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Tubes too close to output transformers?

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dlec84

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Joined 2011
#1
Hi,

After drilling all the holes for my kt88 project (SE class A) I've realized I made a major miscalculation when placing my power tubes and output transformers. I underestimated the spacing and now I've got about a half inch between the kt88's and the output transformers.

I was wondering how big of a problem this is likely to be..

Will the fields of the OPT's passing through the output tubes lead to a positive feed-back loop? how much space do I need? (typically/ideally). I can push the transformers back about a quarter inch, but the tubes cant be moved.

How heat resistant is powder coating? Typical operating temps for kt88s?

Sorry if any of this has been dealt with before, I cant seem to find much info on the issue. Any advice would be greatly appreciated, this is my first build.

Thanks!
 
#2
Will the fields of the OPT's passing through the output tubes lead to a positive feed-back loop?
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No. Don't worry about that.

Heat is more of an issue. According to Wikipedia, powder coatings usually cure at about 390F (200C), so I guess you should be sort of safe. In the long term, I'd expect local degradation of the powder coating though. I wouldn't worry about the transformers though; more about the tubes themselves. They are convection cooled, and a transformer reflecting some of the heat they generate could make the tubes run a bit hotter. If this will be a problem depends largely on the rest of the layout and on how hot you run the tubes to begin with.
 
#3
The general rule of thumb for conventional, air convection cooled power tubes running close to maximum ratings, like in a class A amp, is they should be spaced at least their own, maximum diameter from other non-tube objects. Even then it is not recommended to enclose power tubes on all or most sides with close neighbors. That is surface-to-surface distance, not center to center, so about 2" of spacing would be the starting point in this case.

Double this distance for spacing to other power tubes, assuming you are not trying to make a cluster of them. For a large number of tubes you pretty much need forced air cooling, or a very big stash of spares.

Whether your close spacing will give problems with hot-spotting on the tubes is hard to say, tough I have to say I wouldn't build it like that. I am known for taking the belt-and-suspenders conservative approach, though. 😉

All these rules breaks down if you use radiation cooled transmitter tubes, but fortunately that is not the issue here.

For these tubes the surface temperature of the glass will hopefully not run above 2-250 deg C, about 390-480F.
 
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#4
Thanks for the quick responses!

So I shouldn't have to worry about feedback issues? That is great news.

I'm running the tubes a little cool due to power limitations, ~26.5W vs the 40+ they are rated at, so hopefully this reduces the heat problems slightly. I've raised the tubes about a quarter inch, and pushed them back about the same, so I'm hoping this will allow for more air flow.

As for the powder-coat lifespan.. I'm hoping I don't exceed the cure temp and the color maintains. Unfortunately there is not much I can do about my current situation aside from a new chassis..

Thanks again!
 
#8
I did the same thing, and have about the same spacing between the KT88's and the OT's. I've run this amp for many hours, and I've never felt the OT's even get warm. Of course it probably takes a very long time to fully heat up an 11 lb OT from one side 🙂

EDIT-
I forgot to mention that I do have a convection hole pattern drilled around the tube base. This however doesn't do anything for IR heating.

Glenn
 
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