Presence control barely adjusting presence

[thehoj]

I added adjustable presence to my bogen chb-35a project, basically copied the 5f6a circuitry.
Here's the schematic as it stands.
http://vwtweaked.ca/images/Bogen%20B...%2005nov09.jpg

My problem with this presence control is that the pot doesn't seem to really adjust the presence any noticeable amount.

I have a basic understanding of the nfb, but don't really understand how the pot with the cap in parallel to the nfb resistor adjusts this.

What can I change about the components in this section to make the presence control more functional (more presence when it's turned up)?

I realize there is another presence thread on the first page in this forum, but the topic kind of swayed from the discussion of presence, and I didn't really see what I was looking for in that thread.

Thanks for any help on this.

As an extended thought to this question, how would I wire in resonance control in addition to the presence control?

[loudthud]

Your Presence control works in two ways. When negaive feedback is applied to an amplifier, the apparent gain of the amplifier is reduced. As you turn up the pot, you reduce the feedback at high frequencies. Thus, the gain increases at high frequencies because of the reduction in feedback.

The second way in which your Presence control works is that it acts like small cathode bypass cap. This is important because if you move the Presence pot to the other side of the feedback switch, it will work some even if the switch is open or the Master Volume is turned down.

Does the gain change much when you open the feedback switch?

[thehoj]

That makes sense.

As for the feedback switch, yes, the gain as well as the highs and lows seem to get a decent bit more intense when I open the switch. I like having the option of having that feedback or taking it off too. But with it on I can barely tell the difference between having the pot turned all the way one way or all the way the other way.

Your Presence control works in two ways. When negaive feedback is applied to an amplifier, the apparent gain of the amplifier is reduced. As you turn up the pot, you reduce the feedback at high frequencies. Thus, the gain increases at high frequencies because of the reduction in feedback.

The second way in which your Presence control works is that it acts like small cathode bypass cap. This is important because if you move the Presence pot to the other side of the feedback switch, it will work some even if the switch is open or the Master Volume is turned down.

Does the gain change much when you open the feedback switch?

[Tonewood]

Hey thehoj-

I know nothing about calculating the FB resistor size but the 100K is about 4 times the resistors that I have seen in my amps.

[ThermionicScott]

I'm kind of late to the game, but you might check to make sure that your OT is hooked up properly to provide negative feedback rather than positive feedback. I built a 5E7 Bandmaster clone, and wondered for a year why the Presence control didn't do anything. The amp didn't squeal, so I didn't suspect that the OT was hooked up backwards. I switched the OT leads, and now it works great. Something to try...

- Scott

[guitardude57]

So ThermionicScott,
You had the primaries reversed and it didn't squeal until you adjusted the nfb?

I might have to check that on a recent build on my experimenter chassis, that currently has a P2P Express circuit on it. No board, just terminal strips and tube socket connections. Surprisingly stable. 2 X 7189 output. And loud as hell.

It sounds good, a little hiss/hum due to steel chassis (the real estate in the chassis is great for playing with different circuits, had a Plexi 1986 in it last).

The NFB is on a switch to turn off for no nfb. So when fired up it worked well until I turned the switch to adjust and check presence..... and Wow! big oscillation that changes pitch with the presence control.

Anytime I had the OT reversed primaries....it let you know on turn on. Since Presence has a switch - on - pot, it didn't sqeal until i turned it on.

So this must be my problem. Sorry wasn't trying to hijack the tread. Thanx for the idea to check.