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Hi everyone. I have a question that I haven't seen addressed already, so hopefully this isn't something that's already been addressed.
I have a 67 Pro Reverb that I bought from the second owner in 2001. I used it regularly with no trouble until September 2005. One night I was letting a friend play through it at band practice, and I guess there wasn't enough ventilation in the back. After playing for two hours, the amp just stopped producing any sound. It didn't happen suddenly, but over the course of a few minutes. When I shut it down I remember the power transformer being extremely hot.
Since my wife was about to have a baby and I had other amps to play through, I never got around to taking it in for repair. Every now and then I'd try swapping in new tubes to see if that would fix it, or open it up and look for visible problems, but never saw anything. If I tried to play through the amp, I could hear my guitar, but I had to turn the volume all the way to 10, and it was very quiet, with a lot of crackling and static. This happened on both the normal and vibrato channels.
Recently I decided to get it running again, and figured I'd try doing it myself. I sat down with my multimeter and the schematic, measured everything to see what was out of spec, and ordered some parts. I also got some new preamp and rectifier tubes.
The first thing I did was replace all of the filter caps and the bias cap. After I did that, the amp sounded a little louder, but I still got the crackling and static on both channels. The next thing I did was to replace any bad components around the PI stage. I replaced the 82k resistor and .001uf cap, and after doing that, replaced all of the 25uf cathode bypass caps. When I fired the amp up, the normal channel sounded loud and strong, with no crackling or distortion. The vibrato channel sounded clean too, but was as loud on 10 as the normal channel was on 2. I tried swapping in various tubes that I know to be good in all slots, but nothing changes.
After that, I went through and replaced the other out-of-spec components, and returned a few modded components to the values specified on the schematic.
Even after doing all of this, I still have the volume issue on the vibrato channel. I've also noticed something odd regarding two of the components on the tagboard. There's an 820 ohm resistor that connects a wire leading to the extension speaker output. When I measure the resistor while soldered into the circuit, it measures at 95 ohms. If I desolder it, it reads correctly at 820 ohms. This is without the amp being plugged in.
Similarly, there's a 470k resistor that connects to the brown wire that goes to the reverb control. This one also reads extremely low when soldered in, but normal when removed.
Does anyone have any suggestions for what I should look at next, or what might be going on with those two resistors? I've checked voltages, and everything checks out, although they're a bit high all the way around. But from what I understand, that's normal for blackface amps due to the bump in modern line voltage.
Thanks in advance.
I have a 67 Pro Reverb that I bought from the second owner in 2001. I used it regularly with no trouble until September 2005. One night I was letting a friend play through it at band practice, and I guess there wasn't enough ventilation in the back. After playing for two hours, the amp just stopped producing any sound. It didn't happen suddenly, but over the course of a few minutes. When I shut it down I remember the power transformer being extremely hot.
Since my wife was about to have a baby and I had other amps to play through, I never got around to taking it in for repair. Every now and then I'd try swapping in new tubes to see if that would fix it, or open it up and look for visible problems, but never saw anything. If I tried to play through the amp, I could hear my guitar, but I had to turn the volume all the way to 10, and it was very quiet, with a lot of crackling and static. This happened on both the normal and vibrato channels.
Recently I decided to get it running again, and figured I'd try doing it myself. I sat down with my multimeter and the schematic, measured everything to see what was out of spec, and ordered some parts. I also got some new preamp and rectifier tubes.
The first thing I did was replace all of the filter caps and the bias cap. After I did that, the amp sounded a little louder, but I still got the crackling and static on both channels. The next thing I did was to replace any bad components around the PI stage. I replaced the 82k resistor and .001uf cap, and after doing that, replaced all of the 25uf cathode bypass caps. When I fired the amp up, the normal channel sounded loud and strong, with no crackling or distortion. The vibrato channel sounded clean too, but was as loud on 10 as the normal channel was on 2. I tried swapping in various tubes that I know to be good in all slots, but nothing changes.
After that, I went through and replaced the other out-of-spec components, and returned a few modded components to the values specified on the schematic.
Even after doing all of this, I still have the volume issue on the vibrato channel. I've also noticed something odd regarding two of the components on the tagboard. There's an 820 ohm resistor that connects a wire leading to the extension speaker output. When I measure the resistor while soldered into the circuit, it measures at 95 ohms. If I desolder it, it reads correctly at 820 ohms. This is without the amp being plugged in.
Similarly, there's a 470k resistor that connects to the brown wire that goes to the reverb control. This one also reads extremely low when soldered in, but normal when removed.
Does anyone have any suggestions for what I should look at next, or what might be going on with those two resistors? I've checked voltages, and everything checks out, although they're a bit high all the way around. But from what I understand, that's normal for blackface amps due to the bump in modern line voltage.
Thanks in advance.
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