22-02-2017, 05:52 AM
(This post was last modified: 22-02-2017, 05:56 AM by Diabolical Artificer.)
It's running in fixed bias Trevor, there's -35v next to the 807 grid resistor. 807's are set to 50mA Ia though on my load line they should be at -40/45v. I have a pot I use to set bias, -35v at the moment is about right, but I'll probably set it lower. The 12BH7 runs at -10v bias, 15mA Ia. To get it to run higher I'll need to get some bigger cathode R's as the 10k 2w R's in there at present are getting hot.
The amp has developed a problem, which is a sod, all day Sunday it ran well. Switched on Monday to do the frequency test and I heard a clicking or low frequency buzzing. Checked DC voltages and found one side or one triode of the 12BH7 was low. I suspected a diode were switching. They're there to keep the grid and cathode within about a volt at switch on. Disconnected the diodes, found one triode of the 12BH7 was low gain, bummer, they were NOS valves from the US. So new valve in DC conditions are back to spec, IE biased at -10v center biased, but now getting distorted OP.
When the driver's are disconnected from the OP stage all is well, connected the OP waveform distorts. Sorry camera died so no pic but the sine goes back up prematurly on the negative going part of the cycle. The sine looks like it has a small bite taken out of it, or picture two sines on your scope, both the same amplitude and frequency. Now move the bottom trace up till the peak of the bottom sine is superimposed on the negative going part of the sine above by about 1/5 th, maybe a 1/4 at high amplitude. That's the sine I'm getting, though maybe move the bottom trace 10 degrees or so forward. I thought this might be 2nd harmonic distortion and am not sure whats causing it. I wonder if the 807's are drawing grid current, maybe going into AB2? Or is there some feedback?
Will do some testing today to see if I can find out.
Andy.
The amp has developed a problem, which is a sod, all day Sunday it ran well. Switched on Monday to do the frequency test and I heard a clicking or low frequency buzzing. Checked DC voltages and found one side or one triode of the 12BH7 was low. I suspected a diode were switching. They're there to keep the grid and cathode within about a volt at switch on. Disconnected the diodes, found one triode of the 12BH7 was low gain, bummer, they were NOS valves from the US. So new valve in DC conditions are back to spec, IE biased at -10v center biased, but now getting distorted OP.
When the driver's are disconnected from the OP stage all is well, connected the OP waveform distorts. Sorry camera died so no pic but the sine goes back up prematurly on the negative going part of the cycle. The sine looks like it has a small bite taken out of it, or picture two sines on your scope, both the same amplitude and frequency. Now move the bottom trace up till the peak of the bottom sine is superimposed on the negative going part of the sine above by about 1/5 th, maybe a 1/4 at high amplitude. That's the sine I'm getting, though maybe move the bottom trace 10 degrees or so forward. I thought this might be 2nd harmonic distortion and am not sure whats causing it. I wonder if the 807's are drawing grid current, maybe going into AB2? Or is there some feedback?
Will do some testing today to see if I can find out.
Andy.
