Tapping on wires causes noise

[frank57]

If I tap on certain wires connecting a tube to the main pcb from the tube card pcb,
I get tapping noise in the speaker.
What does it mean?
Bad connection?

[silverfox]

I got the same thing through the input jack of a Peavey Road Master when I plugged in the cable. It is a high gain amp and tapping caused the electrons to move in the wires connected to the jack at a frequency related to the tapping sound. Just a different way of inducing a voltage to the plate of the tube.

Sounds normal to me. Is this a high gain amp?

Silverfox.

[Slimtim]

microphonic or bad tube maybe.have you tried any other tubes?

[Dreams]

I'm not sure if this will help, but I asked a similar question on a different forum some years back:

http://repforums.prosoundweb.com/index.php?topic=22922.0

[cbass]

Sounds pretty normal.If it's cranked up.Is the noise extremely loud or can you just hear it?
Could definately be a tube going microphonic

A bad conection would more likeley to pop and hum.IMO

Is everything else ok?I would suggest just not tapping on the wires

[FranciscoPerez]

Maybe those wires are located in high gain and high impedance circuit points.
The hum may be induced from the room's AC wiring.

Getting close to the wires without tapping them does produce some noise?

[frank57]

Every once in awhile  I trot out this amp from hell, and I decide to poke around in the power section.
The amps power section is actually dead quiet all the way up high noon  then abit of hum and then a lot of noise from 3pm on up on the master volume.

It's the wires connected to the master volume and the tube supplying the bias to the phase splitter.
Maybe a shielded wire on the grids might do something?
But the wires seem awfully tight in there.

[Jack1962]

More info about the amp would be helpful Frank . if it's a high gain amp that's fairly normal , if not chances are the tube is microphonic .

[Slimtim]

hiwatt custom 20 tube?nice

[Slimtim]

look in the bottom left corner where the 3 wires are going to ground.2 of those are not factory,at least the ends don't look it.are those wires original,are they getting good contact,why were they messed with if they were?also that's alot of tubes real close together on a small pcb.

[HotBluePlates]

Every once in awhile  I trot out this amp from hell ...

I'm convinced either that amp either houses demons, or was poorly designed/laid-out.

After years of battling, nothing seems to have gotten it as quiet as the home-brew projects of many of the visitors to this forum.

If I tap on certain wires connecting a tube to the main pcb from the tube card pcb, I get tapping noise in the speaker. What does it mean?

Maybe something, maybe nothing.

Is everything else ok? I would suggest just not tapping on the wires

  This is actually more relevant than you might think.

Tubes that are perfectly fine (i.e., non-microphonic) will pass a tapping noise if you tap on them. When you encounter a truly microphonic tube, you'll recognize how it tries to ring like a bell when you tap it, or it may just howl outright without tapping with enough volume.

I've noticed some coupling caps will pass a tapping noise to the speaker when tapped. Again this might be normal, but an extreme response might be a problem.

You'll find that if a problem only surfaces when you poke at something in the amp, it's likely not a problem... and that you should stop poking.

[frank57]

It seems to be in the preamp and the input jack where the biggest problems are.
I can't get rid of the bzz when I plug in unless I put a 102  cap across the jack.
That's one of the biggest problems.
And the lead dress in the original amp was a horror show.

The second is turning up the gain increases the buzz and hum .
I put a 1m resistor instead of 56k before the efx loop to really hear the power section.

Up to halfway it's pretty quiet then some hum and a lot of noise from 7 or 8 to 10.
i think that part of the amp is pretty much ok.
It actually sounds pretty good in the return.
But it's important to note the amps were modded as they were sold.
They had 15 k on the cathodes not 1k5.
The 1k5 circuit will sound like total crap past halfway on the gain and you will lose note definition.
Better solution is to take the lead 20 circuit or the Soldano atomic 16 circuit and go with that and go around the efx loop maybe.
Another test was taking the english muffin and running it in the efx loop return.
Any buzz? Nope.

I tried a test where I put a 104 ceramic cap to ground at r7 and that seems to kill a lot of the problem.
Is it coming in on the plate before the  22n cap?

[HotBluePlates]

... Is it coming in on the ...

No f-ing clue. It's a p.c. board with some layout issues you've worked on, has some parts that aren't necessary, and some that look from the schematic like they must be in the wrong place to do their job.

You're using the cap to kill highs, but other parts of the amp are trying to boost highs as part of the distortion sound. Meanwhile, the amp has some caps designed to roll out the highest highs to keep the amp from getting harsh.

All this should work; but I bet this amp was designed/prototyped, but not prototyped in its as-built form (or else they just thought the problems still resulted in a "sellable" amp).

[Willabe]

Frank, you've studied and have gottin your feet wet working on this and other amps, do you feel comfortable enough now to gut that thing and put in a turret or eyelet board?


             Brad     

[Slimtim]

willabe has the balls to reccomend what i wanted to but couldn't.it being a hiwatt custom and all i would think that would be on the side of destroying value.but if it's flawed it's flawed and with more know how i would rebuild it if it were mine.

[Willabe]

it being a hiwatt custom and all i would think that would be on the side of destroying value.

It's not, it's a PCB reissue.

A year or 2 ago Frank said he didn't feel comfortable enough, yet, to gut the amp. And that's fine.

But, now maybe he feels different?


                 Brad            

[Slimtim]

oh ok i would really really gut it then.

[frank57]

Yes I'm definitely thinking about gutting it now.
But it's a lot of work because you have the tube sockets way too close.

Short of that the solution is to change the preamp ,maybe axe the efx loop and change the tone stack
and it will be  useable at least.
The pcb circuit values with 1.5k on the cathodes in the preamp definitely is something that does not work well.
It's way too much.

Beyond high noon it's crap so I'm not surprised they changed it,but if they had stuck to a typical Hiwatt circuit the whole thing would be better.That cathode on the second stage is causing a lot of the problem,but it's so badly laid out.

The problem might just be on the tube card with the tubes so stupidly close there might be crosstalk.
Then again the tracks are too close everywhere too and the transformers are in the wrong places.
The one thing I haven't tried is dc power on the filaments.
Every time I trot the amp out I'm sorry I did .

I picked up the volume 3 in the O'connor series and am looking it over.
But it's a lot to digest.

[Willabe]

I picked up the volume 3 in the O'connor series and am looking it over.
But it's a lot to digest.

That's a great book, IMO, I have it too, for learning how to layout an amp. Yes it's a lot to digest but take your time and read it, then re-read it and read it again, you'll get it. There's more than 1 way to layout an amp but IMO, I think that book is solid advice.


          Brad     

   

[plexi50]

I run across a totally original vintage amp (not high gain) and sometimes have this issue of microphonic sensativity. But all the tubes i use are not microphonic and great tubes. I have have yet to explain the reason some amps have an electrical sensativity around each tube socket. Scratch the chassis with a screw diver and all hell breaks loose. Like a phonograph sensativity. Some do some dont. Even when finding a good preamp tube combination it still seems have some electrical voodoo. It's friday night and thats all the voodoo i need to call it friday the 13th. I dont why some chassis are a fire with the smallest change in on the edge pin pricking oscillations.