009
Wholenote
Take me to your leader!
Posts: 521
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Post by 009 on Jan 31, 2020 8:11:44 GMT -5
Question: I have a mid-60s (1965?) GA-5T (from the time they became rectangular in shape). It has two inputs, and having to make a repair on one of them, I know they differ a little with their electronics (e.g., resistor at one jack vs. no resistor at the other jack, etc.). I’ve tried to do a comparison of the two inputs, but with my hearing limitations, I can’t discern any real difference. Can someone here tell me what Gibson intended with these two different inputs? While I was able to find the schematic online, I have never been able to find the user manual.
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Post by Leftee on Jan 31, 2020 10:05:27 GMT -5
I have a ‘64 Explorer that uses a pair of 6BQ5s. AKA EL84.
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Post by LTB on Jan 31, 2020 17:18:44 GMT -5
I have a ‘64 Explorer that uses a pair of 6BQ5s. AKA EL84. Thanks Lefty! did not realize 6BQ5 and EL84 were one in the same
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Post by Vibroluxer on Feb 2, 2020 18:16:13 GMT -5
I thought there was a Gibson amp with EL-84’s? I don't know, but wasn't the GA-5 "reissue" an EL84 amp? Yep! I have one. It was modded to increase headroom. It's a little tone monster.
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pnutz
Halfnote
Posts: 83
Formerly Known As: "Most folks just call me Jimm."
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Post by pnutz on Feb 12, 2020 22:28:10 GMT -5
Puntz, which model is that beauty? It's a GA30RVS ... many find troublesome ... I've had nothing but goodness ...
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Tequila Rob
Wholenote
Posts: 685
Formerly Known As: Guitar Fool
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Post by Tequila Rob on Feb 13, 2020 11:54:39 GMT -5
wow.....you've got an awesome collection there my friend
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Post by jazzguy on Feb 13, 2020 12:15:00 GMT -5
wow.....you've got an awesome collection there my friend thanks, the sad thing is they don't get played anymore, I'm strictly a blackface Fender amp guy these days. but I don't have a 401K or any other retirement fund and these keep going up in value, so....
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Tequila Rob
Wholenote
Posts: 685
Formerly Known As: Guitar Fool
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Post by Tequila Rob on Feb 13, 2020 19:18:48 GMT -5
wow.....you've got an awesome collection there my friend thanks, the sad thing is they don't get played anymore, I'm strictly a blackface Fender amp guy these days. but I don't have a 401K or any other retirement fund and these keep going up in value, so.... You know, I only have the 3 Gibson Amps I started the post with but there are times I miss that Blackface clean.....
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Post by jazzguy on Feb 13, 2020 22:16:44 GMT -5
thanks, the sad thing is they don't get played anymore, I'm strictly a blackface Fender amp guy these days. but I don't have a 401K or any other retirement fund and these keep going up in value, so.... You know, I only have the 3 Gibson Amps I started the post with but there are times I miss that Blackface clean..... I get it, I was playing duo gigs way back then and the low wattage was acceptable but it's been Hammond B3 gigs the last 20 + yrs so it's been blackface Vibrolux Reverbs, more frequently bf Twins
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Post by LTB on Feb 16, 2020 19:27:06 GMT -5
Question: I have a mid-60s (1965?) GA-5T (from the time they became rectangular in shape). It has two inputs, and having to make a repair on one of them, I know they differ a little with their electronics (e.g., resistor at one jack vs. no resistor at the other jack, etc.). I’ve tried to do a comparison of the two inputs, but with my hearing limitations, I can’t discern any real difference. Can someone here tell me what Gibson intended with these two different inputs? While I was able to find the schematic online, I have never been able to find the user manual. Back in the 1950's and early 60's Musicians would sometimes use one amp for two guitars etc so they had 2 jacks. Later models put a resistor divider network on one to cut some of the signal if one person wanted to tame an amp some for whatever reason such as a person playing lead on input 1 and they wanted the 2nd person to be at a lower volume and you only had one volume control on the amp.
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009
Wholenote
Take me to your leader!
Posts: 521
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Post by 009 on Feb 17, 2020 7:20:44 GMT -5
Thanks. From observing the second input jack directly and viewing a schematic, there is a resistor right there, whereas the first input has none. I did not understand the effect and significance.
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Post by LTB on Feb 17, 2020 21:35:01 GMT -5
I don't know, but wasn't the GA-5 "reissue" an EL84 amp? Yep! I have one. It was modded to increase headroom. It's a little tone monster. What exactly did you do to increase the headroom?
