leslie 145 pops and screams

[boye]

After the leslie warms up (10-30 minutes), there is a strange sound coming out of the leslie. It starts with little pops. After a couple of minutes the evolve into longer pops. The longer pops are quite loud. After another 2 or 3 mins the sound evolves into long scream like sounds. This sound is very loud and cuts trough al of the clean organ signal so that it is almost impossible to play. Al the sounds described above ar random. playing louder can stimulate the sound. I think the vibrations are causing that because whenever i slap the tubes in the leslie soft (wich tube dous not matter), the sound appears. When the sound appears I can see the OC3 tube flickering. The pops and screams aren't coming out of the organs speaker (A100) So the problem must be located in the leslie.
I think one of the 6550 tubes ar down or the OC3 Tube is down (wich dousn't happen often:-P). But I am not sure and ordering new tubes is pretty expensive (located in The Netherlands). So my question is: Has anybody had this problem or has an idea of what is causing the pops and screams? That would be great!

cheers
Boye

[johnny b3]

The 145 is likely in the neighborhood of 40 years old! The amp needs attention if stock...General rebuild techniques should be applied, such as new capacitors all around, plate load resistors, etc...

This sounds like two different issues - possibly failing components (caps etc) and poor connections between tube sockets and pins. All should be cleaned with alcohol, sockets cleaned and re-tensioned as well.

Get a qualified tech if you don't know what you're doing, could be unsafe. At a minimum you can clean the tube pins and sockets with alcohol - spray the sockets and seat the tubes a few times.

[boye]

Thanks for your reply, Coming weekend I will take the amp out and measure some voltages to check if I can locate the problem. I will also clean the tube pins and sockets and re-tention them like you said. I might do a rebuild any time soon as the parts aren't really expensive. How would I benefit from replacing the crossover caps? they aren't expensive either...

cheers

[boye]

Allright i checked the the 147 amp voor voltages and a visual check and this is what i found:

The amp already is rebuilt! Every component is replaced except for the cancapacitor/powersuply capacitor. I am expecting that the problem is laying there because whenever i hear a pop or scream as described in the original post, the voltage rises 10 volts and after the next pop/scream my multimeter reads 10 volts lower. So the voltage is sometimes rising 10 volts and a little later back to the orgiginal volts. Also measured that most of my voltages werd about 5-10% lower then on the schematic. Tomorrow i will clean the tube sockets and prins!

Cheers Boye

[David Anderson]

The amp already is rebuilt! Every component is replaced except for the cancapacitor/powersuply capacitor. I am expecting that the problem is laying there because whenever i hear a pop or scream as described in the original post, the voltage rises 10 volts and after the next pop/scream my multimeter reads 10 volts lower.

Your can capacitor may or may not need to be replaced, but that is almost certainly a separate issue from the problem you're experiencing. Popping and squealing is a symptom of a bad connection. I just had to replace a rectifier tube socket in a Fender amp that was exhibiting this behavior. Also the fact that your amp is rebuilt raises other possibilities. I've had to re-do many rebuild jobs. Not everyone has good soldering skills, and it can be easy to miss soldering a joint. I've fixed several pieces of gear that had joints that were wire-wrapped, but never soldered at the factory, including my own Hammond C3 (missing pedal tone, never soldered).

If the can were the problem, your voltage would drop when the problem occurred. The 12AU7 sockets in Leslie amps are prone to bad connections, but it could even be a bad weld inside a tube.

There is a diagnostic technique known as the "chopstick test." You take a non-conductive probe (wood or plastic) and move things around gently with the amp turned on. When you find the point that causes the problem, you can hear it.

[Bodie]

Not wishing to seem alarmist and I could be wrong... I remember my 122 (same circuit?) was doing similar before the main power transformer blew up. In my case it also had earth leakage and occasionally trip the MCB in my house. Am I correct in thinking you operate on 220V and the Les motors are at 110V so have a primary tap as mine did? We run on 230V here (UK) I wonder if the Tx was a little under-rated and with age has finished the job off.
I'd be tempted to dangle my 'scope on the 420V HT and watch for sharp voltage drops.

