Hammond CV with 122 - Can I adjust the CV preamp for more gain?

[glensound]

I would like to know if it is possible to adjust the preamp gain for a little bit of distortion. If an adjustment is not possible, is there a circuit that I could add to this system to accomplish the added distortion?

Thanks!

Hammond CV (1945)
Leslie 122 in 145 cabinet
Trek II percussion

[David Anderson]

The gain of the CV preamps has been discussed extensively on here in the past.

In short, there's no adjustment as there is on the AO-10 or AO-28. The best you can do is to make sure it's working properly. Much of the gain in the CV preamp comes from the 1st stage.

You can choose the highest-input connection at the input to the preamp, if this has not already been done. That's in the rheostat box. Diagrams for finding this connection are in the service manual.

Remember that Hammond wanted to avoid distortion, and the design reflects that.

[glensound]

You can choose the highest-input connection at the input to the preamp, if this has not already been done. That's in the rheostat box.

Thanks for your response David. I did search the forum a bit before I posted but did not find what I was after. I'll check into this. My CV came to me from the original owner (a religious convent) with the DR-20 cabinet. So I have my doubts that any rheostat adjustment has ever been made, but one never knows.

David

Hammond CV (1945)
Leslie 122 in 145 cabinet
Trek II percussion

[David Anderson]

There's a voltage divider string on the signal at the output of the rheostat box, and you can either take the signal from the top of that series of resistors or further down the line for lower input voltage. It's not an adjustment you turn; it's swapping wires from one terminal to another.

Wes and I discussed the output of his CV preamp, but I can't recall everything he did. Perhaps he can chime in.

[glensound]

Can someone please verify the movement of the red wire for the increased gain on the CV rheostat?

[enor]

Can someone please verify the movement of the red wire for the increased gain on the CV rheostat?

Yes. Correct!

[David Anderson]

That is correct. Might want to swap out that 0.001uF paper capacitor while you're in there. No telling what value it's drifted to at this point.

[glensound]

Thanks for the verifications! Will pull that cap and replace. Good idea David. I have some sprague orange drops. What voltage, do you know? Sometimes it's not evident on the old one. Also I don't have an R2 installed. Or at least it seems to me..it is hard to see by the photo. Does anyone agree?

[David Anderson]

Capacitor voltage rating is largely irrelevant here. Use whatever fits best.

I don't know about R2 here because it doesn't look like it was ever installed in your CV. There's no wire to the upper terminal, so even if you installed it, it wouldn't be attached to anything. It's possible it was a running change in the circuit. The changes in the Hammond CV series are not as well-documented as they were on later models. Yours looks early because you have 20k and 50k resistor values, not 22k and 47k.

Maybe R2 wasn't used in yours, or maybe it was located somewhere else on the rheostat assembly.

Perhaps someone who's been inside more BVs/CVs could comment.

[glensound]

I'm happy with the change and need to experiment some more with my volume level on the Leslie 122 etc. It has made a difference due to the gain staging. That cap will have to wait since I need to order one after all.

David

Hammond CV (1945) SN 11382
Leslie 122 in 145 cabinet
Trek II percussion

[glensound]

What I am now noticing is that when the system is idling, I am hearing a low level scratching noise. I am wondering whether this is a result of just moving the rheostat wire to the 'connection for increased output'. Should I ignore this low level noise or maybe think that it is a result of moving the wire. I don't recall this noise before the change. Once again, this is while idling with no volume on the pedal. Any comments? It sort of sounds like tube noise to me.

[David Anderson]

You may simply be paying more attention to it.

The thing I've never entirely liked about the pre-1950 Hammonds is that the entire preamp circuit comes after the volume control. The result is that you always get all the preamp noise at the organ's output regardless of the position of the swell pedal.

Beginning in the AO-10, the early gain stages of the preamp are before the volume control, so most high-gain circuits are muted with the expression pedal down.

The noise in CV preamps can be minimized by rebuilding them and selecting low-noise tubes insofar as that's possible.

In HiFi preamp design, it's best to have the volume control as close to the output as practical.

[glensound]

Well, I did change the gain structure. And I did have the Leslie volume up too loud. Now that I've been playing around with a lower 122 setting and cranking the Hammond, I do like the new improved sound. I had the idea of swapping tubes to see if that changed anything. Do you have any recommends for the 6SN7GT and 6SJ7's? I've got GE's and a KenRad in there from who know's when... What you are saying about the preamp design and volume control makes sense. Also I'm using this setup in a project home studio, so I will be recording not playing in a venue. I may be able to use a gate during recording.

[muckelroy]

The 6SJ7 can me microphonic. If you tap the tube with a pencil eraser while it’s on and hear it like tapping a mic, there’s your answer. Badly microphonic tubes will pick up all sounds from striking the cabinet or the preamp chassis. You may need to just buy a set of 5 or 6 known good tubes and swap until you find one that is quiet enough.

[David Anderson]

Do you have any recommends for the 6SN7GT and 6SJ7's? I've got GE's and a KenRad in there from who know's when...

6SJ7 pentodes are notorious for being noisy and microphonic. I recommend using one that's the least noisy/microphonic one you can find, and, no, one brand does not tend to be better than another. It's the individual tube.

6SN7s, being medium-mu triodes don't tend to be noisy, but they can be. Any tube can fail.

There's a 6J7 under the turret, strapped as a triode. Check the top cap for a good, tight connection.

Carbon composition resistors on plates and screens, and, to a lesser extent, unbypassed cathodes, can also introduce sizzling/popping/rustling noises.

The CV preamp, IMO, is one of Hammond's stranger designs. I get what they were doing. They were trying to avoid the use of electrolytic capacitors for power supply filtering, but they also needed more voltage gain than previous preamps to drive the vibrato/chorus circuit, in which a lot of that gain is lost, so they used symmetrical circuits that would tend to cancel power supply hum from a not-so-well filtered power supply. The problem with losses in the vibrato/chorus circuit also required them to use a an extremely high-impedance input return circuit, necessitating the shielded turret.

The AO-10 is their tacit admission that this approach had reached the limits of its usefulness.