Borbely EB-600/312
I admit it: I was too excited to wait. After dinner and getting the kids to bed, I went down to the basement and fired up the ole soldering iron. I had to build something. Instead of starting the driver or power supply boards, which will take longer to build, I set my sites on the little EB-600/312 circuit. This little guy is a delayed turn-on that Erno designed to prevent large signals being passed to speakers when high-powered amps are turned on. It’s based around a 555 timer which is powered by either 9-12VDC or 12VAC and controlled by a capacitor and resistor. Change the value of the capacitor and resistor to increase or decrease the turn on delay.
Why am I building this circuit for the 15 watt SET? Simple, Erno—and nearly every forum post I could find on building amps around the 6C66C—recommended a two minute delay between applying voltage to the heaters and appying the B+ voltage. This gives the tube time to settle and, apparently, promotes its general health and well-being.
My idea is simple enough: power the EB-600/312 with the 12.6VAC winding on the mains transformer. When the amp is turned on, the heater elements will be powered for two minutes, then the timer will energize a relay on the AC side of the B+ power supply and, voila the amp should be up and running. At least that’s the plan. The wrinkle is that Erno said the EB-600 wouldn’t be suitable for delaying the B+ voltage; he’s right, of course, the relays are only good for about 30v. But the EB-600/312 also sports provisiosn for energizing an external relay. Add something like an Omron solid state relay that can handle a 240V ouput load and Bob’s your uncle.
The build went well—heck, there’s only 13 components and a little jumper—and it powered up right away. Seven seconds after powering up, the on-board relays clicked and, well, it worked. Tried it a few more time and it still worked. Sorry, no pictures of the build, but here’s the finished product:
The time delay is adjusted by changing the values of C1 and R1 (bottom left). I’ll need to make some pretty radical adjustments to change the delay from seven seconds to 120 seconds. The leads to the external relay are to the far right. The overall size is only about 3” x 1.5” so it should fit inside the final case without too much difficulty.
Now on to the driver and power supply boards.