It is the weirdest thing because I have an old Melos SHA that makes the exact same noise in the exact same way. I have never figured out what it is there, though I haven't tried very hard either. I always figured, based on age and the amount of corrosion on the circuit board, that it was a leaky something.
Anyhow, for grounding, just so I know what I am doing, each power suppy currently has what might pass as a star ground. The high voltage center tap and the negative side of all three power supply caps all connect to a point. Is it reasonable to now conect the bass of the cathode resistor/cap as well as the ground connection for the CCS to this same point? Do I also connect input and output grounds to this point? This will leave me with 2 stars. Would I then connect them together?
Also, I currently have the main's earth connected to the chassis and the circuit ground also connected to the chassis in the same spot. There don't sem to be any ground loops, so should I keep this basic setup?
Anyhow, for grounding, just so I know what I am doing, each power suppy currently has what might pass as a star ground. The high voltage center tap and the negative side of all three power supply caps all connect to a point. Is it reasonable to now conect the bass of the cathode resistor/cap as well as the ground connection for the CCS to this same point? Do I also connect input and output grounds to this point? This will leave me with 2 stars. Would I then connect them together?
Also, I currently have the main's earth connected to the chassis and the circuit ground also connected to the chassis in the same spot. There don't sem to be any ground loops, so should I keep this basic setup?
One 0V star for all the supplies. Incoming mains earth directly to the chassis. 0V star positioned as close as possible to the input stage with really short wires from the grid and cathode. Bond 0V star to chassis with really short wire (certainly <2").
Earth loops: Only one bond from 0V to chassis. make the bond at the input of the most sensitive audio amplifier (usually RIAA stage).
Earth loops: Only one bond from 0V to chassis. make the bond at the input of the most sensitive audio amplifier (usually RIAA stage).
This is kind of how it is already. As I said, each PS has a star. Each of these is connected to the terminal strip next to the tube. Each cathode is also connected to this point as are the input and output grounds.
There are some wires going through the air to get there, but other than tidying things, I am not sure what to change?
There are some wires going through the air to get there, but other than tidying things, I am not sure what to change?
I have had several low power guitar amps with the kind of noise you are mentioning. On every one of them the cure was to bias the heater to some positive voltage, say +20 to +40volts. Dead quiet after that.
Try connecting the heater transformer center tap to one of the cathode resistors and see if it helps. For some reason I always use a series resistor when connecting the heater to anything but ground, but really don't think that's necessary.
Try connecting the heater transformer center tap to one of the cathode resistors and see if it helps. For some reason I always use a series resistor when connecting the heater to anything but ground, but really don't think that's necessary.
SY said:
I think that's right. I replaced the original ccs with a different one to see if this might be the problem.
Anyhow, I added a 100uF cap in parallel with the CCS (I think there was an earlier suggestion to do something similar to this) and while the hissing is gone, there is still a little bit of hum, and more importantrly, I seem to have made a low pass filter around a couple hundred Hz. So, I now have a muffled and quiet and bass only pre. Is there a better way to get rid of oscillations, or a different size cap to use?
I tried the heater biasing as well but it didn't do anything.
I've not. I am not sure what it is exactly, other than another CCS, or why it might be beneficial. Is the game here to keep trying different CCSes until one of them doesn't oscillate? I'd rather not do that. I really only tried the LM317 to see if the noise was in the tube, or due to oscillation. Now that I know it to bein the CCS, my strong preference would be to go back to the bottlehead CCS and get it under control somehow. What I really don't know, however, is what to try next or why it oscillates in the first place, so my powers of figuring out a solution are pretty limited.
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