#1
I'm getting a little fed up with my amp here guys. I turned it back on yesterday after a few months and after warm-up it started to make some sort of wierd high pitched oscillating noise. What I mean by that is that it wasn't a constant tone that altered with the volume, it was like a pulsing effect.

Any ideas what it could be?
Guitar:
Dean Vendetta 3 - Dave Mustaine Livewires

Amplifier
Carvin X100B - Bias Mod - Tungsol 12AX7's - JJ KT77's

I have built the most badass 212 that puts all others to shame
#3
Quote by MeanwiththeDean
I'm getting a little fed up with my amp here guys. I turned it back on yesterday after a few months and after warm-up it started to make some sort of wierd high pitched oscillating noise. What I mean by that is that it wasn't a constant tone that altered with the volume, it was like a pulsing effect.

Any ideas what it could be?
Most likely candidate to fit that description would be one of the filter capacitors. Most tube amps use a resistor to drop the voltage going to the plate supply of the phase inverter and all preamp tubes. If the capacitor at this node is not functioning, it allows the voltage at that node to respond quickly to any changes in current. This affects the voltage at the plate resistors of all the preamp tubes. Oscillation is likely. This is called "motorboating" as it usually has a strong low frequency or sub-audio component.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#4
The original post was done a bit hastily so i neglected to mention a few things.

ALL the tubes are brand new and it has been properly biased by myself.

Thank you SYK, I'll have to check it out. The only wierd thing about that though off-hand is that right before I moved, I replaced all of the filter capacitors. They were 20+ years old so I just did it. However, I will check for loose connections. I don't have a capacitance meter, but is there any way to test the filter caps with a DMM to see if they are working properly? I tested the amp after I replaced these caps and they were fine, and i know I installed them with the proper polarity.

Also, if it was a filter cap, wouldn't the oscillation be constant regardless of whether I was playing a note? I forgot to mention it in the original post but this only happened when I played a note. The noise was a constant pitch and volume regardless of how high or low the note played was, if that helps any.

Thank you for your advice so far.
Guitar:
Dean Vendetta 3 - Dave Mustaine Livewires

Amplifier
Carvin X100B - Bias Mod - Tungsol 12AX7's - JJ KT77's

I have built the most badass 212 that puts all others to shame
#5
Quote by MeanwiththeDean
The only wierd thing about that though off-hand is that right before I moved, I replaced all of the filter capacitors. They were 20+ years old so I just did it. However, I will check for loose connections.
Definitely do that. A poorly soldered (or even unsoldered, yeah, sometimes we miss things) joint could have the amp working properly at first, then exhibit problems later.

Quote by MeanwiththeDean
I don't have a capacitance meter, but is there any way to test the filter caps with a DMM to see if they are working properly? I tested the amp after I replaced these caps and they were fine, and i know I installed them with the proper polarity.
Kinda tough with a big electrolytic. You could put an oscilloscope on that point of the circuit and power up. Measure the ripple. Then remove the cap and measure the ripple without the cap in place. No change = cap is open. Or just parallel a cap of the same size. If the ripple decreases by a factor of 2, the cap is fine. If the ripple decrease by a ton, the cap is open.

Quote by MeanwiththeDean
Also, if it was a filter cap, wouldn't the oscillation be constant regardless of whether I was playing a note? I forgot to mention it in the original post but this only happened when I played a note. The noise was a constant pitch and volume regardless of how high or low the note played was, if that helps any.
It's really difficult to predict how a particular amp will act if a de-coupling cap opens. How many stages of gain, which stages are most affected, whether the net result is an increase in gain or decrease, etc. It's possible to have just enough positive feedback, that the amp won't break into oscillation on it's own, but have some very peculiar performance when a signal is applied.

Don't get the idea that the problem must be a de-coupling cap. This is just a likely candidate, ime.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#6
Alright, I have checked the previously mentioned possibilities and none of them are the problem.

I bought a filter cap and swapped it in every possible position with no change. So it's not a filter cap.

All the solder joints are perfect and I even re flowed them just in case.

I did a lot of research and found some information on a website originally intended for older tube radios. But the schematics where very similar around the tube. I don't have the website now though, it's bookmarked at work, will post tomorrow.
Guitar:
Dean Vendetta 3 - Dave Mustaine Livewires

Amplifier
Carvin X100B - Bias Mod - Tungsol 12AX7's - JJ KT77's

I have built the most badass 212 that puts all others to shame