Preamp tube Oscillation

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Lukep
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Preamp tube Oscillation

Post by Lukep »

Hi,

I have been messing around with an amp based on the Marshall 18 watt. It has two channels 1 = EF86 and 2 = 12ax7. Nothing too radical yeah just sussing out how different values and layouts change the tonal character of the amp. I have come across an issue that has me stumped. In channel 2 the tube oscillates like crazy. I have checked the connections looking for positive feedback nothing found. The resistive values are all correct when measured. No obvious shorts. Behaves the same with different 12ax7 tubes insitu. I have attached the schematic sorry it’s a photo but a working drawing with some annotation in pencil. I currently have the capacitor connecting the pre to the splitter lifted in an effort to isolate the issue. Incidentally the EF86 side works well. Thanks for reading. Regards Luke
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JMPGuitars
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Re: Preamp tube Oscillation

Post by JMPGuitars »

Hi Luke,

Quite often oscillations can be caused by lead dress. Post some photos of your build so we can take a look. I also suggest "chopsticking" the wires on the suspect channel.

Thanks,
Josh
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Lukep
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Re: Preamp tube Oscillation

Post by Lukep »

Hi Josh,

Thanks for your response. Attached are some photos. It’s a bit of a rats nest largely as a result of part swapping. The section of concern is the tube base with the orange wiring.

It should be noted this is a rebuild the first build had the same issue.
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Re: Preamp tube Oscillation

Post by JMPGuitars »

I immediately see at least two problems.

1. The shielding on that wire should be connected to the back of the pot, not the ground bus (also make sure only one end is connected to ground). You should read the threads in my signature. Shielding gets its own connection to ground, never the ground bus.

2. What is that giant copper wire looking like an antenna connected to the tube socket?

Besides that, you really need to improve the soldering quality. Watch the videos linked in the soldering thread in my signature. I could point out a LOT of things in those photos, but hopefully watching the videos will help you see the issues for yourself.

Thanks,
Josh
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Re: Preamp tube Oscillation

Post by Lukep »

Thanks Josh, I look through the vids. Excellent tips re shielding, I didn’t know that. Yes the soldering is average it’s just tacked together really. I haven’t even cut the component leads. This is on purpose. Like I said it’s a test frame. The copper bar is a ground it’s not connected to anything other than a wire shield and ground of course. The chassis is my test build chassis it has had a fair number of builds in it. I leave the ground busses insitu.
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Re: Preamp tube Oscillation

Post by JMPGuitars »

Lukep wrote:
Tue 10/12/21 8:51 pm
Thanks Josh, I look through the vids. Excellent tips re shielding, I didn’t know that. Yes the soldering is average it’s just tacked together really. I haven’t even cut the component leads. This is on purpose. Like I said it’s a test frame. The copper bar is a ground it’s not connected to anything other than a wire shield and ground of course. The chassis is my test build chassis it has had a fair number of builds in it. I leave the ground busses insitu.
That copper wire is an antenna. Grounds need to be as short as possible, read the grounding thread too. ;) ...and I would gamble that it's likely the cause of the oscillation, or at least making it worse.

Everything matters in amp building. The lack of proper solder = issues. The unclipped ends = issues. Long copper wires that should be short = issues.

Thanks,
Josh
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Re: Preamp tube Oscillation

Post by Bieworm »

JMPGuitars wrote:
Tue 10/12/21 8:00 pm
I immediately see at least two problems.

1. The shielding on that wire should be connected to the back of the pot, not the ground bus (also make sure only one end is connected to ground). You should read the threads in my signature. Shielding gets its own connection to ground, never the ground bus.

You're right about the shielding from the shielded wire not to be attached at the ground buss. Yesterday I looked for 2 hours where an oscillation came from. On my 6G3 build there was this oscillation coming straight out of nowhere... in the end it appeared to be a shielding I attached to the ground buss. When I removed that one the oscillation stopped instantly.
Now there is another problem... I first desoldered that shielding and noticed the noise stopping. I couldn't attach it to the back of the pot, so I cut off the shielding branch. It's a shielded wire coming from the wiper of the bright vol pot btw... that explains the loud oscillation. But now when I dime the vol pot I get oscillation again.. right at the end of the pot's travel, the last milimeter...
I will make a new shielded wire there, attaching the shield at the back of the pot.
oscillation problem.png
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Re: Preamp tube Oscillation

Post by zaphod_phil »

I don't agree with soldering shielding onto the back of pots, as that's a surefire way of creating ground loops through the chassis. 8O

I am a firm believer in grounding the shield ( at one end only) to the nearest common ground point for that stage (usually at the bottom of the nearest cathode cap). That keeps all the ground currents tightly confined where they belong.
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Re: Preamp tube Oscillation

Post by JMPGuitars »

zaphod_phil wrote:
Fri 10/15/21 3:34 pm
I don't agree with soldering shielding onto the back of pots, as that's a surefire way of creating ground loops through the chassis. 8O

I am a firm believer in grounding the shield ( at one end only) to the nearest common ground point for that stage (usually at the bottom of the nearest cathode cap). That keeps all the ground currents tightly confined where they belong.
Why would it cause a ground loop? As you said, the shielding is only connected on one end...
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Re: Preamp tube Oscillation

Post by TriodeLuvr »

JMPGuitars wrote:
Fri 10/15/21 5:10 pm
zaphod_phil wrote:
Fri 10/15/21 3:34 pm
I don't agree with soldering shielding onto the back of pots, as that's a surefire way of creating ground loops through the chassis. 8O

I am a firm believer in grounding the shield ( at one end only) to the nearest common ground point for that stage (usually at the bottom of the nearest cathode cap). That keeps all the ground currents tightly confined where they belong.
Why would it cause a ground loop? As you said, the shielding is only connected on one end...
The back of the pot isn't a clean ground. It's connected to the chassis, which is connected to the third wire house wiring (ugh). The best reference for the shield is at the amplifier stage itself. Connecting to the pot might not create a ground loop in the usual sense, but avoiding it and using the technique described by zaphod_phil is sound.

Jack
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