Tube Amps / Music Electronics
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|8/5/1999 4:07 AM|
||Another weird noise!|
This must be my week for oddball amp problems! Here's the problem. It's kind of hard to describe in words so here it goes. The amp is generating some subtle distortion overtones that sound sort of like a fuzz box. This happens only momentarily when a note is struck and then fades off. This effect is most pronounced on the low E string and diminishes as you strike the higher strings.
This diminishing effect leads me to believe it has something to do with a coupling cap but I'm not sure. I've never seen this before.
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|8/5/1999 6:53 AM|
Your description of the noise sounds a lot like a problem I was having when I was working up the mods for my Pignose G40V amp (I thought it sounded like a cheap 60's fuzzbox, but there was an "envelope" around the distortion- meaning the distortion would drop off rather abruptly as the note faded away and would sound normal again.)
Turns out that it was some sort of parasitic effect when I moved a switch that boosted the treble cap with a 750pF mica cap over too close to the V2 tube socket. I moved the switch around and the noise disappeared.
While probably not relevant to the amp you are working on, I thought I'd mention it anyway...
|8/5/1999 1:37 PM|
There are two ways I know of that this can happen.
1) one or more stages are biased near but not quite at either limiting or cutoff, so strong, especially bass notes push them into distortion on peaks, and this fades out as the note decays. This is the flip side of a stage being biased all the way into limiting or cutoff and only strong notes pushing it out so the signal appears abruptly and highly distorted when strong notes are played.
2) a signal level dependent parasitic distortion, one that only appears on certain notes.
In this case, I'd bet on 1) until proved otherwise by putting a scope on it. Check the DC levels on the tubes and see if you can find a stage that is atypically biased.
|8/5/1999 7:37 PM|
R.G, the weird thing is that the amp is doing it whether it's set for clean or distorted operation. You can really hear it in the clean channel due to the fact that it's so clean. You get a nice full note then BZZzzz over the top of it that fades away quickly.
Here's the DC bias values for the preamp stages:
B+ = 339v
V1a: Vp = 220v
Ip = 0.99mA
Vk = 1.46v
Ik = 0.97mA
V1b: Vp = 207v
Ip = 1.32mA
Vk = 1.29v
Ik = 1.29mA
V2a: Vp = 215v
Ip = 1.03mA
Vk = 1.53v
Ik = 1.02mA
Are you seeing anything here that I'm missing? The values seem to look pretty good.
|8/5/1999 4:58 PM|
I just had this happen to my Pro Reverb. Turned out to be a broken reverb cable. The vibration would cause intermittant contact which was more severe with the lower notes than the higher ones.
I've also had something similar to this happen (on this same amp actually) and it turned out to be a cold solder joint. Looked like it had been that way from the factory. Again it only happened at higher volumes.
|8/5/1999 9:03 PM|
I had a similar sympton in a tweed deluxe - turned out to be the line cord vibrating around in a cheapo input jack. It occurred playing clean or distorted, but only on lower notes and power chords.
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