Leslie 122 bad sound

[geldo79]

Hi,

once again I hope to find answers concerning the
bad sound of my Leslie 122.

I 'll try to explain the problem:

Compared to my Solton Leslie clone + Neo Vent, the
sound of my 122 is worse. Even if the 122 produces
a much better sounding overdrive, I really don't like the
basic organ sound it produces.

The overall sound is dull on the first impression. But
when I tested the high frequencies only, I think they are
there. But, the signal from the mid and lower tones seem
to be too dull and too loud.

When I pick up the signal from the upper driver with mics,
I can get a nice sound. But without, the deep freuqencies
seem always to worsen the overall sound. When I activate
the percussion, the effect is very weak. Seems that I have
to select the loud percussion volume for the ping to come
through clearly audible. On my solton, I get a really nice
percussion tone in the sound mix.

On the other hand, if I reduce the organ volume to a minimum
using the expression pedal, the overall sound seems to be a
lot better. I always wish I could get that sound at a higher
volume.

What could be the reason for these observations? This can not
be "the sound" I would expect from a 122.

By the way, the upper driver is new, and the crossover is recapped.
These modifications did not change the sound at all.

Could the woofer be the problem? It is a vintage Utah woofer.
I already tend to buy a new Jensen C15k to check the difference.
I am wondering what could be broken in the woofer for behaving
like it does.

I am also wondering what impact on the sound the woofer has. I
think it produces the mid - and low tones, right? So it might
be a bigger impact than I might believe.

Any suggestions?

[crysalis]

Are both upper and lower drivers 16ohms? If not, that can throw the crossover out of whack. You also don't mention what organ or connector kit you are using.

Has the leslie amp been serviced at all? Caps?

[muckelroy]

I've glanced through your previous threads on this same issue.

Which model driver did you have before, and which model driver do you use now?

Also, both drivers should be 16 ohms, but the DC Resistance will almost always be significantly lower (anywhere from 7 ohms to 13 ohms). This should not be a cause for concern. However, if you're uncertain which speaker impedance your driver has, one technique is to measure the peak-to-peak output of both speakers independently using a test tone generator, and an oscilloscope. I've done this a couple times to map out the frequency response of my drivers, when there was doubt as to the speaker impedance.

How to do it:
1. Find source that can play specific frequency of sine waves. You can use your organ, if you figure out which frequency each note plays. http://www.goodeveca.net/RotorOrgan/ToneWheelSpec.html/
2. Connect oscilloscope leads to first the driver terminals. Leave all speakers connected, but also attach your scope leads.
3. Play test tones at uniform volume at 50 Hz intervals. Each time, make note of the peak-to-peak amplitude seen at the driver.
-- Do this from about 400 Hz to about 2 KHz.
4. Do the same test, with the same frequencies, this time with scope attached to woofer leads.
5. Make a graph of these two lines. The place where they intersect is your crossover point. You want to not see any big "holes". (i.e. you do not want the bass to roll off at one point, then the treble to only take over at a much later point.)

The crossover point should be close to 800 Hz. A little higher is OK, but any more than 200 - 300 Hz difference is cause for concern. Mostly you don't want a hole.

Using an 8 ohm driver with a 16 ohm woofer will create a situation where the woofer rolls off at around 800 Hz, but the treble driver does not begin picking up signal until about 1,600 Hz. So you end up with a hole in between.

Conversely, using an 8 ohm woofer will create a hole in the bass, between 400 and 800 Hz.

Using BOTH 8 ohm woofer and driver will create a massive hole between 400 Hz and 1,600 Hz.

[Sweet Pete]

In order of likelihood causing issues:

1. Wrong impedance driver.
2. X-over caps have gone off course.
3. Get a 1.5V battery and check phase of the 15" driver.PITA to open up,but I check the V21 too!
4. Check the 470 ohm? resistor off the OC3 in the 122.(147 for sure has one!)

The best I can coax out of a pristine 122 chassis in a known good box beside a 147 chassis in the box? maybe 75% of the treble......not much more.
122 has always been too tubby for my tastes.Great in a studio or for 888000000 or 888800000 or 858000000......not much more IMO.
I take 147's or 145's or a 770 out to a gig. They do have more hi frequency response. Also,no need to keep 'nice' side from view! My top rotor covers are usually on.

