Gibson GA42 subtle overdrive problem

[EETStudent]

This one is hard to explain so bear with me. I just received a brand new Gibson GA42RVT, and it sounds excellent overall. However, channel 2 is given me a little problem. If I play a chord or pick consecutive notes I get this subtle fuzzy sound along with the clean signal. This channel is supposed to be the clean, little breakup channel so it must not be natural. Its not unbearable, but its bothering me. The amp takes all 12AX7s(two channels, PI, tremolo and reverb). You know the sound you get when a coupling cap is bad? Sort of farty distortion. It sounds like a mixture between that and how it should sound. The tubes are cheap so I suspect that might be a problem. The speakers are also wired in series for 16 ohms, and the 16ohm tap yielded better results compared to the 8 ohm tap. Any idea what this low, subtle growl might be?

[Robert M. Martinelli]

This one is hard to explain so bear with me. I just received a brand new Gibson GA42RVT, and it sounds excellent overall. However, channel 2 is given me a little problem. If I play a chord or pick consecutive notes I get this subtle fuzzy sound along with the clean signal. This channel is supposed to be the clean, little breakup channel so it must not be natural. Its not unbearable, but its bothering me. The amp takes all 12AX7s(two channels, PI, tremolo and reverb). You know the sound you get when a coupling cap is bad? Sort of farty distortion. It sounds like a mixture between that and how it should sound. The tubes are cheap so I suspect that might be a problem. The speakers are also wired in series for 16 ohms, and the 16ohm tap yielded better results compared to the 8 ohm tap. Any idea what this low, subtle growl might be?

Hi,
To me it sounds like you have little, but noticeable, crossover distortion.
Even if the ads say it' s a class "A" amp, ( blah,blah,blah...blurb! ) I think it to actually be just another "AB1" class amp, so, IMHO you could have crossover distortion, probably due to a fairly cold output tube bias, sometimes manufacturers bias amps on the low side to improve reliability, and, of course, this can be mostly noticeable using the clean channel....how ' bout injecting a sine wave at the input and take a look with an o-scope to how the waveform looks like at the output? Bet you' re going to find something wrong around the zero-crossing points....

I expected the problem to get worse using the 8-Ohm tap with 16-Ohm speakers, because you are adding an impedance mismatch to the problem you already have, and that can only make things worse.

Hope this helps

Best regards

Bob

[Steve Conner]

Does it happen at all volumes? Do the tone controls affect it? Does the guitar volume affect it? Do you have a guitar with hot pickups?

I've found that even tube amps can sound kind of fuzzy when the power amp is running clean at low volume and the preamp tubes are providing the dirt. You get a kind of cheesy, buzzy decay on certain notes sometimes. If you were playing quietly at home with hot pickups, this is probably what's happening.

[Rick Erickson]

Without a schematic or the amp on my bench it is difficult to be specific about this kind of problem. There could be an oscillation problem in CH2, or it could be a design flaw - like a preamp stage being overloaded, or your guitar may be pushing the limits of the input circuit, especially if they used a grid-contact bias. arrangement.

RE