Strange presence

[496dart]

Im having my presence control acting weird. It is acting like a "Resonance" control .
I cant hear any change in the high end when i turn it, but when its cranked up the amp gets a HUGE amount of low end .

the pot is wired like this:
looking from the back, left tab to ground, middle tab has a .1 cap going to ground, right tab is connected to the 100k feedback resistor .

What am i missing here?

this is a regular marshall style circuit.

[Enzo]

How about it is backwards? I mean instead of thinking of it as adding bottom when cranked up, think of it as subtracting highs. Does turning it all the way "down" make it brighter? Swap the two ends maybe?

[Ray Ivers]

I wonder if it's actually positive feedback, from incorrect OT primary-lead connections? Normally this would result in massive howling, but that's only if PFB exceeds a certain point. A PFB 'presence' control would work exactly the opposite of a normal presence control; full CCW pot rotation would represent full PA wideband gain with PFB, and turning the control CW would result in a gradual reduction of treble above the cap/pot F3 breakpoint as treble PFB was shunted to ground, leaving only the low-mids and bass still PFB-boosted at full CW rotation. This pot is wired backwards, so it doesn't work that way in this amp.

Interesting! A number of 'golden-age' hi-fi power-amp designs used PFB in conjunction with NFB, usually to control output impedance and damping factor into varying speaker types/loads - apparently some even managed to achieve the behavior of a negative output impedance (what a concept!).

Ray

[496dart]

How about it is backwards? I mean instead of thinking of it as adding bottom when cranked up, think of it as subtracting highs. Does turning it all the way "down" make it brighter? Swap the two ends maybe?

ill try swapping the ends. The highs are not affected when i turn the pot--only gets REAL bassy when turned clockwise

[Enzo]

Well if my idea sinks, try Ray's.

[496dart]

My plate wires were backwards--that cured the feedback problem i was having at high volume

However, the presence doesnt do anything at all now. I even wired the pot backwards.

[Bill Durham]

There you go.. it was positive feedback. I did the same thing with my recent build. Do you have an impedance switch on the output? Your NFB source should be before the switch if you do.

BD

[496dart]

its wired just like a 2204 marshall-- imp switch to 100k then to the pot .

still dont understand why the presence doesnt work....

[Bill Durham]

Sounds to me like you don't have any negative feedback at all. Make sure that the connection at the impedance switch is on the 4 ohm tap of the transformer before the switch. Its supposed to be connected to that tap regardless of where the impedance selector is set.

BD

[496dart]

Heres the pres. control. Right tab goes thru the 100k to the 4 ohm tap.

[Bill Durham]

Please don't be offended by my response, but according to the 2204 schematic, there are three other connections to the junction of the 100k and the presence pot. A 10k resistor that is part of the PI cathode network, a .022 cap to the grid of the PI and a 4700 ohm resistor to ground. I don't see how those could be elsewhere considering where that 100k resistor is located.

Just trying to help
BD

[496dart]

I finally got to work on it today. I went back and wired it just like the scem. Now it works fine --the presence works. I think it was because i had the plate wires reversed ? Because this is how i had it wired in the begining. Thanks for the help anyways guys !

[Ray Ivers]

As apparently your original hookup dialed in positive bass feedback starting from zero, IMO it might make a cool 'resonance'-control option for us zero-NFB guys.

While it's still fresh in your mind... can you recall exactly how you wired it originally? I can't really tell from the photo - both the left-hand white wire and the left-hand cap lead look disconnected to me, and I guess the center white wire goes to the left pot terminal? - but I don't know where they ran to in your amp circuit.


Thanks,
Ray

[496dart]

Originally it was wired just like the schematic. Left 2 tabs connected on the pot, then to ground. wire from the imp switch to a 100k ,then to a .1 ,then to the right tab on the pot.
the junction of the 100k and .1 is where all the other parts are

I tried it like the photo because the presence didnt work, and i was looking at a pic of a jcm800, and thats how it "appeared" to be wired .

But....... i didnt know my plate wires where reversed, because the amp didnt have the telltale Super loud howl !

[Ray Ivers]

OK, cool - thanks for that information. I was figuring it had to be wired differently in some way since it didn't howl, but apparently not.

Ray