high quality copper, both using Nordost and Vandenhul wire
• Signal connections are Audio Consulting pure silver enamel cryogenic wire
• Solder is WBT
• All tube sockets are Yamamoto
• Volume control is TKD top level
• All screws and nuts are stainless steel and/or brass. All nuts are pure copper
• Resistors are Mills and Kiwame
• Capacitors are ASC oil caps, Panasonic, Kelvin-connected Morgan Jones and Russian PIO
• Rectifier is AZ11 Telefunken Mesh plate
• VR tubes are Russian new old stock
• Type #26 tubes are Cunningham globe type
• Iron from Thomas Mayer, Weiss, Lundahl (chokes and OTP transformers) and Hashimoto (chokes)

First impressions

The first thing I am always very fundamentalist about, is the noise, again in this case, DEAD QUIET!!! Maybe due to all heavy artillery I’ve installed, the very good schematics, implementation, ground path, well, all counts.

 

The gain is for me perfect, meaning for my normal hearing level, with CD, and my 300W amplifier, depending on the level of the recording, I tend to go around 12 0’clock on the knob (best linear region of the volume control).

The sound is incredibly clean and tri-dimensional, very clean black back-ground and instrument separation with splendid dynamics and fast response.

All instrument timbres and vocals are very detailed with all the whispers and tonal qualities we could expect from top hi-end equipment, without any special effect normally related to tubes.

Bass presentation is very clear and tight, with great control.

Overall, this pre-amplifier performance is amongst the very best you can find. It was/is really worth the efforts.

My System

Presently I have a system based upon a big 300W transistor amplifier, its a local Hiend project where I also participated, fantastic sounding, I have a CD player front end with CDM Pro2 transport feeding the very good Parasound DAC1500. My present loudspeakers are Wharfdale Steve Denton 80th aniversary (recently sold my Sonus Faber Guarnieri, and in place of locally developing a very high quality monitor like Magico, with o local developer). Speaker cables are Harmonic Technology Pro reference 9, and IC are several own designs with copper and/or silver.

Acknowledgements

To Ale and his efforts to publish this very good design, and for the help during the prototype phase. To Thomas, for the top quality parts and for all the advice. To Rod, for his wonderful filament supply and all the help in the implementation. To Kevin for the help in setting up the CCS. And finally to Salas for his fantastic regulator.