12ax7 and output transformer

[pjd3]

Hi,

I know this has been brought up but wanted to pass along this specific scenario, seeing as that I'm surely no expert yet in transformer magnetic emission fields.

After a good amount of time try to fit two amps in one big chassis while following advised (good advice from the folks at Amp garage) and written layout guidelines the best I can, it ends up that it is difficult to get the V1 12ax7 (or probably 5751) a substantial distance away from the output transformers. As it is, the two output transformers are in the rear middle of the chassis separated by about an inch. If the V1's are to be in a good position for wiring to the board component turrets, that puts the V1's about 1 1/4" away from the "non-bell" side of the output transformers. It may be worth it to mention that these are 20 watt amps that will be run at a relatively low stage volume. Whether that is of any consequence I'm not sure but, figured I'd mention.

The output transformers do have bell covers and I did purchase the Belton 9-pin sockets that allow a metal cover to go over the tubes.

Of course, this could very well be a case of "you won't know until you try it" but your input is surely invited. The principal system engineer here at Zoll Medical who used to design transformers consistently suggests using a galvanized steel plate between transformers and anything else should there be a fear of susceptibility, that although the galvanized steel material doesn't actually block or absorb the magnetic field, but that the field has a proclivity to more or less spread around the plate kind of deviating the magnetic field away from the destination of concern, in my case, the V1 of each channel of the amp.

It wouldn't be much of a problem to do that. I would consider it cheap insurance to even slightly or moderately improve any susceptibility that may occur.

Thank you!
Best,
Phil D.

[B Ingram]

... The principal system engineer here at Zoll Medical who used to design transformers consistently suggests using a galvanized steel plate between transformers and anything else should there be a fear of susceptibility, that although the galvanized steel material doesn't actually block or absorb the magnetic field, but that the field has a proclivity to more or less spread around the plate kind of deviating the magnetic field away from the destination of concern, in my case, the V1 of each channel of the amp. ...

Man, it's almost like that engineer does this stuff for a living!

IF you can't move the tube socket or transformer, then this is the next best thing. It basically "conducts the magnetic field" away from the sensitive stuff you want to protect.

[pjd3]

Thanks B Ingram.

Yes, he is the engineer that all the other engineers report to. He has forgotten 10,000 more things than I will even learn but, he is pretty generous with his knowlege and occassionsallly has time for me. But, its nice to be able to come here to run it by to see what others think about those things. Afterall, this is only my third amp, at least that I will gig with so, I'm really trying to take in what I can.

So it sounds like the right little metal plate in line with the first input tube and the output transformer is likely worth implimenting to help snuff out any chance of a direct hit. I will look into that pronto then. There sure is plenty of room to properly install it. I have spent alot of time trying to come up with and alternate layout that would adhere to all good electrical/magnetic layout guidlines but, this is the best compromise I could come up with. Everything else looks good - all of the Power AC is situated far from anything signal related, components are positioned for short lengths of wires where that is pertenent. Really the only potential issue is that output transformer/V1 proximity. It may not even be an issue but, it makes me uncomfortable and that shield may be just the ticket to both disperse any infliential fields and to allow me to live with myself. Good performance in the end is what I'm looking for and perhaps I will get closer to an ideal layout for the next amp build, which is already in the planning.

Thanks again for your vote of confidence. it goes a long way with helping me move along.
Best,
PJD3

[B Ingram]

... Really the only potential issue is that output transformer/V1 proximity. It may not even be an issue ...

I would build first without, and see where you land. If you suspect coupling from the transformer to the 12AX7, then add the metal plate.

I wouldn't want you to convince yourself it was doing something (if it wasn't) if it was included from the outset (but not needed).

[TUBEDUDE]

It sounds like you can cut a plate, bend a lip for screwing to the chassis, and just move it around. Same as you might do with the transformers.

[pjd3]

Another thought occurred to me.

Even if there isn't an issue with proximity underneath the chassis where the tubes and transformers are mounted, there is the presence of the plate wires inside the chassis that depending on the position of the OT could be close to the input stages of the tube, grid resistors and wires n such. Although it is just a pair of 6V6's, there still are large voltage swings on those wires. That got me thinking to keep that wire entry to the other grommet hole that is closer to the rear of the chassis than the front.

That option is there is desired, or needed.

Best,
Phil D.

[Helmholtz]

^^^^ That's a good point.
OT plate wires are the strongest radiators of electric fields inside an amp.
That's why I always recommend to twist primary (and secondary) OT wires and route them close to the chassis.
While they won't induce hum they can cause oscillation by positive feedback.
As long as those wires run above/outside the chassis they won't cause problems.
The magnetic field radiation of an OT typically is much lower than that of a PT.
(Similar thread over at diyAudio.)

[pjd3]

Thanks for that comment Hemholtz.

I hadn't considered moving the plate wires to the underside of the chassis. Although I don't consider that an optimal situation, it could perhaps be OK if it was situated properely, with good coverage and support. That would indeed create a much larger abyss between OT plate connection and input.

If I were to rotate the OT 180deg that would put the plate wires a few inches further away from the input V1 but it would bring in the speaker taps close to the input. While the voltage isn't terrible on those wires to my knowlege, there would be some good AC currents moving along them. Hmm.

Thank you,
Best,
Phil D

[TUBEDUDE]

/I guess I've been lucky. I've never had any issues with coupling, even with close proximity. See these distances. The O.T. is almost on the first preamp tube, and the hot channel pre preamp tube.

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