Issue with tremolo, "ploc" when switching.

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PhilippeCamus
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Issue with tremolo, "ploc" when switching.

Post by PhilippeCamus »

Hi all,

I've got a 18w 1974x amplifier. It's a kit, I guess from Tube town. Here is the picture.I don't know who built it. I discover a lot of mistake and solve it. The amplifier sound great, no noises, no hum.

Image

The tremolo switch is very noisy. Switching from tremolo on to off make a big "ploc" (not the opposite). You can see the noise on this picture. I plug my oscilloscope on R25 2.2M.

Image

I tried a lot of things, verify capacitors, switch etc... Nothing help. The tremolo sound nice. Just the noise at switching is noisy. Is it a usual issue?
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Re: Issue with tremolo, "ploc" when switching.

Post by Bieworm »

Wow there are some really long wire runs in that amp. The heater wiring... it's a miracle it doesn't hum. And no shielded wires from the inputs..
But I keep my tremolo switching jack as close to the tremolo circuit as possible. That could be part of the problem. Try running a shielded wire there maybe? Ground the shield at 1 side only.
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PhilippeCamus
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Re: Issue with tremolo, "ploc" when switching.

Post by PhilippeCamus »

Yes, agree with you. The wiring is not very nice. Has there is no hum, I will let it like that, or maybe do something if I'm courageous. For the ploc, maybe the ground wire is too fare and is at a different potential, generating the ploc when switched off. I will investigate this option.
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Re: Issue with tremolo, "ploc" when switching.

Post by TriodeLuvr »

I'm new to the idiosyncrasies of guitar amps, but I've been working with tubes for many years. Is this the schematic for the tremolo input of your amp? If so, I'm surprised they don't all do it. Does it pop at all settings of the Intensity control? You might be right about the ground - make sure the ground return on the jack is connected directly to the ground of the intensity pot. A ground loop there might cause this problem. It would also be interesting to know if the pop goes away if the switch is wired between the two caps (at the arrow) where there's no standing DC. Just thinking out loud here...

Jack

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Re: Issue with tremolo, "ploc" when switching.

Post by Bieworm »

TriodeLuvr wrote:
Wed 03/03/21 12:44 pm
I'm new to the idiosyncrasies of guitar amps, but I've been working with tubes for many years. Is this the schematic for the tremolo input of your amp? If so, I'm surprised they don't all do it. Does it pop at all settings of the Intensity control? You might be right about the ground - make sure the ground return on the jack is connected directly to the ground of the intensity pot. A ground loop there might cause this problem. It would also be interesting to know if the pop goes away if the switch is wired between the two caps (at the arrow) where there's no standing DC. Just thinking out loud here...

Jack

Image
Well.. I might be lucky, but I have 3 18W tremolo amps and none of those pop. And they're wired like that...
Long wire runs are prone to cause issues. Especially when there is no shielded wire used.
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PhilippeCamus
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Re: Issue with tremolo, "ploc" when switching.

Post by PhilippeCamus »

TriodeLuvr wrote:
Wed 03/03/21 12:44 pm
I'm new to the idiosyncrasies of guitar amps, but I've been working with tubes for many years. Is this the schematic for the tremolo input of your amp? If so, I'm surprised they don't all do it. Does it pop at all settings of the Intensity control? You might be right about the ground - make sure the ground return on the jack is connected directly to the ground of the intensity pot. A ground loop there might cause this problem. It would also be interesting to know if the pop goes away if the switch is wired between the two caps (at the arrow) where there's no standing DC. Just thinking out loud here...

Jack

Image
Yes this is the good schematic.
My first idea was a capacitor leak. Some DC in the tremolo switch and intensity pot. But it's not the case. I tried to connect the switch as you show between the two cap. This is the same. I discover that the pop appears if you switch at a moment when the tremolo wave form is not at 0. It's easy to see with the osciloscope at slow rate. If I switch at the exact moment the wave form cross the 0v, no pop. Like a sound sample you try to loop in the midle of positive or negative wave and create a pop.
And I'm quite sure there is a Hight frequency loop problem also. Don't know from where it goes. I add a 150p across the switch, but I still have this HF messing around. I will carry on tomorow. Thanks.
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Re: Issue with tremolo, "ploc" when switching.

Post by TriodeLuvr »

Incidentally, I found your intro post interesting regarding your work with tape machines. I performed factory warranty service for a number of years on Teac, Akai, Revox, etc. They were high maintenance compared to modern sources, but great fun to use. I still miss those large reels turning in the living room. :)

Jack
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Re: Issue with tremolo, "ploc" when switching.

