During gig last night amp started to sound metallic and tinny...

[bobbtoz]

Just want to get some feedback on what the problem could be. During the start of the second set I started hearing a very metallic tinny/ trebly sound. The amp did not cut out but I lost all dynamics and bottom end. Very trebly and somewhat buzzy.

I'm using a modified 100 watt JMP. I've owned this amp since mid summer. The previous owner told me the power tubes were new. I just fired up the amp and the 4 power tubes were glowing and looked fine. the preamp tubes on the other hand had various types of glow. The preamp tube in the tube buffered loop glowed similar to the power tubes but the other three preamp tubes were dim and one barley had a glow.

My guess is to replace the preamp tubes, but wanted to see what you guys thought.

Thanks
Bob T.

 

[shredtrash]

Without hearing it or seeing it, I'd say you were right about one or more preamp tubes being shot.

 

[vicdeluca71]

sounds like a power tube issue,loss of bottom end is usually a sign

 

[lespaul6]

when you turn your guitar volume off and listen to the backround (at volume) does it sound like thunder far away in the distance? If so, It might be a power tube. Google the "tube amp debugging page" and follow the order of suspicion based on your symptoms.

 

[bobbtoz]

I tried it agian this afternoon. I noticed the tinny buzzy sound but not as dramatic as last nights gig. I tried a preamp tube I had and the sound seemed to improve. I put the old preamp back in to confirm that this preamp tube was shot and it sounded fine. Now I'm at a loss. I would like to know its a preamp tube before I go and replace them thinking I've solved the problem to discover at my next gig I have the same issue.

It's also possible that the amp does it tinny metallic sound once its warmed up or hot. Last nights gig the amp was in tight corner w/ poor ventilation.

Any other thoughts?

 

[tonedrip]

For what it's worth I recall similar symptons when I have been plugged into bad power. Ater a couple of those occasions , I got myself a Furman voltage regulator and power conditioner unit....... I never play without it now.

 

[bobbtoz]

For what it's worth I recall similar symptons when I have been plugged into bad power. Ater a couple of those occasions , I got myself a Furman voltage regulator and power conditioner unit....... I never play without it now.

Very interesting point for me to consider, although it didn't seem to effect the other guitarists amp.

 

[gldtp99]

Judging a preamp tube's level of health by looking at "how much it glows" is pretty much useless--- unless it's completely dark, no heater glow at all--- different brands/models of preamp tubes emit different levels/shades of heater filiment glow in normal operation.
It sounds like the preamp tube socket had some corrosion and when you swapped tubes, the process of unplugging/plugging the different tubes cleaned some of it off--- allowing the orig tube to operate properly----- if this amp was mine, i'd clean all the tube sockets with D5 (retension as needed) and troubleshoot from there.
Also it's a good idea to keep a few known good preamp tubes handy for a gigging amp---- they're pretty easy to sub in/out at the show..................gldtp99

 

[cswanston]

For what it's worth I recall similar symptons when I have been plugged into bad power. Ater a couple of those occasions , I got myself a Furman voltage regulator and power conditioner unit....... I never play without it now.

I use to have the same problem in a dive I played. Pretty soon, I realized that my tone problems usually started at around 10:30. Turned out that someone in the kitchen would turn on or off something that affected the circuit used on the stage. Also, I play a lot of EL84 amps, and EL84s get hot and mushy and wierd. They rarely sound as good during the last set as they did during the first set. They're just a temperamental tube.

Always a good idea to consider the power source. You never know how these places have been wired--or by whom.

 

[buddaman71]

Low voltage sure has given me that exact same problem a few times. It's very common here in the south in the summer when the big AC units kick in and brown it out.

Similar problem in restaurant/clubs when the cooler compressors kick in.

 

[cap47]

Useless analogy to compare glow of preamp tubes. Sovtec 12ax7 LPS and some others barely glow. The construction on some shields the heater glow from sight. I would bet a power tube getting weak.