6CB6 pentodes and 12AU7 twin triode to replace 7199 in major SCA35 rebuild

wa2ise

wa2ise

Super Member
If one was willing to replace a pair of 7199's with 3 tubes, 2 6CB6s and a twin triode 12AU7 looks to be pretty close in specifications. This would only be practical, say in an SCA35, if you were to replace both output circuit boards with a rectangular piece of metal chassis with holes for tube sockets and a few terminal strips. You'd then punch 3 holes for the 12AU7 and the pair of 6CB6s instead of holes for 7199s. And there'd be four holes for the 6BQ5s. Assuming quality sockets, this would stand up to the tube heat better than the original circuit boards.
 
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Actually,this is a fine idea for virtually any Dynaco amp. I still have a stack of stainless plates (316L #4 brush finish) that I fab'd at work for my MK III's.
Really cleans up the topside of the chassis (and keeps the voltage under the hood) so you can run without covers,and very easy to replicate point to point.

While I've used a 6AU6 & 6C4 (1/2 12AU7) combination,I've often wondered about the 6CB6's for this application.
 
But why? 6gh8a is a single tube, and thus actually viable, not to mention practically free.
 
Or you could use in production 12AX7 & 12AU7 like the diytube version of the ST-35 uses and allows lots of tube rolling.
 
wa2ise said:
If one was willing to replace a pair of 7199's with 3 tubes, 2 6CB6s and a twin triode 12AU7 looks to be pretty close in specifications. This would only be practical, say in an SCA35, if you were to replace both output circuit boards with a rectangular piece of metal chassis with holes for tube sockets and a few terminal strips. You'd then punch 3 holes for the 12AU7 and the pair of 6CB6s instead of holes for 7199s. And there'd be four holes for the 6BQ5s. Assuming quality sockets, this would stand up to the tube heat better than the original circuit boards.
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Why not use a fresh pair of 7199 ? They are still available and total cost would be lower then a rebuild of a working amp.
 
wa2ise said:
If one was willing to replace a pair of 7199's with 3 tubes, 2 6CB6s and a twin triode 12AU7 looks to be pretty close in specifications. This would only be practical, say in an SCA35, if you were to replace both output circuit boards with a rectangular piece of metal chassis with holes for tube sockets and a few terminal strips. You'd then punch 3 holes for the 12AU7 and the pair of 6CB6s instead of holes for 7199s. And there'd be four holes for the 6BQ5s. Assuming quality sockets, this would stand up to the tube heat better than the original circuit boards.
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Cool idea! Are you building this mod or just an idea on paper? the plus side for me would be getting rid of a PC board or two!!
 
Sometimes I think you guys roll this stuff out just to distract me:D. While I'm working on my MK3 rebuild. I fortunately came into a bunch of reasonably priced 6AN8s so ( I've been led to believe by very reliable sources they worked well stock). My curiosity will force me to watch and learn. Hopefully I'll learn and remember more than I forget :lurk:
 
The 6AU6 and 6CB6 aren't designed for low microphonics, may not be ideal. Two 9-pin pentodes designed for audio use are 5879 and 6267 (EF86). The latter is available as new production. Still, I have hundreds of the former numbers, may try them anyway. But the 6GH8 is my most likely replacement for a 7199 - just 3 pins to rewire.
 
knockbill said:
Are you building this mod or just an idea on paper? the plus side for me would be getting rid of a PC board or two!!
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Just a paper idea right now. As these stages are not at levels seen in phono preamps, don't know how important microphonics would be. Maybe the signal is weak after the tone controls, though.

Using a subchassis makes sense for oneie-twosies, PC boards are mainly for mass production in tube electronics anyway
 
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