OK,
I see there is another post just below this where someone is asking about testing amps also. Here goes a few questions of mine.
I recently purchased an old Tek scope T922. I'm not sure how great it is, because I have never used a scope. I can get waves to come close to being stationary on the screen, but not anything I would consider stable enough to analyze. I think there is a simple issue with the screen adjustments, or the triggering circuit. I'll get that schecked out later though.
Anyway, the scope does work well in all other aspects.
I recently avoided buying a function generator by building one based on the 8308 chip with an opamp buffer on the output of the generator chip. I could not justify spending the money on a function generator that I may or may not use that much. The generator cost about $18 to build using high tolerance resistors, and it does square, sine, and triangle.
The buffered output also is set up for gain, thus eliminating another preamp in the signal chain.
Anyway, without further boring everyone with these projects, I tried the methods described to me in several of my other posts throughout the bulletins.
I hooked my generator up to my amps with one scope channel connected to the input to see the signal wave, and the other probe up to my OPT with a 150w dummy load connected to my 8 ohm tap. This, to my surprise makes the OPTs sing quite loudly when enough input voltage is applied.
The amps that should easily make over 100w began to distort the tops of the sine waves at 1K @ 23.8v.
If you do the math: 23.8 * 23.8 / 8 = 70.805w.
This was measured using my RMS meter connected to the dummy load.
Now for the questions.
A) These are my big amps. There are three pairs of U/L KT88. I would think they would make a bit more power than that before clipping.
B) These amps are biased around 58mA per tube, so on par with Shannon's reccomendations for 60W with a single pair of KT88s. I'm using 1,900ohm primary OPTs, so they are not perfect, but take 4,200/3=1,400ohms. I could not find a 1,400 ohm primary OPT.
C) What do you think would be the most likely candidate for this clipping? The 6SL7, or the phase inverter 6SN7 not being able to deliver enough current, or maybe needs to be biased a bit harder? The circuit is essentially Shannon's front end with 6SN7/SL7 tubes.
D) should I leave the dummy load connected and run the same testes clipping on to the signal through the circuit to see at which point I see the clipping happen?
BTW, before I continue with any further testing, I need to get a few more resistors for my dummy load. It gets very HOT!!
Thanks,
Blair