Fuzz Face - exhausted battery DC fluctuation

[Lino22]

Guys did you actually measure the DC fluctuation on an exhausted battery Fuzz Face without a DC filter?

I did it today but no luck. My battery was about 7V (loaded). No LED. The oscilloscope probes were on the battery, and the oscilloscope added a sort of 65kHz parasite signal on the DC, very weak though.
I could see a shy movement of the DC level when i stroke open strings hard, but ... maybe 0.2V both sides at best.

[anotherjim]

I have not tried this experiment.
For some PP3 batteries, 7v is "terminal" but not yet dead, especially with a small load.
With a small signal current, the dying battery still has enough capacitance to shunt out the signal.
It's probably changing in bias and headroom that produce a dead battery tone.
If a positive ground PNP FF, you won't see any action on the battery + referenced to ground.
Mojo lore has it the battery must be an EverReady zinc carbon in a blue wrapper made on or before September the 18th 1970.

[Lino22]

I used a cheap zinc carbon Greencell. The FF is an Si, NPN type.

[Lino22]

Here is what i measured today. A 9V Greencell zinc-carbon battery was exhausted down to 7.75V unloaded, and when i connected a fuzz face (no filter, 1.8mA current draw, LED) the loaded battery went down to 6.94V.
I played hard open strings via Burstbucker pickups (SG) and here is the result. Persistence was on on my oscilloscope.

The DC was fluctuating between 6.8 - 7.45 V.




Then I added a DC power filter (220u//100n) and 220 ohm resistor (on the circuit side of the filter) to simulate the battery's internal resistance. I measured on the circuit side of the resistor.

[anotherjim]

I think you have RF coming in somewhere. Silicon FF probably more prone to this. Have you tried adding small pF caps on Q1 C-B?

[Lino22]

Jim, yes the oscilloscope generated that signal, 65kHz. Probably some kind of oscillation. I ignored it as the whole DC line was jumping up and down when i stroke the guitar.