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Rick S.

Skylark GA-5 Need Info

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Hello everyone!

 

I'm new to the forum. I just purchased a Skylark GA-5 that appears to be 100% original right down to the Gibson branded tubes. The person I purchased it from said it sounded staticy. I haven't plugged it in, because I don't want to do any more damage. I did plug the tubes into a Champ clone I have and I think the 6V6 is bad - it crackles on occasion especially when turned up and hitting bass notes. Also, I tapped (lightly) on the 6V6 and the crackles occur at each tap.

 

How do I find out what year this amp was manufactured? Is there a chart somewhere to lookup the serial number? I figure I'll have to do a complete re-cap, also the 10K resistor in the power supply looks like it's been quite hot and has darkened in the middle. I figure replacing both the 10K & 22K in the power supply, and the 470 ohm in the Output bias. Also, I'm having a hard time finding the 20uf 25V caps for the output and 1st stage bypass would 25uf be OK?? or should I look harder?

 

Any other suggestions or thoughts of areas for possible repair before I power this up??

 

Thanks so much,

 

Rick

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You might be able to see the tube date codes on the tubes. If they're original, then that will give you a hint as to the date. The pot may have a date code on it. The transformers may have date codes but I don't think so.

 

The 6V6 is probably bad but you'll know for sure after the other work is done.

 

Convert to a three prong plug and rewire fuse and plug like this so the new black wire runs to the fuse and then the power switch. The old style wiring had the fuse on the white wire. You might need to drill out the hole in the chassis some to get the new cord and the strain relief grommet in there. Use as skinny a cord/strain relief as you can find. Like this except there's no ground switch -

http://www.recycledsound.net/4_Converting_2_prong_to_3_prong_grounded_plug.pdf

 

Mouser has the 20uf-50V bypass caps. Fifty volts is what you need -

http://www.mouser.com/ProductDetail/Vishay-Sprague/TVA13055/?qs=sGAEpiMZZMtZ1n0r9vR22UzXQwAPJeqE%2fW5e0v9KIU0%3d

 

I'm not sure what kind of filter caps were in there originally or what you can do to replace them. It depends on what kind of mounting clips are in there and what you can fit. You can always tie some caps together with zip ties. There's probably an existing clip screwed into the chassis. You can modify that if you need to.

 

I'd replace the dropping resistors with fireproof 2 watters and replace the bias resistor with a 5 watt wire wound cement. F&T filter caps are excellent and a good value. Sprague Atom and excellent but more expensive. I don't think Mouser carries F&T but Amplified Parts does.

 

The size of the bias resistor will effect how hot the 6V6 runs, so you may want to get some different value resistors and check/adjust the bias while you're at it, depending on how hot the thing's actually running. You might need to use a bigger resistor, like somewhere in the 750 ohm range. I'd get a range of them from 500 to 1K and see what works best to get the bias into the 100% dissipation range. Bias it using the cathode resistor voltage drop method using this calculator -

https://robrobinette.com/Tube_Bias_Calculator.htm

 

That should pretty much cover it but I probably have missed something. Let us know how it goes. How about posting some pix? [thumbup]

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You might be able to find a code on one of the transformers. I'm working on this 1959 Gibsonette GA-8 and there is a code on the output transformer. The code 549941 on this xfrm means that it was made by EIA manufacturer #549 (Midwest XFMR) and it was made during the 41st week of year ending in 9 (1959 in this case.) [thumbup]

 

20160107_123343_zps6apzc2cz.jpg

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I purchased repair parts from Tubedepot.com. I went with 25uf/50v Sprague atoms for the bypass caps and 500V Spragues for the Power Supply filter caps. I ordered new Power supply resistors (2w 10k & 22K) and a new output bias resistor.

 

Here's a few photos in case anyone would like to see it.

 

20160104_195621_zpslanbtxtz.jpg

 

20160105_053248_zpsvvwmwxj1.jpg

 

20160105_053222_zpsmj2paost.jpg

 

20160104_195653_zpsz6yazjjs.jpg

 

20160106_051122_zpsbj8m1kok.jpg

 

20160106_051054_zpsw2jialko.jpg

 

I see this amp has a cap on the AC line to ground that isn't on Gibson's Schematic. I also noticed it's cracked. This could have been a shocking amp had I plugged it in :D

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Great find. I think everyone should have one of these, be it this one, a "lp", a champ, etc.