[geldo79]

Are both upper and lower drivers 16ohms? If not, that can throw the crossover out of whack. You also don't mention what organ or connector kit you are using.

Has the leslie amp been serviced at all? Caps?

The upper driver is 16 Ohms. It is a Monacor KU516. When I replaced the Jensen V21 I did not hear any differences. I replaced the V21 because I didn't want to kill the driver with too much overdrive.
The lower driver is a vintage Utah driver, which measures 8.8 Ohms DC resistance....and should therefore be a 16Ohms driver....I hope so.
I am using a 8010-Leslie kit connected to a M3. The Leslie amp has been completely serviced by the pre-owner and should be ok.

- - - Updated - - -

I've glanced through your previous threads on this same issue.

Which model driver did you have before, and which model driver do you use now?

Also, both drivers should be 16 ohms, but the DC Resistance will almost always be significantly lower (anywhere from 7 ohms to 13 ohms). This should not be a cause for concern. However, if you're uncertain which speaker impedance your driver has, one technique is to measure the peak-to-peak output of both speakers independently using a test tone generator, and an oscilloscope. I've done this a couple times to map out the frequency response of my drivers, when there was doubt as to the speaker impedance.

How to do it:
1. Find source that can play specific frequency of sine waves. You can use your organ, if you figure out which frequency each note plays. http://www.goodeveca.net/RotorOrgan/ToneWheelSpec.html/
2. Connect oscilloscope leads to first the driver terminals. Leave all speakers connected, but also attach your scope leads.
3. Play test tones at uniform volume at 50 Hz intervals. Each time, make note of the peak-to-peak amplitude seen at the driver.
-- Do this from about 400 Hz to about 2 KHz.
4. Do the same test, with the same frequencies, this time with scope attached to woofer leads.
5. Make a graph of these two lines. The place where they intersect is your crossover point. You want to not see any big "holes". (i.e. you do not want the bass to roll off at one point, then the treble to only take over at a much later point.)

The crossover point should be close to 800 Hz. A little higher is OK, but any more than 200 - 300 Hz difference is cause for concern. Mostly you don't want a hole.

Using an 8 ohm driver with a 16 ohm woofer will create a situation where the woofer rolls off at around 800 Hz, but the treble driver does not begin picking up signal until about 1,600 Hz. So you end up with a hole in between.

Conversely, using an 8 ohm woofer will create a hole in the bass, between 400 and 800 Hz.

Using BOTH 8 ohm woofer and driver will create a massive hole between 400 Hz and 1,600 Hz.

Now I use a Monacor KU516, before it was a JensenV21 which I still have (and should also be ok).
Unfortunatley I don't have an oscilloscope or test tone generator :-(

- - - Updated - - -

In order of likelihood causing issues:

1. Wrong impedance driver.
2. X-over caps have gone off course.
3. Get a 1.5V battery and check phase of the 15" driver.PITA to open up,but I check the V21 too!
4. Check the 470 ohm? resistor off the OC3 in the 122.(147 for sure has one!)

The best I can coax out of a pristine 122 chassis in a known good box beside a 147 chassis in the box? maybe 75% of the treble......not much more.
122 has always been too tubby for my tastes.Great in a studio or for 888000000 or 888800000 or 858000000......not much more IMO.
I take 147's or 145's or a 770 out to a gig. They do have more hi frequency response. Also,no need to keep 'nice' side from view! My top rotor covers are usually on.

wrong impedance: Well....as I've mentioned before I measured 8.8 Ohms DC resistance. But I don't know if this definitely means that it is a 16 Ohms speaker.
X-over: I recapped the crossover with high quality audiophile caps.....this should not be the source of the problem.
Driver Phase: Can you explain how to do the test? What can be wrong with the phase?
470 Ohm resistor: I can check that. What effect can a wrong value have?

[muckelroy]

I've never heard a Monacor driver so I can't say what to expect from it.

Sweet Pete referred to the 470 ohm resistor, only because that is the most frequent component that fails in these amps. What often happens is the resistor may open up (both in value, and sometimes, literally, split in half.) When this happens, the screen voltage goes down, which could result in any number of undesirable sounds, and could also cause tremolo switching to no longer work properly, as the kit requires the pin 5 B+ voltage from the Leslie.

