Just thought I would would share my impressions of these valves in my Rogue Audio Stereo 90.
I recently noticed that one of my EH KT88s had started to suffer from bias current drift, and decided to replace them. I temporarily substituted a set of EH 6550s while I decided what to do. The EH 6550s didn't work out well - tubby bass, strangely recessed midrange, loss of detail and curtailed treble response. I was quite suprised that they sounded so inferior to the EH KT88s I was using as they appear identical visually.. maybe there are actual internal differences as they bias quite differently. Interestingly, the factory packaging has a test result label showing that the Gm values of the Golden Lion KT88s is significantly higher than the 6550s - 7700 vs 6700.
I ordered a set of Genelex reissue tubes from eBay, and payed £120 for a PL matched quad.
After installing them in the amp, I dropped the bias current to 25ma and left them to cook for an hour or so before making final bias adjustment to 35ma indicated (40ma actual) after everything was at full operating temperature. I notice that they held their bias very consistently once adjusted and there was no change over the course of an evening's listening. The EH KT88s had an interesting 'random' fluctuation in bias current on valve 2 that doesn't seem to happen at all with the new set... odd.
First listening impressions were quite suprising - I hadn't expected there to be such a signficant improvement across the board, even before anything was burned in. The main differences were as follows:
I immediately noticed a considerable improvement in the impression of bass spectrum 'weight' and energy. Dynamics in the bass were suddenly a lot better, and there was also a significant improvement in the resolution of bass textures, e.g. Electric Bass and synth bass became much more discernable and the envelope of each bass note was more clearly defined in terms of attack and decay into silence. It also became a lot easier to determine pitch intonation for bass instruments.
In the midrange, reproduction of voices was noticably improved so that any studio effects applied were more apparent, and there was less masking of quieter sounds by louder sounds. Reproduction of instrument timbres is considerably better, and there is a sense that separation of sounds was better than before. Previously I had thought that solo guitar, voice etc was quite convincing but the illusion tended to collapse when the recording got more busy. I also had an impression that they decay of notes into silence was more realistic - previously I thought that the timbre seemed to change as notes or level faded. Low level details seem to be better reproduced than previously - it seems as though the noise floor has been lowered, allowing previously masked details to be heard.
In the treble register, percussion instruments have a more realistic tonality and cymbals sound more like cymbals and decay more realistically. Although the more powerful bass made me wonder if the frequency response had been 'tipped' towards the bass end, treble extension is as good or better than with the EH KT88s. Stage width and depth is also improved, probably as a consequence of the improvements I listed above.
All in all, music reporduction seems more involving, natural and detailed. Although you can analyse where specific improvements occur, the most impressive aspect is that it all blends harmoniously into an enjoyable and realistic whole - I think I am listening to music as the artists and producers intended rather than my more usual 'forensic' feeling of listening to HiFi. It is a relaxing and enjoyable rather than occasionally frustrating experience it was previously.
I am very impressed by these valves. It is probably the most rewarding £200 improvement to my system, and I have spent considerably more and gained less elsewhere. I admit to being very pleasurably suprised as the consensus is that power valves make less of a contribution to the sound than small signal or driver stages. It will be interesting to see how these initial impressions are changed as they accumlate more hours - possibly there is even more improvement to come!.
Regards,
Alex
My system:
Rogue Audio Metis pre-amp (EH 6SN7 gold pin)
Rogue Audio Stereo 90 (Mullard 12AX7 phase inverter stage/ EH 6SN7 gold pin drivers, Genalex Gold Lion KT88 re-issue)
Avalon Acoustics NP Evo 2.0 speakers
Cambridge Audio 840C with Audiocom Intl level 3 digital, clock and analogue stage mods)
Logitech Squeezebox Touch as primary source
Technics SL-1200 turntable with Benz Micro Silver
Cardas Audio Quadlink-5 Interconnects and speaker cables