 

Thought I might add, what you describe the 6v6 tube doing, that's one way of checking for a bad tube. This one is clearly bad.

 

One of the charms of these is you only need one tube, so finding various real NOS tubes, or using leftovers, makes life more fun.

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Oh... and that cracked cap is the "Death Cap." Remove it and don't replace it. [thumbup]

 

Yeah, I was kind of surprised to see it there since it wasn't on the schematic. It's definitely leaving the circuit though.

 

I was thinking this morning, should I replace the .02uf coupling caps as well as the electrolytic caps or would they typically be OK? I have a couple of new .022uf 630V Xicon Metalized Polypropylene caps in my parts box I could probably use if need be.

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Yeah, I was kind of surprised to see it there since it wasn't on the schematic. It's definitely leaving the circuit though.

 

I was thinking this morning, should I replace the .02uf coupling caps as well as the electrolytic caps or would they typically be OK? I have a couple of new .022uf 630V Xicon Metalized Polypropylene caps in my parts box I could probably use if need be.

The coupling caps are probably o.k. They don't tend to go bad the way electrolytics do. You generally don't want to replace them unless it's necessary. That retains the originality of the amp. At the same time it can be kind of tough to isolate a bad one. If it sounds good after you do the other work then they're probably fine.

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Did a little more research, and based on the Volume Pot code, the Speaker code, and Output Transformer code, This appears to be a 1960 model. I also checked the coupling caps with a meter and they both appear to be within tolerance and several sites I read said to leave them in to keep it as original as possible and maintain original tone. So I will leave them in circuit.

 

I've always wanted to play but as of now, I don't... At age 50 though, I'm getting the bug strong enough that I plan to begin lessons in the spring. I have built up a small amount of equipment over the past several years. This however is my first True vintage amp. My other amps include a home built Champ Using a PCB from TubeDepot, a VibroChamp XD, and an 80 something Peavy tube amp. Instrument wise, I have a Takamine Acoustic/electric, and an Ibanez electric.

 

I'd like to learn to play music similar to the Eagles, and Kansas. I also love the Bluesy / Jazz guitar sound and would love to figure out the instrument / amp combination that would give me that.

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Looks like you got a real gem there! I have found a few unmolested examples and a couple of badly hacked ones too. These amps are fairly logical in layout and use a minimum of parts. Gibson was fond of the cardboard "party-caps". In the more thrashed examples this large cap is pimpled or fairly near-bursting open. I usually replace it with individual F&T caps. The cathode bypass caps are a must-do. I usually replace badly drifted resistors as required. The interstage caps may hold up or they may fail as the amp is powered up. I try to keep them if at all possible. Do you have a tube tester? If not, you may want to use a known-good set of tubes to start the amp up. It helps to eliminate some of the possible points of failure before you plug it in and smoke-test it. Paul Ruby has a very good start up procedure for tube amps after repairs. I think he recommends starting up without tubes-checking AC voltages (high voltage windings/5v filament and 6.3 volt heaters) from the power transformer at the 5Y3 socket and the 6V6 socket. Only after these voltages are verified does he proceed to install the tubes and continue the testing.

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Looks like you got a real gem there! I have found a few unmolested examples and a couple of badly hacked ones too. These amps are fairly logical in layout and use a minimum of parts. Gibson was fond of the cardboard "party-caps". In the more thrashed examples this large cap is pimpled or fairly near-bursting open. I usually replace it with individual F&T caps. The cathode bypass caps are a must-do. I usually replace badly drifted resistors as required. The interstage caps may hold up or they may fail as the amp is powered up. I try to keep them if at all possible. Do you have a tube tester? If not, you may want to use a known-good set of tubes to start the amp up. It helps to eliminate some of the possible points of failure before you plug it in and smoke-test it. Paul Ruby has a very good start up procedure for tube amps after repairs. I think he recommends starting up without tubes-checking AC voltages (high voltage windings/5v filament and 6.3 volt heaters) from the power transformer at the 5Y3 socket and the 6V6 socket. Only after these voltages are verified does he proceed to install the tubes and continue the testing.

 

I don't have a tube tester, though I've thought of possibly getting one. I wish I had known in the 80's that I'd get into tube amps. I worked for Radio Shack back then and while I worked there we literally threw away the tube testers that were in the stores. I've ordered Sprague Atoms for all the electrolytics and should get them in early next week. I'll replace them all, as well as the power supply and bias resistors. I'm going to leave everything else as is. I've checked all the resistors and they all appear within spec. The two coupling caps also checked within spec with a meter.