You are free to totally ignore the above!

[johnny b3]

How would I benefit from replacing the crossover caps? they aren't expensive either...
cheers

Make sure you get appropriate capacitors (non polarized) and spring for a good set (I use Solen, made specifically for crossovers but there are others, too). As the original caps age they drift upwards in value, most commonly 100% increases... this causes mid/bass frequencies to be diverted from their normal flow to the woofer, instead being sent to the horn. Not only does this provide a risk for damaging the horn, it ruins the hi/lo separation of the cabinet and makes the horn sound muddy. This happens so slowly over the years that the deterioration in tone is hardly noticeable, but replace the caps and you'll hear a world of difference!

Of course...you may want to sort out your original issue! You mention that you have issues when you tap the tubes - start there. Proper tensioning of the preamp socket (easiest to do from underside) and cleaning should clear this up!

[geoelectro]

I recently had a Leslie amp where the tube terminals under one of the 6550 sockets were so close that touching the tube would cause an arc to jump from the plate terminal to the nearby ground terminal. Apparently this damaged the output transformer. After the socket was repaired and new 6550's installed, it would suddenly burst into a harsh noise and just as quickly quit, or you could make it quit temporarily by powering it down and then back up. One of the plate voltages would drop about 20~30 Volts during the noise.
Replaced the output transformer and all was well.

Geo

[boye]

Thanks for all your replies!!

Your can capacitor may or may not need to be replaced, but that is almost certainly a separate issue from the problem you're experiencing. Popping and squealing is a symptom of a bad connection. I just had to replace a rectifier tube socket in a Fender amp that was exhibiting this behavior. Also the fact that your amp is rebuilt raises other possibilities. I've had to re-do many rebuild jobs. Not everyone has good soldering skills, and it can be easy to miss soldering a joint. I've fixed several pieces of gear that had joints that were wire-wrapped, but never soldered at the factory, including my own Hammond C3 (missing pedal tone, never soldered).

If the can were the problem, your voltage would drop when the problem occurred. The 12AU7 sockets in Leslie amps are prone to bad connections, but it could even be a bad weld inside a tube.

There is a diagnostic technique known as the "chopstick test." You take a non-conductive probe (wood or plastic) and move things around gently with the amp turned on. When you find the point that causes the problem, you can hear it.

I will definately try the ''chopstick test", The amp is rebuilt by an Hammond/Leslie tech (I thought he had only rebuilt the motors) so everything is soldered well. Dispite that, there still could be a bad solder joint but at the moment I am suspecting the tube sockets as I can see that they are a little loose. Tomorrow I wil clean the sockets and tube pins with alcohol en re-tention the tube sockets. Also besides the screaming sound problem, after the laslie warms up, there is an inconstant hum coming out of the speakers but I am almost for sure that is the can capacitor so he will be replaced (:

Not wishing to seem alarmist and I could be wrong... I remember my 122 (same circuit?) was doing similar before the main power transformer blew up. In my case it also had earth leakage and occasionally trip the MCB in my house. Am I correct in thinking you operate on 220V and the Les motors are at 110V so have a primary tap as mine did? We run on 230V here (UK) I wonder if the Tx was a little under-rated and with age has finished the job off.
I'd be tempted to dangle my 'scope on the 420V HT and watch for sharp voltage drops.

You are free to totally ignore the above!

I think mine run on 230V but I will check that!

Make sure you get appropriate capacitors (non polarized) and spring for a good set (I use Solen, made specifically for crossovers but there are others, too). As the original caps age they drift upwards in value, most commonly 100% increases... this causes mid/bass frequencies to be diverted from their normal flow to the woofer, instead being sent to the horn. Not only does this provide a risk for damaging the horn, it ruins the hi/lo separation of the cabinet and makes the horn sound muddy. This happens so slowly over the years that the deterioration in tone is hardly noticeable, but replace the caps and you'll hear a world of difference!

