Good afternoon everyone, I have a strange problem with my homemade tube amp it consists of a 12ax7 input long tail pair followed by a 6sn7 driver stage into x4 807 push pull output stage. The problem is that when I input a signal the amp it begins to clip in and out at a certain volume. But what is strange is that when when I flip the off switch and there is still charge in the main caps, I can increase the volume on the input and get a much louder output without clipping (sound coming in and out of existence).
I replicated the situation in a way by putting a light bulb in series to the line voltage coming in and it drops it down to roughly half. With the amp basically running at half line voltage I can get a much louder output than I would if it were running at full line voltage before it does the audio cutting in and out thing. Needless to say, I genuinely have no idea what is causing this I know it is not anything in the preamp because I tested it and no cutting in and out. Could this be arcing inside transformer or something with the power supply? I am for the most part new to amp design but I have been into electronics for a while so this may be an issue derived from shear incompetence. Any input is appreciated.
I replicated the situation in a way by putting a light bulb in series to the line voltage coming in and it drops it down to roughly half. With the amp basically running at half line voltage I can get a much louder output than I would if it were running at full line voltage before it does the audio cutting in and out thing. Needless to say, I genuinely have no idea what is causing this I know it is not anything in the preamp because I tested it and no cutting in and out. Could this be arcing inside transformer or something with the power supply? I am for the most part new to amp design but I have been into electronics for a while so this may be an issue derived from shear incompetence. Any input is appreciated.
This sounds very much like too much negative bias voltage on the 807's control grid causing them to run too near cutoff. As the signal voltage increases, the tubes are driven further into cutoff by the negative peaks and that's what you are hearing. If you have an electronic voltmeter, measure the G1 voltage (no signal). Depending on how much plate and screen voltage you are using you should have about minus 37 (or less) volts.
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“in and out of existence” is a very curious description, not what I would expect as a description of regular clipping distortion. Schematic definitely helpful. Wonder if some kind of blocking distortion.
Well this is embarrassing to admit but I just found that cathode bypass cap on the 807 stage had its negative lead in the wrong place by virtue of being grounded to input. The problem has since subsided but I did use my friends ocilliscope to see that there was some distortion at the out but so that is my next big task, thanks anyway for your help.
CosmoKramer3,
So, to help us help you solve your new problem (some distortion at the out), would you post a complete schematic,
with operating voltages and currents, etc.
And describe the nature of the distortion, or a picture showing the shape of the waveform, and how many volts into the (8 Ohm?) load resistor.
Are the 4 plate currents of the 807 tubes identical?
So, to help us help you solve your new problem (some distortion at the out), would you post a complete schematic,
with operating voltages and currents, etc.
And describe the nature of the distortion, or a picture showing the shape of the waveform, and how many volts into the (8 Ohm?) load resistor.
Are the 4 plate currents of the 807 tubes identical?