i would say I like them. I sold my X5 before I bought this amp. I am using them to drive a pair of JBL 4341 now. I only have 3 power amps to compare: a custom built 300B by Otomon Lab from Japan using Luxman mq-300 schematics, the ANK, and Coincident 211 Dragon. I then sold the Otomon Lab. The ANK sound as good as the Coincident yet different. The ANK are faster, a bit lighter, better mid range, and a bit more holographic. The Coincident also sound very good. It is a 211 push-pull delivering 75W per channel. It produce a lot more power but also sound very lively and holographic. I had a McIntosh MC 275 IV before. It is a KT-77 push-pull, and it sounds boring and flat, which leave me an impression that is the characteristic of a push-pull amp. It is not the case for the Coincident. On my Harbeth 40.1, it will be clear it needs the Coincident 211. But for my JBL, both sounds great.
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Just want to provide an update. I tried adding the diodes but then one of the Sylvania 5931 (which is a rugged 5U4G) dies. I had one of these died died before adding the diodes. And they didn’t arch, but just died with no sound. I think it is not conclusive if the diodes works or not because the tube and symptom are different. But I am concerned and took out the diodes.
then I bought 3 pairs of 5U4GB, 2 pairs of TungSol and 1 pair of Westinghouse. So far so good. I will keep observing. Maybe the amps are just more demanding on the tubes, and some of the old tubes I tried are just bad.
Thanks for the update, @gte357s
You mentioned one of your 5931 dies after adding diodes that lead me to restudy the schematics and found that the peak inverse voltage(PIV) of a single 1N4007/UF4007 is not high enough for the job. If in the future you decide to retry the diode mods please use two 1N4007/UF4007 in series to double the PIV or use a diode with higher PIV rating.
The 5U4, being a directly heated rectifier, is a far simpler tube than a KT88 or like; I really doubt the ones you had were bad. These tubes are generally pretty reliable because they are so simple! Keep in mind that tubes, especially older NOS tubes, can be quite forgiving when their specs are exceeded as long as those specs are not exceeded for too long.
Because this is a directly heated rectifier, one way the tube can fail due to arcing is the filament can open up- causing the tube to simply die with no (apparent) fireworks. This is because its two cathodes serving the two plates are in series.
Given your prior experience thus far I expect if you didn't make the changes in the power supply previously suggested that you'll be seeing this problem again.
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