Gibson GA-5 vs Fender 5F1: Circuit Analysis
We’ve written about the history of the Gibson GA-5 before. Basically, the GA-5 was Gibson’s first practice amp offering. Not only was the circuit nearly identical to the Fender Champ, but the GA-5’s cabinet was suspiciously similar as well. Eventually, Gibson adopted a more original exterior design, but the circuit remained pretty much the same. This means that a GA-5 of any vintage is an extremely affordable equivalent to a 5F1 Champ.
So, both the 5F1 and the GA-5 are small-wattage, single-ended amplifiers. They use the same tubes and the same topology. But what specifically are the differences between the GA-5 and the 5F1?
To learn how to convert a GA-5 to a 5F1, or vice versa, check out this article.
An original tweed Champ on the right. (Vesper amp on the left.)
Input. The 5F1 has a classic Fender hi/lo input topology. Each input features a 68k grid stopper resistor leading to the grid of the first half of the 12AX7. Input 1 is the hi input and features an additional 1M resistor to ground. (Hence the name: the 1M resistor makes this input the high-impedance input.) The tip of Input 1 is also connected to Input 2’s switching lug. This means that when a guitar is plugged into Input 1, the signal will see both 68k resistors in parallel - so, effectively, a 34k resistor. But when a guitar is plugged into Input 2, the lo input, the connection at the switching lug is broken. Instead, the 68k resistors form a voltage divider that attenuates the signal.
In contrast, the GA-5 uses two 47k grid stoppers at the input. Like the 5F1, the switch on the second input is connected to ground, so when plugged into the first input the signal will see a voltage divider and will therefore be attenuated. However, the first input lacks the 5F1’s 1M resistor to ground, so it is not strictly a high-impedance input. In contrast, when plugged into Input 2, the 47k resistor on Input 1 is taken out of circuit (since it is connected to nothing but the tip) and the signal will see just one 47k grid stopper resistor.
The 1M resistor on the hi input does offer a slightly different character compared to a lower-impedance input. It is slightly brighter and a little more intense in volume. However, if you miss that, it is very simple to convert the GA-5’s second input to a Fender-style hi input. Just add a 1M resistor between the tip and sleeve of the input jack.
The difference between the 47k and 68k resistors are negligible. These resistors are meant to stop radio frequencies from reaching the tube grid, where they would enter the signal path and add audible (or even inaudible) noise. Either value should be equally effective. However, all resistors create something called Johnson noise - a sort of hiss as they dissipate heat. Smaller values of resistor have less Johnson noise, so if that is a concern the 47k resistor is a slightly better bet.
Preamp tube bias. Both the 5F1 and the GA-5 use a 12AX7 in the preamp position, but it is biased slightly differently in both amps. The 5F1 uses a 100k plate resistor and a 1.5k cathode resistor, which, assuming roughly 300v supply voltage, places the 12AX7 in the most linear region of its operation. In contrast, the GA-5 uses a 220k plate resistor and a 2.2k cathode resistor. The larger plate resistor allows the tube to achieve more gain. The tube’s cooler bias (due to its larger cathode resistor) will allow the amp start clipping at a somewhat earlier point than the 5F1.
Both amps are known for clean bedroom tones as well as rich overdrive. If overdrive is your goal, the GA-5 will get you there at earlier positions of the volume knob. You may also hear more compression compared to the 5F1.
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Negative feedback. Negative feedback sends a portion of the amp’s output back to the preamp. This reduces undesirable distortion at the expense of gain. Negative feedback circuits in guitar amps are very simple and typically consist of a single resistor. In some amps, this resistor is variable and is called the presence control.
The 5F1 has a 22k negative feedback resistor. In the GA-5, that resistor is 47k. The lower the value of the resistor, the more negative feedback reaches the preamp. More negative feedback is associated with more stability, flatter frequency response, and less gain.
This model of Gibson GA-5 has a chassis that slides out. At some point, someone replaced the filter caps, but the circuit is otherwise pretty original.
Cathode bypass capacitors. The resistor at the cathode of a tube helps set the tube’s DC bias. However, once the tube starts conducting, audio signals will also appear across the cathode resistor. This creates a local negative feedback loop (called cathode degeneration in this context). Just like the global negative feedback mentioned in the last section, cathode degeneration improves stability at the expense of gain.
Adding a capacitor in parallel to the cathode resistor allows audio signals to bypass the resistor, removing the potential for negative feedback. The value of the capacitor determines which frequencies are bypassed. A 25 uf capacitor will more or less bypass all useful frequencies, while a 1 uf capacitor will bypass only treble frequencies, giving the amp a treble boost.
The GA-5 will typically have a 20 uf capacitor on the input triode, which boosts the gain of the amp. The preamp cathodes of early 5F1s were unbypassed, although at some point Fender started adding cathode bypass caps to the input triode. Stability isn’t too much of an issue in circuits like the GA-5/5F1, but some people prefer the input stage to be unbypassed because it offers a mellower tone.
One common mod to this circuit is to put the input stage’s cathode bypass capacitor on a switch. Engaging the capacitor allows the user to access more drive. We performed this mod on a GA-5 circuit that we installed in a vintage Harmony cabinet.
Power supply. The GA-5 has slightly larger filter capacitors (20uf/10uf/10uf) than the 5F1 (16uf/8uf/8uf). More capacitance in the power supply is associated with more tonal responsiveness and somewhat stiffer bass. However, these values are very similar and actually are within the 20% tolerance that is typical for filter capacitors, so in practice the difference between the two amps’ power supply is probably negligible.
Further Reading
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