Blues Custom Humming?

[spaaarky21]

I have the Blues Custom 30 and recently had it in for service. The amp was producing no sound in the 30W mode. In the 15W mode, it hummed while playing for about a minute and then starting humming and buzzing loudly. The problem turned out to be the output tubes. Now I have a set of Sovtek 6L6WXT+ tubes and it's playable again. However, when I first turn the amp from "standby" to "play," I get a noticeably loud hum that seems to fade away shortly after. I have heard that tube amps can hum when they startup (I assume until they warm up.) Has anyone else had this problem with non-stock tubes or is this something I should be worried about? And on the subject of replacing tubes, does anyone know if the Blues Custom needs to be biased after replacing tubes? I have heard that the amp uses tubes in a configuration where they are auto-biasing.

 

-Brandon

[MikeRom]

If you replace the tubes with those of the same kind (i.e. 6L6) it shouldn't need to be rebiased. The hum issue you're having now doesn't sound like anything that will hurt it. How long do you let it warm up for?

[spaaarky21]

If you replace the tubes with those of the same kind (i.e. 6L6) it shouldn't need to be rebiased. The hum issue you're having now doesn't sound like anything that will hurt it. How long do you let it warm up for?

 

Unless I get sidetracked setting up other stuff, I try to keep the "mains" power on for at least 30 seconds before switching from standby to play. That should be enough time to get things going. And like I said, the humming seems to fade out after a minute or so.

 

Since the problem I had before switching the tubes involved a low hum, I just wanted to make sure that startup hum wasn't too out of the ordinary. The amp didn't used to do that before this last round of trouble. I was kind of wondering if that might have to do with the preamp or rectifier tubes (since they aren't new like the power tubes.) But since preamp tubes usually last quite a while longer and most amps warm those tubes while in standby mode, I didn't figure they would be the cause.

[MikeRom]

Do you still have the old tubes? Switch them back for a test if you do. I usually let mine warm up for a few minutes before kicking it out of standby.

 

When I pulled the original power tube, the little black plastic guide thingy snapped right off. You have that happen?

[spaaarky21]

Do you still have the old tubes? Switch them back for a test if you do. I usually let mine warm up for a few minutes before kicking it out of standby.

 

When I pulled the original power tube' date=' the little black plastic guide thingy snapped right off. You have that happen?[/quote']

I do have the original power tubes but they were replaced because they went bad and had some major humming/buzzing/squealing issues. I was really just wondering how normal a little humming is at startup since the hum was almost non-existent before the power tube were switched. I don't think the warmup time should matter though, should it? I thought standby mode only heated the preamp tubes since they aren't handling as much power and take longer to heat up. I thought power tubes heated up relatively quickly. Or does standby mode heat all the tubes but just not connect the signal to them?

 

And to answer your question, the store replaced the tubes for me but when I was trying to troubleshoot it before that, I took the power tubes out and didn't have any trouble with it. What part are you talking about? The tab that makes sure it is installed correctly?

 

Brandon

[MikeRom]

Yeah, the black tab that makes sure they are oriented correctly. Warm up quckly? No, not in my experience. Try letting it go a little longer before switching on the standby and see if that makes any difference.

[lostindesert]

Did you buy a matched pair of output tubes ?

Sounds to me your amp needs to be rebiased.

Also try 1 to 2 min. on standby before switching the power on.

Do the same when switching off.

[spaaarky21]

Did you buy a matched pair of output tubes ?

Sounds to me your amp needs to be rebiased.

Also try 1 to 2 min. on standby before switching the power on.

Do the same when switching off.

Since I wasn't sure if the tubes were the problem before I brought it in, I didn't buy them myself. The shop (which was a certified Gibson/Epiphone service center before they stopped selling Gibson) put them in. The tubes' boxes came taped together and the receipt says they are a matched pair... but who really knows.

 

If the problem was biasing, is the hum something that you would expect to fade over time? From my electronics experience with opamps, I wouldn't expect bias in an amplifier to "right itself" but maybe tubes are different. Aren't all of the current Epiphone amps self-biasing?

[lostindesert]

If your amp runs in A/B configuration, that is one tube on and one tube off it can be biased.

My Vox AC15 runs in A, all tubes on at the same time and needs no biasing.

I noticed when changing tubes that the old ones hummed and the new ones didn't (my Vox).

I do now that changing to a different tube compagny with L6L, like Sovtek to Philips, your

amp sometimes needs to be rebiased.

With tube amps it's allways a good advice to have it checked by a pro amp tech.

Humming could also be bad caps or something else.

 

Peter

[cGil]

Any wire carrying 60Hz AC can induce a 60Hz hum in another wire if it's close enough. Humming could simply be from the rats nest of bundled PT wires the factory leaves behind. Try twisting the common pairs and get the foreign voltage taps off the board, properly insulated, coiled, and tied into a corner. Be sure to insulate those unused voltage taps. The slip boots will slip and if the quick connect contacts the chassis, the PT is toast! So use shrink tube over the quick connect, and then pull the slip boot down over that. Works great. And when twisting the pairs (Red-Red, Blu-Blu, Brn-Brn, Blk-Blk, etc.), keep them away from everything except for where they connect to the board as best as possible.

 

Gil...