Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: PharmRock on March 27, 2021, 07:06:49 pm

Title: Lar/Mar master volume - ground lug on potentiometer
Post by: PharmRock on March 27, 2021, 07:06:49 pm

I can't seem to find it anywhere at the moment, but somewhere in this forum there was a post about wiring up a Lar/Mar PPIMV in a Plexi style circuit.  In the schematics I've found, the shields for the cables that run to/from the dual 250k pot for the MV connects to the bias supply voltage.  I recall reading (perhaps sluckey??) that it would be better to connect the shields directly to a "proper" ground, such as the power supply ground. 

I was hoping someone could explain to me the benefits of grounding to the PS ground vs the bias supply.  Most all schematics (e.g. Metro Amp and others) I've seen for the Lar/Mar PPIMV have done via the bias supply, and the overall satisfaction with this set up has been very favorable.  thanks for any info.

Here is a link I found for the Lar/Mar PPIMV (accessed via Valve Storm's website):
http://www.maxless.nl/PPIMV.jpg (http://www.maxless.nl/PPIMV.jpg)

Title: Re: Lar/Mar master volume - ground lug on potentiometer
Post by: texwest on March 31, 2021, 02:17:57 pm

I installed mine as it is on this layout and it works great.

Title: Re: Lar/Mar master volume - ground lug on potentiometer
Post by: thetragichero on March 31, 2021, 05:08:54 pm

shield is connected (on one end) to ground as there's no reason to inject any possible noise into your bias supply. less convenient than the pot lug connected to the bias supply but if we were concerned about convenience we'd probably just buy an amp off the shelf instead of investing hours of our time building em

Title: Re: Lar/Mar master volume - ground lug on potentiometer
Post by: sluckey on March 31, 2021, 06:57:37 pm

Quote from: thetragichero on March 31, 2021, 05:08:54 pm

shield is connected (on one end) to ground as there's no reason to inject any possible noise into your bias supply. less convenient than the pot lug connected to the bias supply but if we were concerned about convenience we'd probably just buy an amp off the shelf instead of investing hours of our time building em

Not an issue. The bias filter cap holds the bias supply at ac ground.