Help with prescence control 5f6a

[Kee22]

Hi
Trying to install a presence control in a 5f6a circuit. Not 100 procent sure about the circuit no my original build. Tried to mod from rob robinett with a 25k lin pot, 0.1 uf cap and 4.7k resistor. When I insert it in the circuit the amp goes dead quite. Can anyone give me some pointers, and yes Im a newbie so my soldering needs improvement but im getting better even if it still looks like a year old.

[martin manning]

Did you have a feedback loop connected before you tried this? Dead quiet might mean it's oscillating at some ultrasonic frequency, and I'm wondering if you need to reverse the primary leads on your OT.

[Kee22]

Hi
There is a negative feedbackloop its the resistor and the white cable thats connected to the 8ohm tap. Dead quiet is an exaggeration, theres some noise from the amp almost sounds like the the loop is canceling out all the sound. But theres no difference if you turn the pot.

[martin manning]

Maybe the lead routing of the added circuit is causing the problem?

[Kee22]

Its strange the 27K NFB is connected to the tailresistor from the PI so the insertpoint should be correct. But when the prescensce control is inserted the amp has no sound? This over my understanding.

[sluckey]

Its strange the 27K NFB is connected to the tailresistor...

The resistor on the board with the white wire connected is NOT a 27K resistor. Looks like a 680Ω (possibly 68K). Replace it with a proper 27K and see if it behaves any better.

[Kee22]

My multimeter says it 2.6k when i measure across it?

[sluckey]

My multimeter says it 2.6k when i measure across it?

You must disconnect one end of that resistor to get an accurate resistance reading.

[Kee22]

You were right 68k and this i what i found under the board.

[sluckey]

Did this amp have a functional standard Fender presence pot before you started modding it? That last pic you posted is not a typical 5F6A circuit. How do you know this is even a 5F6A amp?

I'd like to see a hi-rez gutshot of the amp that clearly shows the entire board, tube sockets, and control panel.

[Kee22]

You`re right I dont no if thats the case Its probaly some modded version of an 5F6A and I dont have a schematic for it. I unsolderd the board today and measured every component on it. Going to change it all out with a better board and new components. Here are some pics. The master volym is disconnected it whas there the prescence control was meant to be.