Post by TriodeLuvr »

Bieworm wrote:
Wed 03/03/21 12:51 pm
TriodeLuvr wrote:
Wed 03/03/21 12:44 pm
I'm new to the idiosyncrasies of guitar amps, but I've been working with tubes for many years. Is this the schematic for the tremolo input of your amp? If so, I'm surprised they don't all do it. Does it pop at all settings of the Intensity control? You might be right about the ground - make sure the ground return on the jack is connected directly to the ground of the intensity pot. A ground loop there might cause this problem. It would also be interesting to know if the pop goes away if the switch is wired between the two caps (at the arrow) where there's no standing DC. Just thinking out loud here...

Jack

Image
Well.. I might be lucky, but I have 3 18W tremolo amps and none of those pop. And they're wired like that...
Long wire runs are prone to cause issues. Especially when there is no shielded wire used.
The reason for me wondering why they don't all do it is the transient that occurs when the switch shorts the grid to ground. The transient would be even larger when the tremolo oscillator is swinging. Maybe the frequency response of the amp is supposed to block it. Hammond did that in their older organs (key click filters). Or maybe the 220K at the anode of V3A effectively blocks it...

Jack
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Re: Issue with tremolo, "ploc" when switching.

Post by JMPGuitars »

The lead dress could certainly be a contributing issue. I don't see any shielded wires in there either.

Compare your tremolo to the later layouts in the download section, and also the tremolo section of the Tremolo TMB build docs. You may find some clues in there.

Thanks,
Josh
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PhilippeCamus
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Re: Issue with tremolo, "ploc" when switching.

Post by PhilippeCamus »

TriodeLuvr wrote:
Wed 03/03/21 5:57 pm
Incidentally, I found your intro post interesting regarding your work with tape machines. I performed factory warranty service for a number of years on Teac, Akai, Revox, etc. They were high maintenance compared to modern sources, but great fun to use. I still miss those large reels turning in the living room. :)

Jack
Oh yes, it was an interesting time for the audio. My father was technical manager of the Pathé Marconi/EMI sound studio at Boulogne (near Paris). After electronic studies I worked for Studer/Revox (thank you dad) and differents audio gear importer. It was an exciting time. But with the fall of big studio in the 90's I moved in the video gears maintenance. Home studio were not very interesting for me. Video, and particularly specials effects was the place to be in the 90's and 2000's. I'm half retired now and with the vintage fashion, some musicians gave me old gears to repair. By words of mouth, one musician, 2, 3 and so on ask for repair... And I'm back in audio gears. It's unbelivable for me to be back in the tape recorder, echo tape etc... In 2021! Strange world... It's a lot of fun and nostalgia for me. I started a Youtube channel (not very good, it's difficult...) about all these old gears with the help of the French Guitar channel. If your search youtube with my name you will find it. The last video is the modification of a Wurlizer 214 for the guitarist Kamyl Rustam.
Repairing tube amplifier was not my speciality, I had, from time to time, some to repair. I still learn tube and guitar amplifier with books of Merlin Blencowe, Gerald Weber etc... At 63 years old I'm still a student.
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PhilippeCamus
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Re: Issue with tremolo, "ploc" when switching.

Post by PhilippeCamus »

JMPGuitars wrote:
Wed 03/03/21 11:22 pm
The lead dress could certainly be a contributing issue. I don't see any shielded wires in there either.

Compare your tremolo to the later layouts in the download section, and also the tremolo section of the Tremolo TMB build docs. You may find some clues in there.

Thanks,
Josh
Yes. This morning i'm full of courage and decided to redo the wiring to get off to a good start. And I will study your documentation.
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PhilippeCamus
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Re: Issue with tremolo, "ploc" when switching.

Post by PhilippeCamus »

Hello every body,

I shielded the wire comming from the tremolo tube to the jack socket. It was better. Than I discover that the Marshall switch wire was not shielded. I change it for a good one with shield. It's much better. The ploc is still there, but it's acceptable. I have to say that I make my tests with the volume, the intensity, and the speed at the maximum. So it's ok for me. As the tremolo is much less noisy, I discover a motor boat noise... Again at full intensity, volume and speed. I will see tomorow if I can do something else.
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Re: Issue with tremolo, "ploc" when switching.

Post by Bieworm »

PhilippeCamus wrote:
Thu 03/04/21 2:33 pm
Hello every body,

I shielded the wire comming from the tremolo tube to the jack socket. It was better. Than I discover that the Marshall switch wire was not shielded. I change it for a good one with shield. It's much better. The ploc is still there, but it's acceptable. I have to say that I make my tests with the volume, the intensity, and the speed at the maximum. So it's ok for me. As the tremolo is much less noisy, I discover a motor boat noise... Again at full intensity, volume and speed. I will see tomorow if I can do something else.
That motorboating woof woof sound is very very hard to cure. I have it a little on my modern classic, but it's fine. I stopped chasing that long time ago. My tremolo TMB 's don't suffer from it though...
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Re: Issue with tremolo, "ploc" when switching.

