Choke Placement next to Power Transformer

[MarshallPlexi]

Does the proximity of the choke in reference to the power transformer make any difference? I'm working on a design and the most logical placement of the choke is next to the PT with the laminations parallel with the PT's laminations. I know this is a huge no-no with Output Transformers but I was curious to know if it would make any difference with a choke.

Thanks!

[Wombaticus]

I don't actually have a good answer for this, but have been wondering the same thing myself. I will offer the observation that on the Bassman amps that Marshall copied, the choke was indeed positioned between the PT and the OT. But Marshall moved the choke to the other side of the OT -- whether to reduce noise or to facilitate layout, I'm uncertain. Note also that there is probably a difference if you are using a laydown type PT, as if the PT is Z mounted the OT and the choke fields should both always be orthogonal to that of the PT (assuming they are of the standup type). So it may be more critical if the PT is also of the standup type... That said, I'm not sure how big a deal choke placement is in the first place.

[tubeswell]

On all the amps I've done with chokes, I orient the laminations at right angle to the PT and the OT if possible - (which works better if you have a Z-mount PT). Otherwise try turning it around a bit if you get unwanted EMF coupling.

[bob p]

I know this is a huge no-no with Output Transformers ...

placing no two stacks of laminations in the same plane is a good rule to follow. we obsess on it a lot here. but i can't tell you how many HiFi amps i've seen that get away with placing the lams in the same plane, with the transformers snugged right up against one another.

Here's an example:




Interestingly, as "badly" as this amp is designed, the HiFi press at Stereophile commended the amp as being "the best amp they've ever heard" in their review. Go figure.

[mooreamps]

In some of my earlier builds, the chassis's were so small, all the transformers, chokes and tubes were just butted up against each other. But no audiable hum in the loud speaker, with the input shorted, at full volume. Perhaps this misalignment of the power transformer with respect to the output transformer is only a second order correction. I never worried about it since the output impendance of the output transformer is so low.

-g

[octal]

One of my earlier builds included an interstage transformer. Try puzzling out how to place a power X-mer, a choke, an interstage X mer, and an output x-mer so that none of them couple with each other! If I'd had a reverb driver X-mer too, it'd have been the perfect pent-fecta of potential unwanted coupling. I have attached headphones to the speaker wiring of an OPT and moved it around the chassis (with the power X-mer energized) to find the best orientation, but as Gary noted, it's really not that critical- I was just being a perfectionist.

[R.G.]

I have seen one example of an amp where the speakers emitted hum with the power switch on but all tubes removed. The OT picked up enough stray magnetics from the PT to cause a hum. That one caused a minor stir when it was found.