Repairing a 1964 Gibson Skylark GA 5T Tremolo Tube Amplifier

“Adopt a son,” was the tagline under the Instagram photo. A local music shop had a vintage Gibson amplifier listed for sale in remarkably good cosmetic shape — except for the logo.

1964 Gibson Crestline GA 5T Skylark Tremolo

I had a quick chat with Mary about the rising value of vintage instruments that veered into a promise to sell the large Line6 AX2-212 amplifier sitting in my study, and I was off to retrieve the Gibson.

The amplifier had the original speaker, the original leather handle, no tears in the tolex or cloth, and very minimal rust on the faceplate. Unfortunately, the amp didn’t play quite as good as it looked. We still brokered a deal, and I brought it home. That’s when I took the week long deep dive into vintage radio and amplifier repair.

Yes, we have gone way beyond banana bread.

The Death Cap

It wasn’t until 1969 that Underwriters Laboratories mandated three-prong plugs on appliances. Amplifiers from the 1950s and 1960s came with a two prong power cord, which could be plugged in either way. The lack of earth ground made those amplifiers susceptible to RF noise. To combat this, designers added a capacitor between the negative terminal of the power cable and the chassis ground of the amplifiers. It was well-known and accepted at the time that if a musician was touching the guitar strings and touched another reverse grounded object such as a microphone, he or she would receive a noticeable shock. The problem with having a capacitor coming from the cable to ground was that if it failed open, it would deliver the full 120 volts AC to the musician.

The “paper caps” and the “death cap” on the unmolested board.

While I wanted to keep the amplifier as original as possible, I decided removing the “death cap” and adding a three-prong power cable with ground to earth was the way to go. I also relocated the positive cable lead to run through the fuse before the switch for a little added protection of the amplifier internals.

Three strand power cable with earth ground installed.

120hz Hum

The most noticeable issue with the amplifier was a very loud hum coming through the speakers even with the volume turned to zero. A lower 60hz hum can be an indicator of poor shielding, but a 120hz hum is usually an indicator of bad filter capacitors.

From what I could tell, the amplifier internals had never been touched, so the circuit was sporting two paper-wrapped electrolytic capacitors — a Maximite and a Minimite. I didn’t have a way to test those capacitors, but it’s generally accepted that the lifespan for a paper-wrapped electrolytic capacitor is 6 – 10 years. Being 56 years old, it was a pretty solid bet that both of them needed to be replaced.

I replaced the Maximite with two modern 22mf 450volt capacitors, and the Minimite was swapped for one of the same. I flipped the amplifier back on, and the hum was gone.

A pair of 22mf 400Volt capacitors in place of the Maximite.

As a side note, if someone is selling you a tube amp and says, “It has a hum, but it still plays great,” walk away. You can’t play great battling that hum. You can’t record with that hum. You can’t perform with that hum. Also, it’s just a matter of time before the leaking capacitors fry the power transformer and cause more damage to the amplifier.

The Mysterious Disappearing Tremolo

This amplifier has a built-in tremolo circuit powered by the oscillations of a vacuum tube. When I purchased the amplifier, it wasn’t working at all. Later at home you could hear the oscillations in the 120hz hum, but it wasn’t evident in the actual guitar sound. Then I tapped on a few connections and spread some crowded wires apart, and the tremolo disappeared completely.

My online searches turned up conflicting diagrams, some indicating a 6EU7 tube and some indicating a 6C4. As I researched both, the 6C4 was noted for it’s oscillations, so I ordered one thinking I had the wrong tube. Turns out a 6C4 isn’t even the right size for the plug. I tried a new 6EU7, and the tremolo was back.

Matched Power Tubes

When I purchased the amplifier, the dates and makes of the tubes varied greatly. The schematic called for two 6AQ5 power tubes, but one was a much more recent 6005. Power tubes are supposed to be electrically matched, so that they have the same plate current and amplification characteristics. The performance of vacuum tubes can vary wildly, so matched tubes were allegedly manufactured at the same time and more rigorously tested to meet the same specifications. When I ordered replacements from Amplified Parts, they sent me a pair of matched new old stock 6AQ5 tubes that were manufactured together in France in 1963. It kind of blows my mind that there’s still parts for these amplifiers sitting in warehouses.

Does the amp sound better with the matched tubes? Maybe I’m just a auditory plebeian, but I can’t tell a difference.

With the three-strand cable, new filter capacitors and new tubes, the amp is playing well, and I imagine it should be able to handle another 50 years. However, if I get the chance to upgrade my testing equipment I’d like to take some measurement and see how far off the original values the other capacitors and various resistors have wandered.

