Hissing disappears when putting multimeter on DC blocking cap

[BK1985]

Hi everyone,

Last year I built an AX84 Hi-Octane amp and truly love it. Everything was working great aside from some hiss when my two gain pots were turned past 3/4. I decided to rebuild it in a different chassis to have more space, and to arrage the wires better.

Well, I powered it up last night. Everything works, but the hiss I get got worse. Here's a quick rundown of the situation:

Gain 1 anywhere with Gain 2 at 0 : No hiss
Gain 2 at 3/4, get hiss when Gain 1 is anywhere from 1/2 to full
Gain 1 at 3/4, hiss from Gain 2 anywhere from 1/2 to full

The hiss' pitch goes up the further the Gain pots are turned up.

I did a little poking around with a multimeter, and I had it setup as a voltmeter across the DC blocking cap between stages 2 and 3, and the hiss stops immediately. If I put the voltmeter across the DC blocking cap between stages 1 and 2, the hiss gets worse.

The voltages I read were within 4 volts of what I'm supposed to get.

The hiss also changes intensity when I move my shieled signal wires in the pre-amp stage, but it never goes away completly.

What do you guys think, bad cap?

Here's a link to the schematic: http://www.ax84.com/static/hioctane/AX84_Hi-Octane_101004.pdf

Any help is much appreciated :)

Also I have checked the library of info, but couldn't find anything that's exactly the symptoms I'm seeing...

-Brian

[shooter]

voltmeter across the DC blocking cap

typically blocking caps are measured either side to ground, your way you put a ~10meg R in || with the cap.  swap the cap and see.

I move my shielded signal wires

that's a good start, verify grounds are only 1 side, and up-stream.  start pullin tubes V1 1st, where does the hiss quit?

[BK1985]

voltmeter across the DC blocking cap

typically blocking caps are measured either side to ground, your way you put a ~10meg R in || with the cap.  swap the cap and see.

I move my shielded signal wires

that's a good start, verify grounds are only 1 side, and up-stream.  start pullin tubes V1 1st, where does the hiss quit?

Okay so I found one mistake I had made, the 2nd gain knob had no lead going to ground so 2nd stage was always at full gain, fixed that.

Now I'm exactly where I was before rebuilding the amp hiss-wise. New pots/hiss values:

G1 3/4 and G2 3/4 : Hiss starts
G1 FULL and G2 1/2 : Hiss starts
G1 1/2 and G2 FULL: No hiss

Anything below that and no hiss. I've double checked all my wiring and everything is good.

Same thing as before though, if I put my voltmeter across the blocking cap between stages 2 and 3, the hiss stops.

I truly am at a loss :( I can live with the limits the hiss puts on my gain settings, but I would've like to try this bad-boy at decent volume and full gain!

[dude]

Sounds like a "lead dress" issue, start chop sticking with a wooden stick, maybe a Popsicle stick.


Keep OT secondaries away from PI.



Try shielded wire on grids to gain pots. 

[BK1985]

Took some additional measurements between the C11/G2 junction and ground. I held the voltmeter there while I fooled around with the pots. As soon as the hissing starts I'm seeing negative DC voltage on the meter, up to -0.5VDC.

Is it not supposed to block all DC voltage and be at 0V? Could this mean I have a ground loop somewhere feeding DC voltage back into the signal path?

-Brian

[shooter]

G1 3/4 and G2 3/4 : Hiss starts

what value on your gitar volume makes it go away and music begins?  My plexish hisses til I get my guitar to 1.5 from then on it's all rocknroll till I turn the guitar to 0, but then I can't hear the hiss till I play again

[BK1985]

G1 3/4 and G2 3/4 : Hiss starts

what value on your gitar volume makes it go away and music begins?  My plexish hisses til I get my guitar to 1.5 from then on it's all rocknroll till I turn the guitar to 0, but then I can't hear the hiss till I play again

Sadly the hiss is there even when no instrument is plugged in :( I'm probably paranoid but I can still hear it while I'm playing at full guitar volume 

[PRR]

> voltmeter across the DC blocking cap between stages 2 and 3, and the hiss stops immediately

*Across* the cap? Why?

That seems to put 10Meg of DC leakage from 2nd plate to 3rd grid, lifting it WAY positive probably to saturation (won't amplify).

Less so on the 1-2 cap because of different values and added attenuation.

I don't know why you put the meter there, and don't think this "result" is any sort of clue.

I would say a tube is hissy. However moving wires changes sound suggests supersonic oscillation which will sometimes induce hiss, and suggests layout could be better.

[BK1985]

> voltmeter across the DC blocking cap between stages 2 and 3, and the hiss stops immediately

*Across* the cap? Why?

That seems to put 10Meg of DC leakage from 2nd plate to 3rd grid, lifting it WAY positive probably to saturation (won't amplify).

Less so on the 1-2 cap because of different values and added attenuation.

I don't know why you put the meter there, and don't think this "result" is any sort of clue.

I would say a tube is hissy. However moving wires changes sound suggests supersonic oscillation which will sometimes induce hiss, and suggests layout could be better.

I thought of a bad tube too, and swapped my two 12AX7s between each other, but the hiss is at the exact same spot on the pots. I thought layout problem, which is why I rebuilt the amp, but the hiss is at the same exact spot as before...

Guess I'll go buy a brand new pair of preamp tubes and see if that helps...

EDIT: I'll try and get a video of the hissing uploaded to give you guys a better idea of what's going on.

[jjasilli]

Always suspect tubes 1st.  Then go through a documented checklist, e.g. http://geofex.com/ > tube amp debug page > hiss

[BK1985]

I got rid of most of the hiss by swapping the standard wires going from the Blocking Caps to the pots with shielded wires. Now I only have some hiss at a very minimal pot adjustement range, the rest is most probably wire routing issues which I'll address this weekend when I have more time.

However, now I've got some motorboat sounds going on, a putt-putt-putt-putt. From the debugging page it seems to be the decoupling capacitors going bad...but I haven't even went close to them and they were working fine 1 hour ago.

I suspect I may have accidently desoldered my ground bus that's underneath my board. Could a bad ground connection on one of the bias caps cause this too?

Thank you all for all the help so far, it's very much appreciated. I can say that when I go for my next amp build, I'm going to go about things much differently.

-Brian

[shooter]

I'm going to go about things much differently.

We all start out pee yourself excited about the 1st amp to slow and methodical, test as you go life-time amp