Princeton Non-reverb low output and scope question

[judgeamps]

Hey everyone, I'm working on a Black panel era Fender Princeton (non-reverb) that has very low output. I have a question about my trouble shooting technique using a signal generator and scope to track down where I lose the gain. What I'm seeing is that I have excellent gain increases all the way up until the output transformer primaries. Both 6v6 plates have a strong signal, but at the output transformer secondary, I lose almost all of it. I happened to have another transformer on hand so I tried that, I also replaced the speaker jack as a test, and disconnected the NFB loop. All voltages match the schematic, just a little higher which is what I expect. I'm reading 6.5vac at the output jack into an 8ohm load when the volume is at 10 with a 1khz sine wav. Using what is probably a dodgy formula I read about in some Gerald Weber article, that is telling me an output of 5-6 watts and thats probably generous because the output is distorted at about 8-9 on the volume knob.

My first question is, am I using my scope properly? Are the high gain inverted signals on the 6v6 plates sending me in the wrong direction? Should the waveform on each plate look different on the scope? I expected one to be inverted, but it's not.

Second question is, am I missing anything?

Thanks in advance!

[sluckey]

My first question is, am I using my scope properly? Are the high gain inverted signals on the 6v6 plates sending me in the wrong direction? Should the waveform on each plate look different on the scope? I expected one to be inverted, but it's not.

That's all a matter of how you are triggering the scope.

[judgeamps]

My first question is, am I using my scope properly? Are the high gain inverted signals on the 6v6 plates sending me in the wrong direction? Should the waveform on each plate look different on the scope? I expected one to be inverted, but it's not.

That's all a matter of how you are triggering the scope.

Thanks for the reply, sluckey. I'm using a Tek 2235 set to p-p Auto and internally triggered.

[sluckey]

Thanks for the reply, sluckey. I'm using a Tek 2235 set to p-p Auto and internally triggered.

And I'm assuming you are only using one probe? You will need two probes to see the inversion when using internal trigger. Set scope as follows...

Connect CH 1 probe to one 6V6 grid (pin 5). Connect CH2 probe to the other 6V6 grid. Set vertical mode to BOTH and ALT. Adjust position and gain so you can see both traces.

Set A TRIGGER to auto or normal if you want to adjust the trigger level yourself. Set source to INT, CH1. Now the scope will always trigger on the signal on CH1, even when you are looking at CH2 and the phase inversion will be obvious.

My preferred method of using the scope with a sig gen is to connect the sig gento the amp input and also connect CH 1 probe to the input. Trigger on INT, CH1 and set CH1 vertical for a visible trace on the top portion of the display. This will serve as a good reference and will provide a constant trigger so you will not have to be constantly changing trigger levels as you move CH 2 probe around in the amp. Now use another probe connected to CH 2 to look at the signal at various points in the amp. You will be able to easily compare the CH 2 signal to the CH 1 reference. Signal gain and phase inversions will be obvious.

[PRR]

An OT is a 30:1 step-down, so you expect MUCH less voltage on the secondary. 6Vrms sec may be 180Vrms 250V Peak on the primary which is in the right ballpark.

Phase inversion is *relative". Relative to what? As Sluckey is saying, with one probe on Auto it will always trigger on the rising side of whatever wave it is looking at. You can use a second input for triggering left on the same point for all readings. Or you can understand that the phasing just can't be wrong, and even gross problems like a dead tube will look "OK" due to transformer action.

Very few 'scopes will stand the hundreds of volts in a tube power stage. Even a 10:1 probe rated for 500V will smoke in a 400V amp driven hard, because the 400V B+ plus the 300V swing is 700V peaks. (My probes had melt-marks where the electricity jumped out.) There is almost NO reason to ever be poking 6V6/6L6/etc plates, especially with signal. Either they work or they don't; poking a 'scope for a look just makes more problems to fix.

[judgeamps]

Thanks PRR and sluckey. My troubleshooting techniques are now improved! I used both of sluckeys techniques to confirm the phase inverter is producing an out of phase signal and to easily see the gain increase from source to each stage with the 2 probes. It's a great way to work.

I will go back to my initial problem I'm trying to solve. I'm seeing good gain increases all the way up to the power tubes and then a huge drop off at the output transformer secondary/speaker/dummy load. Like just barely louder than the first gain stage at the v1 plate. To compare, I have a Vibro champ of the same era at the moment and its much louder.

Already tried new tubes, new output xformer, tightening and cleaning socket pins, new jack, removed the NFB resistor. Voltages seem correct, about 15-20vdc higher than the schematic. Anything else come to mind?

Thanks!

[octal]

Have you verified that both output tubes are actually conducting? (Can stick a 1 ohm resistor in series with each cathode to monitor the bias/idle current, or carefully measure the resistance of each plate to the B+ CT of the output transformer with the amp off, then measure the voltage drop across each half of the primary winding with the amp on and use ohm's law to figure out your plate current.)

[judgeamps]

Hi octal, yes I did do that using a 1 ohm resistor (amp had been modded with them in place as well as an adjustable bias.) I also used my bias probe.