Name: Steve Kennedy
Email: steve@pacair.com
Subject: 210 SIxty-Five mod?
Thread: 152
Time: Thu, 30 Aug 2001 22:38:18 UTC
The story I heard was that the 12AX7A phase splitter circuit achilles heal... if it failed it could take the output tubes and possibly an output transformer with it!
This apparently was happening often enough that their warranty costs in the repair of these failures dictated a new design (which turned out to be a solid-state board that was retrofitable).
Units that were sent back to the factory had the tube socket hole plugged and the solid-state phase-splitter circuit added. Brand-new amps also had this plug & board installed on the assembly line until all the old chassis were used up. The newer chassis have no hole for the tube and hence, no plug plate.
So, your amp could have been made that way. I have two 410-65 amps. The 1974 has the 12AX7A, the 1979 model has the plugged hole and the solid-state board. Both appear to be built that way from the factory and both amps sound similar UNTIL you overdrive the output tubes.
When overdriven (Master Volume control turned all the way up, then dial in the input Volume control until you reach overload), the 12AX7A phase splitter sounds warmer and more "bluesy", while the solid-state unit seems to be harder to overload (more headroom)and a bit more "aggressive" when you finally get there.
Steve