How to Convert a Gibson GA-5 to a Fender 5F1 (Or Vice Versa)
Please note that tube amplifiers contain high voltages that persist even when the amp is turned off and unplugged. Do not attempt to repair or modify an amplifier without reviewing the appropriate safety protocol. If in doubt, bring it to a tech.
But just because you can, does that mean that you should? We’re going to remain neutral on this question. All we’re talking about is replacing passive components. It’s not like we’re suggesting that you gut the chassis and install a Bluetooth speaker instead.
At the same time, both of these amps are pretty valuable, and modifying an original vintage amp (particularly if it’s a tweed Champ) can reduce its resale value, if that’s a concern for you. Tweed Champs in decent condition are generally sold for around $2500. On the the other hand, Gibson GA-5 amps are relatively plentiful and can be found at a fraction of that price. But that could change at any time. What if Kendall Jenner is photographed wearing a GA-5 as a backpack? What if a warehouse is discovered somewhere in the California desert containing thousands of tweed Champs in perfectly-preserved condition, and suddenly the market is flooded with NOS 5F1s?
Another thing: there are other factors in the construction of the amp that will prevent you from ever fully 100% converting one circuit to the other. For instance, the GA-5 has a point-to-point layout while the 5F1 uses turret board. Both of these wiring schemes should be equivalent, but you can’t exactly swap one for the other. (Well, I guess you could, but then you'd definitely have a brand new amp at the end.) Also, this list doesn’t account for any differences in larger components including the transformers. Any difference in spec between transformers should be very minor, and anyway we don’t recommend replacing a working vintage transformer with a new one. (It’s a lot of work, you may need to add mounting holes, old transformers are cool, and a new transformer will definitely reduce your amp’s resale value.)
How to convert a GA-5 to a 5F1
Replace the 47k input resistors with 68k 1/2W resistors.
Connect the switching lug of Input 1 to the tip of Input 2.
Add a 1M 1/2W resistor to Input 2 between tip and ground.
Replace both of V1’s 220k plate resistors with 100k 2W resistors.
Replace both of V1’s 2.2k cathode resistors with 1.5k 1/2W resistors.
Remove the 20uf capacitor at the cathode of V1A.
Replace the 47k negative feedback resistor with a 22k resistor.
Replace the input filter capacitor with a 10uf 450v electrolytic capacitor.
Replace the remaining two filter capacitors with 8uf 450v electrolytic capacitors.
How to convert a 5F1 to a GA-5.
Replace the 68k input resistors with 47k 1/2w input resistors.
Clip the lead that connects the switching lug of Input 2 with the tip of Input 1.
Remove the 1M resistor on Input 1 between tip and ground.
Replace both of V1’s 100k plate resistors with 220k 2W resistors.
Replace both of V1’s 1.5k cathode resistors with 2.2k 1/2W resistors.
Add a 20uf capacitor in parallel with V1A’s cathode resistor.
Replace the 22k negative feedback resistor with a 47k resistor.
Replace the 10uf input filter capacitor with a 20uf 450v electrolytic capacitor.
Replace the remaining two filter capacitor with 10uf 450v electrolytic capacitors.
If you don't care about following either circuit exactly, but still want to do something interesting to your 5F1 or GA-5, here’s a list of common mods:
Remove the negative feedback resistor. Removing the negative feedback increases the gain at the expense of stability. As long as your amp is functional, “less stability” means that you may hear some odd/interesting frequency behavior at certain volumes. (If removing negative feedback makes the amp oscillate, something is wrong with it.) Basically, the amp will sound a little louder and more unruly, with less headroom. Some people put the negative feedback resistor on a switch, so the user can engage or disengage it as needed.
Adjust the value of the cathode bypass capacitor on V1A. The value of the cathode bypass capacitor determines which frequencies are bypassed. A value of 25uf is common because it bypasses all useful audio frequencies, which provides an across-the-board volume boost. (The GA-5 uses a 20uf capacitor, but the difference should be subtle.) Lowering this capacitor bypasses treble frequencies only, resulting in a treble boost. The second gain stage can also be bypassed.
Swap the 12AX7 for a 12AY7. The 12AY7 has less gain than a 12AX7, so it will give the amp a slight overall volume reduction.
Add a bright cap. Many amps sound darker when played at low volumes, so it's common to add a small capacitor between the input and output of the volume pot. This capacitor allows high frequencies to bypass the pot, so you’ll hear more treble at low to mid volumes. (At high volumes, the bright cap isn't doing much, because most of the signal is moving through the pot anyway.) The higher the value of the capacitor, the more frequencies bypass the pot. 47pf is a common value, but higher values up to 500pf or so can also be used, according to your preference.
Switched mods. Many mods can be placed on a switch, so the user can take them out of circuit if desired. Bright caps, cathode bypass caps, and the negative feedback resistor are often switched.
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