I have a Webcor 161-1 amplifier my son wants to use as a guitar amp. It is a nice unit, and did work, but now has low output. It has dual 6v6 output and a 12ax7 for the phase splitter and a 6t6 or something as a preamp tube.
It sort of works, but if I connect a guitar I don't hear it coming through the amp. If I touch the input I can hear the hum coming through, so it seems to work, just have way too little gain.
The output tube voltages seem ok, but the phase splitter 12ax7 is supposed to have 105vdc on both plates, and I read only about 50vdc. The cathode is supposed to be 1.5 vdc and is only 0.7. The 2200 ohm cathode resistor measures about 2500 ohms, but that would cause the bias current to be too low and the plate voltage higher, not lower.
Would a leaky cathode cap cause the low voltage and also lower the gain of the signal?
I don't think I have encountered a bad cathode bypass cap before, so I'm not sure what the symptoms would be. But it may measure ok with an ohmeter and have a lower resistance when voltage is applied?
It sort of works, but if I connect a guitar I don't hear it coming through the amp. If I touch the input I can hear the hum coming through, so it seems to work, just have way too little gain.
The output tube voltages seem ok, but the phase splitter 12ax7 is supposed to have 105vdc on both plates, and I read only about 50vdc. The cathode is supposed to be 1.5 vdc and is only 0.7. The 2200 ohm cathode resistor measures about 2500 ohms, but that would cause the bias current to be too low and the plate voltage higher, not lower.
Would a leaky cathode cap cause the low voltage and also lower the gain of the signal?
I don't think I have encountered a bad cathode bypass cap before, so I'm not sure what the symptoms would be. But it may measure ok with an ohmeter and have a lower resistance when voltage is applied?
Here's a schematic:
I would start out by making sure the power supply is sufficient. With these older models, it is simple, since they contain the big silver filter cap cans. This means all your HV power supply nodes are on a few tabs, located right next to each other. Measuring these voltages should be done both before the heaters warm up and after. What the individual nodes measure will lead you in the right direction.
If your power supply voltages are good, you can follow each node to the resistor that provides plate current to its respective tube stage. Measuring the voltage drop across the plate resistor, you can quickly calculate plate current for each stage to find out what's going on.
If you are talking about C2 in the above schematic, yes it is possible for old electrolytics to read correctly and still not work. If you have a good cap checker with esr measurement, it should read low esr as well as close to its nominal value, but sometimes the good instruments lie too. You can lift a leg of C2 and fire up the unit to see what happens without fear of destroying something.
I would start out by making sure the power supply is sufficient. With these older models, it is simple, since they contain the big silver filter cap cans. This means all your HV power supply nodes are on a few tabs, located right next to each other. Measuring these voltages should be done both before the heaters warm up and after. What the individual nodes measure will lead you in the right direction.
If your power supply voltages are good, you can follow each node to the resistor that provides plate current to its respective tube stage. Measuring the voltage drop across the plate resistor, you can quickly calculate plate current for each stage to find out what's going on.
If you are talking about C2 in the above schematic, yes it is possible for old electrolytics to read correctly and still not work. If you have a good cap checker with esr measurement, it should read low esr as well as close to its nominal value, but sometimes the good instruments lie too. You can lift a leg of C2 and fire up the unit to see what happens without fear of destroying something.
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There is no voltage shown for C1-D, but the plate voltage on the first tube is about correct, so I assume the voltage at C1-D is also correct. The voltage at both plates of the 12ax7 are low, around 70v and should be 102V. So the 12ax7 is drawing too much current.
I will try replacing C2, which is actually a 25uF cap, and appears to be a haphazard replacement job so it was probably not the original cap.
I don't have a cap checker.
I will try replacing C2, which is actually a 25uF cap, and appears to be a haphazard replacement job so it was probably not the original cap.
I don't have a cap checker.
Fixed it!
I tried removing the cathode bypass cap, no change.
I repaired my oscilloscope so I could use that. That helped a lot. As I was tracing the signal through the amp I saw about 5V on the grid of the second tube? It should have been 0v.
I clipped the lead and the DC voltage went to zero. It was a large wax red cap, 0.022uF. I happened to have some ceramic caps of the same value and 1000V withstand so I replaced it. That fixed the DC offset and I could now hear some sound coming through, but it was really quiet still, but at least audible.