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Post by Vibroluxer on Feb 18, 2020 17:54:00 GMT -5
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Post by Vibroluxer on Feb 18, 2020 18:32:19 GMT -5
There's a video in the link
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pdf64
Wholenote
Posts: 557
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Post by pdf64 on Feb 19, 2020 7:09:37 GMT -5
Question: I have a mid-60s (1965?) GA-5T (from the time they became rectangular in shape). It has two inputs, and having to make a repair on one of them, I know they differ a little with their electronics (e.g., resistor at one jack vs. no resistor at the other jack, etc.). I’ve tried to do a comparison of the two inputs, but with my hearing limitations, I can’t discern any real difference. Can someone here tell me what Gibson intended with these two different inputs? While I was able to find the schematic online, I have never been able to find the user manual. It's probably just a grid leak resistor; only need one as both inputs feed the same input stage grid. But a link to the schematic you mention would be helpful here. There are numerous versions of this amp model A grid leak resistor is necessary to ensure reliable operation of the tube, which must have a dc path between its grid and cathode www.aikenamps.com/index.php/designing-common-cathode-triode-amplifiersSome super cheapo 50s designs rely on the instrument plugged into the input to provide that dc path. If it's a regular guitar, then that works out ok, but try that with a capacitor mic, piezo pickup or some effect pedals (eg Rangemaster treble booster) and the grid voltage may wll drift off and the input stage be stuck deep into saturation or cut off, and no signal can get through.
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pdf64
Wholenote
Posts: 557
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Post by pdf64 on Feb 19, 2020 7:21:41 GMT -5
There's a video in the link The video commentary just says that the gain was reduced with a few cap changes, no further detail. Guitarists are want to describe a change that results in the gain/volume control being set higher before distortion kicks in as being an 'increase in headroom' but technically that's incorrect, the headroom the exactly the same as it was before, it's just the user interface is a little more to their liking. Whatever
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009
Wholenote
Take me to your leader!
Posts: 521
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Post by 009 on Feb 19, 2020 7:43:05 GMT -5
Pdf64 said: ”But a link to the schematic you mention would be helpful here. There are numerous versions of this amp model ” Yeah, there are a lot of versions, and as I discovered this, I realized that I can’t be sure that the schematic I saw is the correct one. But the info I’ve gleaned in this thread is enough for me to understand that it’s not some sort of effects alternative, e.g., clean vs. dirty.... My grasp of electronics is rudimentary, at best.
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pdf64
Wholenote
Posts: 557
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Post by pdf64 on Feb 19, 2020 8:42:56 GMT -5
Haha, ok. With many designs the prime reason for 2 inputs was probably to allow 2 things to be plugged in at the same time. As a historical perspective, Jack Darr’s book says a whole string band could manage with the 4 inputs provided by a twin reverb etc.
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009
Wholenote
Take me to your leader!
Posts: 521
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Post by 009 on Feb 19, 2020 15:55:48 GMT -5
So, I poke around my HDD and found two main schematics. One had two parallel resistors (47k), one on each input jack, but I know this is not the one for my GA-5T. The other is here (and is the only one provided for GA-5T: EDIT: YOU WILL ONLY SEE A SMALL BOX HERE (OR MAYBE NOTHING, BUT CLICK THE BOX OR THE SPACE, ANYWAY - THE IMAGE WILL APPEAR. (The link for the complete list of schematics is at the bottom of this post.) archive.gibson.com/Files/schematics/GA-5T.jpgYou can see that big 1M resistor on one of the jacks. The jacks/inputs are not identified, #1 vs. #2, but I have looked inside my amp and could see that resistor on input jack #2 --- This jack (connection) did not work when I received the amp, so I went looking for something that didn't look correct. I got really lucky right away when I noticed that one end of the resistor on #2 had broken free from its solder joint on the jack; apparently someone tightened up the jack using the nut on the outside of the control panel and the multi-tooth locking washer on the inside of the jack's assembly did not prevent the jack from rotating, so the resistor's wire broke. I simply repositioned the jack's rotational position and resoldered the resistor at its original solder joint (after tightening the jack properly). Anyway, this schematic is probably correct, and it does come from the Gibson archive. Here's the Gibson archive site in case someone is interested.... archive.gibson.com/Service/Tech/Schematics/
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Post by Vibroluxer on Feb 20, 2020 17:18:08 GMT -5
Sorry, that's all I got. But I'm sure you write the guy in my posted video and her tell ya what he did.
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