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.

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