Quote from: Kaz Kylheku on July 05, 2013, 03:04:50 PMI made that audio probe!Quote from: belleraphon88 on July 05, 2013, 05:09:04 AM
So here is what i did.
Did the tests with the send into my pc speakers, used 3 different tubes. It didnt change anything.
Next i took the tube out and did the same test, still sounded the same.
So cant be the power amp then or tube
Yes; the tube stage is after the preamp and effect send, so it makes no difference.
This is lucky for you because you don't have to mess around in the power amp where you have higher voltages (especially given the tube).
The thing to do now is to make yourself some kind of adapter so that you can use those same PC speakers as an audio probe. A simple setup would be to have a cable which has an alligator clip for ground, and a multimeter probe for signal. Clip the ground somewhere to a ground point in the amp, and then use the probe to pick up the audio from various sections of the preamp based on following the schematic. You can find out the first stage where it is happening. It would really help you if you reproduce the issue with some signal source other than your guitar, like using your PC's audio line out to play some continuous tones into the amp's input.
You said earlier that the issue occurs whether or not you use the gain channel; that you can hear it on a distorted tone that is produced by a pedal going into the clean channel. Assuming this is really the case, the problem is really narrowed down. There is very little circuitry between the input and the effect send that is common to both channels. What is common to both channels is the IC1A op-amp input stage, and and the channel switching and muting FETs Q4, Q5 and Q6. (Well, Q4 and Q5 are specific to their respective channels, but the circuits could interact in some way since channel switching is supposed to be mutually exclusive). I can't find the logic that drives the channel switching in that schematic, by the way.
If the voltage at the base of Q6 oscillates for some reason, then you will get volume fluctuations or periodic cutouts. It's not obvious why that would be the case, since the Q6 gate is driven by a voltage divider that is straight off the power supply. Still, if you had an oscilloscope you could easily put a probe on the bases of these transistors to see what the voltage is doing. If there is such a thing happening, it is probably a little too fast to register on a multimeter, but a scope trace will react instantly.
There is the possibility that it's an issue with the preamp power supply: that basically the entire preamp cuts out due to a periodic voltage brown-out. Your tech would almost certainly have picked up on such a glaring thing!
just took a while, had to drive pretty far just to buy that capacitor for the probe.
I will do the testing tomorrow.
Ive noticed 2day that if i scoop the mids the tone is pretty nuch normal when i play riffs and palmute, as soon as i turn the mids up i do get blocking distortion.
Just thought id bring this up.