PT and OT correct mounting

[danch]

Hello to everybody

I have classic tone PT and OT for 50W build. I can see that on PT there is plastic washer between nuts/screws and outer bell covers. Why is that when there still is continuity between bells and laminations? Is there supposed to be continuity? Also when mounting PT on chassis should I use plastic washer to isolate it from chassis or no.
Same question for OT. I can see plastic washer on one side of transformer but not on the other side bell cover. There is continuity between bells and laminations, is that ok? Also does OT needs to be isolated from chassis or not?

What about old marshall laydown PT with copper foil around coils and laminations, what needs to be grounded with chassis and what not?
Now that laydown PT on jmp100 is mounted directly on chassis with laminations and bells all in continuity with chassis and it is working like that since I bought it, just checkin is that the right way
Thanks

[danman]

I had the same issue with my PT and I took a file and ground the plastic down flush with the transformer so that it would sit flat against the chassis with no gaps. I have not had any problems with mine doing this and I have been using the amp for over a year now.

[neikeel]

There is no current in the transformer lams and no need for continuity.

Some people say that using washers helps insulate from vibration but in general the transformers are best tightly clamped (some people even advocate heating theirs in an oven and re-clamp them in a vice before use to get them tighter!).