Time for another tube family to try in Crack!
The Dull Emitter Transmitter 20 or DET20.
Also known as CV6, E1148, 7193, CV3601, 2C22, VR135, CV1135, NR80, CV2920, VT252 and by some other names. It had many!
Back in the 1900’/10’s, the first tubes were developed (Fleming diode -> R triode). You had bright emitters, those were the early tubes that could function as a light bulb because of the pure Tungsten filaments. Later-on came the dull emitters, you’ve guessed it, not so much of a light source and far more economic for use with a battery, as was common in the 1920’s.
This Dull Emitter name since stuck a bit in England. So when de E1148 prototype was released in the USA for UHF use, the British had to get their counterpart. These tubes go by various names and have one thing in common, both anode and grid are connected via top caps. Starting out as the 6J5 (a 12AU7 predecessor), i guess this family was developed with the prevention of oscillations at high frequencies in mind. This tube (named military Common Valve 6) played a major role in military equipement during WWII. DET20 was the name given to civil versions of this tube, sold by the Marconi company, but picked from both the GEC as Mullard production line.
Being so close to the 6J5 (6SN7/12AU7), these make for a perfect fit in our beloved OTL headphone amps. Maybe even with some extra special “sparkling highs†(Because VHF tube, haha, bad joke... .... ).
https://ep1.pinkbike.org/p5pb17858503/p5pb17858503.jpg
Top: E1148 (Hytron) and 7193 (Ken Rad)
Bottom: CV6 (Mitcham) and CV6 (Hammersmith)
The E1148 then is a one of the first in this family and was build in the USA and Hytron branded tube. It has it’s inners mounted horizontally. It’s British and other American relatives show a normal vertical construction. Here’s an example from a 1942 group of this type. As you see, these all show a pan getter. Later versions show a different mica construction over that getter of no getter? I might have to break one to find out exactly.
In the CV6 type, tubes can be found from various factories and most of them were Mullards made in Mitcham (D) or Blackburn (A). My only Marconi tube looks exactly the same as the CV6’s from the Mullard line. I can’t spot any difference in internal structure. The plates of these area smaller then 6J5’s and charcoaled black for better heat radiating performance being welded together.
The 7193 is the American sibling, with the same construction/look as a 6J5 metal base (the normal ribbed plates), but with the same top-caps. They are fysically smaller compared to the EU counterparts and E1148 predecessors. In 7193’s there’s 2 versions i’ve seen. The Ken-rad, distinguischable via the top mica spacer from the National Union version (round mica).
When using these, please take extra care for the correct position of both the anode cap and grid cap!! Don’t ever swap them! Mishaps possibly harm the tube and your amplifier.
Always look up the datasheet before putting them in your amp to check both the parameters and connections. Then put the guide-pin correctly in the socket. After that you place the top caps according to the datasheet. Before power-on, visually double-check if the datasheet is correct for your tube (just look trough the glass and follow each connection).
Because i couldn’t find any datasheet on the E1148 (only the mention of it in the CV6 datasheet), i played around with it a bit longer before amplifier testing. For this, "used" specimens from 1942-1944 era were tested. All tests were done with 250V anode, -5,5v grid and show +- 8,2Kohm plate resistance, amplification factor 21, mutual conductance 2,5-3 ma/V with 13-15ma output. So close enough to CV6 (and same pinout) for the use of that same datasheet (+-NOS). So, these tubes are interchangeable and very usable as driver in Crack!
https://ep1.pinkbike.org/p5pb17858501/p5pb17858501.jpg
Picture: Test setup E1148.
CV6 datasheet:
http://www.r-type.org/pdfs/cv6.pdf
On page 3, mounting position is specified.
Note that the E1148’s and CV6’s in my collection all share the same mounting positions.
7193 datasheet:
http://www.r-type.org/pdfs/2c22.pdf
On page 2, mounting position is specified.
Note the differences!
Oh.
Sound!
These are very, very, very good sounding tubes. These add layers and layers of texture to the music bringing out soooooo many details. Wow, just wow. I’ve got tons of tubes to choose from for my Crack, and these enter the top-10 out of nowhere. This makes me want to explore more odd-ball UHF over-constructed tubes with the right specs.
I’ve tried 6 or 7 pairs by now in my Crack and none have been microphonic or problematic with interference due to long wires. All test strong and perform even better!
For the money this family is impossible to beat! You can find a pair for 25€ or less and i found a “made in China†adapter for 25€ that is performing perfectly.
It’s more worthwhile buying a pair of these with an adapter vs any 6SN7/12AU7 that goes for the same price. In the states the 7193 Ken Rad would be the one i’d recommend. In Europe the CV6’s can be found at bigger retailers, for very convenient prices (a few Euro/pound each) but you have to ask for supply (as nobody uses them). My personal favorite is the Mitcham factory CV6.
https://ep1.pinkbike.org/p5pb17858502/p5pb17858502.jpg
Picture: Kings of the bunch, the Mitcham CV6. Used with Bendix 6080 driving AKG K340 electret/dynamic headphones.
Yes, i still need to rework the adapter and rotate it 90 degrees.
All-in-all these tubes sound so good that i’m planning on researching further developments of the UHF group for a next, completely nuts, tube adventure!
. No magical appears ......flat; dull; mushy , thin , so i confess i was devastated!!! :'(
