5F1 vs. 5E1 cap question

[cubhead]

I see there is a 25uF/25V cap in parallel with a 1.5k ohm resistor on pin 3 of the 12AX7 on 5E1 amps but the same cap is missing on the 5F1 amps. Does anyone know what this cap does? Can it be added to a 5F1 amp?

[Jazz P Bass]

12AX7 Cathode bypass capacitor

The 25uf capacitor is called a cathode resistor bypass capacitor.

Quote from Aikenamps:Designing Common-Cathode Amplifiers

"Capacitor Ck is used to bypass the cathode resistance to ground for AC signals, which results in a higher gain. Without Ck, there is negative feedback, or degeneration, which reduces the gain of the stage and increases the output impedance. If Ck is not large in comparison to Rk, it will affect the frequency response of the stage, by introducing a "shelving" response, where the stage gain is boosted at higher frequencies compared to lower frequencies. The "breakpoint" of the frequency response is controlled by the value of Ck, in conjunction with the cathode impedance."

As far as I can tell the from Fender schematics, the only difference between the 5E1 & the 5F1 is that 25uf bypass capacitor & on the 5F1 they omitted the B+ choke in favor of a 10K resistor.(where they changed the filter capacitor to 16uf from 8uf)

[Wombaticus]

I see there is a 25uF/25V cap in parallel with a 1.5k ohm resistor on pin 3 of the 12AX7 on 5E1 amps but the same cap is missing on the 5F1 amps. Does anyone know what this cap does? Can it be added to a 5F1 amp?

My understanding is that, despite the schematics, a fair proportion of 5F1's actually did ship with this cap in place back in the day. It's a large cap, so you will percieve an increase in gain across pretty much all frequencies. It is a very simple mod indeed, if you don't like it you can always take it back out.

The 5F1 is pretty much just a 5E1 slightly reengineered to save Leo a couple of bucks on the choke. I doubt there's too much difference in the tone between the two, although interestingly, the 5E1 runs the screens at slightly higher voltage than the plates, which is slightly unusual.

[cubhead]

Thanks for the feedback. I put the cap in!

[Casey4s]

Although that cap is not on any Fender schematic, in reality that cap is the same as a 5F2. If you find an original vintage amp you will likely fiond that bypass cap. I put mine on a toggle switch to get a little flexability.

[cubhead]

Thanks!