In my D'Lite build, I have twice run the amp when a low hum would start. The first time it happened was right after a gig, when some announcements were being made. At the time, it was easy enough to just reach over and turn the amp off as soon as I started hearing it. Got home and fired up the amp, and nothing. Left it running for over an hour listening from the next room, and no problems at all.
Just now, again after the amp was on for a couple of hours, the hum started up. This time I took a moment to look at the tubes. One of the Tung Sol KT-66s had a plate that was glowing orange.
Plug in a set of old 6L6's and nothing so far. I had changed Rg's to 5.1k, since I had read somewhere that KT-66s preferred higher values there. Any suggestions as to how to track this down? Intermittent problems...shoot.
As you're not having any problems with the 6L6's I have to ask ,are you using pin 1 as a tie to pin 5 for your grid resistors ala Fender? here's a quote from John Templeton on JJ KT-66's
The large flat base is just like a 6550 including the metal base ring that is internally connected to pin one. This is a red flag for Fender owners. A 6L6 doesn’t use pin one and Fender usually ties the input grid resistors from pin 1 to 5 for easy mounting. If this tube is inserted into a Fender or other amp using pin one as a tie off point problems will result. In this particular case shorting the input grid signal and bias voltage to ground if you have the standard retaining clip.
Look for problems (cold solder joint, dying electrolytic cap) with the bias circuit. I had a similar problem which turned out to be a bad Sprague Atom.
gtr777 wrote:As you're not having any problems with the 6L6's I have to ask ,are you using pin 1 as a tie to pin 5 for your grid resistors ala Fender? here's a quote from John Templeton on JJ KT-66's
The large flat base is just like a 6550 including the metal base ring that is internally connected to pin one. This is a red flag for Fender owners. A 6L6 doesn’t use pin one and Fender usually ties the input grid resistors from pin 1 to 5 for easy mounting. If this tube is inserted into a Fender or other amp using pin one as a tie off point problems will result. In this particular case shorting the input grid signal and bias voltage to ground if you have the standard retaining clip.
This should only be an issue if I am using the metal (grounded) tube retainers, correct? And I would expect to have problems instantly, and to not be able to measure correct bias?
jaysg wrote:Look for problems (cold solder joint, dying electrolytic cap) with the bias circuit. I had a similar problem which turned out to be a bad Sprague Atom.
Thanks, Seth and Jay. I fired it back up with the KT-66s in it, and same story as before: nothing happened. Intermittent issues like this really are hard to track down. After that test, I did go over solder joints again just to make sure. Guess I'll let 'er run for awhile and see what happens.
if its just one tube replating.. try swapping the tubes in the sockets.. see if the problem stays with the same socket..or if the tube red plates in the other socket.. You probably just have a bad tube.,at least thats one other thing to check.
Richie wrote:if its just one tube replating.. try swapping the tubes in the sockets.. see if the problem stays with the same socket..or if the tube red plates in the other socket.. You probably just have a bad tube.,at least thats one other thing to check.
Yeah, swapping them is one thing I thought of, but this happens so seldom that it's going to be hard to track down. Shoot, these tubes have less than 6 hours on them total, I'd say. Just got them. They sound good, and bias within 3mA of one another.
Richie wrote:if its just one tube replating.. try swapping the tubes in the sockets.. see if the problem stays with the same socket..or if the tube red plates in the other socket.. You probably just have a bad tube.,at least thats one other thing to check.
Yeah, swapping them is one thing I thought of, but this happens so seldom that it's going to be hard to track down. Shoot, these tubes have less than 6 hours on them total, I'd say. Just got them. They sound good, and bias within 3mA of one another.
There's always DOA's. But since it doesn't happen all the time I'm thinking there is a loose connection or something is shorting out.