I should also probably spend a little time working on my guitar skills, so they do justice to this amplifier.

Guitar Comparison: Gibson Hummingbird versus Epiphone Hummingbird Artist

The Gibson Hummingbird has always been my dream guitar. It had that rock and roll pedigree, mellow mahogany tone, and just enough flamboyance to make it a legendary instrument. There’s just one catch, it’s really expensive.

Epiphone_vs_Gibson_01

I fell in love with the Gibson back in my teens, and more than 20 years later, I finally have one (used, of course, I’m not crazy). However, needing another guitar for boating and camping, I was very curious as to the real differences between the Gibson and the very affordable Epiphone Hummingbird Artists. In fact, I found a blueburst B-stock Epiphone Hummingbird Artist for only $169.

Aside from the headstock you’d think the Epiphone would be a spitting image of the Gibson, but it’s definitely not. First off, their bodies, while both mahogany, are not quite the same size. The Gibson is slightly wider and deeper than the Epiphone with a more pronounced curve to the back.

Epiphone_vs_Gibson_03

Both guitars have a 24.75″ scale neck, which is probably my favorite aspect of the guitar. It really helps me reach some of those chords with wide spreads. While the Gibson neck does feel more refined, when switching back and forth between the two guitars, you essentially feel like you’re playing the same instrument.

The Epiphone has a synthetic bone nut and a truss rod cover with three screws while the Gibson has a real bone nut and a truss rod cover with only two screws.

The rosewood bridges are similar, but once again, the Epiphone has a synthetic saddle while the Gibson has a real bone saddle. However, the Gibson still has cheap plastic pegs to hold in the strings. Being outside of the saddle, I know they don’t affect tone, but for the price, you’d think Gibson would spend $1 for real bone there as well.

There’s a HUGE difference in the tuners. My Hummingbird has sealed grover tuners, and the newer Gibson models have sealed Gotoh tuners. Epiphone doesn’t even mention the brand of their cheapo tuners in any of their collateral. They’re pretty terrible. I had some serious trouble keeping the Epiphone in tune for the first few weeks I owned it, although it has gotten better. With the Gibson, it’s usually in tune when I open the case, and it never goes out. With the Epiphone, I have to make sure and tune it before I start playing, and I might need to readjust it once or twice throughout the course of a three-hour jam. (This is about on par with every sub-$400 guitar I’ve ever owned.)

Of course, the real signature of a guitar is it’s tone, so I made a short video comparing the Gibson Hummingbird to the Epiphone Hummingbird Pro. Both guitars have Elixir Custom Light strings, and the audio was recorded on a Zoom H2n set to 4 channel mode. If you’re reading/watching this on a phone or laptop, you’ll probably have to plug in some headphones to really hear the difference.

So there you have it, a detailed look at the differences between a Gibson Hummingbird and an Epiphone Hummingbird Artist.

Want to see my G-string?

Any luthiers or guitar experts out there?

Gstring

I always keep a cheap guitar on the boat. I want it to sound decent, but if worse comes to worst, I don’t want to feel bad if I have to use it as a paddle.

My first boat guitar was an Epiphone Dove with a cracked neck that I procured for $30.

6319817649_ec09eec893_b

A little wood glue, and it was a nice player for about a year. Then the same spot on the neck broke again, so I stripped it for hardware and junked the body.

My current guitar is an Epiphone AJ. It was a B stock guitar with a few blemishes that I picked up new for $89.

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For the past two years or so, it played and sounded great. Unfortunately it has suddenly started popping G-strings. The last two times I’ve played it the G string has broken at the saddle with less than two hours of use. Then tonight I swapped the strings again, and the G broke while I was just tuning it.

I use Elixir strings, so they don’t rust out in a week on the boat, but they’re about $15 a set. When the guitar is worth less than $100, even $15 is a large investment, especially if the strings are now only lasting for an hour or two of play time. They were previously lasting for up to six months of weekend jams.

My Gibson has a bone saddle, and I see lots of them available on eBay for anywhere from 99 cents to $20. But how do I know they’ll fit, and is a 99 cent bone saddle really any good? Should I search for a replacement nylon/plastic saddle? Should I gift this guitar to a starving artist and find another sub-$100 boat instrument?

Right now I have a B string replacing the G string, so that there’s less tension, but it makes the tone a little funny. Using two B strings is definitely not a long-term solution.

Come on guitar experts, I know you’re out there. What in the world is going on with my guitar and how do I fix it (cheaply)?