Mojotone Princeton B+ voltage way low

[Randall]

FWIW, trying another rectifier tube would have been the first thing I would have tried. I would consider 20- 50 vdc for B+ on a rectifier tube to be "no voltage". And so you know, rectifiers, once they are warmed up for a few seconds, do not slowly climb up in voltage like yours is doing. They get to where they should be pretty quickly. You can try a 5Y3 in there, but don't expect as high a dc voltage as the 5U4/GZ34, as it is designed for a lower output voltage. But, if it puts out something close to 300 volts or so, then you have a bad GZ34. That would be my guess as to what the problem is.

[Enzo]

Just about any of the basic 5v rectifier tubes will work well enough to see if everything else is working. At this point it doesn't matter if the resulting B+ is 20-30 v different, we are just looking for go or no-go.

[RadioGuySD]

Well.. I put the 5Y3 in there and got nearly 600vdc B+ voltage, so I put the preamp tubes in and checked voltages and all seemed good.. hooked up the speaker and put the 6V6's in and plugged in a guitar and it sounded fantastic. I fiddled with it for about 20 minutes and then cranked the volume up to see if it would breakup. About three notes in (volume at 8 or so, guitar volume turned down) I started seeing smoke and the amp was definitely breaking up. I switched it off, gave it a moment and turned it back on to see if I could identify the area of the smoke. Plugged a guitar cord in and tapped my thumb on it a few times to ground it out and it made the 'buzz buzz buzz' sound and then POP one of the 25uF 50V caps popped. The one that feeds the reverb driver tube, I'd checked the voltage on it before playing through it, schematic said it should have been 8vdc and it was metering 7.5vdc.. I assumed slightly low because I was running the lower power 5Y3 rectifier. I do not have the reverb can plugged in, and the reverb setting was at 1. I wouldn't expect a filter cap to pop unless it was getting way more voltage than it should or the polarity was reversed. I guess my next step is to get a few new caps and the right rectifier and give it a go again and see what happens.

[g1]

That sounds like a cathode cap rather than a filter cap, but you need to post the schematic (or a link to it).

[Enzo]

And make sure the cap was not wired in backwards.

[RadioGuySD]

The cap in question is the one running to 'reverb send' and the polarity of the cap was correct. The voltage at the board was 7.5vdc before I started playing through the amp and the sheet says it should have been 8v. The amp played fine till I cranked up the volume.

[RadioGuySD]

I cut the bad cap out and thought I'd check the voltage across where it had been to see if it was high.. but it was way low. I pulled all the tubes out besides the rectifier and was back down to 50v on B+. I disconnected the B+ and now I have 100vdc off the tube with no load.. so.. another dead rectifier tube. Maddening, because the amp was playing fine then.. bleh.

[Helmholtz]

Are you absolutely sure that the multi-section can cap is wired correctly, i.e. you're using the correct negative lug?

[RadioGuySD]

Are you absolutely sure that the multi-section can cap is wired correctly, i.e. you're using the correct negative lug?

It's wired in the same orientation as all the other caps and identical to the diagram.. AND I had 7vdc out of it and it ran fine for over 30 minutes. If the cap was wired backward it wouldn't run for a half hour I wouldn't think.

[Helmholtz]

It's wired in the same orientation as all the other caps and identical to the diagram.. AND I had 7vdc out of it and it ran fine for over 30 minutes. If the cap was wired backward it wouldn't run for a half hour I wouldn't think.

See if the negative lug connects to ground.

[g1]

mojotone schematic

https://www.mojotone.com/BF-Princeton-Reverb-SCH.png

[RadioGuySD]

See if the negative lug connects to ground.

It was the left hand cap that popped in this diagram.. there's no question the negative lead went to ground and that the polarity was correct.

[Helmholtz]

Always speaking of the large can cap. It contains 4 HV capacitor sections and has on common negative lug.

The small axial caps as shown in your picture don't even connect to high voltage.

[Helmholtz]

Well.. I put the 5Y3 in there and got nearly 600vdc B+ voltage

That's impossible with your transformer voltages.

Is your meter battery good?

[Enzo]

Just an idle thought. A really cheap meter set for AC volts, can be confused by DC vvoltage.

A basic test of your meter is to measure a 9v battery with meter set for AC volts. A good meter will briefly peak then slide down to zero volts. A cheap meter will read somethng like 12vAC from the BATTERY.

If that happens to be the case, "600v" on the AC scale implies about 420vDC.