Troubleshooting Carvin X100B Tube Amp

[Skizzly]

Hello I am trying to troubleshoot a tube amp I got from my friend. He says it was very distorted and strange when he would play it. When i opened it up I found that the PCB had gotten so hot that some resistors across the tubes had desoldered themselves and fallen out of place. This was an easy fix and I replaced them with new ones of the same value. I also replaced the power tubes because I assume the old ones were not happy after all of this abuse. I have a set of 8ohm speakers that I wired up in series and connected them with the OT set to the 16ohm winding. The sound that came out was HEAVILY distorted and had volume spikes associated with how hard i would strum. It sounds like a noise gate opening up, with some bit-crush level breakup and artifacts. 

 

After this I tried connecting it to two 8ohm wirewound resistors, also connected in series, so I could run a function generator into it and inspect the output with a scope. Sure enough it folded the {censored} out of the waves and it looks like a heavy distortion effect. Also i noticed the amp would create its own whine in the same freq as the input from the function generator. Is this normal for tube amps?

 

I am a certified electronics tech but i an not very familiar with tube amps, any help would be greatly appreciated!

 

Thanks,

Skizzly

[SteinbergerHack]

1 hour ago, Skizzly said:

Hello I am trying to troubleshoot a tube amp I got from my friend. He says it was very distorted and strange when he would play it. When i opened it up I found that the PCB had gotten so hot that some resistors across the tubes had desoldered themselves and fallen out of place. This was an easy fix and I replaced them with new ones of the same value. I also replaced the power tubes because I assume the old ones were not happy after all of this abuse. I have a set of 8ohm speakers that I wired up in series and connected them with the OT set to the 16ohm winding. The sound that came out was HEAVILY distorted and had volume spikes associated with how hard i would strum. It sounds like a noise gate opening up, with some bit-crush level breakup and artifacts. 

 

After this I tried connecting it to two 8ohm wirewound resistors, also connected in series, so I could run a function generator into it and inspect the output with a scope. Sure enough it folded the {censored} out of the waves and it looks like a heavy distortion effect. Also i noticed the amp would create its own whine in the same freq as the input from the function generator. Is this normal for tube amps?

 

I am a certified electronics tech but i an not very familiar with tube amps, any help would be greatly appreciated!

 

Thanks,

Skizzly

Have you scoped the signal before the output transformer?  What about the V+ rail - is it stable when playing?

[onelife]

Based on the symptoms you describe, the first place I would look would be the grid bias on the output tubes. The grid bias may have its own power supply or it may be achieved by means of resistors from cathode to ground. 

What make and model is the amp and what does it use for power tubes?

[Skizzly]

4 hours ago, SteinbergerHack said:

Have you scoped the signal before the output transformer?  What about the V+ rail - is it stable when playing?

I checked it out a bit more and found a ~4Vpp sawtooth wave on the power rail, which was also visible on the OT primary windings, so I am guessing one of the power filter caps is leaking. Its the original caps, plastic wrapped electrolytics, so they probably should be changed anyway. I ordered some replacements so ill check back in when I get those installed. You all think thats enough noise to cause this weirdness? No 60hz hum, just weird volume/distortion problems. I also have some spare 12AX7s laying around, this amp uses three in the preamp section which i have not changed yet.

[Skizzly]

2 hours ago, onelife said:

Based on the symptoms you describe, the first place I would look would be the grid bias on the output tubes. The grid bias may have its own power supply or it may be achieved by means of resistors from cathode to ground. 

What make and model is the amp and what does it use for power tubes?

Its a Carvin X100B amp and it uses EL34s for the power tubes and 12AX7s for the pre.

I will measure the bias tomorrow when I am back in the shop.

[onelife]

I looked at the schematic. The negative grid bias to the power tubes is supplied from a separate tap of the power transformer.

You can help isolate the problem by using the Pre Amp Out and Power Amp In jacks. Try plugging a guitar into the Power Amp In jack (you may need to boost the signal with an effects pedal that has some gain) and see if you get the same type of distortion.

You can also take the Pre Amp Out and run it into another amp (be careful, the signal may be quite high) or take a look at it with your oscilloscope.