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I have a Sunn Model T reissue (the Fender one). It has basically the exact same power stage as the newer Fender Pro Tube Twin Amp. Plate voltage +489V, bias voltage ranges from -63V to about -50V.
About 6 months ago I bought a matched quad of JJ 6L6GCs and put them in the amp. Biased them to 36mA per tube (roughly 60% dissipation?), and they were great. I played them LOUD for about 3 months, about 6 hours weekly. I decided to try JJ KT66s in the amp, bought a matched quad, biased them to 30mA per tube (roughly 60%), no issues, sounded better to me than the 6L6GCs. Played those for about the same amount of time with the same regularity. Switched back and forth between the 6L6s and KT66s a few times, no issues biasing up. Last time I swapped was about 2 months ago, and the KT66s were in there until today. Hadn't checked the bias since the last swap.
Since I'm headed out on a 5-week tour this summer, I decided to swap again and put the 6L6GCs back in the amp, since they're cheaper. I wanted to use 'em up on tour, then put the more expensive KT66s back in the amp when I get home. If I had a blowout on the road, I didn't want to lose a $140 quad of tubes.
ANYWAY: Today I had a lot of trouble biasing the 6L6GCs when I put them back in. Wild mismatches between different sides, weird readings. So I verified that all the socket voltages were correct, and checked that each tube drew the same current in each socket. Here's what my matched quads are drawing now, at -63V bias voltage, when I tried them one at a time in the same socket:
6L6 #1: 9.3mA
6L6 #2: 8.2mA
6L6 #3: 16.1mA
6L6 #4: 29.6mA
KT66 #1: 53mA (!!!)
KT66 #2: 18.2mA
KT66 #3: 16.4mA
KT66 #4: 20.7mA
What's going on here? The weird thing is, the amp sounded fine at shows on Saturday and Sunday nights, with the KT66s in it. These tubes aren't old at all and haven't been mistreated. I found an even older crappy set of GT 6L6GCs (matched quad) and stuck them in, and they bias up fine. Is this normal for this much drift in bias grade over time?
In both sets, there's one tube that's drawing about 3x the others (although the 16mA 6L6 is supect too). Could this be a problem in the amp that's causing this? I label them by socket though, for consistency between swaps, and it wasn't a tube on the same socket going bad in each case. JJ quality control issue?
About 6 months ago I bought a matched quad of JJ 6L6GCs and put them in the amp. Biased them to 36mA per tube (roughly 60% dissipation?), and they were great. I played them LOUD for about 3 months, about 6 hours weekly. I decided to try JJ KT66s in the amp, bought a matched quad, biased them to 30mA per tube (roughly 60%), no issues, sounded better to me than the 6L6GCs. Played those for about the same amount of time with the same regularity. Switched back and forth between the 6L6s and KT66s a few times, no issues biasing up. Last time I swapped was about 2 months ago, and the KT66s were in there until today. Hadn't checked the bias since the last swap.
Since I'm headed out on a 5-week tour this summer, I decided to swap again and put the 6L6GCs back in the amp, since they're cheaper. I wanted to use 'em up on tour, then put the more expensive KT66s back in the amp when I get home. If I had a blowout on the road, I didn't want to lose a $140 quad of tubes.
ANYWAY: Today I had a lot of trouble biasing the 6L6GCs when I put them back in. Wild mismatches between different sides, weird readings. So I verified that all the socket voltages were correct, and checked that each tube drew the same current in each socket. Here's what my matched quads are drawing now, at -63V bias voltage, when I tried them one at a time in the same socket:
6L6 #1: 9.3mA
6L6 #2: 8.2mA
6L6 #3: 16.1mA
6L6 #4: 29.6mA
KT66 #1: 53mA (!!!)
KT66 #2: 18.2mA
KT66 #3: 16.4mA
KT66 #4: 20.7mA
What's going on here? The weird thing is, the amp sounded fine at shows on Saturday and Sunday nights, with the KT66s in it. These tubes aren't old at all and haven't been mistreated. I found an even older crappy set of GT 6L6GCs (matched quad) and stuck them in, and they bias up fine. Is this normal for this much drift in bias grade over time?
In both sets, there's one tube that's drawing about 3x the others (although the 16mA 6L6 is supect too). Could this be a problem in the amp that's causing this? I label them by socket though, for consistency between swaps, and it wasn't a tube on the same socket going bad in each case. JJ quality control issue?
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