Presence on 5f6a

[tylerrussell]

Hi, all. First post. Looked high and low and can't find an answer. Go easy on me, please, there's no local tech help in my area. Had a Bassman LTD that was not working, a guy that was local installed a Hoffman board (I believe) using the existing chassis and trannies. It's been working great for a couple of years~the presence control does nothing. (http://s1291.photobucket.com/user/tylerrusswell/media/5f6aOld_zpsad65ecbe.gif.html?sort=3&o=8) Across the two lugs on the presence pot (the one's with the .1 cap), sweeping the pot takes the meter from 0 to 4.2 (ohms, I believe) So the pot's doing something, but no change audibly. I'd like to correct this but don't know where to start. I know enough to be safe and have biased, changed bias resistor(s), and would like to tackle this with your help. Could someone look at the link to the pic off this particular amp and steer me? Other that the presence control not working the amp is very quiet and screams (albeit loudly) rather well. Thank you for any assistance. On another note, is there a diagram that shows labeling~like which are the coupling caps, cathode thingies, etc? I feel that knowing which components are what may help immensely in my quest for somewhat understanding these things. Sorry or the length of this post, but, hey~~~I'm trying.

[tylerrussell]

Oooops, here's the actual amp pic:
http://s1291.photobucket.com/user/tylerrusswell/media/Bassman_zps64c869f6.jpg.html?sort=3&o=9

[sluckey]

It's wired wrong. Follow the fat cloth covered wire from the presence pot to a 27KΩ resistor on the board (next to the big orange cap). Now look at the other end of that resistor. There is a small grey wire that goes to the speaker jacks. Those two wires are reversed at that 27K resistor. Unsolder the wires from that resistor and swap them. Unfortunately you'll probably have to actually replace both wires because they're not long enough to reach. Refer to pic...

[tylerrussell]

Yes, the wires were exactly as you have shown. I did reverse them, and I've got to say~~~you are brilliant! The presence control now works! I've put up with it not working all this time because the guy that wired it has moved location and I have no idea where he's at. Sluckey--if you're ever in my neck of the woods~~~beers' on me. I do notice a "hiss" or "whooshing" when the presence knob is rotated-but only while rotating it.
The best to you and this site. Thank you very, very much.

[HotBluePlates]

... I do notice a "hiss" or "whooshing" when the presence knob is rotated-but only while rotating it. ...

That is due to d.c. on the pot, and is normal for the presence control in this circuit.

Other, later, amps have a redesigned presence circuit that keeps d.c. off the pot and eliminates the "whoosh" or scratchy sound.

[tylerrussell]

So.....the "whooshy" sound is no cause for alarm then. I started browsing this site after buying a biasing "kit" from here, and, at 61 years old, want to understand some of these things. I see a 5e3 in my future.

[tubeswell]

So.....the "whooshy" sound is no cause for alarm then.

Perfectly normal in a 5F6A circuit.