My experience was with 6SL7’s in my Cayin Integrated Tube Amp A88T.
Sounded: Great, terrible, great again, future covered!
Made in 2001, I Bought it with used tubes 2019 on eBay with:
4 Gold Aero 6550 power tubes
2 Cayin branded 6SN7
2 Cayin Branded 6SL7 (identical size/appearance to 6SN7s)
all tubes tested good and matched strengths on my tube tester when received.
sounded darn good, I changed to 4 Matched KT88 Power Tubes,
JJ KT88 Blue on sale at Aliexpress, $98. free shipping quad matched $367.
I prefer them to 6550 and the amp’s ultra-linear sound to it’s triode option. I also like having the reserve power even though my 16 ohm horn speakers are incredibly efficient. My model, MK1 with 16 ohm taps, has dangerous internal bias adjusters, I paid VAS to do it, luckily I live only 35 minutes from them. MK2 version had meters and external adjusters, but they dropped the 16 ohm taps.
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2020 Yearly pre-holiday test tubes, all still ok, I mistakenly reversed a 6SL7 and a 6SN7, blew one 6SL7.
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tried a pair of matched NOS, from Brent Jesse, 6SL7WGT Sylvania Brownbase, military matched pair. different look chrome top I thought was smart.
hated their sound. Brent had me break them in (just leave on for 3 full days, no signal needed, 60+ hrs), still hated them.
Exchanged for matched pair of 6SL7GT General Electric, side getters, cylindrical plates, half the price, Brent gave me a full refund, I just paid the shipping.
Happy Days returned.
Since then, found a pair of used Cayin brand 6SN7 and 1 Cayin 6SL7 on eBay (I still have 1 good one), bought them for future use.
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TUBE LIFE: that’s 18 yr old OEM tubes with good matched strength. Signal tubes can last 10,000 hrs, Every year since 1973 I test all my tubes just before Thanksgiving, problems are more likely a short, occasionally a weak one.