Hey all!
Got this broken Princeton reverb for cheap to fix up. I found the components differed from fenders own schematic, so I decided to build in Mojo tones, blackface Princeton reverb kit.
I’ve had success building kits before, but this one has me stumped. Admittedly, I’m not electronics pro but I have a basic grasp of what not to grasp.!
The 1K resistor keeps burning up when I get to about halfway on the variac. (I have left the new one unsoldered to try and measure voltages) Not much of a chance to measure voltages in the circuit if it fails this early in the path! Also, if I read the voltages off of the yellow leads of the PT, together they read about 6 V, but if I read one lead or the other to ground, the voltage goes off the chart if that helps anyone.
I do have an amp tech scheduled to look at it this weekend. It would be awesome if I could figure it out before then!
68 Princeton reverb reissue rebuild To mojo tone
Moderators: pompeiisneaks, Colossal
68 Princeton reverb reissue rebuild To mojo tone
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Re: 68 Princeton reverb reissue rebuild To mojo tone
Your bias circuit looks a little odd to my eye. Is the grey wire coming off the pot under the board connected, or is it just kinda stuck between the resistor and cap leads there?
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Stevem
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Re: 68 Princeton reverb reissue rebuild To mojo tone
Does that resistor get hot if your output tubes are not in their sockets?
Do you have at least a minus 28 volts on pin 5 of each output tube socket with the intensity pot down?
Do you have at least a minus 28 volts on pin 5 of each output tube socket with the intensity pot down?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: 68 Princeton reverb reissue rebuild To mojo tone
That 1k in question is the first B+ dropping resistor that connects the plates' node to the screens' node, right? If so, I'd want to ensure there's infinite resistance between both of the following nodes and ground with the amp off. I also agree about looking into the bias circuit -- could you show a close-up of the whole bias board?
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stratomaster
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Re: 68 Princeton reverb reissue rebuild To mojo tone
This is a very rough build. A burnt 1k dropper indicates a short of some kind after it. Given the amount of burns to the insulation it's possible that the orange wire going to your screens has burnt though insulation and is contacting the chassis, hardware, or nearby pins on the power tubes. I'd recommend replacing this wire (and adding 470 ohm screen resistors while you're at it). It's carrying high voltage and should not have the insulation integrity compromised.
Also I can't tell how your board is insulated from the chassis, but there's a bare wire going off that node to the eyelet with the screen wire and what looks to be the PI (this typically takes HT from a later node on Princetons) or reverb--hard to tell from your photo. Reverb is correct at this node. Make sure that bare wire can't contact anything conductive.
The 1k is limiting the shorting current below the threshold of blowing a fuse, but it's burning up in the process.
Beyond that one of the tubes may have an internal short. You can confirm if this is the case by powering the amp up without power tubes. If the 1k remains intact and has a reasonable voltage drop across it then you can be reasonably confident the tubes are the culprit. You should still redo the wiring in your amp and be very careful about collateral damage with your iron.
Also I can't tell how your board is insulated from the chassis, but there's a bare wire going off that node to the eyelet with the screen wire and what looks to be the PI (this typically takes HT from a later node on Princetons) or reverb--hard to tell from your photo. Reverb is correct at this node. Make sure that bare wire can't contact anything conductive.
The 1k is limiting the shorting current below the threshold of blowing a fuse, but it's burning up in the process.
Beyond that one of the tubes may have an internal short. You can confirm if this is the case by powering the amp up without power tubes. If the 1k remains intact and has a reasonable voltage drop across it then you can be reasonably confident the tubes are the culprit. You should still redo the wiring in your amp and be very careful about collateral damage with your iron.