I first posted to AK in 2018 about a 500c my parents bought new, and I pulled from the box I had saved it in 12 years earlier. At the time I had little experience restoring vintage audio. I asked a bunch of noob questions which gave me direction and motivation. Four years later, I still have not restored the 500c, which is a ‘64, but have been learning, building my bench, and have done several other tube amp restorations, so I think I have the skills to do this right. Since 2018 I have also acquired a second 500c (1966) and an 800c Executive from 1964. Two years ago I bought a set of Hayseed Hamfest caps, and a “kit” for the remaining parts from Jim McShane. In a few cases, I don’t understand the rationale for the parts Jim included – they are all high quality parts, but some of them are different from recommendations I have seen discussed in the threads.
I spent the past week with the two 500c’s belly-up and side-by-side, looking and taking notes. The ’66 came from eBay with a few spare parts and after examination, I can see it has had some work done to it that is definitely not correct, it also has a few factory differences from the ‘64 (mostly in the FM section). The family heirloom ’64, in the photos on the right with the large number dial, looks like it has not been touched inside, except for me replacing a broken fuse holder and adding a CL-80 in 2018.
I have decided to work on the ’66 first and then replicate the effort on the ’64. The 800c will come later.
I have been reading about 500c’s on AK for four years, and been through the stickies and the various posts many times over, and I’m still finding new stuff. From reading these posts, and from posts related to my other projects, I feel like I know several of you very well.
Even with all the info, I have come across only one or two lists of what to do and how to do it to a 500c that are not complicated by the fog of variations for the 400 or early s/n 800c Executive - sorry Larry D, I couldn't resist. To that end, I have spent a good part of a week putting together my modification plan for the 500c, which reflects my goal to do most, but not all of mods that are possible. This plan is broken down by section of the receiver: power input, high voltage supply, bias supply (including individual bias adjustment), phase inverter, output stage, phono/input stage, tone control section, and FM/MPX. These are listed in the rough order I plan to do the work. I have also listed cosmetic, mechanical, and miscellaneous modifications. For each of these sections I have listed the changes or mods, major steps for more complicated changes, and the parts required.
See attached restoration plan below.
I have a few detailed questions, which are grouped by the sections of my plan. I will ask questions a few at a time to keep the discussion on topic. The question numbers are from the restoration plan. There are gaps in the number/letter sequence, because I only have questions on some topics.
Questions:
1 Power Input
1C – I intend to replace C102 and C103 black beauties. Jim McShane's kit provided .01uF 1.6kV polypropylene as replacements. C103, is on the primary side of the power transformer and looks like a “death cap.” C102 is on the secondary. Would an X1/Y2 rated disc cap be a better choice for either or both of these locations than the polypropylene caps?
1E – Jim’s kit included a pair of UF5408 fast diodes (rated at 1kv 3A) intended to replace CR5 and CR7. These seem adequate, but I don’t know the rating of the stock diodes. Please confirm that Is this an appropriate upgrade.
2 High Voltage Supply
2F – Jim’s kit provided a 1.2k ohm 5W resistor to replace R143. The specified stock value which is in my ’64 is 1.2k ohm 7W. My ’66 receiver has a 1k ohm 10 watt in it, that looks like it could be original. I would like to use a 1.2k ohm 10W, any reason not to?
3 Bias Supply
3B – Jim’s kit supplied two more UF5408 diodes to be added to the + and – DC outputs from the bridge rectifier before the bias caps (C97 A and B). He calls these “noise buffers”. I'm not exactly sure of their purpose, perhaps extra rectification. Are these a good idea to install in this location?
More questions to follow after a few responses to these questions have been received.
Thanks in advance,
Gary
I spent the past week with the two 500c’s belly-up and side-by-side, looking and taking notes. The ’66 came from eBay with a few spare parts and after examination, I can see it has had some work done to it that is definitely not correct, it also has a few factory differences from the ‘64 (mostly in the FM section). The family heirloom ’64, in the photos on the right with the large number dial, looks like it has not been touched inside, except for me replacing a broken fuse holder and adding a CL-80 in 2018.
I have decided to work on the ’66 first and then replicate the effort on the ’64. The 800c will come later.
I have been reading about 500c’s on AK for four years, and been through the stickies and the various posts many times over, and I’m still finding new stuff. From reading these posts, and from posts related to my other projects, I feel like I know several of you very well.
Even with all the info, I have come across only one or two lists of what to do and how to do it to a 500c that are not complicated by the fog of variations for the 400 or early s/n 800c Executive - sorry Larry D, I couldn't resist. To that end, I have spent a good part of a week putting together my modification plan for the 500c, which reflects my goal to do most, but not all of mods that are possible. This plan is broken down by section of the receiver: power input, high voltage supply, bias supply (including individual bias adjustment), phase inverter, output stage, phono/input stage, tone control section, and FM/MPX. These are listed in the rough order I plan to do the work. I have also listed cosmetic, mechanical, and miscellaneous modifications. For each of these sections I have listed the changes or mods, major steps for more complicated changes, and the parts required.
See attached restoration plan below.
I have a few detailed questions, which are grouped by the sections of my plan. I will ask questions a few at a time to keep the discussion on topic. The question numbers are from the restoration plan. There are gaps in the number/letter sequence, because I only have questions on some topics.
Questions:
1 Power Input
1C – I intend to replace C102 and C103 black beauties. Jim McShane's kit provided .01uF 1.6kV polypropylene as replacements. C103, is on the primary side of the power transformer and looks like a “death cap.” C102 is on the secondary. Would an X1/Y2 rated disc cap be a better choice for either or both of these locations than the polypropylene caps?
1E – Jim’s kit included a pair of UF5408 fast diodes (rated at 1kv 3A) intended to replace CR5 and CR7. These seem adequate, but I don’t know the rating of the stock diodes. Please confirm that Is this an appropriate upgrade.
2 High Voltage Supply
2F – Jim’s kit provided a 1.2k ohm 5W resistor to replace R143. The specified stock value which is in my ’64 is 1.2k ohm 7W. My ’66 receiver has a 1k ohm 10 watt in it, that looks like it could be original. I would like to use a 1.2k ohm 10W, any reason not to?
3 Bias Supply
3B – Jim’s kit supplied two more UF5408 diodes to be added to the + and – DC outputs from the bridge rectifier before the bias caps (C97 A and B). He calls these “noise buffers”. I'm not exactly sure of their purpose, perhaps extra rectification. Are these a good idea to install in this location?
More questions to follow after a few responses to these questions have been received.
Thanks in advance,
Gary
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