problem with marshall clone: presence control HELP!

[drz400]

I've tried a lot things, some with sense and others (I admit) with no sense, but really this issue is driving me crazy:
- I changed presence cap
- I changed 0.1u PI caps
- removed feedback
- I removed 47pF cap between anodes of V3 (PI)
- Moreover I did some other tests on output tubes section: I removed 8K before screen resitors and so I burnt 470 ohm output screen resistor (only 2W). replaced with 5W

Make a shield can around the input jacks and redress the plate wires to the power tubes to the front away from the impedance selector. Had the same problem once and these two took care of it, basically the chassis is too small

[flatart]

after lot of tries and tests I eventually limited the issue to reasonable levels, even if the problem didn't disappear completely.
Here the main changes that helped me:
- shorten ground presence directly to pots ground wire
- shielded cable for some lead signal wires
- shorten wires that were too long
- moving wires to get a better layout
My target was to get a JTM45 clone that even with all the pots wound up at 10, it should sound good with no hiss.
I know that I will never use this amp completely wound up, but this is a way to measure the accuracy of circuit building and layout.

Now the question is: do you think that this is an achievable goal with this kind of amp?
I hope everyone of you can answer based on his own bulding experience

For example with my clone if volume is equal or lower than 6 I have no problems whatever tone and presence I set. If volume is between 6 and almost 9 I start to get some hiss and with presence over 7. If volume is at 10 (max) I can't get a good sound out of the amp, whatever setting I use.

here there 3 samples, I hope that you can download and hear them (problamatic link):
http://mio.discoremoto.alice.it/flatart
- test JTM45: volume1 at 6, volume2 at 3 (chained), bass 5, mid 10, treble 10, presence 5, just a strato plugged in, no effects, nothing, just playing with strato volume pot
- test JTM45-2: volume1 at 8, volume2 at 3 (chained), bass 5, mid 10, treble 10, presence 5, just a strato plugged in, no effects, nothing, just playing with strato volume pot
- test JTM45-problem: everything wound up at 10

thanks everybody for your preciuos help

[KT66]

Hey Flatart, I think that the best grounding point for the presence pot is not to mix it with the preamp grounds, but to put it near the power amp section and to have a wire from the common of the output jacks grounded at this point also. I always do this with my Plexi style builds and never have a problem, though I use a star ground scheme.

Here's a bit from Aiken's site on grounding that explains it :

"If the amplifier uses global negative feedback, there will have to be a ground path from the secondary common back to the phase inverter tail ground (or wherever the feedback is returned to). This should be done via a wire from the ground of the output jack back to the ground of the stage the feedback is applied to. The idea is for the feedback amp to amplify the difference between the feedback "hot" lead and the feedback ground lead, but nothing else. Note that there is very little current flowing in this ground wire. The total secondary current only flows in a loop around the secondary and the speaker, not back through this wire, so it is a voltage sense wire only. Even if global negative feedback is not used, it is sometimes necessary to ground the secondary common side to prevent noise or oscillations."

Hope that helps.