Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Cabinets-Speakers => Topic started by: tubenit on August 21, 2009, 10:44:38 am
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What type of clearance above the power tubes do you guys usually like to see in a cab head that is unvented? Such as with EL84 tubes?
Do you ever put RTV silicon on the wood "ceiling" above the tubes?
Tubenit
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All the ones I've done have vents if they're Marshall-style with the chassis sitting on the floor of the cab. I haven't built any in the blackface-style as heads, but it sounds like you're talking about Marshall-style anyway.
The RTV silicone sounds like a good idea to me if there was some reason I absolutely couldn't cut a vent. Even so, vent or not, I'd allow for airspace that at least equals the center-to-center distance between tubes as recommended on the data sheet. That would be at least enough, I would think.
The shortest head cab I've made had about 4" of relief between the top of the power tubes and any part of the cab/vent. That was a head cab with a 10" overall height (26-1/2"W x 10"H x 10"D). The vent was a big one - I think something like 3-1/2" x 10" or something like that, with a plastic vent insert like on a Marshall. I wanted space enough for any KT-series power tubes.
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Thanks for the response! I was just curious. I have used vented and none vented heads. On the none vented I just RTV on the wood and vented the back panel well which was somewhat close to the power tubes.
Not had any heat issues/concerns that I am aware of on any amp I've built.
Thanks, Tubenit
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If the RTV is to protect the wood, I would use a heat shield in stead. Piece of sheet metal with 1/4" standoffs will work very well.
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There is one thing I do that I wasn't thinking about: The black paint I use inside the cab is the high heat automotive variety. I only use it so the prolonged heat doesn't make the paint flake off, but maybe it's protecting the wood somewhat, too? I don't know. I guess we're all generally trying to avoid a fire hazard and I don't know if the paint hurts or helps that situation. I know it doesn't flake off or turn colors inside the cab... like it does if you use it on what it's supposed to go on, like headers, exhaust manifolds, etc.
Isn't there an anti-flame wood treatment somewhere we could use? Seems like I've read or heard of something like this. I guess I'll try google and see if it's still my friend. If I find something plausible, I'll post it.
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You are NOT going to set fire to wood with tubes!!
Try it.
I've had a 6550 at full rating for hours, threw a Kleenex on it, hardly browned.
Receiving tube temperature is limited, partly by material limits, and mostly because "low" temperature materials (seals, glass) are cheaper.
Yes, the wood will warp, the glue will turn brittle, and somewhere in there your caps will cook, die years earlier than necessary. * Vent the cabinet! * But not for "fire risk".
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Here's how Sundhy did my Sceptre back in the '60s/'70s. There's about 2.25" clearance between the top of the 6550s and the cab. But wait. There's also a reverb pan sitting up top too!
The tubes are mounted toward the rear edge of the chassis though and the back panel allows for pretty good ventilation.
http://home.comcast.net/~seluckey/sunn/sunn.htm