Leslie cabinet hum. any ideas?

[rjd2]

hi folks, i just picked up a second leslie cab, and tested it out on my leslie preamp pedal. the speeds change properly, and it passes signal just fine, but there is a really loud hum. KIND of like ground hum, but seems higher in pitch.(the hum is a low B natural, if that helps).

its independent of the volume level, although the 10% extra hum i get with the pedal and leslie amp at 10 seems to be the exact same pitch.

i dont believe its the preamp pedal, as my other leslie cab doesnt hum when i use it with the preamp pedal.

the guy i bought it from said he JUST changed the tubes. any ideas where i should start? i havent tested it yet on my hammond(hate pulling it away from the wall), but that's a possibility. thanks for the help.

[b3groover]

What model of Leslie is it? What models does your preamp pedal convert?

[Milkmansound]

what did you test it with?

many of the leslie speakers are pinned out differently - and there are many preamp pedals for many purposes to interface different organ pinouts and signal types.

we need more info!

[rjd2]

ahh, sorry guys. just assumed that all 6-pin leslies and preamp pedals are all compatible. i'll look this up, and also check my m3 organ and see if that changes anything. the leslie is a 145. thanks.

[???????]

ahh, sorry guys. just assumed that all 6-pin leslies and preamp pedals are all compatible. i'll look this up, and also check my m3 organ and see if that changes anything. the leslie is a 145. thanks.

When is the last time the filter capacitors have been replaced?

What you have is a 120Hz hum, sounds like, which is a sure sign of dead filter caps.

Also, if he "JUST changed the tubes," then that would be among the first places I would look, not the last. Just because a tube is "new" doesn't mean it's "good," especially if it's a new-production Chinese or Russian tube. Typically, when you make some changes that need troubleshooting, you look at what was done since it was last working.

[casey campbell]

ahh, sorry guys. just assumed that all 6-pin leslies and preamp pedals are all compatible. i'll look this up, and also check my m3 organ and see if that changes anything. the leslie is a 145. thanks.

When is the last time the filter capacitors have been replaced?

What you have is a 120Hz hum, sounds like, which is a sure sign of dead filter caps.

Also, if he "JUST changed the tubes," then that would be among the first places I would look, not the last. Just because a tube is "new" doesn't mean it's "good," especially if it's a new-production Chinese or Russian tube. Typically, when you make some changes that need troubleshooting, you look at what was done since it was last working.

+1 on the filter caps. if you dont know what you are doing with electronics, get a professional to replace them. an electrolytic capacitor can hold around 450 volts even when the unit is switched off and unplugged. if you touch those caps you could either -

a. die

or

b. lose control of your bowels and have to clean out your drawers

or

c. die and lose control of your bowels. this is not fun.

im not kidding either. i've heard of a guy that was working on a guitar tube amp by laying it on a table and leaning over it. his friend had left the room for a second, and when he came back, his buddy was dead laying on the amp. the amp was unplugged.

even if you know what you are doing, please be careful. i myself repair electronics and have done alot of tube amps before. i always discharge the caps, and even then, i work with one hand in my back pocket and the other i work on the amp with. this helps not getting killed in case of an accident so that the voltage doesnt go straight through your heart from one hand to the other.

my guess is that you are not familiar with electronics or you would have checked the filter caps to see if they are leaky. i would encourage you to seek a professional electronic repairman's help. not to scare you, but i hope i have.

[rjd2]

thanks alot for the leads, guys. im pretty competent with a soldering gun, but i have done 90% of my work on pcb's and solid state synth-based stuff. i am aware of the fact that discharging caps needs to be done, and the lethal nature of tube-based work. i will read up on the proper discharging technique before i dive into this thing(is there ANY sort of safety period after which the charge on a cap will have dissipated, or can it last for weeks on end?).

[???????]

Think of it like a gun.

If you have a gun that's definitely loaded, you want to make sure it's unloaded before you do anything else. You need to know how to safely do this.

