Originally Posted by gibosi
No, I wasn't able to discern any difference between the GE 6AU6A and the 8425A either. That said, I can easily hear differences between the various 6AK5s. And recently, it was easy for me to detect differences between major tube families, for example, 6CB6, 6WE6, 6GX6, 6GM6, 8136 and most recently, 6AU6. So I have to assume that any differences that might exist between these 6AU6s must be fairly subtle, requiring better ears and better gear than I have..... and more experience...
The LD1+ is my very first headphone amp and I have had it for only three months. I bought my first full-sized budget headphones shortly thereafter. Before that, I listened to Pandora on a cell phone with IEMs...
I try to contribute what I can, but I am truly a freshman in the company of graduate students. :)
Well, I guess not all tube families show as much variety as the 6AK5 or EF91 families, or even all the different types we've been testing on lately.
In the meantime, I had ordered and just got in the mail 3 pairs of 6AH6 tubes. Not a single plain Jane one (for $2/3 a tube who cares?), and not just GE for once:
- JAN Raytheon 6AH6WA - 1973
- JAN Sylvania 6AH6WA - 1970
- JAN GE 6485 - 1978 (that was before my conclusion about 70/80s GEs, oh well, we'll see if these are good or not...)
The 6AH6WA and 6485 are, as usual, the special quality versions of the 6AH6, which is considered and even recommended as a replacement of the 6AU6 in many circuits. So if the 6AU6 works so well in the LD amps, why wouldn't these be as good or better? Note that the 6AH6 has similar recommended plate voltage and cathode resistance triode-strapped compared to a 6AK5 -just like the 6AU6- which means the amp should be very well suited and biased for this type, unlike many types I tried in the last two weeks, that had different cathode resistance recommendations and other recommend settings (could explains why some tubes didn't fare so well).
I put the Raytheons for burn-in this morning and first impressions are very favorable; I'll wait the usual 10h to give more details.
The 6AH6 are sharp-cutoff pentodes with a 450 mA filament current (6AU6 has a 300 mA heater). These should have almost the same voltage gain/amplification factor/mu (40 instead of 36) as the 6AU6, so I didn't expect much more volume and didn't find a significant difference in level compared to the 6AU6; and they have more than double the transconductance/gm (11000 mmhos instead of ~5000) the 6AU6 has, which I'm still struggling to find out what effect that would have on the sound, should mean more mA on the anode -and into the power tubes- for every V on the control grid, so higher output current and lower output impedance if I understand... Apart from that, these tubes have similar characteristics and are known to work well in audio circuits triode-strapped (the datasheets even have triode curves for both), so it should interesting to see how they fare compared to the 6AU6.
I also ordered and got a -final- 6AG5 tube with the 3 pairs of 6AH6, last try for this tube family. A Sylvania JAN 6AG5WA/6186 dated 1967, how bad can it be really, it's mil spec, special type and from the 60s (so final improved construction and getter structure). Just so I can reach some kind of closure on the 6AG5 family after the disappointment the Russian 6J3P-E were (not 6AG5 really, but the point remains)...
I'll try to put some pics up at some point when I start reviewing these tubes.