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SCA-35 Project

albusark

albusark

Dynaco Fan
Hello again all,

I'm embarking on an SCA-35 rebuild. The base amp is in excellent condition. Coupling capacitors have been replaced with extremely cheap Chinese poly caps. Anyone have a good argument for why I should not toss them?

I'll be replacing filter cans, bias circuit, resistors, and whatever else is needed to make this a rock solid amp for the next 60 years, will document the process.

I also have some fun EL-84 matched tube sets to test, including a nos JAN green letter kit and some Amperex bugle boys. This will be my keeper SCA-35, but will be offering fully restored amplifiers in the marketplace as I consolidate my collection.
 

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If by the base amp is in excellent condition, you mean this thing already plays great, I wouldn't go overboard and fix anything that isn't broken. That said, yeah, if I was going to start someplace I would replace the orange caps.
 
Plainly said:
If by the base amp is in excellent condition, you mean this thing already plays great, I wouldn't go overboard and fix anything that isn't broken. That said, yeah, if I was going to start someplace I would replace the orange caps.
Click to expand...
They're cheap Chinese poly caps, not orange caps. Anyway I was asking if anyone had an argument for keeping the caps, thanks for the input.
 
I read a post in another forum where someone did some testing and comparing various film caps. It was a pretty long list too.
Some of those Chinese poly caps tested and supposedly sounded pretty decent. Some, not so much. Those red "Audiophile" caps tested pretty good according to that article. Those orange Chinese caps don't sound horrible.
They came with a Geozone M-7 preamp board kit I bought.
It sounds pretty good. Way better than the $34 I payed for the board and a bag of parts.
If I found them in a SCA-35, or ST-35 I would change them out for something better though myself.
 
albusark said:
Hello again all,

I'm embarking on an SCA-35 rebuild. The base amp is in excellent condition. Coupling capacitors have been replaced with extremely cheap Chinese poly caps. Anyone have a good argument for why I should not toss them?

I'll be replacing filter cans, bias circuit, resistors, and whatever else is needed to make this a rock solid amp for the next 60 years, will document the process.

I also have some fun EL-84 matched tube sets to test, including a nos JAN green letter kit and some Amperex bugle boys. This will be my keeper SCA-35, but will be offering fully restored amplifiers in the marketplace as I consolidate my collection.
Click to expand...
If your budget allows, replacement of the pc boards with modern fiberglass (and new tube sockets and components (if desired)), will position the amplifier best for the "next 60 years" vs the original phenolic.
 
Ya, those old phenolic boards tend to get dried out and crumbly, especially around the tube sockets from the heat. I've seen some where the boards were burnt black around the sockets.
Dave's boards are nice. His power supply is a great upgrade for sure.
It's just one of the reasons I went with a Dynakit ST-35 kit instead of buying an original Dynaco. If I were going to keep a SCA-35, I would definitely buy the better boards.
 
Some further reading in case you're up for more involved mods to improve things beyond a stock refresh

Hidden Treasure: Bringing Out The Very Best of Dynaco's SCA-35

This thread presents a significant work for this amplifier, examining all of its virtues and warts, providing explanation of all the various issues, and presents real modifications for resolve. It will be presented in a number of sections as the thread develops, which will allow for questions or...
audiokarma.org
 
Lavane said:
I read a post in another forum where someone did some testing and comparing various film caps. It was a pretty long list too.
Some of those Chinese poly caps tested and supposedly sounded pretty decent. Some, not so much. Those red "Audiophile" caps tested pretty good according to that article. Those orange Chinese caps don't sound horrible.
They came with a Geozone M-7 preamp board kit I bought.
It sounds pretty good. Way better than the $34 I payed for the board and a bag of parts.
If I found them in a SCA-35, or ST-35 I would change them out for something better though myself.
Click to expand...
Thanks Lavane
 
Lavane said:
Ya, those old phenolic boards tend to get dried out and crumbly, especially around the tube sockets from the heat. I've seen some where the boards were burnt black around the sockets.
Dave's boards are nice. His power supply is a great upgrade for sure.
It's just one of the reasons I went with a Dynakit ST-35 kit instead of buying an original Dynaco. If I were going to keep a SCA-35, I would definitely buy the better boards.
Click to expand...
Well I already have lots of components to work on SCA-35's but I guess I need to make a digikey/mouser order to populate the Gillespie boards lol.
 
Echoing that the DG boards are high quality. I have not finished mine yet, but I've owned multiple SCA35 (and other Dynaco stuff) and IME the phenolic boards frequently chip along the edges and can come loose in the chassis. I don't want to sound too alarmist but loose voltage makes me feel funny
 
I've never seen any discussion on Packaged Electronic Circuits, the proto IC's that are in various Dynaco amps. They contain capacitors and resistors that I assume set values for the tone controls. Should they be replaced? I always wondered about these but never have seen any focus on them. You can find NOS with PN 555003. Safe to assume that the capacitors inside are some sort of ceramic type build? I ordered some to investigate. Maybe I can pull some values from them and we can experiment with wiring up discrete components instead.

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