Soaring DC on power tubes

[Choan]

I posted a week or two back on a blown xfrmr on a Princeton reissue. Welp, trouble continues.

Originally, there was also a roasted filter cap resistor, and a filter cap that was on the edge of out of spec, so I decided to recap, correct the resistor, install new Hammond xfmr. I put new power and rectifier tubes in to just make sure they were known working tubes (I replaced GZ34 w 5u4gb since it was laying around).

Filter voltages read fine although slightly lower, which accounts for 5u4 voltage drop. I got excited. Checked DC on power tubes- holy smokes!! Jumping to 500vdc and then into overload within seconds. Hope I didn’t roast the new caps.

I gave up, then rechecked my wiring. 5v to rectifier, 6.3 to power tubes, 350v tap to filters. I’m second guessing myself, and now I’m afraid to power up with those voltages. I found a bias resistor that read open, replaced it, same thing. Pray upon my amateur tech soul. If anyone has any hunches, I’m all ears. Is the professional protocol to check every single component value before firing up to ensure there aren’t any big red flags, or to replace what you’ve found obviously wrong and then retest?

[pdf64]

Fit the other valves.
Use a lightbulb limiter till it’s proven good.

[PRR]

It would be reaonable to have a voltmeter on a new-PT-job smoke-test, because you can do it wrong.

I once replaced a PT, wrong. Expected 450V but it was going past 600V when I got my finger in gear on the off switch.

So did the power bottles ever heat-up? 350VAC makes 490V or more unloaded. Power tubes hot, even with hollow rectifier you may be flirting with 450V on the first cap.

[tubeswell]

So you metered the -ve DC bias voltage on pin 5 of each output tube socket before you put any 6V6s in, right?


(And you hooked the bias supply circuit up when you put the new PT in, right?)

[Choan]

Tubeswell

I foolishly didn’t check the negative voltage first. I did connect it to the bias supply circuit. However, after seeing the DC on the power tubes I check the bias supply and the 470 resistor read open. Bias capacitor read good too.

PRR, I didn’t check the tubes for to see if they were heating up, I saw the voltage surge and immediately powered down.

[tubeswell]

the 470 resistor read open.

Pretty sure that the bias supply limiting resistor on a 65 Princeton Reverb Reissue is meant to be 120k. Which schematic are you looking at? This one?
https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-65-Princeton-Reverb-Schematic.pdf

If you’ve got 470R dropping off the High Tension winding in that amp, the Zener (D4) will be clamping the bottom leg of the voltage divider (to protect the bias output voltage) and the rest of the 300v or so will be dropping across 470R.  Most 1/2W resistors are rated for 350v maximum. 1/4W for 250V. (But you’d be lucky for those resistors to hold for long at those voltages*). So if you’ve put 470R in where it’s meant to be 100k or so, chances are you’ll smoke the resistor.


*i.e., 300V/470R = 0.64A. 0.64A x 300V = 191W

(On the other hand, if you’re talking about one of the screen resistors being open, that will kill the tube current on that tube - if the bias is operating properly- but it won’t cause the tube current to soar. Which resistor are you talking about?)

Please confirm which schematic/amp you’re looking at?