Troubleshooting my Express combo build: noise and hiss

[rooster]

Wow, nice work, and quick.

I realize your Presence pot layout is imitating a known Express, but still I would change the grounding for this pot as I described before.

However, moving on, I would now suggest a sheilded wire off the Volume pot back to V1. Also, I don't see any grid stop resistor off the input jack at V1, you need to try something there, maybe 10K. It can also help on the wire from the Volume pot, too. You need to try these ressitors I think.

Check out what Glen is saying, too. He has been all over these amps and understands their issues better than most.

All of this said, maybe your new build here is actually only dealing with V1 problems, as you say? Eh, then focus here maybe. Add the grid stop resistors, one at a time and see what happens. Also try other tubes, right?

[vincenz]

ok, I tried all of that:

-Changed the lock washers on the pots, input jacks, bright switch. Cleaned all of this and the area with rubbing alcohol. Re-checked all solder joint on pots. No change.
-Tried a shielded wire for V1-to-volume-pot. Underneath and over the board. No change.
-Tried grid stopper res (10k, 64k) on this pin. As well as on the input grid pin. No change.


Do you think the cause of that ccchhhhh noise would be the short distance between V1 and speaker?

Here is a sound clip with this noise. Unplugged guitar is for audio level reference.
http://www.youtube.com/watch?v=e6fUtQ9ucYQ

[rooster]

OK, I get your problem I think - you are not using a jack that shorts out when the instrument is unplugged. So to test this properly, just like it is, you need to plug in the instrument and turn the INSTRUMENT volume pot to zero. This will effectively short the input. Now, you might still get Rf from the cable, but the noise factor should be reduced quite a bit.

If I were to plug a short cable in with no instrument attached, my Express's would buzz like this, too.

And I am seeing this pic correctly? You are not using a shorting jack on the input? Very important for this kind of test.

[vincenz]

Rooster: YOU WIN !
I pay the beers for all forum members

Hehe I was closer than I expected.
Now the amp is stable, no hiss, just a reasonable background noise for a high gain amp.

BIG THANKS TO ALL TOPIC'S CONTRIBUTORS


I made a clean topic to show you my Express combo build -->
https://tubeamparchive.com/viewtopic.php?p=158844