Speaker Sound while in Standby?

[TD_Madden]

I noticed that when turning the amp on (with standby engaged), I still am getting a (low) hum through the speakers...it's not a mechanical noise from the PT humming, but definitely the speaker.

Since the output section isn't getting high-tension, where is this hum coming from...just the filaments?

[Steve Conner]

It's coming from magnetic coupling between the OT and the PT. I have an amp that started doing it when I rebuilt it with the transformers in different orientations.

It actually stops when I turn the HT on: I assume because when the power stage is fired up, the negative feedback rejects it.

[TD_Madden]

actually, it doesn't go away.....I just don't notice it as much!!

[bob p]

It's coming from magnetic coupling between the OT and the PT. I have an amp that started doing it when I rebuilt it with the transformers in different orientations.

Not intending to hijack, I've got a related question for you Steve.

We all know that its best not to orient the PT and OT so that their lams lie in parallel planes. Its easy enough to rotate one of them 90 degrees in some direction to minimize the problem.

My question then, is what's the best orientation for the choke? Optimally, I guess you'd want to orient the PT, choke and OT in three different XYZ planes. This is easy enough to do if you've got a laydown type PT, and a standup choke and OT. But what orientation is best for the choke if your only option is all standup style parts? Is it best to couple the choke to the PT?

[TD_Madden]

actually, that's no hijack..I was wondering the same thing!

[Enzo]

And what is the amp we are talking about. Not all amps remove the B+ in standby.

[TD_Madden]

a Plexi 50....1987 circuit.

[DirtyGeorge]

Bob p and TD_Madden,

I had no problem with orienting the PT and choke in the same plane. I reasoned that the coupling between the PT and choke would be less harmful than allowing the choke to couple with the OT.

But then I have only built one amp with a choke so I hope the more experienced builders will correct me if I am wrong.

DG

[TD_Madden]

I double-checked.....all three transformers are on different planes....

Enzo had mentioned another possibility being that the B+ may not be disconnected in standby.....

[Steve Conner]

Haha Bob, in actual fact, I don't really know

I'm currently trying to fit a 50W PT and OT, a big choke that carries the full HT current, and a reverb driver transformer, onto a 5E3 sized chassis, and I was pondering the same thing, except the 4 pieces of iron give another degree of freedom to mess up. :|

My plan is to copy the layout in the amp I got the PT, OT and choke from, and then find the position for the reverb driver tranny using a pair of headphones hooked to it while the PS is powered and loaded. It needs to be loaded to get the choke pumping out whatever EMI it's going to, though since it's a CLC filter, I expect the choke will be much quieter than in a choke-input filter.

The donor amp had a drop-through PT, stand-up OT, and stand-up choke, but the latter two stood up in different planes, so they did the three-plane thing. The PT and choke were also close together, but the OT was along at the other end of the chassis as far away from them as possible.

[TD_Madden]

turns out it does disconnect the B+ when standby is on.

I wonder if the CT could be lifted from ground as an alternative...or maybe I'm just grasping at straws and accept the sound...