Fender Princeton AA1164 1K b+ dropping resistor cooks

[fredcapo]

Working on a silverface Princeton Reverb that was sitting for a few years. New 4 section electrolytic was put in in 2006 but amp didnt work. I discovered the 18K B+ dropping resistor that supplies the preamp tubes was never installed when the filter was changed. So I installed one and amp worked great for about 3 minutes. Then the 1K 1W resistor that drops the B+ to the screens started burning along with hum.

Replaced it, amp worked albeit with some distortion when played loud, and burnt again after several minutes. I didnt have a pair of 6V6's to swap in so not sure what could be causing the burning. Seems like it could be either the tube or the cap section.
Would appreciate any suggestions. Will try tube tomorrow. The B+ voltages are all about 5-10% low when amp is running. Thanks.

[Jazz P Bass]

Replace the 6V6 tubes & report back.

[Enzo]

Or just fire it up without power tubes. If the tubes are causing this, then the resistor should remain cool. If something else is causing this, then the resistor will get hot anyway, even without the tubes.

As to voltages being off a ce3rtain percent, read the fist note upper right on this schematic:

http://www.webphix.com/schematic%20h...z34_aa1164.pdf

[fredcapo]

Thanks. Borrowed the 6v6s from my Deluxe and the princeton has been running fine for 30 mins or so. Voltages pretty steady. 20% huh. Dont have a tube tester handy but anxious to test the questionable 6v6's to see if the short/defect is picked up.

Dont have a tube tester in my shop...any suggestions as per a reasonably priced and functionally accurate tube tester? Would predfer to find one used.

[Tom Phillips]

...anxious to test the questionable 6v6's to see if the short/defect is picked up. Dont have a tube tester in my shop...

If I wanted to sort out which of the original 6V6s was bad I'd do the following using the amp itself as a diagnostic tool

1. Monitor the voltage across the 1KΩ screen dropping resistor. You are actually indirectly monitoring the current through that resistor.
2. Plug in just one of the known good 6V6s. It doesn’t matter which power tube socket.
3. Turn the amp on and watch the voltage across the resistor. There is no need to put a signal through the amp. After the voltage stabilizes make note of the value. Calculate the current flowing through the resistor using Ohm’s law if you want to but it’s not really necessary for this test. The voltage across the resistor is your baseline value for this test. Switch off the amp and proceed to the next step.
4. Pull the “good” 6V6, insert one of the suspected bad tubes and switch the amp back on.
   1. If the voltage stabilizes near the baseline value from step 3 and stays there then the tube is probably OK.
   2. If the voltage rises higher (like twice as high or more) then switch off the amp immediately. You have found the bad tube.
   
   
5. If no problem is found in step 4 then repeat with the other suspected bad tube.

Based on the symptoms you have reported in your previous posts the bad tube should be easy to find with this method. Note that you get a soft start as the tubes warm up. I find that there is plenty of time to observe the rising voltage and determine good or bad situation without causing any additional damage. But read on…

Hope this makes sense to you. If not just ask questions. Like I said, this is what I would do. If you’re not comfortable then you could try a tube tester some day or just trash both of the suspected bad tubes. That’s a common solution in many shops. Also note that a short exposure to the higher current through the 1kΩ resistor won’t damage it. You just need to constantly monitor it during the test and be ready to switch off the amp if needed.

Cheers,
Tom

[fredcapo]

Tom - Thanks for the info. Understood. I am just trying to weigh the benefits of having a tube tester in the shop. Dont really think I need one but thought this would be a useful way to
collect some data. Dont know if testers typically run at plate voltages and currents found in the field.

I dont stock many tubes, but have a set of golden tubes that I use when troubleshooting, then acquire tubes as per the customers needs.

[Tom Phillips]

Just about all tube testers run the tubes at voltages well below those used in guitar amps.
There is a saying..."If the tube tests good it MAY still be bad. If the tube tests bad then it IS bad."
Testers that give useful information have gotten expensive and they need maintenance and calibration if you want quantitative information. I have a TV-7 tube tester and the info I get from it is nice to have but it is just one step in evaluating the tube. Unless the tube has a hard short I usually end up finding that kind of problem with the tube in the amp circuit. Often by isolating them one at a time as described above. Intermittent problems are a whole different problem and the time dedicated to isolate the exact tube depends on the value of the tube set. Pre-amp tubes that test strong still need a listening audition in the amp of course.

[Enzo]

A tester can tell you a tube is BAD, but cannot tell you it is good. I rarely admit it, because then people expect me to use it, but I do have a tube tester. If it comes out once in five years, that is a lot.

If someone walks into my shop and asks if I have a tube tester, I reply, "Yes, you are talking to him."

The best tube tester is a working amp. Stick the tube in the amp and see what it does. A real world test for a real world tube. Which is what Tom just described.

I picked up a small tube tester for cheap, and I have used it lately, but just to play with it to see if it works.

[fredcapo]

Understood. Thanks. I just would not want to risk damaging a nice amp, as per the subject of this topic, say when going thru a batch of pulled and otherwise untested vintage power tubes. I guess I could open up an appropriate amp and test a batch of pulled used tubes with a few voltages being monitored and a smoke detector above...