I like to first summarize the functional principle of what which strap does to heptodes. By doing this, I repeat Audiofanboy and myself but I found it useful to put recent findings into a bigger picture.
Let’s remind ourselves on how the LD strips 6AK5 pentodes to triodes:
Anode = LD PCB wires pin 5 (anode) to pin 6 (grid 2); in EF91/92 operation, jumper connects pin 7 (grid 3) to anode and grid 2. Effectively, pins 5, 6 and 7 are connected.
Grid = Pin 1 (grid 3)
Cathode = Pin 2 (cathode), 6AK5 tubes wire internally pin 7 (grid 3) to pin 2.
For heptodes, this allows four modes of operation to utilize pin 7 (q3, mixer grid), without heavy mod operations:
- “Grid Strap” (1/7 strap), Audiofanboy’s suggestion to connect pin 7 (q3, mixer grid) to pin 1 (regular LD grid, q1). This results in a double grid, which is neither cathode nor anode strapped. Sounds great.
- Floating grid. Mixer grid, pin 7 is not connected, which would in the original mode of operation be used to add the oscillator frequency to the radio frequency. Works fine and yields great sound.
- Anode strap in EF91/92 setting, found to yield sub optimal sound performance.
- Cathode strap (2/7 strap), found to give poor sound performance.
It would be nice to find a general consensus on the strapology of heptodes but we might end up just saying tube A is the best with this setting and B is close with its preferred strap.
Initial remarks: I haven't made this easy for myself but obviously the transferability to other tubes may not be given.All observations were done using the Siemens EK90 (6BE6) in EF95 setting.
1/7 strap: better definition / resolution, more forward, harder, drier, faster, bass is deeper, better articulation, more treble presence, more bass punch, better instrument separation, higher gain. Width of stage is not negatively affected.
Float: slightly more diffuse, softer, wetter, airier, more ambience, more instrument body, sweeter, more bass presence, bass goes a bit deeper, “tubier”. The plus of ambience makes the stage feel deeper and the room itself more present.
1/7-Triode touch: Yes, it is there. Transparency, definition of imaging, drier character / lesser perception of ambience.
1/7-Gain: I find the tube also slightly louder with the 1/7 strap compared to float setting. For the operation with an external oscillator, Brimmar suggests that highest gain would be achieved with a voltage of – 2 V on the oscillator.
Overall, the Siemens EK90 was already revealing with the float setting, which is like an easy-to-listen-to setting giving lots of ambience. After strapping it becomes an analytical audio instrument with the only disadvantage that it reduces the ambience perception, which - in full reproduction mode - is quite nice on my Senns. Either 1/7-strap or no strap, the tube sounds great. At the moment, if I had to choose, my favour would go to the 1/7-strap because of its definition / resolution but I would have to do a few even longer listening sessions for ultimate conclusion. This would be my best sounding setup overall.
1/7-strap comparison between tubes
Siemens EK90 (6BE6) sounds more transparent, airier, more treble presence (“triode touch”), bass most differentiated, Tung Sol 6BE6 gentler with more ambience, more relative bass presence, more tube like, between Siemens and Sylvania. Sylvania JAN 5750, most relative bass presence, sounds darkest.
In order to compare cross family differences, I sought to find a Siemens 6BY6 (EH900, EH900S, EH960, E91H), but couldn't find one. In contrast, the 6CS6 (EH90) are easily available, which aren't yet in my collection. I bought a pair of those.
Gibosi, excellent work reviewing the heptodes on page 105. I was starting to go through my heptodes as mentioned above but I have to come back later to draw more detailed conclusions between tubes. Funny, I just came up with this post after you strapped your Siemens EK90.
Mordy, I think you would probably favour a 1/7-strapped heptode, because it yields more treble presence, sounds more forward. I am guessing you just need the right manufacturer to suit your sweet tooth.
Audiofanboy, The depth of stage / ambience effect is IMO probably differently highlighted depending on headphones, especially considering that the HE-500 is known to not have a very deep stage and the HD800 produces one of the largest volumes of stage spaces. I might use the HE-500 for comparison as well at one point. As for your larger quantity of the positive treble energy, this is probably what I have described as treble presence and it looks like we agree on this one.
Update: Added EF95 setting info.
Edited by Acapella11 - 6/23/13 at 2:28pm