Post by PhilippeCamus »

Yes, I got it. The motor boating appear when I shield the connection between the grid (V3-2) and the jack socket. Shielded one side only. Since the begining I have a problem with this connection. Too noisy, very unstable. Il put a 150pF on the jack socket as mentionned on schematics without succès. I decided to weld the 150pF next to the tube on pin 2. Motor boating desappear. As usual, using the capacitor with a short connection is the best solution.

For the ploc, I found a solution. TriodeLuvr Mention it above. Switching after the second capacitor. This is the solution of the Marshall 1930 popular. Changing the 1M by a 680K with the switch in serie. Now the tremolo is dead silent. The funny side of this modification is the delay when you switch on and off. With the usual schematic, the tremolo appear with a small delay. Modified, the tremolo desappear with a small delay. This is the opposite. Not a big deal.

The amplifier is now completly quiet and sound great. I just have to balance the level between tremolo on and off. Not difficult. Thank you for your help.
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Re: Issue with tremolo, "ploc" when switching.

Post by Bieworm »

Try different tubes with the tremolo. And see that the plate voltage of the tremolo triode is spot on or a little higher. That should cure the delay
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Re: Issue with tremolo, "ploc" when switching.

Post by TriodeLuvr »

PhilippeCamus wrote:
Sun 03/07/21 4:24 am

I just have to balance the level between tremolo on and off.
I wasn't aware of the 1930 circuit until you mentioned it (found it on the Tube Store Website). Could the on/off balance be the purpose of the additional pot at the cathode? That pot will set the DC idle voltage at the anode of the oscillator tube, which in turn modifies the gain of the preamp stage. If you have owned or used a 1930, you probably know whether that's its purpose. I'm no expert on tremolo circuits, so I could be wrong about this.

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Re: Issue with tremolo, "ploc" when switching.

Post by PhilippeCamus »

TriodeLuvr wrote:
Sun 03/07/21 2:18 pm
PhilippeCamus wrote:
Sun 03/07/21 4:24 am

I just have to balance the level between tremolo on and off.
I wasn't aware of the 1930 circuit until you mentioned it (found it on the Tube Store Website). Could the on/off balance be the purpose of the additional pot at the cathode? That pot will set the DC idle voltage at the anode of the oscillator tube, which in turn modifies the gain of the preamp stage. If you have owned or used a 1930, you probably know whether that's its purpose. I'm no expert on tremolo circuits, so I could be wrong about this.

Image
I would say, yes and no. As the cathode are common, it has an action on the first triode, but this is not the purpose of this pot (the 5k). I don't know in English, in French we say "résistance talon", I would translate by position resistor.
With the switch modification the oscilator signal is much stronger. Too strong (bad sound after 8 on the intensity pot). This pot is for tweaking the max intensity. The 480 is also important.
To balance the 2 triodes I change the 220k or the 100k, I don't remember.
(Sorry, it's dificult for me to speak English this morning :D )
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Re: Issue with tremolo, "ploc" when switching.

Post by TriodeLuvr »

Your English is fine. Certainly better than my French. :) Yes, I understand. Maybe the 100K resistor between the two anodes can be used to trim the gain when tremolo is off. There isn't much else in the circuit.
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Re: Issue with tremolo, "ploc" when switching.

Post by Bieworm »

I'm confused that all that tweaking is necessary. The schematic as is has been built for decades... it's almost certain a layout problem. I'd be digging into that instead of adding more components...
A good and deep tremolo is going to make some noise to a modest level.. understanding exactly what it does to your signal...
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Re: Issue with tremolo, "ploc" when switching.

Post by PhilippeCamus »

Bieworm wrote:
Mon 03/08/21 12:29 pm
I'm confused that all that tweaking is necessary. The schematic as is has been built for decades... it's almost certain a layout problem. I'd be digging into that instead of adding more components...
A good and deep tremolo is going to make some noise to a modest level.. understanding exactly what it does to your signal...
I don't know. There are no error compare to schematic. The tube voltages are good. Tube, resistors, capacitors are good (measured with kelvin precision LCR and my tube meter). The only problem is the messy wiring. In fact, the tremolo is very unstable with HF feedback and motor boating. This amplifier belong to my guitar teacher and he don't want to spend money to rewire the amplifier (and I also don't want to waste too much time for free, even for my venerated teacher). The 1930 popular Marshall schematic mod works fine for a small amount of work. I agree with you, and I'm not very happy with this solution, but it works. My teacher is a stage professional musicien, and actualy with the covid he is economically weak. Maybe for the furure I will redo it for him.
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