Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: rafe on March 30, 2023, 01:44:58 am
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First look inside there was a repair receipt from 82 a cap or two replaced (The receipt was folded to a small size and the corners were sketchy when unfolded. $9.50 for parts 10.50 for labor and a dollar for the gov'ner. Not as tight as the Les Paul but there will be some questions. There is a box of caps, I have never come across one of them before I wonder if that was the repair?I am going to replace all the electrolytics. I think the orange ones (non-electrlytic) are early Sprague's anyone familiar with their quality? The resistors look like allen B's so I'll test them ....I have a question on the GA40t the plate resistors on the power tubes drifted from 470 to 570 It sounds good am I playing with fire leaving them in place?
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Odd things marked
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" I have a question on the GA40t the plate resistors on the power tubes drifted from 470 to 570 It sounds good am I playing with fire leaving them in place?"
- rafe
No, tube will run colder.
I'll replace it to have amp stock. Snd to avoid that in 1 years or more(?) the resistor still increases or worse opens= no sound
If amp sound good, leave orange caps in the amp.
The what you call "box of caps"; IMO it is a coil and original . Look at schematic below 30 mH and 10 mh coil in power supply
The multi section capacitor is a repair IMO
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https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga50t.pdf
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Your letters are too small to see them. I see some by chance.
I have re-written your question about the pot.
Can you read schematic to see where the pot are wired ?
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More
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The what you call "box of caps"; IMO it is a coil and original . Look at schematic below 30 mH and 10 mh coil in power supply
No, it's a "bathtub" oil filled cap. Looks to be a stack of more than one cap. Bathtub style caps (https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=bathtub+capacitor&_sacat=0&LH_TitleDesc=0&_odkw=dual+bathtub+capacitor&_osacat=0&LH_PrefLoc=2) were very common during that time. You look closely at the schematic. What you call 30mH and 10mH coils are actually 30hy and 10hy choke coils. (hy = H = Henry) The two chokes are mounted at each end of the chassis.
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The what you call "box of caps"; IMO it is a coil and original . Look at schematic below 30 mH and 10 mh coil in power supply
No, it's a "bathtub" oil filled cap. Looks to be a stack of more than one cap. Bathtub style caps (https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=bathtub+capacitor&_sacat=0&LH_TitleDesc=0&_odkw=dual+bathtub+capacitor&_osacat=0&LH_PrefLoc=2) were very common during that time. You look closely at the schematic. What you call 30mH and 10mH coils are actually 30hy and 10hy choke coils. (hy = H = Henry) The two chokes are mounted at each end of the chassis.
You are right !
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Very interesting amp - especially that tremolo circuit with the ganged pots. This sold listing on Reverb has some useful pics of the guts - restored, but it appears that the work was done close to the original layout.
https://reverb.com/item/29273842-completely-restored-1949-gibson-ga-50-vintage-tube-amp-combo-jensen-alnico-speakers (https://reverb.com/item/29273842-completely-restored-1949-gibson-ga-50-vintage-tube-amp-combo-jensen-alnico-speakers)
It'll be a fun challenge.
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...unless you're really OCD with a touch of germaphobia :icon_biggrin:
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Very interesting amp - especially that tremolo circuit with the ganged pots. This sold listing on Reverb has some useful pics of the guts - restored, but it appears that the work was done close to the original layout.
That's a different amp.
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"Your letters are too small to see them. I see some by chance.I have re-written your question about the pot."
Sorry about that , In checking my post I was able to click on the picture which enlarged it , I then could enlarge it again and it was easy to see. I knew that the tremolo had stacked pots (Lower left corner of schematic) but was surprised that one had 3 pots stacked together with the shaft inside the chassis????
"No, it's a "bathtub" oil filled cap. Looks to be a stack of more than one cap."
It appears to be 4 - 25 @400 volts It's mounted pretty nicely so I'm glad it is original, I may refill that one.
