adding a presence control to a bogen cha33 question

[Regis]

I want to add presence control to a Bogen CHA33 I am modding for guitar.

I looked at a couple of old Fender bassman schems and see values of 5k and 25k for the pot, any preference?

The 5k setup just has a pot and cap, the 25k setup has a pot and cap across a 4.7k resistor.

Are there other factors involved, such as the value of the feedback resistor?

The bogen has an 82k feedback resistor. I assume the feedback control would be attached to the junction of c12, r21 and r27.

thanks guys

[Enzo]

Did the old Fender circuits inject the NFB at the same point in the PI? Was the PI circuit the same as the Bogen? Offhand, the Bogen circuit doesn;t look very Fendery.

Is R33 actually wired like that? or does the upper end of it go to ground?

NFB is a whole circuit, not just a pot. Pot value preference should take into consideration the circuit it is being used in.

[MWJB]

R33 looks OK as it is, it ties to R28 & R29, which is grounded in a self-balancing paraphase PI set up (5D# Fender style).

R23 that looks a little odd, B+ applied to the NFB loop? Looks to be relevant to the 70v/150ohm tap that you are not going to be using?

C13 should be providing a degree of presence anyway?

I'd lose C33 & R23. R21 is your load for the NFB loop, replace with 2200ohms - I'd aim for a pot around 5x this value (10KL), wired in reverse (0ohms at full clockwise) as a variable resistor with a 0.1uf cap in series with the pot to ground. Pot & cap would go from the ungrounded end of R21, in parallel with R21 to ground.

That would give you and NFB ratio of around 40:1, high-ish so try bypassing R22 with another 82K and see how you like that?

I can't see where any dc is being applied to the phase inverter pin 2 (check), so you probably don't even need C14 (plain wire will do) or R24 (redundant without C14)?

Man! Those 6L6s look like they're being driven hard? What plate current do you have?

[Regis]

R33 looks OK as it is, it ties to R28 & R29, which is grounded in a self-balancing paraphase PI set up (5D# Fender style).

R23 that looks a little odd, B+ applied to the NFB loop? Looks to be relevant to the 70v/150ohm tap that you are not going to be using?

C13 should be providing a degree of presence anyway?

I'd lose C33 & R23. R21 is your load for the NFB loop, replace with 2200ohms - I'd aim for a pot around 5x this value (10KL), wired in reverse (0ohms at full clockwise) as a variable resistor with a 0.1uf cap in series with the pot to ground. Pot & cap would go from the ungrounded end of R21, in parallel with R21 to ground.

That would give you and NFB ratio of around 40:1, high-ish so try bypassing R22 with another 82K and see how you like that?

I can't see where any dc is being applied to the phase inverter pin 2 (check), so you probably don't even need C14 (plain wire will do) or R24 (redundant without C14)?

Man! Those 6L6s look like they're being driven hard? What plate current do you have?

Thanks for the replies.

I haven't measured the plate current, I need to do that. The amp actually sounds pretty good as is, and I'm running an old pair of Sovtek 5881's I had in my junk box.

MWJB, did you mean C13, rather than C33? If so should I remove or replace with a jumper?

thanks, regis

[MWJB]

"I'd lose C33 & R23." Yes, sorry I meant C13, remove it (the .1uf on the presence pot will be doing its job).

[Enzo]

R23 and R21 form a voltage divider to bias the upper triode. otherwise 390 ohms seems a little low for the cathode.

[Regis]

R23 and R21 form a voltage divider to bias the upper triode. otherwise 390 ohms seems a little low for the cathode.

Enzo I don't understand, do you think I should leave R23, R21 as is, and still install the pot and cap like MWJB says?

[Enzo]

This isn't my area, really. I was mainly just speaking to what the purpose of the resistor was. I can't recall which amps, but this method of biasing the cathode is used here and there. My suspicion is that they wanted that low 390 ohm cathode resistor for a reason, possibly as part of a voltage divider with the NFB resistor. In any case, just removing R23 leaves the 390 ohm cathode resistor all alone. And that means to get the desired 1.3v at the cathode, about 3.3ma would have to flow through the 12AX7. That is more than I would normally expect. Especially with the 220k plate load. COnsidering the B+ is 270, and they expect 125v on the plate through that 220k, then they are expecting 0.6ma through the tube. That current looks to me like the cathode resistor would need to be more like 2200 ohms. The same value MWJB suggested. Point being that this is a whole circuit, not just individual parts. You yank a part out, the rest of the circuit then needs to be considered.

As to the NFB, you may not be using that 70v/150 ohm winding, but the NFB IS using it. The voltage there will be a lot larger than it is at the 8 ohm tap. The circuit expects that level of NFB coming back around. You could move the NFB to a more familiar spot, but then you'd have to adjust the circuit where it winds up. On the other hand, if you just removed the NFB completely, you wouldn;t be the first guy to do it. Of course, no presence control then. As to MWJBs 40:1 NFB, that is from the 70v levels. You might not want that much.

But his idea might be exactly what you need. Tack it together and find out.

[Regis]

Ok, I begin to understand. Theory and design is not my strong point.

I think I have all the parts, I'll try to get to it over the weekend and see what happens.

Thanks to the both of you. I'll let y'all know how I make out.

[Regis]

I've modified the schematic to reflect your suggestions and marked them in red, if this is what you meant I'll go ahead with the mod.

I removed C14 and R24 from the schem, I haven't checked for DC there yet but I will before removing them.

Thanks guys....

regis

[Enzo]

SHouldn't R23 now be gone?

[MWJB]

"SHouldn't R23 now be gone?" Yes, so should C13 (replace with plain wire).

Feedback loop connects to hot side of the new R21 2200ohm resistor (12AX pin 8). Other end of R21 is grounded. (You currently have no ground reference for R21 & R27)

Second thoughts on the presence pot, keep the cap grounded at one end but lose the ground connection on the pot itself.

[Regis]

Oops yes I forgot about c13 and r23, gone now but I'm not clear whether r23 should be replaced with a jumper or gone completely? (Red arrow)

Also I ungrounded the pot as you said.

Sorry to be so slow understanding, thanks for all the help!

Made the changes on the schem here it is.

[MWJB]

"but I'm not clear whether r23 should be replaced with a jumper or gone completely?" R23 should be gone completely, no jumper.

[Enzo]

Originally, R23 and R21 made a voltage divider across B+ to establish the cathode voltage for V3A. Now you are using the 2200 ohm cathode resistor you installed to do that, so R23 needs to be gone. A jumper wire there would just connect the cathode to B+ and nothing good would result from that.