Bugera Lar Mar PPIMV mod

[fatboy757]

Hello!! Before I get thrashed for not searching, I HAVE. I have not found the answer I need. Im not as advanced as some folks here but im not totally inept. Im just not properly schooled in electronics.

Is there anyone who has performed this on a Bugera 1960 amp head? It has a pc board and Im not confident without a picture and explanation of where this mod goes. Im sure I could do this on a turret board. Just not sure how to perform this on the pc board. I know one side of the dual gang is split to pin 5 of the power tube sides.

Can anyone assist me further with performing this mod on this amp? Pictures possibly? Ive ask others who have modded and they can seem to help me for some reason.

Thanks in advance.

Justin

[danman]

If you can post a link to a schematic for this amp, I should be able to call out the tie in points that you will need.

[fatboy757]

Unfortunately I cannot locate one at all. Ive searched several message boards. The one lead I had, the members inbox was full and didnt publicly divulge the schematic.

I can take pictures of everything on the preamp board. The power amp section and preamp section are on separate boards. I dont know if that would help you in any way. Just trying to help you help me.

Thanks!

[fatboy757]

I retract that last statement. Ive found a schematic finally. If you could assist me from here Id appreciate it!!

https://www.talkbass.com/attachments/bu ... df.921634/

[danman]

Being that the power tubes are pcb mounted, this mod will require major surgery that could seriously devalue or ruin the amp. Looking at the schematic, it appears that the tie in point at the two 220k (r15/r23) resistors is part of the power amp pcb board. This will require you to remove the board so that you can remove those two resistors. You would also need to cut the traces on the board to break the connection and add your new connections to and from the MV pot. This is very tricky work and could likely damage the board. Its difficult to give you step by step instructions over the internet but if you look at the power amp schematic you linked too, you will see the tie in point where the bias supply ties in between the two 220k resistors. The same steps apply here as they would if you were working with a turret board. Just be very careful when soldering and unsoldering on the pcb as it can be damaged very easily.

[fatboy757]

It does help that the tube pins protrude through the bottom of the pc board and have plenty of post to solder to. Im not real clear on where the traces should be cut. Ive already performed the RR/ channel cascade mod by changing position of a 47k resistor on the preamp/ potentiometer pc board. So removing resistors is something ive already done to this amp. I have a dual gang all set up with 2 lengths of shielded cable for the job.

Is there any chance you could enlighten further and maybe highlight where to cut traces?

https://s30.postimg.org/mc2j6mnwh/power_amp_zoom.jpg

[danman]

Without being there with you I have no way to show you the exact spot on the board. The two resistors in your power amp schematic marked r15 and r23 will need to be removed from the board. In their empty holes you will need to solder 4 wires...two leading to the new MV pot and two leading from the pot back to the board. You are essentially breaking the signal by removing the resistors, sending the signal to the pot and then from the pot back to the trace on the board. You also have a bias supply trace that connects between the two 220k resistors. You will likely need to cut this trace and solder a wire from the trace on the board over to the MV pot where it connects to the far left solder lugs. If you look at the instructions for the Larmar MV, you will see this bias supply wire that runs from the board over to the MV pot. This would be a simple mod on a turret board but trying to explain how to do it to a PCB will be difficult. Try to study the instructions and maybe some pics of other Larmar MV installations and then try to see how it will connect in to your board. The principal of the installation is the same as a turret board but you will need to add some wires to your board where the two resistors are removed. I wish that I could be of more help and draw up something for you to go by but my computer skills are subpar at best.

[fatboy757]

I greatly appreciate your assistance. I will pull the board and do my best to take some pictures that could help. It may take a day or so. If I can take some photos Id be grateful for you to further assist me in the endeavor.


I feel like im missing something. I thought lug 2 of each pot goes to pin 5 of the v4 and v5 tube sockets. I may be visualizing this wrong from all the pictures ive seen and saved to study.

[danman]

If you study the power amp schematic, you will see that r15 connects to pin 5 on one pair of power tubes and r23 connects to the other pair. Between these two resistors is the point that the bias supply connects. The audio signal travels from the phase inverter tube, and through the junction where the 220k and 1.5k grid resistors meet. From there, the audio signal travels on to pin 5 of each power tube pair. The bias supply connects between the 220k bias splitters. From this point, the negative DC bias voltage splits and travels through each of the 220k's. The bias voltage rides along the same circuit trace as the audio signal so you end up with an AC signal voltage and a DC bias voltage at pin 5 of each power tube pair.

You will need to get creative and find a way to break the circuit trace and connect flying leads to the board which will connect to the MV pot. You are basically diverting the signal from the board, over to the MV pot and then back to the board again. This will involve cutting the traces and then soldering wires to the board. There is no way for me to communicate to you how to do this over the internet so you will need to understand the process completely before you begin. I'm not sure that I would try this on my own amp as the chance for screwing up the board is very high. An attenuator would be a much better solution that wouldn't cause any irreversible damage to the amp.