It is definitaly a different amp Like the ga5 and ga5t they do however look the same from the front.....This is the GA-50T and there is less room to work in there than the GA-50
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...unless you're really OCD with a touch of germaphobia :icon_biggrin:
I agree, when I lifted the chassis and saw the folded up repair receipt it was covered with dust and I thought it was a sheet of asbestos (Like the ones I found under the heat shields on my supro thunderbolt). So rather than suiting up in hazmat garb, I just closed my eyes when I picked it up. There is going to be a good cleaning m
needed on this one .
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(https://el34world.com/Forum/index.php?action=dlattach;topic=30146.0;attach=104946;image)
OMG :huh: :huh: :huh:
Franco
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...I think the orange ones (non-electrlytic) are early Sprague's anyone familiar with their quality?...
If they are actual Orange Drops (can't see in this angle) then they are good forever. And NOT original next to a 1953 OT: Orange Drops start around 1960 and were mostly sold for replacement, not factory.
Most of the other caps may be past their best-by dates.
Most of the resistors look post-1953 to my eye.
Me, I'd shellac it and hang it on the wall. But maybe to someone it would be worth a tedious restoration.
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^^^^ what he said, wall hanger
make sure you're tetanus shot is up to date :icon_biggrin:
looks to be a Monday build after a hard week at the Gibson plant
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That's a different amp.
And so it is - I jumped too quick.
Here is a thread that includes some pics on the second page.
https://www.jazzguitar.be/forum/guitar-amps-gizmos/58335-gibson-ga-50t.html (https://www.jazzguitar.be/forum/guitar-amps-gizmos/58335-gibson-ga-50t.html)
I would be one of those that PRR mentions would be foolish enough to tackle the restoration - despite the fact that Gibson stuffed way too much into that narrow chassis - typical of them.
Tetanus - Heck I bought an old Gretsch Electromatic on Ebay that was listed as "works great." I opened it up and there was a dead mouse with its teeth around B+ and he had managed to ruin most of the lower voltage wiring before he met his match. That amp is making noise again.
You can do it Rafe.
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(https://el34world.com/Forum/index.php?action=dlattach;topic=30146.0;attach=104946;image)
OMG :huh: :huh: :huh:
Franco
Toss in a few more assorted caps, resistors and some hook up wire and it would look like a drawer full of used parts! :help: :w2: :dontknow: :BangHead:
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"Me, I'd shellac it and hang it on the wall. But maybe to someone it would be worth a tedious restoration."
PRR do you remember that amp I did many moons ago where I had to draw the layout for you? It was from the 30's and looked like perhaps a homemade copy of an early gibson or some kind of prototype? #42 power tubes and a #80 rectifier. Now that was quite a challenge for me. I sold that to a guy in a 30's band in Europe ....He paid me well for it. I have heard that Jazz guitarists like this model .....a lot! I believe this will be easier than the ga40 les paul I just did and these are both players amps ..... .....We will see ........Stand by I may need your help again!
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"make sure you're tetanus shot is up to date "
It is I was rooting around for a drill bit in my nephews shed and managed to get a treble hook about an inch into my right index finger. My nephew is an EMS and says; I'm going to have to push it through and cut it. I says; like hell you are and spent the next ten or fifteen minutes taking it out as close as i could figure to the path it took in . I hadn't had a tetnus shot in over 30 years so I got one. But on another note seeing's how you all would just toss this one out :l2: I guess it's in the right place. This one is magic, I promise To give it my best shot, I'm even going to get a new soldering iron. I will put up some recordings with it and the GA40 when they are complete. Thanks for the support Brian
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Before anyone tries to give you the old - figure your time and the cost of parts and the value... blah blah. This is not resource economics - these are pieces of history. In this case, a piece of jazz guitar history. Jim Hall and Bernie Kessel played through 50 & 50Ts. Sure - there has been some questionable work done. But document what you have. Remove what you know must go. Clean it really well. Install new electrolytics. Test it using a variac. I do not think its as bad as first glance would leave one to believe. Ever watch Maine Cabin Masters? Onward.
