Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: theundeadelvis on February 02, 2014, 02:59:20 pm
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Hey guys! I've been away from the forum for a long time (two kids, new job, and new house will do that to you), but I've recently had the amp itch again. I've also developed an unhealthy Craigslist addiction. :help: Well, I came across an ad for a Gibson Thor, not working. I made an offer for $75, the guy said yes, and he brought it to me today. I got it home, opened it up, and it looks pretty much all original, with the expected collection of dust of and dirt, handles are shot, and the there are a few tolex nicks.
Pretty interesting little amp. The thing I find most interesting is, the controls are not mounted to the chassis. There are 2 umbilicals that run to a panel on the front of the amp where the controls are. Amp uses to 6EU7's and 2 6CA7's, is supposedly 50 watts, and has the two original (and perfect) Jensen C10N's. Speaker cabinet is completely enclosed. Also, worth noting, this is basically the same amp as the Kalamazoo 50 (so the internet has told me).
Now the fun part! :guitar1 I plugged it in and fired it up (carefully). It powers on, tubes glow, and once it was warmed up, I can get some faint guitar tones out of it, BUT with a lot of crackling and hiss and hum always present (makes no difference if the guitar is plugged in or not). Once it had been on a couple more minutes, it then went pretty much silent, with no guitar sound at all. And then it smelled like something was burning, so I shut it off. Chassis is out, and I plan to start taking some measurements, voltages, etc.
If anyone has any suggestions, please feel free to make them. I thought some might find this interesting, since there's not a ton of info on them out there.
:argue: And of course, THIS THREAD IS WORTHLESS WITHOUT PICTURES! So here they are: http://imgur.com/a/XlfXU (http://imgur.com/a/XlfXU)
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Schematic here ; Gibson call it Thor Bass.
http://bmamps.com/Schematics/gibson/thorbass.pdf (http://bmamps.com/Schematics/gibson/thorbass.pdf)
First is to check tubes . Tube tester is useless for amps tubes .
Second , check fuse value . Many second amp have wrong fuse value and you can damaged your amp if short happened .
If tubes are good , check voltages reading.
You can check bias cathode current to see if Output tubes are short or drive to much current . Bias too hot or Output Transformer little bit short
Check speaker DC resistance to be sure they are not short .
You can keep Output tubes removed to see if it smell burn . If something is wrong ( a burn may come from short circuit ) voltage reading will show you .
Or burn smell is dust burning ?
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Hey, pretty nice!
I have to tell you, from looks only, I would have suspected that era Gibson amp was solid state.
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Thanks guys!
stratele52 - good call on the fuse. Someone stuck a 5 amp in there, when it should be a 2 amp slo-blow. I'll probably order a new set tubes, and set the old ones aside while I'm working on it. While I'm at it, I'll probably go ahead and order new cap cans, and replace those guys. I also need to throw a 3 prong cord on it. My first goal is to get it safe, then we'll get it fine tuned and sounding sweet (hopefully).
eleventeen - it definitely has a 70's solid state vibe to it. They actually did make a solid state version a few years later.
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I'm starting to wonder if all it needs is new tubes. I got the chassis to my bench, wipe it down, and fired it up again with some different tubes. Unfortunately, I have no 6EU7 tubes, and I only had one good EL34. I fired it up with the original 6EU7's, my known good EL34, and one original EL34. The amp was dead quiet (noise wise), and I had good guitar tone, but not enough output. It sounded good, but cranked all the way up, it was a comfortable volume with no breakup. For giggles, I pulled the old EL34 and threw in a new JJ 6v6, and it sounded about the same. So I am wondering, if maybe some new 6EU7's might do wonders?
I think I'll get some tubes ordered, and go ahead and replace the cap cans, and hope for the best. As soon as I have some more time, I'll gather some voltages.
For the interested, here's a good page on the Thor: http://johnkvintageguitars.homestead.com/Thor.html (http://johnkvintageguitars.homestead.com/Thor.html)
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I would highly recomend using some contact cleaner to clean the tube sockets, pots and jacks. Bad contacts alone at these points can make an amp perform poorly. Platefire
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"For giggles, I pulled the old EL34 and threw in a new JJ 6v6"
I wouldn't recommend that!
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I wouldn't recommend that!
Yeah, I wouldn't actually run it that way for any extended period of time. But Eurotubes boasts about the robustness of their 6V6's, so I thought I'd give it the smoke test. :think1:
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I would highly recomend using some contact cleaner to clean the tube sockets, pots and jacks. Bad contacts alone at these points can make an amp perform poorly. Platefire
I will definitely be cleaning all of the mentioned. Surprisingly though, the pots aren't all that scratchy.
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This is a very cool amp with a couple of mods to make it guitar friendly. I had one. Don't be surprised if you install the new tubes and the thing is very clean and loud. Great find and for $75 is killer.
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Thanks Ed! Once I get it reliable and safe, I do plan on making it a bit more guitar focused.
This is a very cool amp with a couple of mods to make it guitar friendly. I had one. Don't be surprised if you install the new tubes and the thing is very clean and loud. Great find and for $75 is killer.
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Finally got around to getting this guy up and running. I replaced both cap cans, a coupling cap, and the tubes. This guy sounds pretty great. Really good with my PRS w/P-90s, not as great with my strat. Surprisingly not loud for 50 watts.
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Thanks guys!
stratele52 - good call on the fuse. Someone stuck a 5 amp in there, when it should be a 2 amp slo-blow.
+1
I see that often as I fix many amps. Same using guitar cable to plug a head to a cab = power output tubes blows . :BangHead:
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two 8ohm speakers in parallel.. I wonder what's on the primary side of that OT.
Didn't Gibson/Kalamazoo have a close relationship with Lowrey Organ? a lot of that amp starts to make sense when you think it terms of it being built along side organs. molex style plastic connector with umbilical, umbilical itself, the generic chassis (note that generic shape of the chassis, without having to contain an amp faceplate, or some exterior design component meant the the same chassis could be fitted to different amps, and/or organs. google up Lowrey organ chassis'es, i bet you'll see strong resemblances.
also, the construction of the cabinet is organ-esque. two tall sides sandwiching the horizontal sections. Estey did this same thing with its organs, and guitar amps (first the cheapo starlites, 411,421,etc) and eventually the solid state amps. If you think in terms of cutting costs in the cabinet shop, ditching eight tolexed corners in favor of two wrap around push-fit rubber weatherstrip would seem to cut build time considerably.
also noticed on the schematic the filament's reference to ground is on one leg, maybe via the chassis. If you have power supply noise, and you want to tweak, that might be a place to poke at.
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Gibson seemed to like those 6EU7 tubes..