Power transformer installation.

[wozt]

Hi. When installing a drop through type power transformer should it be isolated from the chassis? I though I remembered it should have been but there is continuity from all exposed metal to chassis ground. There are red fibre washers under the top posidrive screw heads and under the nuts but i can't see any on the bottom of the exposed transformer where it sits on the chassis. Thanks.

[Chuck H]

In most guitar amps I've seen the transformer is not isolated. The washers are probably there for another reason like vibration or fatigue.

[Tom Phillips]

The insulating washers are there to electrically isolate the bolts passing through the transformer lamination stack. Better quality transformers also use insulating sleeves the whole length of the bolts. The idea is to prevent the bolts from shorting all the laminations to each other. There is supposed to be one conductive path left, such as one uninsulated nut on one bolt, to ground the transformer to the chassis. In theory this is good practice, especially for output transforms. However, in practice, it is often not done for guitar amp PTs. The ultimate "not done" approach is Peavy's practice of arc welding across the whole lamination stack. The lams are thus totally dead shorted and the transformer performs OK for their purposes.

[Chuck H]

I didn't include this in my response. I stopped short when I considered that anyone with the know how to design a product of any kind should be able to see that a single washer on one side and no sleeve falls far short of isolating the laminates. It seems highly unlikely to me that anyone would choose a single flat washer on one side to achieve that result.

WRT shorting laminates... This goes beyond my pay grade as to practical figures, but I've read that shorting the laminates at the outside of the stack is of little significance unless the transformer is saturating the entire stack and that this is rarely the case. Still seems like good practice to NOT short them to me for ideal reasons. But practicality based on actual measured figures is eminently better than idealism. If Peavey has decided that welding the lams together on the outside edge doesn't detriment performance and improves mechanical stability then it's probably true. Why include a process that no one else does if there's no advantage to it? Peavey is definitely a company that wouldn't do that.

[52 Bill]

If Peavey has decided that welding the lams together on the outside edge doesn't detriment performance and improves mechanical stability then it's probably true. Why include a process that no one else does if there's no advantage to it? Peavey is definitely a company that wouldn't do that.

I think that Tom is referring to the way that Peavey power transformer laminations are welded to a flat metal plate that is used for mounting to the bottom of the head cabinet.

Definitely hard core mounting, with no shot at rewinding.