After about 20 seconds of switching the amp on it makes a light crackling scuffling type noise... it doesn't get any louder as turn up the volume, and as I keep it turned down low it's more audible. I've noticed it for a little while, it could have always been there I've just overlooked it. It stops after about 10-15 seconds from memory.
I removed the tube cage earlier and gave everything a dust and jiggle... as well as the knobs on top, as it sound like dirty pot type noise... and this particular noise made me think of dust, still it's not changed anything despite having a clean looking amp. Just a cosmetic clean really. I don't think it's a interference thing either, as I've tried different plug / setup etc.
Anything to be concerned about... ?
Is this an issue ? Classic 30 diagnosis...
Moderated By: mods
So, to clarify:
- ~20 seconds after the amp is turned on it makes a dusty, crackly noise that does not increase with the volume knob
- after 10-15 seconds of noise it stops and does not return
Correct?
The fact that it does not increase with volume suggests it originates after the preamp stage, before the phase inverter section. 90% of the time, any problem in a tube amp is caused by the tubes. so you would be looking at 1 12AX7 and 4 EL84s that could be the culprits. If you have some spare tubes, swap them in and see if the noise changes or goes away. It may just be an older tube acting weird as it warms up.
When the noise occurs suggests it's related to heat in some way. A possibility is a tube socket that is a little too loose when cold, but gains a grip as it heats up. In any case, if it goes away it is probably not a major issue.
- ~20 seconds after the amp is turned on it makes a dusty, crackly noise that does not increase with the volume knob
- after 10-15 seconds of noise it stops and does not return
Correct?
The fact that it does not increase with volume suggests it originates after the preamp stage, before the phase inverter section. 90% of the time, any problem in a tube amp is caused by the tubes. so you would be looking at 1 12AX7 and 4 EL84s that could be the culprits. If you have some spare tubes, swap them in and see if the noise changes or goes away. It may just be an older tube acting weird as it warms up.
When the noise occurs suggests it's related to heat in some way. A possibility is a tube socket that is a little too loose when cold, but gains a grip as it heats up. In any case, if it goes away it is probably not a major issue.
Thanks for the info, you we're correct in your summary. O.k. I will have another look at the tubes....Will wrote:So, to clarify:
- ~20 seconds after the amp is turned on it makes a dusty, crackly noise that does not increase with the volume knob
- after 10-15 seconds of noise it stops and does not return
Correct?
The fact that it does not increase with volume suggests it originates after the preamp stage, before the phase inverter section. 90% of the time, any problem in a tube amp is caused by the tubes. so you would be looking at 1 12AX7 and 4 EL84s that could be the culprits. If you have some spare tubes, swap them in and see if the noise changes or goes away. It may just be an older tube acting weird as it warms up.
When the noise occurs suggests it's related to heat in some way. A possibility is a tube socket that is a little too loose when cold, but gains a grip as it heats up. In any case, if it goes away it is probably not a major issue.
Thought of this after I hit submit:
The Classic 30 is Fixed Biased. Unlike a cathode biased amp, in which the bias parameters of the tube automatically adjust to respond to changes in conductance, a fixed biased amp is running the tubes within the same parameters regardless of tube performance. As the tube heats up its conductance is gradually increasing from 0 to whatever its appropriate value is, but in the case of fixed bias there is no compensation as this occurs. Basically, while warming up there is a period when the tube is drastically mis-biased. It's possible that your's is amplifying some of the resulting noise before it reaches the designed operating temperature.
In any case, I don't think what you're hearing is a big deal.
The Classic 30 is Fixed Biased. Unlike a cathode biased amp, in which the bias parameters of the tube automatically adjust to respond to changes in conductance, a fixed biased amp is running the tubes within the same parameters regardless of tube performance. As the tube heats up its conductance is gradually increasing from 0 to whatever its appropriate value is, but in the case of fixed bias there is no compensation as this occurs. Basically, while warming up there is a period when the tube is drastically mis-biased. It's possible that your's is amplifying some of the resulting noise before it reaches the designed operating temperature.
In any case, I don't think what you're hearing is a big deal.