GM-70  Audio Grade Tube Sockets by deHavilland 

Custom engineered Teflon socket for the Gm-70 Ulyanov Power Triode . This is the socket being used in the deHavilland Gm70  Single Ended Amplifier, now available for sale as a separate item.  The Gm70 is such a fine audio tube that we decided to make special sockets to get the best possible sound out of the Gm70.

The socket base is precision machined out of a bar of 7/8" thick virgin Teflon. Teflon has the lowest dielectric constant of any available engineering material. Our socket terminals are machined out of 3/8"x 3/8" OFC pure copper billet.   Most tube sockets use a Beryllium-Copper alloy for the tube contacts, and the conductivity of the best Beryllium-Copper is only 45% the conductivity of pure copper.

Pure Copper billet before machining

Computer controlled precision machining

In designing our Gm-70 SE amplifier we found that the Russian socket was poorly made and led to endless  plate current  instability problems. Also the sockets were not user friendly in terms of installing or removing the tube.  

We have found a great improvement sonically with our  pure electrical grade Copper/ Teflon socket. The bass and dynamics is markedly better , the mids and highs are better focused , and overall the amplifier sounds like it has considerably more headroom. 

Now available at $395.00 a pair 

Best quality tube socket made on this planet!!!

Contact       e-mail 6sn7@abac.com       Tel 360-891-6570

 Gm-70 Tube Socket, Engineering White Paper 

By Kara Chaffee, Chief Engineer, deHavilland Electric Amplifier Co.

Something that is not widely understood is the inherent level of compromise built into most tube sockets for audio service. We have researched the materials and design features that would need to be incorporated into a socket design, to obtain the best possible audio performance. What follows are our conclusions based on available manufacturing and materials science.

COPPER SOCKET TERMINALS

Nearly all tube socket pins use sheet copper that has been formed or stamped into a shape, which engages the tube pin, and also provides spring tension to maintain close contact with the tube pin. Pure copper is a dead soft metal with almost no "spring" to it. It resembles lead in this respect, and is useless as a spring. Also pure copper is surprisingly difficult to machine due to its gummy character. Thus conventional tube socket pins are made of alloys of copper like Alloy 172, Beryllium Copper or Alloy 1751 Beryllium Copper. The alloy content gives the copper mechanical strength and spring resilience , but Alloy 172 has only 22% the conductivity of pure copper, and Alloy 1751, which is the most conductive copper alloy, has only 45% the conductivity of pure copper.

We have designed a pure OFC  electrical grade copper terminal with a unique spring- loaded system to keep the copper terminal in contact with the tube pin at all times. Our terminal is a 3/8" by 3/8" pure electrical grade copper bar that is over an inch long. You also solder directly to the outside end of each socket terminal, eliminating any welds, rivets or intermediate fastener connections to the actual terminal.

SOCKET BASE and INSULATING MATERIAL

We chose virgin teflon blocks for the mechanical base of our socket. Theoretically, one would desire a material with excellent insulation properties and a low level of electrical interaction with the tube circuit at hand. One important measure of insulation material is its dielectric constant. The dielectric constant can be thought of a material’s tendency to interact with a charged electrical field. Air has the lowest dielectric constant, and has been defined as the reference for the constant at 1.0. What follows is a chart of dielectric constants for some common insulating materials, with air as a reference.

Air 1.0

Teflon 2.2

Polystyrene 2.5

Fused Quartz 4.0

Phenolic 5.5

Porcelain 7.0

Micalex 6-8 .

Because of its extremely low dielectric constant, Teflon is widely used in low loss premium insulation applications. We also found that by using a very thick (7/8") block of Teflon, the mechanical Q of this assembly is quite low. That is important in terms of damping the transmission of vibration to the tube, which contribute at some level, to the background microphonic characteristics of the tube in service. Between the heavy cross-section copper terminals, and the mass of the Teflon base plate, the socket is very solid and acoustically dead.

MECHANICAL DESIGN

We designed the billet copper terminals in our socket to grip the tube pins under all conditions. The socket terminals float slightly in the Teflon block such that their location is completely determined by the tube pin itself. As the tube and socket heat up or expand the copper terminal has no difficulty maintaining an intimate spring loaded contact with the tube. One of the difficulties in utilizing the Gm-70 tube was plate current instability. We determined that it was due to poor socket connections . We obtained actual Gm-70 sockets from Russia, and the quality was poor, and the manufacturing was crude. Given the outstanding audio potential of the Gm-70 tube, we saw that a premium quality socket was needed and would be worthwhile.

CONCLUSION

The Gm-70 power triode is certainly on the short list of excellent power triodes for SE service. We have designed an amplifier around this tube with exceptional sonics and power output. But we kept having circuit instability and finally found that modified sockets and the Russian socket left a lot to be desired. Moreover, the copper alloys used in conventional sockets are not my first choice for quality audio purposes.

    With our Pure Electrical Grade Copper/Teflon socket we have eliminated the plate current wandering problem, and achieved a new level of sonic performance. The bass and dynamics are notably better. I also found an increase in the midrange focus and depth and sweetness. Overall, the amplifier sounds like it gained 3dB in headroom. Not bad for a socket! 

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