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The camera does not do it justice, it's dusty and needs a clean-up but it will be fine. The link you posted is great. This amp is pretty
close to ready to go. biggest issue is that the 8" alnico 5 PM is missing I have 2 from 1960 that I have had reconed in the ga5's I can borrow one for the time being ...I have not checked the 12"alnico5pm concert yet hopefully it is intact and working. if not off to miller sound. These are not only historic they are :worthy1: marvelous I have a P12n (1950) in the Ga40 right now that sounds great at low volume but it does have a voice coil issue above 1/3 volume on the low e-string up to about the b note ,funny though the a string is fine ....I will probably have it reconed at some point.
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If you have the time, patience, and parts to restore something like that, all the power to you. It will be well worth it.
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oh, I'm cheering for you using negative reinforcement :laugh:
the Plymouth radio that i'm bring back, my brother cleaned out the rats nests before he gave it to me, striping down the chassis lots of black rice was wedged into the wires, under the tuners...
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8" alnico 5 PM is missing
I think the Weber Alnico 8s are great replacements for that speaker.
They work well in old Plymouths too. :icon_biggrin:
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"No, it's a "bathtub" oil filled cap. Looks to be a stack of more than one cap."
It appears to be 4 - 25 @400 volts It's mounted pretty nicely so I'm glad it is original, I may refill that one.
I see 4 terminal lugs. That means only two caps. Remember, each cap has two connections. Those caps are most likely perfectly good.
The odd triple stack frequency pot is really just a dual pot as shown on the schematic. The third pot is the oscillator bias pot. It's just a single pot that is soldered to the back of the dual frequency pot. Clever??? :dontknow:
This amp is very restorable. Probably can't make it look any better than it looks right now. Gibson is known for ugly guts. I've attached one restoration...
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"I see 4 terminal lugs. That means only two caps. Remember, each cap has two connections. Those caps are most likely perfectly good."
Thanks Steve, I thought it was like a can where the ground was the metal outside. Good info . That box might be good??
"This amp is very restorable. Probably can't make it look any better than it looks right now. Gibson is known for ugly guts. I've attached one restoration..."
Chrome won't get you home .....I want to listen to it not look at it lol, That photo of the restoration is good to have to compare. I am waiting for my phone to send pictures of the Sprague orange drops in the amp....I have to send to e-mail then download but most of the non-electrolytics are SOD's
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The chassis is out and on the bench Speaker has been removed and put in a safe place in a box OT was removed and wiring left intact. The speaked looks perfect We will see and hear more about that . The 2 stacked boxes of caps are.25 and part of the tremolo. V7 is tremolo tube, and schematic calls for a 6sl7 the only markings on that tube in the amp are on the top (It's a Tung-Sul brown base) The numbers are 3223 6138 There is also a .1 cap (also tremolo) that appears to have been lifted off that socket at some time(pin pin8, heater). I see a 6188 as a substitute but this is definitely 3223 *6138* I can get another 6SL7 but there are still questions as to what and why.- https://www.digikey.com/htmldatasheets/production/5184447/0/0/1/media/bg1.jpg Some of the spragues have cracks should the be replaced they appear to be early I would guess they were before the TV shop left their bill in it but could be later.
A lot old dust bunnies for sure, I left some shredded carrots in a bowl inside a hav-a hart trapped overnight next to the chassis. done! It looks a lot better Both cap cans were replace and I am tempted .....but not overly to leave them in the chassis and test while still on the stand ....but it's probably more work in the long run
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This has the original 3 wire cord on it ....What a shocker :laugh:
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Carrots in a hav-a-heart :laugh:
I think you can be confident that that tube is (or should be) a 6SL7.
Cracks in the OD caps? At the lead exit points? Pretty common. Other than that, I'd likely replace. Caps are cheap.
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Thank you steve (Sluckey) , Your photo has answered a couple questions for me First the disconnected cap, they bypassed that cap with another cap and left the other cap in it's holder so as not to disturb the ground reference to another cap that grounds to the band bolt. Also it looks like the original power cord was left?? Mine has to go the insulation is brittle. The ground reference is on the V1 power tube with a resistor to pin 5 and another from pin 5 to pin 4 (G1&2). I am not sure how to approach that one with a new 3 prong . Green to chassis ground and give pin 1 a chassis ground ? or just leave it disconnected at socket? I also see a brown stranded lamp wire coming off yours as a(footswitch?) Mine has the same that was cut, shorted and left inside the chassis. The footswitch on this one was missing . I have one with the GA-40 that I can clone , But I am considering losing one of the instrument inputs and putting in a switch or putting an RCA through the existing hole for the original footswitch or just leaving it as is??? There is corrosion (AKA Rust) around the cap can inside the chassis and some of the paint has flaked off in that area shiny metal under it I can clean it up and paint it the rest the paint is good. Emily Remler would have loved this one !
