Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BK1985 on May 31, 2017, 07:49:00 am
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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 (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
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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?
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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!
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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.
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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
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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 :laugh:
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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 :laugh:
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 :BangHead:
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> 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.
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> 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.
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Always suspect tubes 1st. Then go through a documented checklist, e.g. http://geofex.com/ (http://geofex.com/) > tube amp debug page > hiss
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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
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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 :laugh: