Was it common practice for amp makers to push output tubes beyond their specs?
I'm fairly new to tubes, but I have restored a half dozen pre-war AM radios. I've recently done a couple post war HiFi amplifiers. The first was an 1962 AMI Jukebox amplifier. The 6973 output tubes were missing, but a quick Google search suggested 6CZ5 as a substitution. These over heated and made the plates glow red.
It was evident from the spec sheet that the plate and screen voltage were too high for this tube. BUT, it was also some what beyond the max for a real 6973. I ordered a set of Russian 6973s and they play fine.
The next amplifier I worked on is a 50's Pilot AA-903B. I got this working with the original tubes. The output tubes are PP EL84s. The two tubes were different brands (GE and "M Valve Art"). I inherited a tube collection that has a dozen NOS Raytheon EL84s, so I figured what the heck, two of the Raytheon would surely match better than what's in there. I put two of the NOS Raytheons in, and they played, but after a while the plates got red hot.
The plates and screens are running at 324V the max spec in the data sheet is 300V. Grid one is -10V just like the schematic shows. I put the original tubes back in, and it's been playing just fine for the past couple days.
Back when I was a teenager, I had a 60's vintage Fisher stereo amplifier. I don't remember the model number or the tube numbers. For some reason, I decided to change the output tubes. I didn't know what I was doing back then, but I do remember that I replaced the old ones with the exact same tube number. Guess what, all four got cherry plate.
I put the old tubes back in and every thing was fine again.
So, I have not had good luck with output tube replacement. Do these things have to be hand picked? Does everybody run into this? What gives?
BTW, the AMI and Pilot amps both have fixed non-adjustable bias, and the coupling caps are new.
Bobby Dipole
I'm fairly new to tubes, but I have restored a half dozen pre-war AM radios. I've recently done a couple post war HiFi amplifiers. The first was an 1962 AMI Jukebox amplifier. The 6973 output tubes were missing, but a quick Google search suggested 6CZ5 as a substitution. These over heated and made the plates glow red.
The next amplifier I worked on is a 50's Pilot AA-903B. I got this working with the original tubes. The output tubes are PP EL84s. The two tubes were different brands (GE and "M Valve Art"). I inherited a tube collection that has a dozen NOS Raytheon EL84s, so I figured what the heck, two of the Raytheon would surely match better than what's in there. I put two of the NOS Raytheons in, and they played, but after a while the plates got red hot.
Back when I was a teenager, I had a 60's vintage Fisher stereo amplifier. I don't remember the model number or the tube numbers. For some reason, I decided to change the output tubes. I didn't know what I was doing back then, but I do remember that I replaced the old ones with the exact same tube number. Guess what, all four got cherry plate.
So, I have not had good luck with output tube replacement. Do these things have to be hand picked? Does everybody run into this? What gives?
BTW, the AMI and Pilot amps both have fixed non-adjustable bias, and the coupling caps are new.
Bobby Dipole