Shorted output transformer? 1962 Princeton problems

hepular

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just to see if i'm sort of following, wouldn't we exxpect bias to be about -35ish volts? so, 10x that might be causing some problems?
 

D'tar

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just to see if i'm sort of following, wouldn't we exxpect bias to be about -35ish volts? so, 10x that might be causing some problems?
Indeed. From the circuit side of the bias diode to chassis, one would expect an ohm reading approx 22k. We have 100^^. Process of elimination... either a connection, wire, bias pot, limitting resistor or final ground connection failure. It is currently an incomplete circuit which prevents the circuit resistance to effectively drop voltage to the desired level. Luckily you are wired to gain full negative bias voltage (-300vdc)as opposed to lose all voltage =0vdc which would allow the 6v6 to dissipate into thermal meltdown!
 

King Fan

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D'oh.

The -ve side of the diode is 655k now (filters still have some juice). The -ve side of bias cap is 3.92M. The +ve side is 3.96M.

So the bias cap has no ground — *no continuity to chassis* — even at its left end? Does your meter have a continuity beep mode?

Now we basically go back to what LLC was saying about the ground wire solder joints.
 

keithb7

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For quick giggles Alberta, you could remove the two philips head screws on the little bias board. Tilt it up, separating the sandwich. Have a peek at the connections and wires underneath. Would not hurt to re-flow a little solder at all the connections on the bias board. My gut is leaning toward the variable resistor as the root cause.

The beep-noise signal for continuity check on your multimeter could lead you to the promised land.
 

Lowerleftcoast

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Circuit side (-) of the diode reads 635k ohm to chassis.
Connect your meter to read this 635k resistance and then adjust the bias pot rotation. Give us your findings.

If the pot is the culprit, I anticipate a large change in resistance readings as the pot wiper is rotated. (Rotate the wiper several times to assure better contact with the resistive track. Assume there is corrosion/gunk/something making for a poor contact.)

If no change with pot rotation, then one or more of the solder joints circled in the diagram King Fan provided is/are the culprit. I would pay particular attention to the solder joint with the chassis, seconded by the solder joint at the diode because of the color of that joint.

In the unlikely event the pot and the solder joints are good and the 635k resistance does not change, then a broken wire making poor contact in this part of the bias circuit could cause this. Murphy's law.

952410.jpg
 

mfratus2001

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Conclusion: I think the tech's added square resistor in the bias circuit has burned out. It really does not belong there, it is for adjusting the bias, but just take it out and replace the jumper according to the published layout and everything will be fine.

Symptom of a bad output transformer is usually reduced output with distortion. Your problem is more basic.
Voltage readings are more revealing than resistance readings. And if there is any voltage stored in any caps, the resistance readings can be meaningless, if they don't harm your meter.
If your tubes light up, then filament voltage is close enough. You can't go by how bright the filaments are.
On the preamp tubes, important voltages are Pins 1,2,3 and 6,7,8. On output tubes, the important voltages are 3,4, and 5.
 

AlbertaGriff

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Sorry everyone for the delay, I needed to sleep off a night shift.

Thanks to everyone for the help in diagnosing this - you guys are amazing.

I tested for continuity from the bias cap to ground - no beep. Took off the bias board and...

20220617_143122.jpg


...the ground wire stayed right there. Obviously I cut it too short when changing the bias cap and then shoved it through.

It appears as though the mini-pot is fine, but also a good idea.

I'll report back once I've re-soldered this ground!

Thanks again!
 

rschiller

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All right I have some voltages:

Rectifier:
-pins 4-6 (reds): 708 V AC
-pin 2-8 (yellows): 5.3 V AC

Filter cap can:
-tab 1: 407 V DC
-tab 2: 454 V DC
-tab 3: 458 V DC

V4 6V6:
pin 5: -353 V DC (-360 at bias cap)
pin 4: 455 V DC
pin 3: 459 V DC

V3 6V6:
pin 5: -353 V DC (-360 at bias cap)
pin 4: 454 V DC
pin 3: 458 V DC

V2 12AX7:
pins 4-9: 6.77 V AC
1: 295 V DC
3: 2.67 V DC
6: 319 V DC
8: 87.8 V DC (****seems bad)

V1 12AX7:
pins 4-9: 6.77 V AC
1: 267 V DC
3: 2.01 V DC
6: 269 V DC
8: 2.03 V DC

Is there something missing here that would be useful?
6G2 Princeton is fixed bias and Fender Schematic is -35v -353 volts, if correct, likely means the 100k drop resistor for the bias taken off one of the ac winding is blown. The power tube cannot draw current with that much negative bias on the grid and would not amplify.

Also Fender speced out B+ plate voltages for the 6V6 at B+ 315vdc on the plates. If your amp is an actual 6G2 then the voltages are very high and 6V6s really should be run below 420v max and preferably lower in a Tweed or Brown amp.

My two cents worth.
 

D'tar

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If @AlbertaGriff would kindly post an updated voltage chart.... surely things look much different now.

Alberta, Congrats finding your issue. In the future you will find these things in minutes! Had you kept probing for voltage around the bias components you would have seen -300vdc all the way to your broken lead, leaving no question where the issue lies.

Great amp you have there. Enjoy!!!
 

AlbertaGriff

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If @AlbertaGriff would kindly post an updated voltage chart.... surely things look much different now.

Alberta, Congrats finding your issue. In the future you will find these things in minutes! Had you kept probing for voltage around the bias components you would have seen -300vdc all the way to your broken lead, leaving no question where the issue lies.

Great amp you have there. Enjoy!!!

I have sealed it up in advance of a visit from the in-laws, but maybe in a couple of weeks I can read the voltages again. I can tell you it sounds just like it did before - glorious.

Funniky enough, the last time my in-laws came I had a Vibrolux Reverb on my bench and was troubleshooting it the whole time they were here haha. Did not want a repeat, so again I really appreciate the help and the speed at which it came!
 




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