If you have a gun that's definitely unloaded, you want to double-check that it's unloaded still before you do anything.

If you have a gun and you don't know if it's loaded or not, you assume it is and proceed as if it were.

An amp with electrolytic caps that has been off for a long period of time falls into category 3 above. If you don't know whether the caps are discharged or not, then you better discharge them anyway.

It's usually not that hard.

Use a digital voltmeter to check and see how much voltage is on them. Measure voltage between the + side of each cap and ground. If you read near zero volts, you are safe. If you read a high-ish voltage, you need to discharge them before proceeding.

[Nate Dort]

thanks alot for the leads, guys. im pretty competent with a soldering gun, but i have done 90% of my work on pcb's and solid state synth-based stuff. i

Please don't use a soldering gun on a leslie, or anything for that matter.

[???????]

thanks alot for the leads, guys. im pretty competent with a soldering gun, but i have done 90% of my work on pcb's and solid state synth-based stuff. i

Please don't use a soldering gun on a leslie, or anything for that matter.

truth.

You need a nice temperature controlled iron like a Weller WES51 or Hakko 936 or equivalent to do most tube amplifier work. I assumed "soldering gun" was just an error in words, but maybe not...

[The Scum]

(is there ANY sort of safety period after which the charge on a cap will have dissipated, or can it last for weeks on end?).

'fraid not.

Think of it like a gun.

...except that the metaphor breaks down, due to dielectric absorption.

An unloaded gun, sitting on a shelf, doesn't magically reload itself. But a cap, even after being discharged, can magically recharge itself.

The dielectric in the cap can store some current. When the cap is charged, that current stays "hidden" in the dialectric. But when it's discharged, that current can leak back into the cap, recharging it. This is a parasitic effect that is more pronounced on old, large value caps...precisely what you'd find in a Leslie.

And in case you didn't know it, the right way to discharge a cap is to strap a resistor across it. A 300K, 1/4W resistor will do the job. You can actually leave it in the circuit when you're done, and it will help prevent future shock, as well.

Pun fully intended.

[Milkmansound]

its too late to prevent this...

[b3groover]

ahh, sorry guys. just assumed that all 6-pin leslies and preamp pedals are all compatible. i'll look this up, and also check my m3 organ and see if that changes anything. the leslie is a 145. thanks.

If the pedal preamp is for a 122 style Leslie, you probably just fried the motor control circuit (relay). The pins that carry the mains and the ones that carry the audio are reversed in the 122 style 6pin connector as compared to the 147 (and 145) style. The two are not compatable.

[rjd2]

thanks, guys.

possibly fried motor control circuit-sweet! sorry to be leslie daft, but had i checked this a second time last week, and the preamp pedal would change the speed in the leslie from A)both drum and top rotor at fast, to B) top rotor at slow, and bottom drum off. is that correct operation of a 145?

-gun/iron/instant happy fun burn machine-sorry guys, im bad w/ words. yes, i have a weller variable temp IRON, not the $7 radio shack thing.

-cap dissipation-thats good to know. i will make sure to keep my terror alert level at red at all times when working on anything tube, never orange or yellow.

i just removed the amp and had a peek inside(carefully), and the +side of the 80mfd/450v cap is loose. neg side ties to one of the big can cap pins. i guess i need to find the schems to find which pin that + side ties to.

also, is this the only place to get a replacement can-style cap? $80 seems kinda steep:

http://www.hammondb3andleslietips.com/L ... rtskit.htm

thanks for all the help guys. (man, i'd much rather be listening to future shock than dealing with broken shit this week).

[???????]

look for can caps at www.tubesandmore.com or www.tedweber.com

You could also replace with discrete caps.

The + side of a filter being disconnected will mos def cause your hum.

Try re-connecting the old cap. It might be fine, for now, though if they've never been replaced it would be a good idea as preventative maintenance.