If the amp was serviced, it would be expected that this resistor was checked, and if it was replaced, then this is a non-issue. Using a higher value could result in a slightly lower screen voltage.

The thing is, you've spread out your issue across at least 3 or 4 different threads on the forum. My advice would be to stop making new threads, and stick to this one thread for this one issue.

You could always post some close-up well-lit photos of the underside of your amp, if you have any doubts about the quality of the work that was done to it.

I have not ignored your other threads about your M3 effects loop. Are you certain that the effects loop work has not interfered with the frequency response in some way? Are you testing in the stock configuration, or with your effects loop in place?

Your Utah woofer is fine. They all measure around 8 or 9 ohms DCR.

[David Anderson]

Generally speaking, we can sometimes provide answers on this forum when something is clearly broken, but "My amp doesn't sound quite right to me" questions lead to endless guessing games and long lists of things that might be wrong, but may not be. It's like a man with a blindfold on trying to hit a target with a shotgun.

Do you still have professional expert tube amp technicians in Germany? In the end, there is no substitute for having the amplifier in front of you on a bench when it comes to identifying a problem.

[geldo79]

I attached an image if the underside of my amp. What I don't understand is why there is a capacitor (0,01 mF) between pins 2 and 4 of one of the 12au7 (the bottom 12au7 in the image). I can not see that cap in the schematics. What do you think?

[crysalis]

Doesn't make sense to me. On a 12AU7, Pin 2 is grid2 and 4 is heater. Can you lift one lead of the cap and test?

[geldo79]

At the moment I can't test it. Next time i am in the band room i can try. By the way....that 0.001mF cap which should be located between pins 2 and 7 of the other 12au7 i can't find. Hmm...

[Jaim]

What I don't understand is why there is a capacitor (0,01 mF) between pins 2 and 4 of one of the 12au7

Pin 2 is the grid of the switching tube. Pin 4 (heater) is probably ground as one side of the heater circuit is usually grounded. I don't know why that cap would be there I would cut it out.

The Missing .001 mfd cap on pins 2 to 7 is there to to roll off highs and key click.

Jim

[geldo79]

The Missing .001 mfd cap on pins 2 to 7 is there to to roll off highs and key click.
Jim

Yes, but then I'd expect the Leslie sound bright....but it doesn't. Or can it cause problems if that cap is removed?

Maybe you guys could also have a look at the next image. Does the wiring look ok for you? I can not find everything in the schematics,
e.g. the cap between pin 3 of the right 6550 and pin 2 of the left 6550.

[geldo79]

Ah....no. That cap should be ok.

[David Anderson]

There's a capacitor on the switching 12AU7 socket that's not supposed to be there -- on any 122 I've ever seen. Did someone re-engineer this amp?

Make sure the resistor to the switching 12AU7 (also connected to the mystery capacitor) is 1Meg Ohm.

Also, the value of the resistor from the 0C3 socket to the capacitor socket looks like the wrong value. That shouldn't affect the sound, but it's supposed to be 470Ω, 0.5 Watt. It's not supposed to be over 0.5W so that it will blow in the event of a 6550 screen short.

And check the marked value of all the film capacitors, especially the ones on the 6550 plates. They should all be 0.005uF or 0.0047uF at 1600V or higher. For some reason, in the photo, it looks to me like the ones that run between terminals on the 6550 sockets are larger than the ones connected to ground, but that may just be the photo. Are the capacitors from the volume pot to the 12AU7 0.015uF?

And there appears to be no 1000pF capacitor on the audio 12AU7. That should actually make it sound brighter.

Because I can't examine it directly, I can't tell for sure, but my intuition is that someone who's not really familiar with Leslie 122 amps rebuilt this amp and decided to make a few changes. It follows my Diagnostic Rule #1: If there is a problem and there is previous work done to the amp, look at the previous work first.

[geldo79]

The 470 Ohms is ok. There are 2 in parallel.....it measures about 465 Ohms.

BUT.....what I have found is that the 1.2k resistor on the 12au7 is actually a 2.2k resistor. That should make a difference....or not?