Measure once, cut twice. Didn't measure at all? Call the fire department.
Not really, but you get the idea.
Had an educational (spelled expen$ive) experience last Saturday. A customer brought in a "Telestar" base linear. That's "tell-uh-star", not "tell-star". The only one I had ever seen previously was converted to a single GU74b/4CX800 tube. This one was original, with sockets for two 4CX250-type tubes. Customer states power is low. I'm in the habit of testing this kind of tube before applying power to the customer's stuff. Both his tubes checked dead as a post. Open heaters. Didn't ask the obvious question "why?". Just shrugged at two chinese tubes. I did make sure there was proper control-grid and screen bias. High Voltage showed 2200 Volts.
Plugged in my matched pair of 4CX250B tubes.
Nada. Not so much as a fraction of a Watt. Wasted a bunch of time rechecking the tubes' operating voltages. Checked the grid post of the socket to see full drive from the radio really was reaching the tubes. Puzzling....... Until..
Until I put the clamp-on meter to the tubes' heater-supply wire. No current draw at all.
My idiot light is just beginning to flicker dimly.
Checked the voltage at the heater pin of a tube socket. Cue the idiot light. Full brightness.
26.5 Volts. Yep, they built it with military-surplus tubes intended for airborne use. Aircraft traditionally use a 24-Volt system to get twice as many Watts from a pound of wiring as you can with 12 Volts. Reducing weight is a big incentive when you design an aircraft and its systems.
Plucked my two tubes from their sockets and sure enough. Open circuit across the heater pins of both tubes. They're dead, Jim.
Felt pretty stupid by this point. Checked, and found exactly one "4CX250F" tube on the shelf. Takes two for this box. Won't get it on the air today with the original-type tubes.
The solution was to mount a 12-Volt center tapped transformer on the outside of the rear panel. Just wasn't an empty space inside where it would fit. Grounded the center tap and ran each of the 6-Volt wires to one socket.
Now it worked great with my last matched pair of 4CX250B tubes. This is one kind of tube where a suffix letter really means something. The "B" indicates a six-Volt heater. The "F" suffix is for the 26.5-Volt version. This guy had paid no attention to that suffix letter when he bought tubes to replace the older set. I'm sure he had no idea he would poof them the instant he turned on the power.
Oops.
Bottom line was the customer went home happy with a working amplifier. And the sale price for one matched pair of tubes almost got me up to the break-even point on my cost for four tubes.
And if that's the worst thing to happen the whole week, I can count it as a blessing in disguise. You can be sure we won't plug 6-Volt tubes into a "Tell-uh-star" box without measuring at least once, so to speak.
73
Not really, but you get the idea.
Had an educational (spelled expen$ive) experience last Saturday. A customer brought in a "Telestar" base linear. That's "tell-uh-star", not "tell-star". The only one I had ever seen previously was converted to a single GU74b/4CX800 tube. This one was original, with sockets for two 4CX250-type tubes. Customer states power is low. I'm in the habit of testing this kind of tube before applying power to the customer's stuff. Both his tubes checked dead as a post. Open heaters. Didn't ask the obvious question "why?". Just shrugged at two chinese tubes. I did make sure there was proper control-grid and screen bias. High Voltage showed 2200 Volts.
Plugged in my matched pair of 4CX250B tubes.
Nada. Not so much as a fraction of a Watt. Wasted a bunch of time rechecking the tubes' operating voltages. Checked the grid post of the socket to see full drive from the radio really was reaching the tubes. Puzzling....... Until..
Until I put the clamp-on meter to the tubes' heater-supply wire. No current draw at all.
My idiot light is just beginning to flicker dimly.
Checked the voltage at the heater pin of a tube socket. Cue the idiot light. Full brightness.
26.5 Volts. Yep, they built it with military-surplus tubes intended for airborne use. Aircraft traditionally use a 24-Volt system to get twice as many Watts from a pound of wiring as you can with 12 Volts. Reducing weight is a big incentive when you design an aircraft and its systems.
Plucked my two tubes from their sockets and sure enough. Open circuit across the heater pins of both tubes. They're dead, Jim.
Felt pretty stupid by this point. Checked, and found exactly one "4CX250F" tube on the shelf. Takes two for this box. Won't get it on the air today with the original-type tubes.
The solution was to mount a 12-Volt center tapped transformer on the outside of the rear panel. Just wasn't an empty space inside where it would fit. Grounded the center tap and ran each of the 6-Volt wires to one socket.
Now it worked great with my last matched pair of 4CX250B tubes. This is one kind of tube where a suffix letter really means something. The "B" indicates a six-Volt heater. The "F" suffix is for the 26.5-Volt version. This guy had paid no attention to that suffix letter when he bought tubes to replace the older set. I'm sure he had no idea he would poof them the instant he turned on the power.
Oops.
Bottom line was the customer went home happy with a working amplifier. And the sale price for one matched pair of tubes almost got me up to the break-even point on my cost for four tubes.
And if that's the worst thing to happen the whole week, I can count it as a blessing in disguise. You can be sure we won't plug 6-Volt tubes into a "Tell-uh-star" box without measuring at least once, so to speak.
73
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