[marcomeloni1979]

Hi, i see that here is mostly German but ill give it a try. I just completed a PX18 and it sounds great. I have one issue with the presence control that when its turned all the way up makes like a frying noise. Anybody has any experience with it?

[stephan61]

Hi,
Don't mind, this is typical for Marshall like amps.
It is always applying DC voltage on the presence pot. This DC produces that noise.

regards
Stephan

[marcomeloni1979]

Thank you for your insight  Stephan.
It gets especially intense when i turn the treble volume all the way up. With the normal volume is barely noticeable. O talked to somebody who built the jtm45 from TT but he said he didnt have that problem. He said it could be a leaky cap on the presence pot. On the other hand i was also reading that the Marshall SV20 are also very noisy...

[stephan61]

Just for clarification, the noise occurs while rotating the presence pot or is it in the signal when the pot has a certain static  position?

[marcomeloni1979]

Its really pronounced when its on 10. I know it makes a scratchy noise when turned but this happens in the signal.

Marco

[stephan61]

Ok, now I understand. It has nothing to do with the DC on the pot. Disregard my first post. Sorry for that.
The root cause may be parasitic oscillations, caused by unproper lead dress. Do you have the 47pF cap between the anodes of V3?

regards
Stephan

[marcomeloni1979]

Hi Stephan,
no, in the tube town layout theres is not a resistor between plates in V3. There is one in V2, but its a 100k.
This is the TT layout of the px18 https://www.tube-town.net/cms/userfiles/downloads/px18 -layout.pdf
Thank you for the help!
Marco

[marcomeloni1979]

https://www.tube-town.net/cms/userfiles/downloads/px18-layout.pdf
The previous link didnt work

[Dirk]

Hi ! the PX18 got build many times so far and the layout and schematics are known to be correct so if you have a problem with your build then this could be because you made a mistake just like a missing ground, a wrong component etc. I highly recommend to check all components, the connections and also make sure that the wires are not to long. Also check the pictures on the project page of the PX18 kit to get an idea where to place the wires in the chassis.
Must of the preamp is just like the original but some modifications had to be done around the PI because of the EL84PP powerstage. So be careful if you change something around the PI, this could lead to other problems.

Best regards, Dirk

[marcomeloni1979]

Thank you Dirk! I followed the schematic and layout faithfully. I mean the amplifier sounds amazing and this kind of sound happens only when both the high treble volume and presence are on 10. Then theres the normal floor noise of course, but thats just normal. Here is the link to a video that documents the noise
https://youtube.com/shorts/lO0U-3GLuBw?feature=share

Marco

[marcomeloni1979]

I could send pictures but im not sure how to....

[stephan61]

Marco,
In your video in the beginning your are talking about a "master volume all way up".
This amp originally does not have a master volume. Is the amp modified?

[marcomeloni1979]

Yes. I put in a type 3 from the trainwreck pages. Ive taken it away now and the amp is back to stock. No change in the problem. I can put up some pics later so maybe if theres a mistake it can be spotted.

[Dirk]

sounds like that the noise only happen with an unusual / extrem potentiometer setup. Is this you standards setup which you use ?

Best regards, Dirk

[marcomeloni1979]

Hey Dirk, yes. Like i said earlier i followed the tube town layout faithfully. Then i installed this that can also be done with alligator clips but ive since removed. Ill put up some detailed pics later. Maybe its just a matter of lead dress...