Power issues for the new BE board

[zgrav]

I have put together a new BE board and it is not working for me. With a 9v supply that runs the other sushibox pedals with no issues, I am only getting about 113 volts on each tube socket. Cleaned the board with IPA and I have confirmed all part values and connections between the components on the power schematic and the audio schematic. Also replaced the mosfet and the 2n3904. Also switched out the two resistors that set the middle leg of the transistor for a 5K trim pot that is similar to the layout for the standalone power PCB. With that in place I can dial in the power for the tubes at 130v.

I know I am missing something here, because the incoming 9v power is dropping to 4.5v when it is plugged in to the board, and that seems to account for why the voltages at the tubes are only half of what they should be. But I don't know what is causing that voltage drop for the incoming power.

 

[vigilante398]

I respectfully request as many pictures as we can get. A schematic including the markup of the modification(s) will also be helpful.

Was the 113V measure with or without tubes present? It's normal for voltage on the tube to be a little lower than the B+ value because of the plate resistors.

Any particular reason you felt the need to drop the B+ down to 130V?

 

[zgrav]

voltages were with the empty sockets, but were similar at 113 when the tubes were present.

I put the 5K trimmer in to see what the voltage range would be because I wanted to see if I could get it higher than 113. it peaks at about 130. I am going to play with it a bit more this weekend and will post pics if it remains unsolved.

Seems like the key is finding out what is creating a voltage divider that is pulling the incoming voltage from 9v down to 4.4v

 

[zgrav]

Here are some pics of the Black Eye Board. As noted above, when it is plugged in the incoming 9v power drops to 4.4v, something that does not happen on the first version of the BE (or any of my other sushibox builds). Note that I am not wiring in a footswitch but I think the 3 sets of connections match how the footswitch is set when the pedal is engaged. Ideally something simple that I missed will stand out here. : ^ )

 

[zgrav]

Also, the trim pot replaces the 560 R and 4K7 resistors, with the middle leg connecting to the middle leg of the transistor and each of the other legs to the 220K resistor and to ground, respectively.

 

[Betty Wont]

Is it possible your polarity protection diode is dropping the voltage? Bad/wrong part? I’d start there.

 

[giovanni]

Is it possible your polarity protection diode is dropping the voltage? Bad/wrong part? I’d start there.

Good call. You can also try connecting the power directly after the diode using alligator clips so you don’t have to desolder it to find out.

 

[zgrav]

Good call. You can also try connecting the power directly after the diode using alligator clips so you don’t have to desolder it to find out.

Thanks for the suggestion. Will check it out.

 

[zgrav]

Sorry to report that bypassing the diode did not stop the voltage drop on the power. More sleuthing ahead.

 

[vigilante398]

Sorry to report that bypassing the diode did not stop the voltage drop on the power. More sleuthing ahead.

Your 2N3904 is backwards my good sir.

 

[zgrav]

Thanks. It was just replaced with a socket so easily switched or replaced. Only put in the socket to simplify trouble shooting, and the first one was oriented correctly. Still, hope springs eternal. : ^ )

 

[zgrav]

I put the 2n3904 in the correct orientation. no change in the voltage drop going into the pedal. also swapped in a new transistor just in case the other had been damaged. still no change.

the odd thing seems to be that the voltage boost through the pcb still works as intended, except that it is only seeing 4.4 volts instead of 9. that explains why I get about about half of the expected voltage on the tube sockets. I just need to find the short or the compromised part.

 

[giovanni]

So at which point does the voltage go from 9V to 4.4V? I mean if you bypass the protection diode, that node should be at 9V because of the power adapter. Do you not read 9V? Did you test the power supply by itself?

 

[zgrav]

Thanks for asking those questions. It sent me back to "think outside the sushibox. No problem with the power supply, but I did isolate the problem to my small test rig, although I am puzzled still. I was plugging the PCB into a small testing rig I use with in/out/power jack/footswitch and status LED on it, where the power adapter plugs into the test rig and I have screw terminals for in/out/power and ground. I connect the ground and power to a modular power plug that mates with the ones I use on PCBs. I just tried using a separate power jack on the modular plug with the 9v adapter and the board is showing 9v and is boosting the power needed for the tubes to the expected levels.

I have not used this test rig with the sushibox boards before, and instead waited to test the other tube boards in the pedal enclosures. This PCB is going in a different enclosure with a small power amp so I was testing it differently. Assuming this board is OK (have not tested it with tubes yet) I will see if the test rig does the same thing with any of my other sushi box pedals.

 

[zgrav]

After testing the board with the tubes in the sockets with audio, everything is OK. With the trimmer on the board there is a wide range of possible voltages for the tubes. They seem OK running with about 160 volts when powered on, which as Nathan noted is a bit lower than the voltages tested with no tubes in the sockets.

I will take a closer look later to try to understand what the test rig was doing to drop the voltage to the pedal.

Thanks again to folks for taking a look at the pics and sharing suggestions.