• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

Amp driving me nuts

BlacknWhite

BlacknWhite

Jun 22, 2012
1,027
2,446
46
Beersville, New Brunswick, Canada
equipboard.com
#1
Hey guys I'm working on a Bugera 6262 and it is driving me crazy. The issue is that intermittently the volume drops right out and then comes right back and it works fine again. Sometimes its within 15 mins of powering it up and other times I can play it for an hour or more with no issues. When I first got it I cleaned all the input and output Jack's. I biased the tubes as they were new. All pre amp tubes were replaced as well. I plugged into the effects return and the issue was happening there so narrowed it down to the poweramp section. Cleaned all the power tube sockets. Pulled the board and checked all the solder connections and touched up a couple. I have a meter on the plate voltage, bias voltage and plate current and when the volume drops out I see no change. I've poked around on the power section board at everything including the lead to the output transformer primary and the secondary out to the output jacks. I'm going to pull the board again tonight and double check a few things. Any other suggestions or is there something I'm missing? Thanks in advance for any input.

@agedhorse @beans-on-toast @BassmanPaul
 

Attachments

#3
If you have confirmed that the symptoms are occurring in the preamp section before the effects send jack, then looking elsewhere (output transformer) is a waste of time. Staring at it and inspecting it over and over is also a waste of time unless you can see inside the components.

Now it the time to use a scope and look at the signal stage by stage before the effects send jack to determine where the signal fails. Otherwise you are just guessing.
 
  • Like
Reactions: ranbush, bbh and byacey
#4
agedhorse said:
If you have confirmed that the symptoms are occurring in the preamp section before the effects send jack, then looking elsewhere (output transformer) is a waste of time. Staring at it and inspecting it over and over is also a waste of time unless you can see inside the components.

Now it the time to use a scope and look at the signal stage by stage before the effects send jack to determine where the signal fails. Otherwise you are just guessing.
Click to expand...

No I plugged into the effects return and the issue was still there. So I was looking at the power amp section.
 
#5
BlacknWhite said:
No I plugged into the effects return and the issue was still there. So I was looking at the power amp section.
Click to expand...
Ok, I mis-read your statement on my phone screen.

Now, you would want to look at the signal through the power amp. Stage by stage, and sometimes you need to remove tubes in order to do this without the global feedback causing opposing signals.
 
  • Like
Reactions: ranbush and bbh
#6
The tube socket tension should be checked. Each tube should not slip in and out too easily. There should be firm contact. Any jumpers need to be checked and reseated.

Separate the preamp and the power amp. Test the preamp out and look for the intermittent issue. Then repeat for the power amp. Hopefully, this will isolate the source to one or the other. If it occurs in both, look to the power supply.

As mentioned, time for test equipment. Isolate the sub circuits, inject a source signal and trace through with a scope.
 
  • Like
Reactions: ranbush
#14
From the description of the problem, it could be an intermittent coupling cap in the power amp section, or perhaps a problem with the bias supply. Usually loose tube pins cause loud pops and crackling unless it's one of the heater pins.

Edit: I overlooked you stating that you were monitoring the bias voltage.
 
#16
beans-on-toast said:
You have to sweat all the details, one step at a time. Doing the simple things first. Switching out the cab and cable is one of those simple things.
Click to expand...

+1

Never skip past the simplest things. They're often worth checking, and then double-checking.

I know a guy who was working on a classic car with his dad. It wouldn't start, and they'd determined that it was due to not getting fuel. He troubleshot the fuel delivery system all the way back to the point where he determined the only thing left was to drop the tank to replace the fuel pump. When he did, he found out it was empty. When it first wouldn't start, he had asked his dad "there's gas in it, right?", to which his dad laughed at him and told him "of course there is. I'm not stupid!". By taking his dad's word for it and skipping the simplest thing, he wasted a ton of time.
 
  • Like
Reactions: BooDoggie, ranbush, wave rider and 2 others
#18
It could be the impedance switch that's a bit wonky. It could also be the relay that switches the effects loop into the circuit.

A classic test method for finding intermittent issues is tapping various components lightly with the handle end of a screw driver, to see if that can cause the problem to manifest itself (or disappear - that's essentially what the age old "whack the telly" move does).

Also, freeze spray and/or heat gun is sometimes useful for the same purpose, if there's a thermal issue in a component, or a solder joint that's cratered up around a component leg.

An oscilloscope or signal tracer is your friend in cases like this.
 
#20
This is kinda weird, but I've had a similar sounding issue on a Bugera 333XL. In my case, after some minutes of being powered up, it cuts out and comes back on. In my case, the amp is actually resetting completely (it always returns to default settings, such as returning to the clean channel if you're on the lead channel). I never fully diagnosed the problem (I don't use the amp much these days) but I'm looking to fix it if I can.

It kinda seems to me like a thermal interrupt, like somewhere in the amp is a circuit that turns the amp off if the temperature of some component goes above a set value. I would recommend looking into that and seeing if that is the case. Hope this info can be of assistance.
 

Latest posts

Top Bottom
Back