The bias voltage on the output tubes is supposed to be 15V and I measured 24V. The resistor seemed OK, and so was the cap? So I though, what is the voltage on the grid of the power tubes? The first one I checked it was zero as it should be. It was hard to figure out what was what, because the cap is different on the two output tubes. The first one was a small brown mica cap. The second tube was another big red wax job, and it had a voltage at the grid of it's power tube of 30V. I guess that really messed the bias up by having such a large DC voltage on the grid.
Replaced that cap with another tiny little ceramic and viola, sound! It sounds pretty distorted, but that's how my son likes it. It needs a boost pedal, but I expect it will get plenty loud with 2 6v6 tubes.
I will probably put a 3 wire grounded plug on it for safety, but it looks like it's done now.
Coolio!
I tried removing the cathode bypass cap, no change.
I repaired my oscilloscope so I could use that. That helped a lot. As I was tracing the signal through the amp I saw about 5V on the grid of the second tube? It should have been 0v.
I clipped the lead and the DC voltage went to zero. It was a large wax red cap, 0.022uF. I happened to have some ceramic caps of the same value and 1000V withstand so I replaced it. That fixed the DC offset and I could now hear some sound coming through, but it was really quiet still, but at least audible.
The bias voltage on the output tubes is supposed to be 15V and I measured 24V. The resistor seemed OK, and so was the cap? So I though, what is the voltage on the grid of the power tubes? The first one I checked it was zero as it should be. It was hard to figure out what was what, because the cap is different on the two output tubes. The first one was a small brown mica cap. The second tube was another big red wax job, and it had a voltage at the grid of it's power tube of 30V. I guess that really messed the bias up by having such a large DC voltage on the grid.
Replaced that cap with another tiny little ceramic and viola, sound! It sounds pretty distorted, but that's how my son likes it. It needs a boost pedal, but I expect it will get plenty loud with 2 6v6 tubes.
I will probably put a 3 wire grounded plug on it for safety, but it looks like it's done now.
Coolio!
Working but...
I got the amp working. I went ahead and replaced the metal can cap and the extra caps that somebody put in there. It sounds nice, but isn't very loud and has a lot of growl. I also replaced the coupling caps that were bad with metalized polypropylene instead of the ceramic caps I had handy when troubleshooting. Ceramic caps lose capacitance when you have voltage across them, which I only recently discovered. These are similar to what was there originally.
I was using a 16 ohm speaker because that's what was easy to hookup, but I doubt that is correct. I just don't have a 4 or 8 ohm speaker in a cab? Could that be the cause of the low volume?
Anybody have any idea what the speaker impedance should be on a Webster Chicago 166-1 ?
I got the amp working. I went ahead and replaced the metal can cap and the extra caps that somebody put in there. It sounds nice, but isn't very loud and has a lot of growl. I also replaced the coupling caps that were bad with metalized polypropylene instead of the ceramic caps I had handy when troubleshooting. Ceramic caps lose capacitance when you have voltage across them, which I only recently discovered. These are similar to what was there originally.
I was using a 16 ohm speaker because that's what was easy to hookup, but I doubt that is correct. I just don't have a 4 or 8 ohm speaker in a cab? Could that be the cause of the low volume?
Anybody have any idea what the speaker impedance should be on a Webster Chicago 166-1 ?
Stuff of that vintage, when speaker is in the same box, was almost always 4 Ohm. (Sometimes noted as DCR 3.2 Ohms.)
This is the cheapest winding. If wires were very long, 16 Ohms would reduce line loss; but when speaker is in box or same-room 4 Ohms was the way to go.
Get the kid a basic guitar speaker at 4 Ohms. It is again the most common impedance in tube guitar amps, so it will find other uses.
This is the cheapest winding. If wires were very long, 16 Ohms would reduce line loss; but when speaker is in box or same-room 4 Ohms was the way to go.
Get the kid a basic guitar speaker at 4 Ohms. It is again the most common impedance in tube guitar amps, so it will find other uses.
I tried an 8 ohm speaker in a combo that somebody dropped off for me to rework, and it was a lot louder than the 16 ohm speaker.
I will try a 4 ohm.
What will happen if it is supposed to be 8 ohms?
I will try a 4 ohm.
What will happen if it is supposed to be 8 ohms?
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