unusual noise on AB763 Super reverb clone - diyAudio
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Old 12th February 2012, 12:12 AM   #1
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Default unusual noise on AB763 Super reverb clone

I've built and repaired my own amps for a long time and I've seen some strange things. but this one has me stumped as to how to fix it.

I'm building myself a AB763 super reverb amp useing one of the Taiwanese chassis found on ebay.. I've built several over tha last 3-4 years and they all have worked just perfect first time fired up.
Bear in mind that everythign on this amp is brand new and I'm trying out the Sozo caps on this build. I've never used them before. so maybe I'm not aware of something they may require different. i did read on the web site that they sound best after about 100 hours of break in.


this amp works and plays fine first time fired.. and every voltage is within 5 volts on the low side of the AB763 schematic. and I'm useing a GZ34 rect. instead of the 5U4... the reverb works great.. the tremolo works, but could be a little stronger for my taste and its REALLY fast on the speed control. I mention this in case its a symtom of something else.

BUT, my real problem is, it has this very low, kind of under the note distortion/buzz that is constant, it seemed at first to be a microphonic first tube.. so I replaced it with a brand new tube... then I noticed that the input wire to the first tube seemed to be making the noise come and go when it was moved or bumped. So, I replaced all 4 of those shielded wires. it came back again within a minute of fireing it up.

so thinking that I had a bad batch of pre-amp tubes, I replaced the first tube.. several times... always useing brand new tubes.. yet it comes back.

now here is where it gets really strange.. at first, I started wiggling the tube thinking it was not makeing good contact in the socket. it would go completely away, but comes back as soon as I let go of the tube.. so I'm thinking may be a bad socket..

but before i replace the socket.. I started trying to see how light a touch I could use to make it stop... I discovered can just place my fingers about an 1/8" away from the top of the tube (not touching the tube at all) and it goes completely away.. only to come back if I move away from the tube a fraction. and it very solidily repeatable... when the noise goes away, the amp sound perfect and very clean and quiet. just like it should.

I'm now thinking this is some sort of parasitic occillation maybe but I have no idea what could cause it and how to get rid of it.

any help would be greatly appreciated.

Thank you in advance.
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Old 12th February 2012, 12:17 AM   #2
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Do you have grid stoppers on output tubes? Also try a different speaker or extension cabinet to see if that helps. I had a Blackface Princeton that had a similiar problem.
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Old 12th February 2012, 01:19 AM   #3
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EVERYTHING is brand new speakers and cab included.. and I've already tried that no change. I even used another amp to check the new speakers.. they sound perfect. and even do with the problem amp....

the amp is perfecty clean and quiet sounding as long as My finger remains on or near, the top of the first pre-amp tube...

so why would I need a Grid stopper on the output tubes?
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Old 12th February 2012, 02:00 AM   #4
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Originally Posted by krayon View Post
so why would I need a Grid stopper on the output tubes?
They will help the battle against oscillation, maybe 99 out of a 100 you will not need them but in cases like this they are a very good and cost effective design feature. Just don't go to large a value and it will not affect the sound. On this circuit the standard value would be 1.5k in series with control grid, as close to the socket as possible.

I think for some reason the most sensitive gain stage in your amp is picking up some sort of RF, vibration or something and that is why when you place your finger near the first tube the noise diminishes.

Try the grid stoppers and see if it helps, if not it is a good idea to have them anyway.
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Old 12th February 2012, 03:01 AM   #5
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I Just found it!

I was useing a piece of Shielded wire from the input jacks to the first tube for each channel and back from the volume pots too, the shielding was grounded at the jacks/pots but not on the other end near the tube that was just shrink tubed and left open similar to later silverface models....

I was researching the grid stopper idea and ran across a really heart sinking post about "board moisture" and voltage leaks... which is what this was sounding like, because all the volume/treble/bass pots were scratchy at weird spots in their rotation.. so i checked the board.. nothing... no voltage at all.. I just randomly checked the Shielding and Bingo!.. 5 MV Dc fluctating right along with my Bias King meter as I played.

Somehow there was a low voltage developeing across the shielding. and it was causeing what sounded like "cable rustle".. but the voltage in the shielding would rise and fall with the voltage in the amp as it was being played..

I'm Still not sure how this happened, but it completely stopped after i cut the shielding loose.

now the amp sounds like a brand new blackface.

anyone know why this occured? and do i still need to do further work to avoid any future issues?

Last edited by krayon; 12th February 2012 at 03:06 AM.
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Old 12th February 2012, 03:15 AM   #6
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Still might have been a high frequency oscillation charging up the shield and the meter rectifying it. Glad you solved the problem, now thread to be sent to the MI section.
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Old 12th February 2012, 03:32 AM   #7
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Quote:
Originally Posted by krayon View Post
I Just found it!

I was useing a piece of Shielded wire from the input jacks to the first tube for each channel and back from the volume pots too, the shielding was grounded at the jacks/pots but not on the other end near the tube that was just shrink tubed and left open similar to later silverface models....

I was researching the grid stopper idea and ran across a really heart sinking post about "board moisture" and voltage leaks... which is what this was sounding like, because all the volume/treble/bass pots were scratchy at weird spots in their rotation.. so i checked the board.. nothing... no voltage at all.. I just randomly checked the Shielding and Bingo!.. 5 MV Dc fluctating right along with my Bias King meter as I played.

Somehow there was a low voltage developeing across the shielding. and it was causeing what sounded like "cable rustle".. but the voltage in the shielding would rise and fall with the voltage in the amp as it was being played..

I'm Still not sure how this happened, but it completely stopped after i cut the shielding loose.

now the amp sounds like a brand new blackface.

anyone know why this occured? and do i still need to do further work to avoid any future issues?

Cool! Glad you resolved your issue, although it can be a pain in the butt these things will only make us better at what we do in the long run.
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Old 13th February 2012, 03:45 AM   #8
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further info on the problem.

I have a brass Plate behind the control pots and I have always used this as a grounding point for my board grounds..
this time around I used it also for my Filter cap grounds.. it turns out that this is Bad idea.

While I was checking around looking for this myterious voltage I found that there was a small 4-5 ohm resistance bewteen my brass Plate and the Chassis. which I guess was rasieing my "Ground pontential" above the chassis therefore allowing a small voltage to develope on my shield whcih was solder on the ground tab of the input jack ON the brass Plate..

so i relocated the Filter caps Grounds to the actual chassis and no more problem.
below is a photo of another amp I built last year showing the brass plate and how my board grounds go over to it.. I've built several amps this way with no issues. but all the Filter cap grounds were to the chassis. hopefully someone else will beneifit from this tidbit.

Click the image to open in full size.

Last edited by krayon; 13th February 2012 at 03:50 AM.
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Old 13th February 2012, 04:43 AM   #9
kevinkr is offline kevinkr  United States
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Originally Posted by Printer2 View Post
<snip> Glad you solved the problem, now thread to be sent to the MI section.
Happy to oblige since it does belong in I&A per forum policy.
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