I've been wanting to try an Almarro A318B in my system (Audiokinesis Jazz Module speakers) for a while. Finally, one came up for sale in "like new" original condition and I was fortunate to get it a few months back.
The sound didn't disappoint. It i a great match for my speakers. I just replaced the 6SL7 and 6SN7 tubes with NOS ones (RCA and Ken-Rad). Then, after a while I decided to upgrade the 0.47 uF coupling capacitors with Russian PIO K40Y-9 ones. I should say that there were periods when I thought I made a mistake because of sibilance issues. However, after about 100 hours those resolved.
This is when I noticed a peculiar problem. After at least 2-3 hours of continuous playing there was wind-like noise of varying loudness coming from the left speaker. By that time the metal top surface of the amp was to hot to touch for more than a few seconds so I thought it must have something to do with the temperature. I cleaned the pins and switched the 6C33 output tubes to rule out a tube issue but the noise remained on the left side. I should also note that the noise was present with the amp input selector on mute.
My next thought was a cold solder joint where I replaced the 0.47 uF capacitors. So, I went back in and reflowed the solder joints. All seemed fine for a period but again after the amp was on for a while the noise was back. That's when I decided to gently wiggle the 6SN7 and 6SL7 tubes while the amplifier was playing with the noise present. Touching the 6SL7 tube could make the noise worse or completely eliminate it. I thought there must still be a cold solder joint on that tube socket. But just in case I decided to try the original 6H9C (6SL7) tube that came with the amp. To my surprise this completely eliminated the noise problem.
So, at this point I believe that it was a 6SL7 tube issue. What surprises me is that for 2-3 hours it would perform perfectly and only then would exhibit this problem (? temperature related). Also, I had paid a good price for this NOS RCA that came with great test results. Somewhere I read that Russian 6H9C tubes may have slightly larger pins than the western 6SL7 variants. So, could it be that the sockets were expanded by the original tube and somehow the pin contact was not as good with the western tube afterwards. All I know is that inserting the 6SL7 tube didn't seem too easy. Any thoughts?
I do want to say that I really enjoy this amplifier in my system.