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Connect the new power cord exactly as the old one is connected.
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!0-4, I need to find a cord with the same diameter or close to reuse the clamp ....Great info thanks
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Solder or bolt the AC green direct to chassis.
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"Connect the new power cord exactly as the old one is connected."
Steve,I have seen a few different restorations and they do all seem to be as built . Could you tell me why this is the proper way as I do believe it is? The original had a wire jumper coming out of the plug as I am sure you know. When you say hook it up as it was do you take into consideration that the hot goes to switch then to PT and common goes to fuse and then PT. We usually go common direct to PT Hot to fuse to switch to PT . Not in the GA-50T ? Why there is a reason ,yes.
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"Connect the new power cord exactly as the old one is connected."
I suggested this because it is the easiest way to replace the power cord. And anything you can do to make working inside that cramped chassis easier is a blessing. It will work just fine wired according to the schematic.
We usually go common direct to PT, Hot to fuse to switch to PT.
Yes we do. And if you were building a new amp, I would recommend doing this. But you are restoring a Gibson rat nest. It's probably best to not disturb the chaos too much. :icon_biggrin:
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This is kind of funny. One of my Gibsons is a GA-8T (2x6BM8) I was playing it yesterday and noted that the Vol pot was scratchy. When I opened it up I noted that I had never done best practice on the power cord - neutral was connected to the fuse. So I looked at it - the fuse, switch, and pilot lamp are all crowded together - and thought, "well how hard can it be." The answer is :cussing: Now it's done, but it was no fun.
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...We usually go common direct to PT Hot to fuse to switch to PT.
People think that is safer.
It's not.
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Oh sure, rub it in that I wasted my time :laugh:
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People think that is safer.It's not.
Inquiring minds and all, that leads to a question. Is it just as safe, less safe OR "You are never really safe"......... :w2:
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GA-8T (2x6BM8)
I really like those 6BM8's have one in a GA-5t and 4 in a conrad stereo. I picked up new russian tubes some years back and wasn't as happy as NOS though. Bet that gibson GA-8T is a screamer
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People think that is safer.It's not.
Inquiring minds and all, that leads to a question. Is it just as safe, less safe OR "You are never really safe".........
When you connect the power cord hot wire directly to the fuse (and nothing else), and the fuse blows or you remove it, then the only place inside the amp that has any voltage present is the power cord hot wire at the fuse holder. I think this is a good idea. Don't you?
When the power cord neutral is connected to the fuse (BTW, this was done as a matter of convenience to the manufacturer), and the fuse blows or you remove it, the 120VAC is still present through the switch, PT winding, and right up to the fuse holder. This leaves several exposed electrical terminals with 120VAC on them. I think this is a bad idea. Don't you?
Either way, I will not trust the wiring of any device that I didn't build. I will always verify presence of voltage with a meter before I stick a finger on any primary circuit terminal. How about you?
Modern manufacturers seem to have adopted the "hot wire to fuse then switch" idea and it's beginning to emerge as an unspoken standard. This probably stems from a NEC safety rule and standard that says the neutral conductor SHALL not be broken, ie, no switches, fuses, or other interrupter devices. The neutral SHALL be continuous all the way back to the breaker panel. NEC does not cover your amp.
