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Dynaco lore - Differences between factory wired and kit built ST-70 - photos .. on Fri Dec 26, 2008 9:01 am
Bob Latino
Admin
Dynaco offered the ST-70 in both kit and factory wired versions. Although over 350,000 ST-70's were sold during the run of this amp, it is estimated that less than 5% of all ST-70's were factory wired units. Only a few ever show up for sale on Ebay. Are there any differences between the kit built units and those that were factory wired ? Although the circuit is the same there were some cosmetic and at least one one wiring difference ...
Bob
1. The wired ST-70 was called the "70-A" (A = assembled) and came with a different user manual. This 6 inch by 9 inch manual was smaller in size than the standard 8 1/2 by 11 inch manual that came with all kit amps.
2. The 70-A had its chassis components attached with rivets instead of 4-40 pan head screws. The exceptions were the front stereo/mono switch and the power switch on the rear of the amp. Reason - those two switches were threaded and the rivets wouldn't pass through the threaded hole.
3. The driver board on the ST-70-A had its own separate "driver board cage"
4. The rear of the amp had a small "factory assembled" tag.
5. The chassis had a factory installed "spanner screw" to discourage the user from removing the bottom cover. You could remove the tube cage for bias adjustments because the cage had a slot that slid over the spanner screw but in order to remove the bottom cover and get at the internal wiring you needed a spanner wrench.
6. On KIT ST-70's the assembly manual tells you to install a 1000 ohm resistor between pins 5 and 6 of each output tube. Pin 6 is not used on an EL34 and is just used as a tie off point for one side of the resistor. On the factory wired units the 1000 ohm resistor goes directly from pin 5 to the driver board and there is no connection for pin 6
Bob
1. The wired ST-70 was called the "70-A" (A = assembled) and came with a different user manual. This 6 inch by 9 inch manual was smaller in size than the standard 8 1/2 by 11 inch manual that came with all kit amps.
2. The 70-A had its chassis components attached with rivets instead of 4-40 pan head screws. The exceptions were the front stereo/mono switch and the power switch on the rear of the amp. Reason - those two switches were threaded and the rivets wouldn't pass through the threaded hole.
3. The driver board on the ST-70-A had its own separate "driver board cage"
4. The rear of the amp had a small "factory assembled" tag.
5. The chassis had a factory installed "spanner screw" to discourage the user from removing the bottom cover. You could remove the tube cage for bias adjustments because the cage had a slot that slid over the spanner screw but in order to remove the bottom cover and get at the internal wiring you needed a spanner wrench.
6. On KIT ST-70's the assembly manual tells you to install a 1000 ohm resistor between pins 5 and 6 of each output tube. Pin 6 is not used on an EL34 and is just used as a tie off point for one side of the resistor. On the factory wired units the 1000 ohm resistor goes directly from pin 5 to the driver board and there is no connection for pin 6
