Scratchy noise when I touch pickup, screws, use brass slide.

[mauiboy81]

Here are a couple pics of the preamp.... There are a few things tacked in there that I will clean up before I close it up but maybe you can spot something.

[martin manning]

So I just checked the amp and with a cord plugged into the first channel I get <1mV on the tip. On the second channel with the cord plugged in I got -31mV on the tip. Could this be the problem?

Does the noise appear just on the second channel or both? With the -31 mV seems there is a grounding issue there. This is a new build, correct? Does it follow an established schematic or is it a new design?

[Reeltarded]

Although problematic, it looks very nice.

[thejaf]

Would it matter if you isolated the jacks from the chassis and then ran ground wires to the buss? Appears that the jacks are grounded directly to the chassis in the pictures.

[mauiboy81]

Thanks guys!

Channel one is not from a schematic or layout but is a real simple 12ay7 triode-coupling cap switch-500kA vol-12ay7 triode-coupling cap and 500k to ground-mixed with channel two before pi with 220k's. Channel two is pretty standard top boost but has a treble cap switch with 50pf/250pf/500pf. I get the noise on both channels. Pretty standard rocket voltages in the preamp. Reverb is taken after 220k mixing resistors and returned at the other side of the pi. I'm running kt66's. It sounds great!
The scratchiness when i use a slide is only when i am not touching the strings or metal parts with anything else.
After changing the 12ax7 I am getting a smaller negative voltage than before. When the guitar cable is plugged in with no guitar I get the voltage on the tip. Then when I plug into the guitar I get 0V at the grid. This is on the top boost channel. When I measure the resistance to ground with just the cable plugged in I get 1Meg.
I resoldered all the jacks and retightened them. Resoldered the 68k's on the sockets. I cleaned out the sockets.

[BTF]

If you can't figure out any other solution, here's a possible:

You might solder either a 1uF 250V or .1uF 250V metal film cap between the input wire and the grid of the first stage. Of course, the 1M grid to ground resistor must go on the GRID side of the capacitor. This will allow the signal to pass through from the jacks to the grid essentially unaffected, but block the space charge which is appearing on the grid.

If nothing else, this cap will tell you whether the charge on the grid is causing your problem. If the problem stops and the amp works well otherwise, you can always leave the cap in. Some companies (Peavey, for instance) put a cap in the input line of some of their tube amps and preamps.

Just a suggestion. Hope this helps.

[Structo]

Have you used those types of jacks before?

If they are the ones I am thinking of I would not use them.

I like either the open Switchcraft jacks or Cliff jacks.

Cliff jacks if you want to isolate the jack ground from the chassis.

Although your symptoms does bring static electricity to mind.

[mauiboy81]

Thanks for the suggestions. I have been out of commission for days as my entire family has been floored by the flu.

The jacks are Switchcraft closed type. I was thinking of trying some open type. I also was thinking of using isolation washers but I don't know if I want to drill larger holes. I don't have any cliff type available.

My brother left an old 70's peavy classic here that I plugged into and it does the same staticky thing. I don't think I am going to worry about it any more.

The amp sounds good. I don't think it is as loud as the last top boost I built but that one was single channel with no voltage divider before the pi and this one has the two channels mixed there.

What kind if noise floor have you guys got on top boost circuits when cranked?

I appreciate you all!

[martin manning]

I wonder if there is some issue with the earth ground in your house?

[Reeltarded]

Yep