Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jojokeo on November 11, 2012, 07:05:57 pm
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Looking for info regarding what's best or minimum for height clearance for a chassis when mounted inside of a custom head-only cabinet? Can you just give a couple of inches above the highest tubes or is there some other way? I don't really want to have an opening w/ screen like on Marshall heads. Something more like Fender's Bandmaster or Bassman heads...how much clearance do they give for those? Thanks for the help as always, Keo
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The blackface bassman has just a bit more than enough room to get the 6L6 in vertically before plugging it into the socket. See pic below. By the way, the tubes pictured look like they might be short-bottle variants, like a 6L6WGB/5881 (thinking 70's-80's G.E. or Sylvania JAN 6L6WGB).
Recommendations?
Well, the "best" way is always to mount the tube upright, though we all know they aren't always done that way. Having a vent on the top is a good thing, though Fender only kept the back open.
As long as there is room/openings for air to circulate, the tubes will probably be happy. I'd prefer to err on the cautious side, because I like to avoid self-inflicted issues.
(http://www.oldenburger-music-station.de/Fender_Bassman_Blackface_1964_Back.jpg)
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I wasn't clear sorry, the chassis' will be mounted on the bottom a la Marshall types and w/ the standard back plate piece (the pic below doesn't show this). I've got several sizes of Hammond chassis' so I was thinking 2" minimum clearance from top of either tubes or PT for 6aq5 & 6dl5 type of bottles or w/ 3" being better for El84s or 6V6s and maybe 4" min for tall 6L6 or EL34s. Does that sound like enough to ventilate these types? Also w/ no top vent, would a piece of metal flashing or = be advised stapled to the top underside or does that really matter? It would be something similar to this:
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If that back is staying open, you have plenty of ventilation. There's more than adequate room for air currents to circulate.
I don't think you'll need anything on the underside of the top. If the tubes were a *lot* closer, and you had some kind of finish that might be flammable or easily marred by heat, then maybe. But I think there's enough distance there that the inside top will simply get a little warm and not much else.
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That's only an example and not the actual situation but just to give the visual. There will be back panels mounted also similar to a Marshall head, HiWatt, etc...
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I'd keep the power tubes in the back closest to the vent opening in the back panel, like Fender and Marshall.
Brad :icon_biggrin:
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Long as the back panel has opening at the top & bottom... let's cool air in the bottom and hot out the top.
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I would staple/screw a piece of aluminum flashing on the inside of that front panel to stop the wood from getting too hot from that tube. (Looks kinda close in the picture) You could get fancy and put angle bends on the side edges of the flashing to maintain a gap between the wood and the flashing.
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Okay I'll start over - Prior picture is NOT what I'm actually using or doing.
Look at picture below. I will be using something like this standard cabinet head a la Marshall, HiWatt, and others.
Power tubes will be closest to the back of cab, standard stuff. I'm looking for minimum height recommendation for adequate ventilation of varioius power tubes in this type of cab.
My intial thought is this:
6AQ5, 6DL5 - would need 1" minimum?
6V6 & EL84 - would need 2" minimum?
6L6 & EL34 - would need 3" minimum?
Do these clearances seem okay with enough room for cooling if using the type of cab in picture below???
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Yes, I have built many head cabinets of that style. I usually leave enough room to put a bias probe on the tube and still be able to get the tube in the socket. Never had any problems with over heating. I wouldnt go less than 2" for the smaller tubes.
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I seem to remember a loooong time ago someone here built a cab with holes over the power tubes so the cab would be more compact :dontknow:
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Such a small valve as the EL34, dissipating a large amount of power at the anode and screen grid as heat, needs reasonable ventilation. It should be mounted vertically, and the air should be able to circulate freely. The distance between two EL34's should be at least 40 mm, and the distance from the cabinet and other components at least 30 mm. Wirewound resistors, mains transformer, and rectifier should not be in the immediate vicinity.
A New 25 W Audio Output Pentode
An advert from Mullard Ltd 1952 for the EL34.
http://www.r-type.org/articles/art-016.htm (http://www.r-type.org/articles/art-016.htm)
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Thanks, that was a nice xf2 in that photo also :icon_biggrin: