5f6a Presence Control no Joy

[The New Steve H]

A day or so ago I realized my presence control was not connected. I got it wired up, but it still does not function. I'm pretty sure there is zero treble reduction, as if the control were not there.

I've been getting advice on it, but nothing has helped. I can't say it's a major issue, as I have had no interest in using the control.

I have a 27K resistor in the circuit, just like Fender. There are 3 output jacks for 2, 4, and 8 ohms. The feedback circuit is connected to the hot side of the 2 ohm output. I have tried it on the 8 ohm tap, but it doesn't work. I use an 8-ohm speaker, and obviously I use it on the 8 ohm jack.

I'm going to test the pot tomorrow and make sure I don't have some ridiculous value in there.

[The New Steve H]

OT schematic.

[Cliff Schecht]

Have you checked the pot for functionality? Have you verified it's connected properly and has a solid ground? Does your hearing go up high enough to hear anything?

[The New Steve H]

Haven't checked the pot yet. I figured my own ineptitude was the first place to look. But as I said, I will check it tomorrow.

People say the presence effect is obvious, so I assume I would be able to hear it.

[The New Steve H]

If I had the wires to the 6L6 control grids swapped, that would explain this, right? Wouldn't that put the wrong polarity in the feedback circuit?

[Structo]

About all I can add is make sure the feedback wire to the OT is connected to the 2 ohm tap, since that is what the Bassman had for a OT back then.

[Firestorm]

If I had the wires to the 6L6 control grids swapped, that would explain this, right? Wouldn't that put the wrong polarity in the feedback circuit?

If you had the leads swapped, the amp would squeal. What cap do you have on the presence pot? Awhile back, you had a ceramic that may not have been the right value.

Also check resistance to ground from the bottom of the tail resistor. Should be 5K.

[The New Steve H]

It's a .1 uF Orange Drop. I think it's a 400V job.

I guess this will sound stupid, but I don't know what "tail resistor" means.

Now that I think about it, I have never seen any explanation for the term "long-tailed pair."

[martin manning]

You should get Merlin's book... and read it!

[The New Steve H]

I got the books! I swear! Give me a year or two to digest them!

[Firestorm]

In this case, the tail resistor is the 10K. One end to the 470, one to the presence pot (and the .1 grounded grid cap).

[marcoloco961]

I got the books! I swear! Give me a year or two to digest them!

LOL.

+1

[tubeswell]

Hi Steve

I'd check for a wiring error. (Esp if you are already using the 2R OT tap and the 27k and 5k pot in the NFB loop).

[Zippy]

It's a .1 uF Orange Drop. I think it's a 400V job.

I guess this will sound stupid, but I don't know what "tail resistor" means.

Now that I think about it, I have never seen any explanation for the term "long-tailed pair."

http://www.aikenamps.com/LongTailPair.htm

Look 'round his website for his tech articles - very nicely done.

http://www.aikenamps.com/

[The New Steve H]

I found it in one of Merlin Blencowe's books last night, but I DIDN'T SEE NOTHING ABOUT WHY IT SAYS "LONG TAIL"!!!

I get the tail part, but I don't get the "long."

Jargon always makes my head pound.

Oh, well. The rest of my Herzog parts arrived, so off to work.