Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: alerich on November 08, 2013, 10:04:23 am
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You may remember a thread I started earlier this year on chassis heat and fans:
http://www.el34world.com/Forum/index.php?topic=15482.msg149277#msg149277 (http://www.el34world.com/Forum/index.php?topic=15482.msg149277#msg149277)
Originally the amp was running way hot. The fan worked really well. Now I am wondering... maybe too well? This is a 2xEL34 / 3x12AX7 amp (50 watt Marshall JCM 800 clone) with the tubes hanging upside down in a small 8" practice amp donor cabinet/chassis. Before the fan after 5 minutes you could not touch the chassis for more than a second it was so hot. I installed a 12VDC computer fan running on a 9VDC wall wart blowing across the bottom of the cabinet. It's not blowing a tremendous amount of air. Now the amp can literally be left on all day and at any time you wish you can touch the EL34 glass and leave your fingers there all day. They are warm to the touch but they will not come anywhere close to burning your fingers.
I began to wonder if there is some actual physical temperature that the glass has to reach for proper tube function. Obviously, the heaters are hot. The amp works fine. What started out as simply a fun "I wonder if this would work" project has turned into one of my favorite "go to" amps. My guitar buddies all comment that it sounds like every great stock JCM 800 they have ever heard.
If it sounds good it is good, right?
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> temperature that the glass has to reach for proper tube function
No.
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...with the tubes hanging upside down in a small 8" practice amp donor cabinet/chassis. I installed a 12VDC computer fan running on a 9VDC wall wart blowing across the bottom of the cabinet.
How did you place & secure the fan in the cab. Does it make any noise or rattling at all?
Are you simply rolling up the wallwart transformer into the back of the cab when transporting or is it hooked up to the 117VAC in some way?
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I mounted the fan blowing sideways left to right (looking from the rear) underneath the tubes. It is held in place with two small aluminum L brackets. The wall wart is held in place with velcro. I wired an AC plug to the AC supply after the power switch. The fan is 12VDC and the wall wart is 9VDC so the fan runs about 75% speed. Not the most elegant implementation by any means but considering the project in question it works and suits my needs. The speaker opening is covered with a piece of masonite that I painted black.
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Boy oh boy, I hopped up a SF Deluxe Rev to 4 qty 6L6 back in the day and I thought that was pretty tight. I am surprised (based on your report) how effective your fan is. Well done!
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Thanks, was wondering how all that stuff was also going to contain a speaker in the cab also. Nice. I've seen where installing a fan can be tricky at times w/ vibration and noise issues.