Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ed_Chambley on October 19, 2011, 10:00:46 pm

Title: jtm 45 negative voltage v2
Post by: Ed_Chambley on October 19, 2011, 10:00:46 pm

I am attempting to startup the amp for the first time.  Every voltage measurement, v1,v3, v4, v5 and v6 look good.  The problem I have is with v2.  I can get no positive voltage on any of the vdc pins.  The heaters are fine.  I have swapped tubes.  I cannot figure out where to look.  I have given up for the evening as the only thing I have left is to remove the board and check the underboard wiring, but before I befin that process I thought I would ask in anyone had any ideas other than underboard wiring causing the problem.  I am getting around -40vdc on pretty much all pins except the heaters.

It is a mojo layout and uses 2 32 cans.  Anyone have any ideas?

Thanks,

Title: Re: jtm 45 negative voltage v2
Post by: sluckey on October 19, 2011, 10:19:46 pm

Gotta be a wiring error...

V2-6 has a wire that connects to a 10K/2w on the board. If that's missing, there won't be any B+ on either side of the tube.There should be B+ on both ends of that 10K. Looking at the mojo layout, there is a jumper wire from the top side of that 10K/2w over to another 10K/2W. Is that jumper in place?

Did you put VVR in this amp? If so, you may have dropped a connection somewhere that wont show up in the mojo layout.

Title: Re: jtm 45 negative voltage v2
Post by: tubeswell on October 20, 2011, 02:05:55 am

Check for DC continuity with your r-meter (for parts of the circuit that should be connected) cathode-to-ground resistance, plate-to-B+ (relevant supply node) resistance, etc

Title: Re: jtm 45 negative voltage v2
Post by: sluckey on October 20, 2011, 09:36:50 am

I re-read you post and based on the fact that V1 voltages are fine, the only "logical" conclusion is the wire that connects to V2 pin 6. See attached pic...

Title: Re: jtm 45 negative voltage v2
Post by: Ed_Chambley on October 20, 2011, 09:55:14 am

Quote from: sluckey on October 19, 2011, 10:19:46 pm

Gotta be a wiring error...

V2-6 has a wire that connects to a 10K/2w on the board. If that's missing, there won't be any B+ on either side of the tube.There should be B+ on both ends of that 10K. Looking at the mojo layout, there is a jumper wire from the top side of that 10K/2w over to another 10K/2W. Is that jumper in place?

Did you put VVR in this amp? If so, you may have dropped a connection somewhere that wont show up in the mojo layout.

No I have not wired the VVR.  It is bolted it and heatsinked, but I did not want to add mods until I have a running amp.  Been down that road before.  Kind of stupid thing to do unless you have done it many times before.

Thanks for your reply.

Title: Re: jtm 45 negative voltage v2
Post by: Ed_Chambley on October 20, 2011, 10:22:53 am

Quote from: sluckey on October 20, 2011, 09:36:50 am

I re-read you post and based on the fact that V1 voltages are fine, the only "logical" conclusion is the wire that connects to V2 pin 6. See attached pic...

Thank you for the drawing.  I am not at the amp currently, but I am quite sure there is no wire, just the resistor.  I am feeling kinda bumd right at the moment, but I think I should have a running amp this evening.

Title: Re: jtm 45 negative voltage v2
Post by: tubeswell on October 20, 2011, 01:41:28 pm

Another missing wire possibility

Title: Re: jtm 45 negative voltage v2
Post by: Ed_Chambley on October 21, 2011, 09:49:01 am

Missing wire was the problem.  Thanks for the help.