Real music has dynamics. Macro-dynamics – with unrestricted headroom in crescendoes. Micro-dynamics – with subtle low-level information.
Music should be reproduced also with a ‘time’ dimension. Not just tempo, but each sound accompanied by an harmonic ‘envelope’, where overtones are produced and decay naturally – providing a sense of ambience, dimension and space. Some vacuum tubes simply seem to be able to generate or preserve these characteristics better than others.
Tubes like the Type 76 triode are in this category. The Type 76 single-triode vacuum tube was introduced in 1933 and is a 6.3 Volt version of the earlier 2.5 Volt battery Type 56. These tubes fit the American UX5-pin socket. In the JEL Linestage, the 76 has an clear, spacious sound, with an ample ambient ‘envelope’.
These characteristics may exist in a similar way, in other the large glass envelope tubes with large internal support structures. (My opinion only.)
When you compare the anode plate design of the 76 (and similar round plate tubes), there may be a clue to why the 76 sounds this way.
Compared with the design of another medium mu tube, the 12AU7/ECC82 (rectangular anode with round cathode tube), the 76 has a rounded anode to match the round cathode.
Electron paths from the cathode to the anode seem to be more consistent – this could arguably mean lower distortion.
Possibly also, the density of emissions is lower in the bigger tube, and low level signal information is possibly better preserved, with less interference with ‘secondary emissions’ from the anode. However, the construction of the 76 tube does have one disadvantage – microphony.
This contributes to the sense of ‘magic’ in the sound of this tube, but can be problematic in a line-level signal stage.
Sonically, whilst the sound is attractively ‘airy’, open and delicate….. high frequencies can be slightly too excitable and uncontrolled at dynamic peaks. (Some HF ringing was previously identified on the ‘scope, here.)
The 76 here benefits from some shielding and/or damping. My solution is to use PEARL Tube Coolers over the 76 glass envelopes. I don’t like over-damping things in audio, too much damping can kill the life of the music.
The PEARL coolers work adequately, although they don’t exactly fit the ‘ST’ shaped glass.
High frequency energy is better controlled, but the sound is still ‘alive’ and still has good definition and ‘presence’.
I will now need to ‘re-balance’ other system components to suit – cartridge loading and VTA, loudspeaker HF level, amplifier PSU capacitor parts, etc.
JEL Preamp Update: Cathode bypass capacitor has been reinstalled on the 76 (as per the original JEL schematic – using Elna Silmic II 100uF/16V) – more dynamic power, rhythm and ‘leading edge’ definition. This suits the ‘damped’ 76s. (See updated schematic below.) Earlier JEL Labs preamp posts: JEL Labs Linestage and JEL Labs Linestage Pt 2