My guitar amp's drive channels are fuzzy for a while?

Question

I have been playing a Laney TF400 guitar amp for about 4 years. It has two channels, clean and dirty, but each channel also has switchable drive, making it as if it has 4 different channels. Recently, I'll turn it on, and while the clean channel will play fine, the three 'drive' channels will be hugely muffled, and give a loud ground noise hum when selected. There is no change in tone or volume to the clean channel at this point. After an undetermined amount of time, the drive channels 'clean up', so to speak, and the amp seems to play as normal.

This has only started happening recently. The amp has one vacuum tube in it, and I'm wondering whether it's just a case of letting it 'warm up' or whether it's something more sinister?

Any ideas? Gigging tonight, have the amp opened up and I'm poking around looking for obvious problems so, fingers crossed!

Answer 1

I actually found a schematic of your amp online, and was able to go through the signal path.



The tube is in line all the time (I was thinking that the clean channel might bypass it, but it doesn't). So, I don't think that's it. However, if you have another tube that's definitely an easy thing to check.



As far as warm up time goes, a 12AX7 (or ECC83) reaches it's operating temperature in less than one minute (power tubes take a little longer, but that's not the case here). As long as you're giving your amp at least a minute to come up, I don't think that's the issue.



The fact that it "fixes" itself after a while tends to suggest some type of intermittent connection. As everything heats up and expands, the connection gets better.



I don't have any illustrations of the inside of your amp, but based on the schematic there appears to be several cables interconnecting the PCBs. I can't tell if they connections are soldered directly to the board of if they are using some kind of connector.



Peavey does the same thing (using connectors), and I've had issues with those connections oxidizing over time.



If the connections between the PCBs in your Laney are on connectors, then you might try unplugging them and then plugging them back in 2 or 3 times each. If there's any oxidation on them, that might give the connections some fresh metal to bite in to. I've brought many a Peavey back to life like that.



If that does fix your problem, later on you will want to get some DeOxit contact cleaner and give all the connections a good cleaning.



That all said, the problem could also be a solder joint that's failing. Again, those type of problems usually show up with temperature change.



But, try the tube and reseating the interconnection cables between the PCBs if you can.



Good luck.



Greetings from Austin, TX



Ken

Answer 2

Sounds like your "pre amp" tube(s) is getting ready to fail on ya,OR the amp wasn't "warmed up" in stand by mode long enough.,I tend to think it's the stand by mode myself. Seeing how it cleans up after a "undertermined amount of time",which would be the same thing if you left it on "stand by" for a bit longer.

I'm gonna star this question,there's a contact of mine "Ken C" who is way more experienced with tube amps and diagnosing these types of problems.Hope I helped in some way..that freekin blows when the amp leaves you wondering.Especially before a gig tonight.Best of luck to ya...hopefully Ken C can help ya out more.

Answer 3

Change the tube and see if that does the trick. If not ya better take a spare amp with you tonight just in case.