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Buzz in preamp (please help)


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5/22/2003 6:15 PM
Carlos Buzz in preamp (please help)
I've been trying to eliminate an unwelcome buzz in my new build. There is a thread in Design & Construction that one may review to see what's happened to date.  
Dai Hirokawa has been a great help to me in rethinking several aspects, including my grounding.  
 
The latest schematic and pics are at:  
 
aol.members.com/xtalale/Tweedy_Lowatt  
 
At this point, I believe I've corrected all my grounding issues and isolated the buzz to somewhere in the preamp. Here's the symptoms:  
 
- Buzz present when volume all the way down, diminishes some (almost disappears) between 1 & 2 on the volume, then gets increasingly louder as volume turned up,  
 
- Hard-ground the input at the jack or the 1st stage grid - no change without guitar plugged in - buzz diminishes with guitar plugged in,  
 
- Hard-ground the 2nd stage grid - buzz diminishes quite a bit (more than when the 1st stage grid is grounded),  
 
Disappears completely when the coupling cap to the 200k PA grid R is grounded (of course so does any signal :-) This is how I've isolated it to the oreamp.  
 
I observed that the 1M resistor at the input jack measured 0.00 ohms, but when I took it off to replace it, it measured 1M. Is this because it was connected in series?  
 
I believe I'm close to getting rid of the buzz, I just need a little more help figuring out where it's being generated.  
 
Thanks in advance for taking the time to look and providing some insight.  
 
Carlos
 
5/23/2003 9:58 AM
Dai Hirokawa

quote:
"- Hard-ground the 2nd stage grid - buzz diminishes quite a bit (more than when the 1st stage grid is grounded),"
 
 
Try shielding that wire to the grid (i.e. try a shielded cable). The copper wire (which makes up the shield) will help provide electrostatic shielding, which will block out stuff at higher frequencies (like squealing and buzz--buzz is the higher harmonics of hum). It won't block out hum BTW, you need electro*magnetic* shielding for that like steel or mu-metal.  
 
quote:
"I observed that the 1M resistor at the input jack measured 0.00 ohms, but when I took it off to replace it, it measured 1M. Is this because it was connected in series?"
 
 
Don't you have it wired so it shorts out to ground when there is no plug inserted?  
 
Dai
 
5/23/2003 1:53 PM
Dai Hirokawa

Hey Carlos, try reading this also--there is a thread going on with some info on grounding on the ax84 forum:  
 
http://www.ax84.com/bbs/dm2.code?id=67520  
 
Dai
 
5/23/2003 1:56 PM
Dai Hirokawa
LOL. Nevermind...
I see you're already familar with the forum...  
 
:D  
 
Dai
 
5/23/2003 4:39 PM
Carlos Re: Buzz in preamp (please help)
Hello again Dai. Thanks for hanging ;-)  
 
I have a shielded cable from the volume to the input of the 2nd stage grid. By hard-grounding, I mean I put a probe with a clip to ground on the copper inner wire and the buzz almost disappeared.  
 
Actually, in the interim, I've been working on the preamp and the buzz is still there in the background, but it only gets loud enough to be really obnoxious above about 5 on the volume. I think it's probably because I've ended up lowering the overall gain in the 12AX7 stages.  
 
To bring you up to date, I've:  
 
- tried a different 0.022uF coupling cap after the first gain stage (no difference),  
- shortened the shielded cable from the 1st stage to the volume pot (no difference),  
- added a 0.01uF cap from the input jack ground lug to the center ground lug on the nearest terminal strip (this is a R. Aiken suggestion for shunting off RF riding on the guitar cable - but no difference),  
- replaced the wire from the coupling cap between the 12AX7 and the 200k grid R with a shielded cable (no difference),  
- removed the bypass caps on the 12AX7 gain stages (this was the biggie noise reducer - it mimics the tweed preamp better and sounds good, but also reduces gain quite a bit).  
 
Without the guitar cable pugged in, it's very hard to hear the buzz until about volume 6.5 to 7. With the guitar plugged in (and the guitar is shielded and dead quiet when it's plugged into my Champ), it starts becoming apparent around 2.5 to 3 and obnoxious after 5.  
 
I guess I could live with it the way it is, but I'd really like to turn the volume up to 10 and get some PA distortion going.  
 
BTW, with a little help from AX84, I'm up to speed WRT the 1M resistor shorting to ground w/o a plug (and feeling a little stoopid). Yeah, I sent the call for help far and wide (FDP also).  
 
If I don't hear from ya, have a great weekend! (are you in the US?)
 
5/24/2003 6:46 AM
Dai Hirokawa

Did you try Bruce's suggestion of grounding the OT secondary?  
 
quote:
"- removed the bypass caps on the 12AX7 gain stages (this was the biggie noise reducer - it mimics the tweed preamp better and sounds good, but also reduces gain quite a bit)."
 
 
I don't think this is solving the problem but just reducing the gain (and the buzz that is along with the signal).  
 
quote:
"If I don't hear from ya, have a great weekend! (are you in the US?)"
 
 
Thanks. You too. :) No, Japan.  
 
Dai
 
5/24/2003 10:59 PM
Carlos Re: Grounding OT secondary, etc
Dai,  
 
This is what Bruce wrote that you're referring to:  
 
[QUOTE]I'm not sure if you are trying to "up" the primary zed by using the speaker load and the DI output in series but I'd move that 10 ohm power resistor/.68ohm resistor from under the output jack to in between the switch lug and the tip lug.  
That way, the PA will work with a ground reference and you can play with no speaker connected too.  
After you fix that, use a direct ground wire from the grounded lug of the output jack, right to the same ground point as the cathode biasing resistor of the 6AK6[/QUOTE]  
 
I responded to this but I don't know if it made sense. This part of my circuit was my attempt to mimic the built-in attenuator and headphone output that my Marshall Studio 15 (model 4001) has. I left off the Line Out portion of the circuit. You can see the schematic at:  
 
www1.korksoft.com/~schem/marshallamps/studio15.pdf  
 
I wanted to add this feature to this build so I could practice at night when my wife is home (she really doesn't mind my playing, but I want to spare her the endless repetition as I learn new songs).  
 
I did run a wire from the ground lug on the second jack (the first jack has the OT secondaries connected to it and is connected to the second jack through the 10 ohm power resistor on the outside of the chassis via the white wires) to the ground point at the 6AK6 cathode resistor.  
 
But I don't know what he meant by moving the:  
 
quote:
"10 ohm power resistor/.68ohm resistor from under the output jack to in between the switch lug and the tip lug"
 
 
Do you know how I would wire these plastic Marshall-style jacks for that. Maybe you can see how I've got them wired now. This is pretty much how they're wired in my Studio 15, so it should work. But when I plugged headphones into the second jack, I got no sound and I got worried that I might mess up my OT. Do you know what I can do to get this feature to safely work?  
 
quote:
"I don't think this is solving the problem but just reducing the gain (and the buzz that is along with the signal)."
 
 
Yeah, I know. It's more of a copout than the ideal solution. Since the buzz is controlled by the volume, I'm pretty sure it's getting into the first stage. I just can't figure out how. It's wierd how it gets worse when I plug the guitar in. This guitar is dead quiet with my other amp. Should I ground my input shield at the tube instead of the jack? There's only so much it could be between the jack and the volume pot.  
 
Very frustrating.  
 
Carlos
 

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