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We had a 1473 that came it with the customer claiming that the audio cut out, and was all weak and distorted. Ultimately, it was suffering from a number of issues that are unfortunately common to these amps, so I thought I'd do a little write up on the repair to help people who own, or are tasked with repairing some of these filthy little beauties.
After reading the repair slip, I put the amp up on the bench so I could make the amp exhibit the symptoms outlined by the customer. I confirmed the amps symptoms:
No audio, with the occasional low, nasty distortion indicating a blown speaker.
the 6CG7 socket was miserable, so I knew that it, along with the pots and 12AX7 sockets, were going to need to be thoroughly cleaned. So, I put away the guitar, unplugged the amp, and began to remove the chassis from the cabinet. This is when I observed the first indication that this amp might be suffering from Silvertone's Disease. The screws which mount the chassis to the cab began to spill dust as I removed them, and the sides of the cabinet shelf which the chassis is mounted on had come detached from the walls of the cabinet box. But it was when we laid the amp down of some milk crates to remove the speaker, and the entire nut and bolt assemblies began to just spin in place that I began to become really discouraged.... You see, Silvetone Amps suffer from the worst cabinet quality in the business – hands down. It's one of the main contributing factors which lead to Silvertone's disease. Once the cabinets degrade beyond a certain point, the only solution I can find is complete reconstruction using real plywood (or solid wood for those ambitious enough).
At first I thought I was going to need to fabricate a new baffle. But, by reaching around underneath and pressing against the inserts, we were able to still get enough grip to remove and install a new 15" speaker. But I can't stress enough how important it is to make sure you don't over torque the nuts when mounting them. The material is just strong enough to hold the speaker, but barely. My recommendation would be construction of a new baffle board, but I'm not doing it for free.
With the cabinet in good enough shape, it was time to go through the electronics. The first thing that needed to be done was to correct the Mains wiring. This, unfortunately, is not unique to Silvertone or vintage factory wiring practices. I see it all the time in amps which were supposed have been updated to meet modern safety standards. But I can say from experience, that when I started out learning how to build and repair amps on my own, it was really difficult to know which information to trust as an authority. Very poor and outdated practices keep getting retaught over and over. For example, here is instruction on how to convert a 2-prong power cord found in vintage amps to a 3-prong power cord on schematic heaven.net
3 Prong Power Cable (No ground switch) Mod
Honestly, ^^ that's terrible f*cking advice for something that is posted with the pretext of being informed and instructional. But I digress... Back to our Silvertone 1473.
After re-wiring chassis ground, fuse, and replacing the power switch, I moved on to some of the more routine maintenance. I used deoxit to spray out all the pots and sockets. However, Silvertone 9-pin tube sockets are notoriously poor quality, so I also re-tension'd the pin receptacles on the 6CG7 socket because I was able to determine it was problematic earlier. Now, it was ready to power up and bench test. I had my scope monitoring the speaker output under load, while injecting signal into the input of channel one. After allowing the amp to come up to full power and warm up, there was no output at all. I tried channel two – same thing. I swapped out each tube for known working samples – no change. With the amp still powered up, I checked the voltages at each of the filter nodes. I've known these carbon comp resistors in the power supply and plate loads to commonly go open in many Silvertone models.
The supply at the last filter node was around +40V or so, implicating the 68k dropping resistor. This was indeed the case, so It was replaced by a 3W MOX resistor. Began the signal test again,, and still no output. So back to testing more voltages, this time at the tube pins. The voltage as each of the 12AX7s measured what I expected to see, but the voltages at each plate of the 6CG7 driver was reading the supply voltage. The cathodes were also measuring in the mV range which told me the tube wasn't conducting. I was able to measure 6.3VAC at the heaters, so I knew that the tube socket pins had degraded significantly and the socket needed to be replaced. This looks like a worse job than it is because the sockets are riveted to the chassis, but it is fairly easy to remove the rivets if you have a good pair of flush/semi-flush cut pliers.
You want to flip the amp so you're working on the underside (component side) of the chassis. Here, you can leverage the cutting edge of the pliers flush in between the chassis and rivet, and when you have them wedged properly, you can use that leverage to cut flush to the chassis and basically sheer off half of the rivet. After this it's easy to push the remainder through and lift the original socket out.
Open resistors and faulty tube sockets are two other major contributors which cause Silvertone's Disease. But one of the interesting dichotomies of these amps is that with all compromises Silvertone made in the materials they used to construct the amps, the quality and competence in the construction itself is as good as you will find in handwired amps of the day. It's almost impossible to remove a lead from the terminal connection, without having to remove some amount of solder, and cut the component lead away because of the redundancy used in wrapping the lead around.
