Deluxe Reverb Tremolo Circuit

[Johnrcurry]

In comparing the tremolo circuits of the Blackface Deluxe Reverb and the Silverface Deluxe Reverb I see there is a difference in the value of the capacitor connected across the 100k resistor from the cathode of second half of V5 to ground. In the Blackface, it’s a 25uf/25V capacitor. In the Silverface it’s a 5uf/50V. I’ve attached the two schematics for comparison. What might be the reason for the difference, and how might the different value effect the behavior of the tremolo?

Thanks

John

[soundguruman]

The 25/25 will make the pulses to the neon lamp a bit stronger than the 5 uF, resulting in a deeper tremolo effect, just a bit more than the newer one.

[pdf64]

My guess is that given the resistances and time constants in the circuit, changing the cap value from 5 to 25uF wouldn't make much difference; the neon either strikes over or it doesn't, and 5uF is sufficient for the circuit to function as intended.
More significant is the voltage rating; the voltage can go over 25V. I've never known it to cause that cap to fail but it's not good practice to push it.
In the 70s, the engineers probably requested an increased voltage rating to increase mean time between failures on that part, the accounts probably blocked it as it would increase cost, then someone worked out that a 5/50 cost and functioned the same as a 25/25, so everyone was happy.
Pete.

[Rick Erickson]

The Blackface Fender amps would lose the tremolo for a moment when the Speed control was advanced quickly from slow to fast. Changing the 25uF cap to 5uF made the recovery time of the circuit faster and reduced this dropout considerably. I consider it a good change - part of my philosophy that not everything changed in the Silverface era was a bad thing. There is no difference in the trem performance other than the reduced dropout.

RE

[Johnrcurry]

Thanks, Rick. If I can't find a 5uf cap to replace it with, would a 4.7 work just fine as well?

[soundguruman]

My guess is that given the resistances and time constants in the circuit, changing the cap value from 5 to 25uF wouldn't make much difference; the neon either strikes over or it doesn't, and 5uF is sufficient for the circuit to function as intended.
More significant is the voltage rating; the voltage can go over 25V. I've never known it to cause that cap to fail but it's not good practice to push it.
In the 70s, the engineers probably requested an increased voltage rating to increase mean time between failures on that part, the accounts probably blocked it as it would increase cost, then someone worked out that a 5/50 cost and functioned the same as a 25/25, so everyone was happy.
Pete.

Time constant yes, but it does make a difference.
The gain and sensitivity of the stage is considerably altered by changing this capacitor.

[Gtr_tech]

4.7u is fine. 5u is no longer a std value.