Runaway Bias

[passfan]

I have a set of Sylvania 6L6GC's that I picked up with an organ chassis off ebay and they tested borderline NOS. I was all excited to hear them so I biased them up and started playing. After a warmup of a couple minutes one tube started drifting upwards and after a couple of re biases still drifting. What causes this and surely "nothing can be done" right ? They started out 5 ma's apart but the gulf is widening

[jjman]

Could be a connection or component problem, or the tube. Have you switched the 2 tubes to locate which it is?

Here's a pic of an RCA 6v6 grid that was doing that pretty quickly to me for obvious reasons:

[Cliff Schecht]

A tube can test great and not work for shit. How do I know this? One of the 12AY7's I was using (GE 5-star) tested fine but sounds terrible in my 5F6A and 5E8A builds. I bought it thinking NOS of course but it wasn't expensive so I'm over it. Point is that a tube can test great and even look great but just not sound right. I wish I knew how to fix this (like how some guys "recharge" their directly heated cathode tubes, if you will) but alas, sometimes the tubes just turn into useless eye candy.

TBH I'm expecting at least one set of the 8 used 1950's 6550s to be bad but I haven't built anything to put them in yet.. Any ideas guys?

[ToneMerc]

I had this problem with V7 of my high voltage(MusicMan style) 100W build upon startup. It started out with about a 10mA delta and over a period of about 5 mins it gradually increased to over a 75mA delta from the other tubes. I changed the order of the tubes and the problem followed. Then I dawned on me to verify that the cathode ground screw was tight, it was loose. As soon put a half turn on the screw, V7 fell within 3mA of its neighbor.

Post Mortem- Early in the build I had changed the lead bend of one of the bias resistors and had loosened that nut. Tubes V6 & V7 were terminated on the same screw and the nut wasn't torqued enough. The V6 lug was on the bottom and made just enough contact to have a decent ground path, while V7's lug was on top and wasn't making a good contact.

TM

[passfan]

Unfortunately for me, they are in another amp that plays just fine with other sets of output tubes. That's the way the ball bounces I guess. They were the big bottle design just like the STR415 set I have and from what I heard they sounded sweet.