Anyways, those are some things to look out for in Silvertone amps of that era.
After reading the repair slip, I put the amp up on the bench so I could make the amp exhibit the symptoms outlined by the customer. I confirmed the amps symptoms:
No audio, with the occasional low, nasty distortion indicating a blown speaker.
the 6CG7 socket was miserable, so I knew that it, along with the pots and 12AX7 sockets, were going to need to be thoroughly cleaned. So, I put away the guitar, unplugged the amp, and began to remove the chassis from the cabinet. This is when I observed the first indication that this amp might be suffering from Silvertone's Disease. The screws which mount the chassis to the cab began to spill dust as I removed them, and the sides of the cabinet shelf which the chassis is mounted on had come detached from the walls of the cabinet box. But it was when we laid the amp down of some milk crates to remove the speaker, and the entire nut and bolt assemblies began to just spin in place that I began to become really discouraged.... You see, Silvetone Amps suffer from the worst cabinet quality in the business – hands down. It's one of the main contributing factors which lead to Silvertone's disease. Once the cabinets degrade beyond a certain point, the only solution I can find is complete reconstruction using real plywood (or solid wood for those ambitious enough).
At first I thought I was going to need to fabricate a new baffle. But, by reaching around underneath and pressing against the inserts, we were able to still get enough grip to remove and install a new 15" speaker. But I can't stress enough how important it is to make sure you don't over torque the nuts when mounting them. The material is just strong enough to hold the speaker, but barely. My recommendation would be construction of a new baffle board, but I'm not doing it for free.
With the cabinet in good enough shape, it was time to go through the electronics. The first thing that needed to be done was to correct the Mains wiring. This, unfortunately, is not unique to Silvertone or vintage factory wiring practices. I see it all the time in amps which were supposed have been updated to meet modern safety standards. But I can say from experience, that when I started out learning how to build and repair amps on my own, it was really difficult to know which information to trust as an authority. Very poor and outdated practices keep getting retaught over and over. For example, here is instruction on how to convert a 2-prong power cord found in vintage amps to a 3-prong power cord on schematic heaven.net
3 Prong Power Cable (No ground switch) Mod
Honestly, ^^ that's terrible f*cking advice for something that is posted with the pretext of being informed and instructional. But I digress... Back to our Silvertone 1473.
After re-wiring chassis ground, fuse, and replacing the power switch, I moved on to some of the more routine maintenance. I used deoxit to spray out all the pots and sockets. However, Silvertone 9-pin tube sockets are notoriously poor quality, so I also re-tension'd the pin receptacles on the 6CG7 socket because I was able to determine it was problematic earlier. Now, it was ready to power up and bench test. I had my scope monitoring the speaker output under load, while injecting signal into the input of channel one. After allowing the amp to come up to full power and warm up, there was no output at all. I tried channel two – same thing. I swapped out each tube for known working samples – no change. With the amp still powered up, I checked the voltages at each of the filter nodes. I've known these carbon comp resistors in the power supply and plate loads to commonly go open in many Silvertone models.
The supply at the last filter node was around +40V or so, implicating the 68k dropping resistor. This was indeed the case, so It was replaced by a 3W MOX resistor. Began the signal test again,, and still no output. So back to testing more voltages, this time at the tube pins. The voltage as each of the 12AX7s measured what I expected to see, but the voltages at each plate of the 6CG7 driver was reading the supply voltage. The cathodes were also measuring in the mV range which told me the tube wasn't conducting. I was able to measure 6.3VAC at the heaters, so I knew that the tube socket pins had degraded significantly and the socket needed to be replaced. This looks like a worse job than it is because the sockets are riveted to the chassis, but it is fairly easy to remove the rivets if you have a good pair of flush/semi-flush cut pliers.
You want to flip the amp so you're working on the underside (component side) of the chassis. Here, you can leverage the cutting edge of the pliers flush in between the chassis and rivet, and when you have them wedged properly, you can use that leverage to cut flush to the chassis and basically sheer off half of the rivet. After this it's easy to push the remainder through and lift the original socket out.
Open resistors and faulty tube sockets are two other major contributors which cause Silvertone's Disease. But one of the interesting dichotomies of these amps is that with all compromises Silvertone made in the materials they used to construct the amps, the quality and competence in the construction itself is as good as you will find in handwired amps of the day. It's almost impossible to remove a lead from the terminal connection, without having to remove some amount of solder, and cut the component lead away because of the redundancy used in wrapping the lead around.
Anyways, those are some things to look out for in Silvertone amps of that era.
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