SMLD MODS: (continued)
Anode/cathode trials -
ANODE/CATHODE listening tests: The Results...
The graph above showed my proposals in green.
I've recalculated the graph with my revised B+ of 185v. The graph shows the region of interest where we are looking for the best operating points.
I tested 20 positions in all including 2 types of 6SL7 driver tubes and high/low gain settings.
I've got my amp up and running again I've been able to finish the anode/cathode experiments for the driver stage that I had started previously so here's what I did for these tests:
I've stuck with the RCA 6AS7G's for the purposes of the experiment because I had already started the experiments using them, before getting the full set of WE's, and I'm used to the sound. For driver tubes I used TS 6SL7's and GE 12SL7's, which are my favoured sets now.
I used the same test tracks, including some high res tracks, for all the tests and the same volume level as far as possible.
I made notes as I went along in brief comprising soundstage, bass, midrange, treble. Initially I also compared high/low gain settings but as I preferred high gain, I decided to stick with this and not evaluate it further after the initial results because I believe there is enthusiasm to see how high we can go with the gain settings in future, I found no downsides with high gain. I desoldered my gain switch due to its unreliability. By the time I had nearly finished the tests I wrote more abbreviated notes referring to other positions rather than repeating myself over and over.
I found that pretty much all of the settings sounded good, with the exceptions of those with scores of -1, and would not discourage anyone from using any of these settings, the difference is mostly a matter of personal preference, and at this level differences are not that great, I used a numerical evaluation, for example starting at 0 for the baseline - Sonic's and my settings - with anything at 1 indicating a preference over the 0 level but with this not indicating a massive difference such as a score of 2 would indicate. I've indicated why I thought one setting was better than another but you have to bear in mind that where there is a difference of only 1 point there is a subjective element in all of this. Only a score of 2 would indicate a definate, objective, indisputable difference. I didn't think any differences justified a score of 2 because there were no massive differences such as is the case for example when changing RCA power tubes for the WE's. Having said that there was one position that I nearly gave a score of 2 to!
One reason all settings sounded pretty good I think was because the voltage swing on this amp is not great as MrCurwen said before, so that even if an operating point was chosen for a tube which was close to a non linear region on the graph it would be unlikely that the swing would go near to the non linear part of the grid curves. This is probably a strength and a weakness of this amp. Another reason is that we are already in the good regions and trying to find the optimum points but I decided not to test the extremes because I wanted to stay in the linear parts, so for that reason there would not really be a huge difference. Anyone who would like to look into those non linear regions, feel free!
We have now looked at a range of operating points and currents for the tubes, we are limited by the voltage we can get so I don't think there's much we have missed concerning the tube operation now. One thing, I believe that the amp sounded better with the 2 last R, 3K3 resistors lowered in value to give a B+ of 185v on my amp.
To summarize my results there was one stand out position which hit me the moment I started listening, the sound was full, palpably real with good clarity and texture which you could almost feel. The sound had a presence, transparency, resolution and dynamism. This for me was the obvious setting to use. I was a bit surprised that my proposed intermediate settings in between Sonic and Maxx's settings were not my favourite, probably because it fell in the middle it was neither one thing nor the other. The best operating point was with an anode of 69K and cathode of 380R, point 7 in the graph. I will scan in my notes page later so you can see in more detail.
The tests also confirmed my choice of driver tubes, I've gone from preferring the TS 6SL7 to the GE 12SL7 because my experience with the other amp made me prefer a sharper, more lively sound which the GE does well, for only a couple of dollars too! The sound of the GE is better for hearing things such as the sound of the plectrum on the guitar string, whereas the TS 6SL7 has a smoother, more liquid yet still detailed sound. The surprising thing about my preferred position is that it happened with the RCA 6AS7G's in, so if it's that good I can't wait to try the new setting with the WE's!
I hope this has been useful for others trying to find the settings which suit them.