Bottom line, if you don't know how the power cord is wired (don't blindly trust the schematic!) it's probably a good idea to unplug the cord and verify for yourself. Then determine the risk level you are comfortable with. :l2:
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Now I am confused again,The white common goes to the fuse then to the PT the hot Black goes to the switch in this amp the to PT. I felt that the hot should go to the fuse and then to the switch and then to the PT and white directly to the PT. If I wire it the same as it was it won't be that . This area is in the least crowded place in the amp and would not be hard at all to do, I have to replace the cap underneath this anyway which gives me even more room .
then the only place inside the amp that has any voltage present is the power cord hot wire at the fuse holder. I think this is a good idea. Don't you? YES
I think this is a bad idea. Don't you? YES
I will always verify presence of voltage with a meter before I stick a finger on any primary circuit terminal. How about you? Yes,I do also
"Connect the new power cord exactly as the old one is connected."???
Herein lies my confusion as I see limited difficulty in wiring it Hot to fuse to switch to PT and it wouldn't be hard at all to put the green on a lug and hook it to the chassis via the metal securing band or to the original spot . This seems like the correct way! What am I missing here ? Thanks
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You're not missing anything. Since it's gonna be easy to change just go for it. I knew I shoulda kept my mouth shut. :l2:
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GA-8T (2x6BM8)
I really like those 6BM8's have one in a GA-5t and 4 in a conrad stereo. I picked up new russian tubes some years back and wasn't as happy as NOS though. Bet that gibson GA-8T is a screamer
Yes - they are sweet tubes, and I agree the old ones are better than the EHX tubes. I have some Telefunkens (these are special), British Phillips, and RCAs. In addition to the Gibson I built a Little Wing Mini-Bassman - great amp, kudos to Geezer and Tubenit. The GA-5T you have is on my build or find list. It would complete my GA-5 collection :laugh:
I also have a KLH model 5 radio - its 2x6BM8 - Henry K knew what he was doing!
I have two Hammond AO-44 chassis and I am thinking about making those into a pair of 6BM8 Monoblocks - I picked up some Edcor OTs for those.
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Steve , I do appreciate the help on this. Sorry for any confusion I might have caused on this, I'm just trying to get it right.
Brian , I actually built a GA-5T board some years ago that's as far as I got, Might finish it some day
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When you connect the power cord hot wire ....
Which is the "hot" wire?
In my last house, outlets were connected randomly. About 60% were 'wrong' H/N. All the lights and appliances worked (well, mostly).
You just don't know unless you check EVERYTHING.
UNPLUG IT before you put your finger inside.
I think I am, publicly, more conservative than you. (What I do in private does not bear repeating.)
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I'm good at home, and when I play other than at home which is not too often anymore, I have one of the plug-in testers that shows if the power is right and have run into situations where it pointed out trouble which was avoided. The only time I work on a live amp is when checking voltages and I keep my left hand behind me (funny that I find myself doing that when working with dangers other than voltage too) I unplug amp and drain caps every time I have to go back in the chassis. My icon is legit too . I was working on a kitchen receptacle with a switch and socket, I tested it and pulled out the 15-amp glass fuse and it was off. Finished the switch and socket then went through the door into the pantry and was doing the same on the inside box which was right behind the first box and was taking out the socket (Keeping my left arm safely away) took out the receptacle and my left elbow touched a copper pipe in the Pantry (small closet) and locked me on and slammed my jaw closed. Luckily no-one had put a penny behind the twenty amp glass fuse on that circuit. I don't know how long it takes to blow one, I do know how long it feels; long. I also know you can 't hold on forever. So be safe....that was 45 years ago ....I sat on the couch for a few hours very relaxed and calm though. :think1:
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Steve , I do appreciate the help on this. Sorry for any confusion I might have caused on this, I'm just trying to get it right.
I'm not confused. Although I think I probably created some confusion. Carry on.
Surely you have that power cord properly changed by now. :icon_biggrin:
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Nahhh I'm waiting on parts to arrive.
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Cord is done , But I need some help . I don't know what the voltages should be for the electrolytics nor how to calculate them .looks like 4-20's 3-10's 1-40's ...my guess would be 3-20's @ 50v 1 20 @500v 3-10's @ 500v and 1-40 @ 500v ....But I don't like to or want to guess.
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That'll work
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Thank you ,Steve.
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Got double sidetracked by the Les Paul (Reconed P12Q is in and awesome ) And the 72 deluxe waiting for de-oxit for pots to arrive. so the GA-50t got bounced out of the chassis stand ....I will continue some time next week.