Metroamp 50 Watt Build + Lar Mar PPIMV

[Webby14]

Hi, Newbie here,

Just about to embark on the adventure of my first amp build! I will be building a '68 era 50 Watt Plexi, from the very excellent, and comprehensive Metroamp 50 Watt build instructions, using the majority of the parts from Valvestorm.

However, I am in a 'state of flux', as to the timing of the installation of the LarMar PPIMV Mod.

Common sense would suggest that the mod should be installed during the build (why fit 2 x 220K Resistors, and route two wires to the Output Tube Socket Grids, only to remove them again later!), or would it be preferable to complete the standard build, and go through all the testing procedures to ensure the amp is working correctly first, and then install the Mod?

I guess what I am trying to say is: Logic would suggest that the test readings should be the same with the Dual 250K Pot 'dimed', as they would be with the amp in standard Non-MV spec, but would that actually be the case? Would the introduction of the mod during the build, cause some unreliable test readings at the end?

Thanks for reading!

Webby

[danman]

I have built two of the 50 watters in the past year myself, and both times I installed the MV during the initial build. As long as you are very careful with the wiring of the MV you shouldn't have any issues during the testing phase. Just set the pot on ten when checking your bias voltages and you will be fine.

[Webby14]

Thanks danman!

Just out of interest, did you route the screened wire underneath the turret board, as the original wires would have been, or did you just run the screened wire across the top, as in most of the common schematics?

Webby

[neikeel]

Thanks danman!

Just out of interest, did you route the screened wire underneath the turret board, as the original wires would have been, or did you just run the screened wire across the top, as in most of the common schematics?

Webby

Under is neater. IMO!

[danman]

In both amps I ran it up over the top without any problems, but looking back I wish I had run it underneath the board just for appearance.

[AndyK]

I put a Lar Mar in my Metro 100 watt kit head (built by someone else).

Why dime the master before checking bias settings? Most bias instructions say keep the volume(s) at zero.

[danman]

You are correct about setting your amps normal volume controls at zero when biasing. The difference with the LarMar ppimv is that you are tapping off of the 220k bias splitter resistors, running through your dual pot and then to the control grid (pin 5) of your power tubes. You must have the MV pot at ten, which means it is basically out of the circuit, before setting your bias voltage when using the LarMar.

[AndyK]

You are correct about setting your amps normal volume controls at zero when biasing. The difference with the LarMar ppimv is that you are tapping off of the 220k bias splitter resistors, running through your dual pot and then to the control grid (pin 5) of your power tubes. You must have the MV pot at ten, which means it is basically out of the circuit, before setting your bias voltage when using the LarMar.

OK, but then after biasing, if I turn the master down low, does the bias fluctuate - or once I get it right with the master dimed, I am good to go?