"Safety" resistors across wrong lugs on Lar/Mar/Fi

[Gaz]

I just realized I've had the safety resistors across the wrong lugs of my Lar/Mar style master volume. In my case I was using a 250K dual pot with 220k resistors across the pot.

So instead of having the resistors wired from the wiper to the bias supply, they were accidentally wired from the wiper to the output of the coupling caps. I intended on lowering the load to about 115k with the 220k resistors, and I know that they weren't doing that! I'm also pretty sure they were providing no safety whatsoever from a failed or intermittent wiper contact.

My question is, what were they doing?

Thanks,

Alex

[Darkbluemurder]

They were basically taking the same place as a bright cap across a gain or volume pot. Wired in that fashion they only alter the taper of the master volume pot, making it come on more quickly.

In order to obtain the safety benefit they must be put between the wiper and the CCW tap (e.g. the bias connection in fixed biased amps or ground in cathode biased amps). The reason for this is that in case the pot wiper fails the safety resistors retain the contact between the power tube grids and the bias source.

Cheers Stephan

[martin manning]

They were protecting against a wiper failure since you would have 470k between the grids and the bias voltage if the wiper lost contact. Since the resistors are close to the pot value, the load on the PI is dropping along with the volume, which reduces PI headroom, and you are creating an odd taper on the pot.

[Gaz]

Thanks guys, I am going to A/B with and without them on the wrong side to see if I can hear the PI loading effect.