Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: duke of earl on July 21, 2010, 09:59:42 pm
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I got a call from a guy who was given a Twin Reverb Head that sat in a paint/body shop for 20+ years. I brought it home and pulled the chassis out for inspection to find it covered in dust. It is an early silverface non master volume that has been blackfaced. He powered it up with a speaker cabinet and it blew the fuse. I pulled the power tubes and it still blows the fuse so it's either the rectifier or power tranny. It is a project but I am going to do a full overhaul but am wondering if the oxide dust from years of sanding cars has destroyed the pots and transformers.
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Pots, quite possibly. Your best bet IMHO is to blow them with compressed air BEFORE YOU PUT ANYTHING LIQUID ON OR IN THEM!!
If it blows fuses w/o tubes installed I would cut one lead of the red HV wires that goes to the 2 * 3 diode stack and see what happens on plug-in. By the way, if I have a real persistent fuse-blower on my hands, I usually take a length of stranded wire and strip it and pick one tiny strand of wire and solder it across the fuseholder....effectively making my own fuse...rather than blowing out half a dozen fuses. All this debris costs money now! When I used to screw around with Fender tube amps in the 70's there were plenty of parts you could buy for 25 cents. Now, every screw is a religious experience.
You are right to suspect the power trans especially because it hangs upside down and might have gathered dust. But in my experience, transformers are practically indestructible.
My first suspect is always caps, and it would pretty much be time to do those anyway. Who knows, the thing might have been electrically bad when it was put away and left there. Could also be a shorted death cap or a short in the filament winding. You'll find it.
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I will keep looking for the problem when I get some time but with 100 degree heat and I am a HVAC service tech, there's not much time for amp work. Thanks for the tips eleventeen. I am going to remove all the components and clean the chassis along with everything else and rebuild accordingly. I will post pics as I make progress. I plan to drop 2 power tubes and rebuild for 50 watts and tweak channel 1 for a plexi-ish tone.
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I will keep looking for the problem when I get some time but with 100 degree heat and I am a HVAC service tech, there's not much time for amp work.
Boy I remember those days. I did that in Houston, Tx. for 8 years. We starved in the winter with gas heat and worked 18 hour days in the summer. I remember one day in particular that started at 6am and I drove up to my last service call at 12:30 that night. I knocked on the door very gently and the door immediately flew open. The guy looks at me with a mean glare and says "It's a little late for this sort of thing isn't it?!" So I said "That's entirely up to you sir. I've been at it since 6am and you're my last call but I'll gladly go home and go to bed. Or if you want, I'll work by drop light and try to get your AC running so you can sleep tonight." He looks at me and says "Let's do it!" I got it fixed and he was one happy camper. You can't sleep in Houston with that heat and humidity with no AC. Sometimes I felt like Mother Theresa. :laugh:
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Did you mean to say Dual Showman Reverb head ?
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The use of a simple bulb limiter will help avoid continuosly blowing fuses. One of the "Must" tools, even for a part timer like me. IMHO
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Nope it's a Twin that someone cut the cabinet down into a head version. Eric Johnson has a few of the same amps (check him out on Austin City Limits). I worked on pulling the circuit board the last couple of nights and noticed the power tranny is melted. I made a current limiter a while back and put a 100 watt bulb in it.
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m-e-l-t-e-d...?
:undecided:
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Or LOOKS melted.Lots of black goo in the holes where the wire come out?
May be normal.Did you try powering up with no rectifier tube?You need to eliminate possibilities before condemning anything.
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Twin Reverbs didn't have a rectifier tube, but the idea is good. Just disconnect the rectifier diodes and see if the tranny gives you somewhere around 340-0-340
Dave
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Oh yeah,TWIN.It still could have a bad recitifier diode,but it won't give 340-0-340 without a filter cap connected.It will be about 240v or so.
Check the diodes.
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Well the jury is in. I have completely disassembled the chassis to clean it and am going to replace tube sockets, power tranny, and maybe the pots. I'm afraid to use the pots because of the oxide dust from years of sandpaper dust collecting in the amp. Oh the amp is a 68 model and is not blackface modded-yet.I may try cathode bias/adjustable fix bias switching to make it a little more versatile. Here's an interesting scenario- (4)EH-6v6's instead of (2)6l6's cathode biased-anyone ever try this? Doug says these tubes will work in a Twin Reverb.
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If I were doing this project, I'd just fix what's broken before considering modifications. I'd also give the pots a good cleaning before making a decision to replace them.
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It still could have a bad recitifier diode,but it won't give 340-0-340 without a filter cap connected.It will be about 240v or so.
The 340-0-340 is AC. You can measure that even with the transformer leads hanging in the air.
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The owner doesn't want the full 100 watts so I can use 2-6l6's or some other alternative.