Hum in left channel after Speedball upgrade. Is it the tube?

[thatguy]

I built my Crack a couple of weeks ago. Everything went fine, and it sounded great. More recently I replaced the stock Phillips 6080 tube  with an RCA JAN CRC 6AS7G. Even better!

I added the Speedball a couple of days ago. Everything looked good and sounded better at first, but after the amp has been on for 15 minutes or so a hum appears on the left channel. It fades in and out, but gets worse the longer the amp is left on. The left channel also picks up a lot of microphonic noise.

The thing is, the hum is only a problem with the RCA tube. The amp is dead silent with the stock tube. What confuses me is the fact that the 6AS7G worked fine prior to the speedball upgrade.

 It is possible that my tube just picked a lousy time to die. What I would like to know is whether some flaw in the speedball’s construction might apparent with the 6AS7G, but not the 6080? or, alternately, could the 6AS7G have a flaw that is only audible with the speedball.

I would rather not go mucking around in my amplifier if the tube is the culprit. Any advice or insight would be very much appreciated.

[Paul Birkeland]

How are the voltage measurements with each tube? 

I would also suggest remeasuring the voltages when the hum is present, just to see if there's an operational change.

Generally speaking, there isn't any mechanism that I'm aware of for the Speedball to interact in such a way with a particular tube, but you may have a cold solder joint or some other installation issue that is jostled when swapping tubes, and that may be at the heart of what you're hearing.

-PB

[thatguy]

Thank you for the reply.

Voltages were fine with the 6080 (original tube) prior to installing the Speedball. With the speedball, measurements on the Speedball boards were fine with both tubes. I did check with the 6AS7g after letting it warm up and start to hum.

Do you think I should check terminals 6 and 10 as well, with the Speedball installed? Should I expect the same values?

Anyway, I may have solved the problem. I was tapping on my Crack this morning, trying to see if I could figure out where the microphonics were coming from, and I blew a fuse. When I inspected the wiring (again!) I saw that some wire sticking off of B7 could make contact with B8 if they were both wiggled just right.

I'll resolder B8 and recheck everything, then give it another go. With any luck it'll solve my problem with the new tube as well. Fingers crossed!

Off to get some fuses.

Thank you, again.