Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: nandrewjackson on July 23, 2020, 10:04:43 am
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Hi all,
In the autumn of last year I did a maintenance job on this 1990 red knob Twin. I did a full electrolytic re-cap, new screen resistors, and got it into fine working order. The owner brought it back to me a few weeks ago, it was making awful lightning/thunder noises when powered on. I dug into it, and found that there was some arcing and carbon scoring on one of the PC boards. Specifically, the tan colored board, underneath the big ceramic resistors.
I scraped away the carbon scored areas, and tidied up the traces, got it back up and running, but the owner has had enough of "The Twin".
To him, the amp is inventory, and with the re-cap last fall, and now this repair job, he's determined that he is "upside down" in the amp, monetarily speaking. He's a music gear dealer. He offered the amp to me at a discount of what he was asking for it, I've agreed to buy it.
I decided to rebuild as a single channel AB763, with trem and reverb using turret boards.
While removing the PC boards, I found that about a quarter of the signal wires were barely hanging on. On the tube sockets, and boards. I started off desoldering them, trying to be nice and neat, but after a few of them broke off with a slight tug with pliers, I put the soldering iron away.
I plan on using Doug's single channel AB763 pdf file as the main resource for this build.
The tube socket and transformer layout is not conventional for BF/SF, so I've decided to leave all that how it is, and change the turret board to fit the chassis layout. I don't have anything drawn up for this yet. It may require 2, or 3, separate turret boards, of unconventional size. One in between the front panel and first row of tube sockets, and another between the power tube sockets and middle row of tubes.
Questions, comments, and concerns welcomed
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Image links
https://ibb.co/Mk7DtWP
https://ibb.co/FhKyyq2
https://ibb.co/X8hxcbr
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https://ibb.co/L81qzQy
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should be a very solid base to work off of. Looking forward to the project. Dougs one channel sounds FREAKING SWEET but note that the layout doesnt include the bright switch thats in the schematic. I put the switch on the front plate of my build but its not attached to anything. Dont miss it yet.
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Hi nandrewjackson,
Really good looking iron and the chassis isn't too bad either.
I repaired a lot of those in late 90's and early 2000's. Very unreliable here in Aussie and R169(?) 30k/20W always crumbles away.
Best thing for it is to do the AB763 with Rev/Trem and it should become a fine amp :icon_biggrin:
I built Doug's one channel AB763 and in the end didn't fit the bright cap either.
Regards
Mirek