Mojotone Princeton B+ voltage way low

[RadioGuySD]

Just put together a Mojotone Princeton blackface kit, following the start up steps, power it with no tubes and check for filament power at the pre and main amp tubes.. all good.. step two, pop in the GZ34 rectifier tube, power it on, and check voltage at pin 8 (B+ feed) and then at the big power cap. It measured 30vdc and seemed to climb about 100mv a second. Seemed way low, particularly after a minute. I went over to grab the wiring diagram to see if it listed what voltage I was supposed to have off pin 8 and the cap.. came back and measured it, the voltage was 50vdc and then I noticed the rectifier tube was flashing.

Looked over things to see if anything looked amiss.. it all LOOKS according to the wiring diagram.. turned it back on it was at 20vdc and creeping up again about 100mv a second. Voltage going IN to the rectifier is 360vac.

I am at a complete loss as to where to even begin to look. Could it have been a bad rectifier tube to begin with? Mojotone tech support said, "Usually a flash is a DC over-voltage or the output tubes drawing way too much current".. but there are no other tubes installed.. so.. would a short somewhere cause that? All my tube sockets look good (no pins shorted).

Any help would be GREATLY appreciated!

[Enzo]

ANy chance a filter cap is wired backwards?

ANY tube can fail at ANY time. Just because a tube is new doesn't mean it hasn't failed. It doesn't have to be working hard to fail, it can just happen.

How about a photo of the guts?

Did you wire the rectifier to the 5vAC for heaters, and NO ground connection?

Your B+ ought to be 300-400 volts. If the amap is struggling to make even 50 AND the poor rectifier is flashing, TURN IT OFF. You will damage the tube and possibly the rest of the circuit.

Are you using a "light bulb limiter"?

[RadioGuySD]

ANy chance a filter cap is wired backwards?

ANY tube can fail at ANY time. Just because a tube is new doesn't mean it hasn't failed. It doesn't have to be working hard to fail, it can just happen.

How about a photo of the guts?

Did you wire the rectifier to the 5vAC for heaters, and NO ground connection?

Your B+ ought to be 300-400 volts. If the amap is struggling to make even 50 AND the poor rectifier is flashing, TURN IT OFF. You will damage the tube and possibly the rest of the circuit.

Are you using a "light bulb limiter"?

Here are the guts. I wired it following the Mojotone wiring diagram which has four wires from the transformer (two red, two yellow) one going up to the "bias" board that has the requisite -31vdc and then the red wire that feeds the main board.

All filter caps have the + side facing "south" as instructed in the Mojotone wiring diagram.

I have no idea what a "light bulb limiter" is. That slang for "turn it off if a tube is flashing"? Because if so, that's pretty much all the protection this circuit has. There is no fuse between the rectifier and the circuit.

The forum won't let me make attachments, so I uploaded two photos here.. http://kbackradio.com/amp/

Thank you for your help!

[Enzo]

You have google? Enter "light bulb limiter" into it.

The whole point of troubleshooting is to isolate the problem.

our B+ circuit starts at the rectifier tube, from pin 8 is the wire to the filter cap. It terminates at the lower left corner eyelet on the board. From that point the wire goes over to the cap, also through a resistor, and the wire up to the output transformer. So what if we disconnect the red wire from pin 8. Either end of the wire will do. Now does the rectifier tube still go nuts? Without the filter cap, your B+ will be low, but not so low as 50v...if it is working. I would guess maybe 250vDC without the filter.

If that holds, return the red wire, and then disconnect the red wire going to the output transformer center tap. Powers up OK or not? Lift the yeellow wire over to the cap from the cap. Does it still freak out or not?

SOmething is repressing your rectifier, it is either the tube itself, the filter cap, the transformer, or something mysterious.

Start there.

[RadioGuySD]

You have google? Enter "light bulb limiter" into it.

The whole point of troubleshooting is to isolate the problem.

our B+ circuit starts at the rectifier tube, from pin 8 is the wire to the filter cap. It terminates at the lower left corner eyelet on the board. From that point the wire goes over to the cap, also through a resistor, and the wire up to the output transformer. So what if we disconnect the red wire from pin 8. Either end of the wire will do. Now does the rectifier tube still go nuts? Without the filter cap, your B+ will be low, but not so low as 50v...if it is working. I would guess maybe 250vDC without the filter.

If that holds, return the red wire, and then disconnect the red wire going to the output transformer center tap. Powers up OK or not? Lift the yeellow wire over to the cap from the cap. Does it still freak out or not?