 

Where do I find Paul Ruby's startup procedure? Is it here on these forums?

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One of these days I'm going to learn how to place links in my replies...but...until then...I used Google to find Paul Ruby. His website has a tech section. In the tech section one of the first things he covers is "I finished my amp and now I want to power it up for the first time".

I am learning how to work on tube amps from the generous help of more experienced people. I started up by purchasing very simple amplifiers that needed to be repaired. I gravitated to Gibson amps because they were not (at the time) in high demand and I could get a fixer for $100 or less. Diagnosing problems and repairing tube amps requires focus and attention to detail. I don't know about you, but these two things do not come naturally to me. And just about the time you start to think you've seen it all, along comes a real head-scratcher that really humbles you. I hope your amp gives you many hours of joy! John King is another Tweed Skylark fan...he wrote several articles about them. When yours is up and roaring, you'll be amazed at how un-Champ like this simple 3 tube amp sounds.

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It's a good idea to make a current limiting device like this using a light bulb. Use it when you start up the amp. The light bulb will blow out if something shorts out in the amp. Then you won't end up frying stuff if something unexpected happens.

AmpCurrentLimiter1.jpg

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I've started the repairs. I removed the two prong cord and death cap. I added a 3 way terminal strip and wired the fuse inline with the power switch. My cord should come in later this week. I thought I had a long one laying around to use, but only had 6 ft long ones and I want a longer cord so I ordered one today. I'll wire the black of the cord to the open terminal on the fuse. The white will attach directly to the transformer on the terminal strip. The green will go to the terminal strip center/chassis ground. I figure this should make this section wiring pretty clean.

 

 

Before:

 

20160110_061011_zpsij4pjw10.jpg

 

After:

 

20160110_092355_zpslmw6ybhy.jpg

 

I'll post more as I go.

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I've now replaced the multi-cap with Sprague Atoms. I haven't fastened them down yet, but the wiring is done on them.

 

Before:

 

20160106_051122_zps9tqjqq8t.jpg

 

After:

 

20160112_053900_zpslapfinqv.jpg

 

20160112_053912_zpsxixqmwle.jpg

 

I used some cloth covering from extra wire I had to cover and color code the leads of the caps. I used the original cap wires to connect to the amp. I'll post more as I go.

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This morning I replaced the 10K & 22K power supply resistors as well as the Bias Resistors and bypass Caps on the 12AX7 & 6V6.

 

20160113_053511_zpsbuhjj5ad.jpg

 

20160113_053529_zpsq3ybc1pz.jpg

 

My power cord should arrive tomorrow so I'll likely install it Friday morning. Then will come initial power up and test time. Assuming that goes well, I'll just need to secure the power supply caps and put everything back together. I'm getting excited to hear what it sounds like now...

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Well, my cord delivery got delayed so I temporarily wired up my short cord and the speaker to the amp just to start initial testing. I did the tests w/o tubes, then with rectifier, then with all tubes. Voltages are good with right at 165 on the plates of the 12AX7 and 370 on the plate of the 6V6. At idle, the cathode of the 6V6 was at 22. I just now realized I forgot to check cathode voltages of the 12AX7, but I know the resistors measured correct earlier on so hopefully that was good as well... I'll check at next power up.

 

I did check the coupling capacitors for leakage at idle. the preamp coupler was a few millivolts DC and the output coupler was at about .5V DC. This was less than 1 volt, so I thought I'd see what it sounded like. One word -- BAD.

 

The amplifier is in a bad sounding distortion at all volume levels especially with a guitar plugged into input 1. The Low's are especially distorted. From what I've read, the sounds I'm getting are indicative of leaking coupling capacitors. So, I have now replaced them with the .022uF 600V orange drops I had in my parts box. Unfortunately, I can't power the amp up right now without waking the rest of the house :D. I'll power it up tonight and see if that makes an improvement.

 

Here's a couple of photo's of the new orange drops installed.

 

20160115_050802_zpsdz9pro2d.jpg

 

20160115_050751_zpsjaex97nd.jpg

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I know it would be cooler looking to have the original interstage caps for appearances sake...but I think almost all of mine were found to be leaky. I chose expensive caps (Auricaps) for one amp ... 716P orange drops for another and I think I used Mallory 150's for a third. None of them sounded bad but they all had unique tone. It sounds like you are moving cautiously and thoughtfully...that is a very wise thing to do.

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