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Before this past week, I'd never seen this model amp before. But a customer brought one it, and the repair slip claimed that it was breaking up very "early" with not much head room. I wasn't there when it arrived, so I didn't witness the demonstration or intake. My boss suggested starting right at the output, this is a common complaint, and often is the result of one side of a push-pull output not conducting.
So, I put load on the amp and monitored the waveform at the output. Initially, it looked like the problem was exactly as was we might expect. The scope was showing what looked like the top half of the cycle almost completely collapsed. But not as sharp as you would expect from one of the output tubes not conducting at all. I figured that the tube had just grown very week. In order to confirm this, I installed a pair of my bench tubes I use for testing. Powered the amp back up, and there was no change at all in the operation. Looked exactly as the pair the amp came in with. I'm like, "hmmm, that's weird". So I start reading idle voltages to look for something out of the ordinary, but as far as the output stage was concerned, everything looked normal. Okay... maybe one of the plate resistors in the PI.. Nope, it's a cathodyne, and both the plate and cathode resistors measured nearly identical.
So I'm thinking, "Where the hell is this thing dropping half the waveform like that?" At that point I wanted track down a schematic and see what was going one, and I suspect you might want to see as well. So here's the schematic:
At first the only schematic we could find was one someone had traced out, so it looked like they had made a mistake in the oscillator circuit. None the less, when were moving around the oscillator tube in the socket, we saw the waveform jump to life for a brief moment with the top half appearing, and with significantly more gain. When we pulled the tube out of the amp completely, we could see both the gain and shape we were expecting to see on the scope. After tracking down the official Fender schematic of the amp, it confirmed what was drawn in the other one: The LFO was being directly coupled into the cathode of the stage feeding the Cathodyne phase inverter. But the way it was done, didn't make any sense to me. (First of all, 2 consecutive DC coupled stages? Settle down, Fender) But if you look at how the stage for V2-A is designed, you can see that they use a 1.5k cathode bias with a 25µF bypass cap. This is bread and butter center biased gain stage. At least, that's clearly what they were expecting. The schematic confirms this. If you look at the expected voltage at TP10, they confidently list +1.25V. But that ain't what's happening. The way the LFO is designed, during "normal" operation, the cathode of V2-A is directly connected to the cathode load resistor of V6-B (the driver for the LFO modulator.) This alters the bias of V2-A because the cathode is now placed at the junction of R13 (1.5k) and the combined series resistances of R35 (3.3k) and R31 (68k), creating a voltage divider.
Instead of the schematic listed 1.25V, the cathode now idles at just over +3V. This is a significant difference for a 12AX7, and pushed the stage almost to the point of cutoff.
Look at this load line graph, and note where the -3V grid location.
Not only is the why half the waveform is lost, it results in a substantial loss of gain. The good news is, this can be corrected fairly easily. I've attached a schematic below, some changes which I think improve the overall sound of the amp, and still provide a deep, rich tremolo effect.
So, I put load on the amp and monitored the waveform at the output. Initially, it looked like the problem was exactly as was we might expect. The scope was showing what looked like the top half of the cycle almost completely collapsed. But not as sharp as you would expect from one of the output tubes not conducting at all. I figured that the tube had just grown very week. In order to confirm this, I installed a pair of my bench tubes I use for testing. Powered the amp back up, and there was no change at all in the operation. Looked exactly as the pair the amp came in with. I'm like, "hmmm, that's weird". So I start reading idle voltages to look for something out of the ordinary, but as far as the output stage was concerned, everything looked normal. Okay... maybe one of the plate resistors in the PI.. Nope, it's a cathodyne, and both the plate and cathode resistors measured nearly identical.
So I'm thinking, "Where the hell is this thing dropping half the waveform like that?" At that point I wanted track down a schematic and see what was going one, and I suspect you might want to see as well. So here's the schematic:
At first the only schematic we could find was one someone had traced out, so it looked like they had made a mistake in the oscillator circuit. None the less, when were moving around the oscillator tube in the socket, we saw the waveform jump to life for a brief moment with the top half appearing, and with significantly more gain. When we pulled the tube out of the amp completely, we could see both the gain and shape we were expecting to see on the scope. After tracking down the official Fender schematic of the amp, it confirmed what was drawn in the other one: The LFO was being directly coupled into the cathode of the stage feeding the Cathodyne phase inverter. But the way it was done, didn't make any sense to me. (First of all, 2 consecutive DC coupled stages? Settle down, Fender) But if you look at how the stage for V2-A is designed, you can see that they use a 1.5k cathode bias with a 25µF bypass cap. This is bread and butter center biased gain stage. At least, that's clearly what they were expecting. The schematic confirms this. If you look at the expected voltage at TP10, they confidently list +1.25V. But that ain't what's happening. The way the LFO is designed, during "normal" operation, the cathode of V2-A is directly connected to the cathode load resistor of V6-B (the driver for the LFO modulator.) This alters the bias of V2-A because the cathode is now placed at the junction of R13 (1.5k) and the combined series resistances of R35 (3.3k) and R31 (68k), creating a voltage divider.
Instead of the schematic listed 1.25V, the cathode now idles at just over +3V. This is a significant difference for a 12AX7, and pushed the stage almost to the point of cutoff.
Look at this load line graph, and note where the -3V grid location.
Not only is the why half the waveform is lost, it results in a substantial loss of gain. The good news is, this can be corrected fairly easily. I've attached a schematic below, some changes which I think improve the overall sound of the amp, and still provide a deep, rich tremolo effect.
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