SOmething is repressing your rectifier, it is either the tube itself, the filter cap, the transformer, or something mysterious.

Start there.

Thank you. I wasn't sure what I should expect from pin 8 without a load but removing the board from it was my first thought.. I just wasn't sure if doing so would make things worse. With the red wire lifted from pin 8, the tube puts out 120vdc. Reconnecting the red wire to pin 8 and removing the output transformer now meters just a few millivolts dc.. same if I remove the yellow wire to the big can cap. So, if 120vdc out of the rectifier with no load is normal, then it's gotta be something not right with the board.. although I tested each component before I installed it and double verified it was going where the wiring diagram said it should go.

[Enzo]

Before we skip too far down the garden path, pull the rectifier tube and measure the AC volts between the red wires on pins 4 and 6. Then also from each of those pins t ground. I link below a spec sheet for your power transformer - I think this is the right one - and it says 330vAC a side or 660vAC between the two red wires. let us see if the transformer is putting out, otherwise you could have a dead rectifier tube

[AttachMan]

Here are the pictures:

[Mick Bailey]

Unrelated, but the parallel cap/resistor on the bias board looks like the resistor leads are twisted but not soldered.

[The Dude]

I think Enzo forgot the link. At least, I don't see it, so here ya go.

[RadioGuySD]

Thanks guys for your ideas and help.

The power transformer meters out at 706vac between the two reds, each red to ground meters at 352vac with the rectifier removed. With the rectifier in place I get 120vdc on pin 8 when disconnected from the board. Removing the output transformer from the circuit has no effect, but removing ALL the leads from the big filter cap gives me 100vdc at the board.

I've tested the big cap and am getting 23uF, 24uF, 24uF and 46uF.. it's supposed to be a 20,20,20,40 so I'd guess that's within functional range. I guess my next step is to connect each of the wires back to the big cap one at a time and watch what my B+ voltage does, and hopefully it only tanks on ONE of them and then I follow that lead into the board and try to figure out what's going on? That sound like a reasonable course of action?

I tested each of the components before I installed them on the board, and I've gone over the board a few times now matching it up to the wiring diagram/print and it looks good. I'd realized my bias board cap/resistor was unsoldered shortly after I started looking things over when I had B+ issues, but figured it was unrelated since it has a good mechanical connection.

I bought this kit with the hopes it would be a learning experience.. and it certainly has lived up to my expectations!

Thanks again for any and all suggestions.

[The Dude]

Sounds to me like you have a bad rectifier tube. Do you have another to try?

[g1]

Sounds to me like you have a bad rectifier tube.

Sounds that way to me too. 120VDC raw out of the rectifier without the cap connected sounds low.

[RadioGuySD]

Well I guess that'll be the next step.. to grab a new rectifier tube and see what I get for voltage. It's possible something else down the line cooked my original tube.. or maybe it was bad to begin with. I suppose the only way I'll be able to eliminate that is to grab a new tube and put the circuit back together and if I don't have my normal voltage out of it right away, shut it down, now that I know what I should be expecting for voltage. Mojotone instructions should say, "Install the rectifier tube, turn it on, check voltage at pin 8, if you don't have at least 300vdc SHUT IT OFF." lol

[Jazz P Bass]

"Mojotone instructions should say, "Install the rectifier tube, turn it on, check voltage at pin 8, if you don't have at least 300vdc SHUT IT OFF." lol

With the tubes removed.

[RadioGuySD]

"Mojotone instructions should say, "Install the rectifier tube, turn it on, check voltage at pin 8, if you don't have at least 300vdc SHUT IT OFF." lol

With the tubes removed.

Yeah.. the only tube that has been in it so far has been the rectifier tube. It was just very unclear what voltage I was supposed to be looking for. It said if you have "no voltage" at those pins shut it off. I had voltage. I had 20 volts and climbing at a rate of 100mv a second and it said to give it a minute to warm up. So basically that tube was plugged in for 60 seconds, I checked voltage and had voltage.. but didn't know at that time that the voltage I had was WAY low. Since it was climbing, I didn't think it was a problem. It got to around 50vdc when I noticed the flashing in the tube and it dropped to 20vdc pretty quickly. A friend locally has a 5Y3 that (from what I understand) is the same tube but draws a bit more power (50v vs 20v) so at the very least it'll tell me if it's a tube problem or a circuit problem.