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This was in the repair queue for FIFTEEN years

 
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Silverface



Joined: 23 Aug 2006
Posts: 324
Location: Hermosa Beach CA

PostPosted: Mon May 25, 2009 4:47 pm    Post subject: This was in the repair queue for FIFTEEN years Reply with quote

I had this lovely 1951 Gibson GA-50T sitting around forever...somehow I never got up the gumption to tackle its rat's-nest of point-to-point wiring (REAL point to point, not like Fender's circuit boards). Casually looking at it I has baffled by the caps - the can, one other multi-section cap, one resistor and a 3-prong plug I installed 10 years ago to safely test it are the only things that were EVER replaced and the original caps were weirdly ALL polarized electrlytics, usually with really high voltage handling; plus two odd rectangular metal caps in the trem circuit.

The trem speed control (a dual-ganged 250k pot pair) was frozen. I found out why when I discovered the obviously original 9.1k resistor was wired wrong and the (I think) bias pot for the trem was a 3.5k...when it should have been 1 meg! The switch was frozen as well, and the 6SL7 oscillator tube was bad - I assume the original owner never used trem,and no subsequent owners tried to fix it.

Well - I dove in and ended up replacing EVERY cap except one "domino" and the already-replaced filter caps which are fine. 100% of the replaced ones leaked - no wonder it had no guts and loads of hiss and hum!

I used NTE poly caps (I've had really good results with these in Fenders) and a couple of orange drops to replace the metal rectangular ones.

A dozen or so Deoxit treatments and turning the speed pots with plies finally got it working just fine, and I put a 500k pot in as the trem bias (I was out of 1 meg pots- figured it was probably close enough). It worked but there were still intermittent cutouts, so I get a 1-meg...same issue. It's the ONLY problem.

My God, what tone! I'm still having trem problems - it cuts in and out - but otherwise it's flawless (except for the new handle I ordered). It has one Jensen Alnico 5 P12P and an 8" P8P. The blend between the speakers is astounding - a really good idea. I put out around 25 watts from two cathode-biased 6L6's and uses 6SJ7's for the preamp tubes and inverter.

The sound is a little hard to describe, because it is NOT only the typical Gibson "clean jazz" sound. That is the case 'til about halfway up on the volume knob - then it starts to smoothly break up and just sings - back off your guitar volume and it cleans right up. As you turn it up you get louder, but the same saturated sound. It almost reminds me of...I'm not kidding...a Dumble Overdrive Special in "Larry Carlton" mode; but at lower levels it's a different amp - more like a wide-panel Super.

I swore I'd build it and just sell it. Now I'm wavering - it sounds SO good it may be a keeper.

Just need to find the solution to the trem problem - very strange circuit (a link to the schematic below - any ideas?)






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Raybob



Joined: 26 Aug 2008
Posts: 98
Location: Kyburz, CA

PostPosted: Mon May 25, 2009 8:38 pm    Post subject: Re: This was in the repair queue for FIFTEEN years Reply with quote

Silverface wrote:


Just need to find the solution to the trem problem - very strange circuit (a link to the schematic below - any ideas?)


Jim, is that bias modulating tremolo or ldr? Where's that link to the schematic?

Raybob
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Silverface



Joined: 23 Aug 2006
Posts: 324
Location: Hermosa Beach CA

PostPosted: Tue May 26, 2009 4:36 am    Post subject: Reply with quote

Ack! My bad - I totally forgot the schematic link:

http://www.gibson.com/Files/schematics/GA-50T.pdf

It's a modulating type, but not one I've encountered before. I think I've run into 3 Gibson amps in the hundreds I've worked on - and two were mine!
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Brian



Joined: 22 Aug 2006
Posts: 1360
Location: Southern California

PostPosted: Tue May 26, 2009 5:18 am    Post subject: Reply with quote

What a cool looking amp. If it sounds like you say ...sounds like a keeper. I've never seen an amp with those 6S7 tubes. You are still able to find replacements for it? re: the cutout...has to be some intermittant connection, bad tube, bad resistor? there's not much else there. Seems like by process of elimination you eventually find it.
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Silverface



Joined: 23 Aug 2006
Posts: 324
Location: Hermosa Beach CA

PostPosted: Tue May 26, 2009 1:27 pm    Post subject: Reply with quote

I have at least a dozen good 6SJ7's.

It helps to have a late father in law who stashed about 2,000 tubes!
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Brian



Joined: 22 Aug 2006
Posts: 1360
Location: Southern California

PostPosted: Wed May 27, 2009 6:13 am    Post subject: Reply with quote

Jim...check out these cool boutique clones of old Gibson amps

http://www.buffaloamps.com/

I wonder what these sound like?
http://www.samedaymusic.com/product--GIBTGA40RVT

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Silverface



Joined: 23 Aug 2006
Posts: 324
Location: Hermosa Beach CA

PostPosted: Wed May 27, 2009 2:34 pm    Post subject: Reply with quote

I've been wondering as well. I haven't seen/heard one yet. I don't see how they can possibly make money on a hand-wired, turret-board amp at $850! ESPECIALLY at Gibson, a company not exactly known for low margins or discount-pricing of hand-crafted items. On some of the amp boards guys have laid out cost/time analyses showing parts costs, minimal labor - and hand-wired "boutique" amps making sense at $2-3k to barely break even.

Definitely going t have to do some more investigating - that type of amp at that price point is REALLY intriguing (and with the power down at the 30 watt level it sounds like they kept within the old Gibson amp traditions - low voltages, cathode bias, etc).
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Silverface



Joined: 23 Aug 2006
Posts: 324
Location: Hermosa Beach CA

PostPosted: Wed May 27, 2009 2:37 pm    Post subject: Reply with quote

Errr..then again:

http://forums.gibson.com/Default.aspx?g=posts&t=1596

Might explain the low price!
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Matt



Joined: 23 Aug 2006
Posts: 182
Location: St Louis, MO

PostPosted: Wed May 27, 2009 4:01 pm    Post subject: Reply with quote

I've heard nothing but nightmares regarding reliability and new Gibson amps. The "goldtone" series sounded great...but weren't dependable by any stretch of the imagination.

Silverface wrote:
On some of the amp boards guys have laid out cost/time analyses showing parts costs, minimal labor - and hand-wired "boutique" amps making sense at $2-3k to barely break even.



Dr Z's are handbuilt/handwired in Ohio and sell on average from $1K-$1500. They have their own thing going on tonally (somewhere between a fender and vox for the most part) which may, or may not float your boat. But they're VERY well built/rugged and ultra-reliable from my direct 5+ years of heavy gigging with them.
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Raybob



Joined: 26 Aug 2008
Posts: 98
Location: Kyburz, CA

PostPosted: Wed May 27, 2009 4:17 pm    Post subject: Reply with quote

I don't know about the Gibson branded hand wired amps but that first link to Buffalo amps is a different thing altogether. Buffalo amps appear to be a quality hand built amp but they are more than twice the price of the Gibson brand.

Raybob
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