Fender Feedback Resistor Question.

[JubileeMan 2555]

I'm testing things in an amp for correctness and found that I can't measure this resistor. It comes up as the same value as the Shunt restistor to ground (47ohms vs. the 820 its supposed to be) I AM measuring with the resistor still in the circuit, so I know I will have to unsolder it to measure it for real... but why is it so low? its acting as if the other side of the 820ohm is going to ground, and I would be then just be measuring the shunt resistor... but this makes no sense since the other end of the 820 isnt going to ground, but rather the positive end of the speaker jack.

Is something possibly wrong with the circuit?

http://www.shelleygrund.com/images/feedbackloop.jpg

[mark norwine]

In a way, you are measuring to ground. The DC resistance of the OT's secondary is very low. You have it in your circuit.....

[pula58]

one end of your multi-meter is connected to right side of the 820 ohm resistor, which means it is pretty much DC grounded through the output transformer. The left side of the resistor in question is also connected to the smaller valued 47 ohm resistor. So, you are measuring the parallel combination of 47ohms and 820 ohms, which should be roughly 44 ohms.

[Old Tele man]

...if you measure from "top" of 47-ohm to ground, you're actually reading:

820||47 = (820*47)/(820+47) = 44.45 ~ 44 ohms

...because the "other" end of 820-ohm connects directly to ground "through" the OT-secondary winding, which itself is probably only 1-2 ohms (rule-of-thumb: Rdc = 0.8*Zac, so 0.8*2 = 1.6-ohms!).

...the "trick" here is to "open" the OT-secondary path to ground by temporarily "lifting" the OT BLACK lead from ground (GRN should be connected to SPKR jack).

...which is what they've already told you.