Of course...you may want to sort out your original issue! You mention that you have issues when you tap the tubes - start there. Proper tensioning of the preamp socket (easiest to do from underside) and cleaning should clear this up!

Thanks for the info! The guy from who I bought the leslie might have also replaced them but I am not sure about that. I will check it! If they are not I will definately replace them. The ones you can get from tonewheelgeneral, are they ok?

Wel actually the sound occurs if I just tap the amp and sometimes even if I slam the leslie cabinet but to my opinion that would still make some sort of loose contact the cause of the sound.

I recently had a Leslie amp where the tube terminals under one of the 6550 sockets were so close that touching the tube would cause an arc to jump from the plate terminal to the nearby ground terminal. Apparently this damaged the output transformer. After the socket was repaired and new 6550's installed, it would suddenly burst into a harsh noise and just as quickly quit, or you could make it quit temporarily by powering it down and then back up. One of the plate voltages would drop about 20~30 Volts during the noise.
Replaced the output transformer and all was well.

Geo

Thanks for this info aswell! This does sound exactly like my problem but I first want to check if a loose contact is the cause and if the plate voltages are dropping 20-30 volts when the sound occurs like you said!


thank you all for your replies, this has been very helpfull!

cheers Boye

[Tim_at_Jonas]

Unlike with guitar amps, most of which are fixed bias, the Leslie noises I've encountered were usually owing to a bad 6550.

[boye]

Allright, I cleaned and re-tentioned the tube socktes and pins. Yesterday evening I have been playing for an hour and heard no pops and screams but this morning when I was playing again, the sound came back. Stil it is random but you can enhance it with vibrations from the woofer and the twisting of the lower rotor (or ofcourse tapping the amp or tubes). I am now suspecting the can cap and therfore ordering a new one so I can check if the problem is solved with the new cap. Still, any other ideas or tips on this problem? Thank you!

My first movie_20150109_163824 (online-audio-converter.com).mp3
Here is a recording of the sound. maybe this is helpfull.

[Guest]

Could be the input tube 12au7. I am afraid that 100pfd long legs left during the recap job are bringing those horrific screams back from death. You would know if you did the Thai chopstick test...

[David Anderson]

I have solved five random noise issues this week (Rhodes Suitcase, Marshall JCM 800, Ampeg R212R, guitar player's pedal board, and Wurlitzer 200A). All were due to bad or dirty connections. As I said above, the filter can may or may not need to be replaced, but a noise that comes and goes by tapping on the amp is a bad connection somewhere--or a short, like a solder bridge. It could even be where the filter can plugs into its socket. I have seen those connections get oxidized and need to be cleaned.

Finding these problems can be difficult. It took me ~3 hours to track down the problem in the Rhodes Suitcase, and that's a circuit I know very well. It was an original 1974 factory solder joint that looked perfectly fine via visual inspection, but it was contaminated somehow.

[boye]

Thanks For your replies,

The chopstick test had nor result.. But I have learned an extra method (:

I noticed that one of the 6550 tubes was alot more sensitive for the pop and scream. sound then the other when tapping it. I wanted to know if it was due to the tube socket or the tube. When I swapped them the same tube but now in the other tube socket was more sensitive for the sound when tapping it then then other.
Also while swapping the tubes I noticed the tube that was sensitive was alot cooler then the other 6550. When i swapped the tubes ,the same tube in the other tube socket was cooler so ons of the two tubes is Cooler no matter wich tube socket.

I then tried pulling the cooler tube and the sound stayed the same (tried the hotter tube in both tube sockets). Also with only the hot tube in the amp, the pop and scream sounds dissapeard. With only the cooler tube in the amp, there was almost no sound, only with Leslie volume on 10 and my ear next to the horn you could hear sound coming out of the speaker.

Might this indicatie that one of the tubes( the cooler one that is sensitieve) is bad?

Something must be interfearing with the tube if the tube is good. Something like a bad contact that is nog located on the 6550 tube sockets.

Cheers Boye

[Tim_at_Jonas]

I think you have found the trouble. Get a new matched pair of 6550s.