Recently, a 1962 Gibson GA-60 Hercules guitar amplifier made its appearance at Cosmic Ampworks. A brief report on how we brought it back in excellent playing condition.
About the amp
Generally, we feel that simple but well-designed amps give you the best and purest tone. With just bass, treble and volume controls and no onboard reverb, the Hercules is indeed simple. Design features that set it apart from other tube amps of the 1960s are an interesting built-in compression circuit, a duo of 7591 output tubes and a 15’’ speaker. Remarkably, a piggyback (top and cabinet) version of the GA-60 was marketed as the Gibson GA-100 Bass Amp. On the exterior, brown tolex upholstery and ditto speaker grille give the Hercules a funky vintage look. Combined with a relatively modest weight and 15’’ speaker, the Hercules is still a cool and flexible amp to bring to jam sessions and gigs today. That is, if you dig clean tones (jazz!) or bring your own overdrive pedals and reverb.
Firing it up
When first fired up, our amp appeared to be still fully functional. With guitar, its sound definitely had a nice vintage vibe to it but lacked sparkle and high end. Moreover, when hitting multiple notes at the same time, a faint and unwelcome fuzz sound could be heard in the background. Connected to an electric bass, the amp just produced a great sound however, with lots of vintage character. Perhaps power sag from the tube rectifier contributed to this. In any case, we decided to re-amp a bass track that we were working on through our amp and record it before making any modifications to it (the track may become available online once finished).
Let’s make things better
Lack of sparkle in vintage amplifiers may be caused by worn-out power tubes. Perhaps, with an amp that is 55 years old, the pre-amp tubes might contribute to the problem as well. Since we found that our Hercules still had all its original tubes (2 x 7591 power tubes, 2 x 6 EU7 preamp tubes and a GZ34/5AR4 rectifier), we set out to test it with new tubes. However, 6EU7 preamp tubes have become quite rare. It would be a tempting alternative to rewire the tube sockets for 12AX7 tubes, as these are electronically identical to 6EU7s (they just have a different pinout) and the choice in 12AX7s being much wider. However, Cosmic Ampworks like to keep things original. Therefore, we decided to go with new tubes. Tung Sol seems one of the few manufacturers that still produce 6EU7s, and we generally like the sound and quality of their tubes; an easy choice. As for the 7591s: based on several reviews on the internet, we also chose Tung Sol here. With a GZ34/5AR4 rectifier from JJ Electronic, the quartet of tubes was turned into a quintet.
The new tubes improved the sound somewhat, but not spectacularly. This is testimony to the quality of the original tubes: after 55 years they were still working fine! Importantly, it also led to the conclusion that replacing the filter capacitors might be necessary. Filter capacitors are electrolytic capacitors that eliminate hum from high voltage DC current in the amp. They deteriorate over time, particularly if an amp is not used on a regular basis. This may also affect the sound, as any current that the tubes draw passes by these caps. When removing the chassis from the cabinet and first opening it up, we found that the inside of the amp was almost completely original and nothing had been serviced any time recent. Upon closer examination, some filter capacitors had been replaced but, judging from the components used, this had probably already been done in the 1960s or 1970s. Replacing these components could make a significant improvement to the sound.
Filter caps in guitar amps are relatively large size components, as they are built to handle voltages up to some 450V. Therefore, they need to be mounted safely and sturdily. In its 1960s amps, Gibson used so-called ‘can-capacitors’; several capacitors put together in an aluminum can or cardboard cylinder with the ground connections tied together (this explains the sometimes odd number of connection points). The advantage of these large can capacitors is that they are easy to mount to the chassis by means of a clamp and a few nuts and bolts.
Replacing the old caps with new cans (instead of loose capacitors) would enable us to mount them safely, respect the originality of the amp and re-use the old drill holes in the chassis as much as possible. ‘Can’ capacitors cost substantially more than normal ‘loose’ capacitors, but they are definitely worth the money in our opinion; they contribute to a safer and more reliable amp! A search for the right components in terms of capacitance and voltage limit led us to use parts from CE Manufacturing and JJ Electronic.
After mounting the new capacitors, the amp sounded indeed a lot better, with more sparkle and high end. However… that annoying fuzz sound could still be heard! Connecting the Hercules to an external speaker led to the conclusion that, unfortunately, the original speaker was the culprit. Apart from the absence of fuzz in the modern (Celestion) speaker, the general difference in tone between the original speaker and the Celestion was spectacular. The original speaker has a very strong mid-range emphasis and relatively little treble and bass whereas the Celestion made the Hercules suddenly sound a more like a twin reverb, with rich bass and treble!
What to do?
Now, this is a dilemma. Should we replace the fuzzy speaker or leave the amp as original as possible? Although it is probably still acceptable for playing live, recording guitars with the current speaker seems not an option. On the other hand, the mid-range heavy voice is probably part of the original character of the amp. Perhaps we could try to source a vintage replacement speaker of the same make. What do you think? Send us a note!