Tube amp noise diagnosing help!

For those building and repairing amps on their own. Learna and share ideas.

Tube amp noise diagnosing help!

Postby msca8h on Wed Dec 25, 2013 12:41 pm

OK I've built my first amp of my life.
A VOX AC15HW with some 1964ish components changes.
So far, It's perfect except some failing EL84 and one channel.
The Top-Boost channel is a mess.

1. Unplugged, quite as a charm. Plugged, some low frequency noise.
So, It's not the Tube catching noise.

2. The channel volume can control the volume of the noise
It is not the PI or the power section, tone stack.

3. Swapping tubes, the same.
Not tube problem.

4. Holding the guitar brings up noise. Maybe grounding?

5. Holding the channel volume brings down noise a bit. However, all pot's exteriors are grounded.

6. Turning the amp off turns off the noise gradually.
It's not heater noise.

7. The Normal channel is quite.
I assume it's not grounding issue.

8. Stepping on an Aux line of my audio amp next to the AC brings a lot of noise.


Those are general noticable things.
Can anybody help me?
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Re: Tube amp noise diagnosing help!

Postby gusfinley on Thu Dec 26, 2013 11:42 pm

Did you use shielded wire in your signal paths? Sometimes its not necessary, but sometimes it is absoultely necessary, escpecially in a top boost circuit.

When waving your hand in a certain area changes the operation of the amplifier, you've definately got some signals crossing that shouldn't be.

It could be noises generated in your output stage coupling to the input. There are lots of things that could go wrong in a handwired tube amp.

Change out you low signal wires (inputs to first gain stages) for shielded wire and see if that makes anything better....

One question. Is there are point on the volume control on the top boost channel where the noise fades away, then comes back louder?

Please post pictures, also!
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Re: Tube amp noise diagnosing help!

Postby msca8h on Fri Dec 27, 2013 5:50 am

[url]
http://s1362.photobucket.com/user/msca8 ... ml?filters[user]=138708364&filters[recent]=1&sort=1&o=1
[/url]

This is the preamp section.

[url]
http://s1362.photobucket.com/user/msca8 ... ml?filters[user]=138708364&filters[recent]=1&sort=1&o=0
[/url]

And this is the poweramp + power section

Did you use shielded wire in your signal paths? Sometimes its not necessary, but sometimes it is absoultely necessary, escpecially in a top boost circuit.

When waving your hand in a certain area changes the operation of the amplifier, you've definately got some signals crossing that shouldn't be.


Everything before the first gain stage that emmited noise was shield-wired before I made this thread.

One question. Is there are point on the volume control on the top boost channel where the noise fades away, then comes back louder?


The noise is pretty small at first but it gradually rises as I turn the control up and it is just a bit quiter than the signal.
So it's really loud when cranked up.
And the frequency doesn't change
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Re: Tube amp noise diagnosing help!

Postby Unit_1 on Fri Dec 27, 2013 6:30 pm

if you click on the QUOTE button you'll be able to see how i posted your pictures. you need to click on the IMG button on photobucket and that will copy the proper link to display remote pictures on this forum.

good luck with your amp and welcome to the forum!



Image

This is the preamp section.

Image
Image
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Re: Tube amp noise diagnosing help!

Postby Papa Dog on Fri Dec 27, 2013 8:35 pm

looks like someone could stand some lessons at the Unit_1 school of amp layout and lead dress.
and loose the sillycone gun.
It's hard for me to see exactly what is doing what there.
Little reason to expect it to NOT be noisy, IMHO.
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Re: Tube amp noise diagnosing help!

Postby Unit_1 on Fri Dec 27, 2013 10:05 pm

first amps are always better if done with a kit, to not have such a huge undertaking - a lot of the thinking has been done for you ahead of time

also while some like to follow schematics, it's very important to also follow layouts - coronal discharge can cause runaway gating - in cases like this where stage two is feeding back into the signal in stage one your sinewave can turn into a squiggly box full of lines and your power drops and the sound is awful

like hooking up three fuzz pedals in a row

many times the layout has been changed several times to arrive at a point where this doesn't happen.

using shielded wire on the most sensitive portions of the beginning of the circuit should help

i have 1000 feet of teflon impregnated shielded cable that i bought for a song on eBay that i'll probably never use since i've switched to mogami shielded input wire. for the price of postage i can send you what you need. just let me know about how many feet you want in a private message

p.s. this is the marshall i did while intending it to be the best looking amp i ever made - son of a gun sounds like hendrix at woodstock - LOUD :

Image

Image
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The ability to play/make music is a gift that not everyone gets. Those of us who have it should use it.

Asimov:Individual science fiction stories may seem as trivial as ever to the blinder critics and philosophers of today — but the core of science fiction, its essence, the concept around which it revolves, has become crucial to our salvation if we are to be saved at all.
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Re: Tube amp noise diagnosing help!

Postby msca8h on Sat Dec 28, 2013 12:05 am

Papa Dog wrote:looks like someone could stand some lessons at the Unit_1 school of amp layout and lead dress.
and loose the sillycone gun.
It's hard for me to see exactly what is doing what there.
Little reason to expect it to NOT be noisy, IMHO.


As I said the normal channel is dead quite.
And I modified the factory schems just a bit. Not much change in layout.

The dirty connections you see connects the eyelet board and other components. They are all checked for noise and have a specific order to not disturb the signal lines. They just look that way ;)
(Okay I might have missed something)

using shielded wire on the most sensitive portions of the beginning of the circuit should help

i have 1000 feet of teflon impregnated shielded cable that i bought for a song on eBay that i'll probably never use since i've switched to mogami shielded input wire. for the price of postage i can send you what you need. just let me know about how many feet you want in a private message


Everything before the first gain stage is belden-shielded 8)
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Re: Tube amp noise diagnosing help!

Postby Unit_1 on Sat Dec 28, 2013 1:14 am

i've also seen people shield the output from the first stage and the NFB

you can also try reflowing the solders just to be sure there's no cold solders - gently tug on the wire to be sure there's good contact before the solder cools

happens to the best of us!
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The ability to play/make music is a gift that not everyone gets. Those of us who have it should use it.

Asimov:Individual science fiction stories may seem as trivial as ever to the blinder critics and philosophers of today — but the core of science fiction, its essence, the concept around which it revolves, has become crucial to our salvation if we are to be saved at all.
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Re: Tube amp noise diagnosing help!

Postby msca8h on Sat Dec 28, 2013 7:46 am

Unit_1 wrote:i've also seen people shield the output from the first stage and the NFB

you can also try reflowing the solders just to be sure there's no cold solders - gently tug on the wire to be sure there's good contact before the solder cools

happens to the best of us!


I'll check the joints.
BTW the volume control attenuates the noise so I don't think I should shield the rest.
And VOX ACs don't have NFBs!
it's a big part of their natural sound
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Re: Tube amp noise diagnosing help!

Postby Papa Dog on Sat Dec 28, 2013 3:06 pm

seriously...whats with the inline fuses inside the chassis? Isn't that AC hanging around very close to the signal path?
Seems there would be a better way of mounting that.
Radial lead caps? Most of those would fit better if they were axial lead.

Sometimes you need to "dry fit" things. Lay them out and see how to get to the tag board with a shirt run and then link between tags, turrets, eyelettes or terminals (such as you are running: those are terminal strips, not eyelette boards).
As in most things, neatness does count. Sure...you built it and know where everything is. Thing is, I can make neither heads nor tails of that from the pictures. I see some joints whewre 2 or more bits connect in seeming mid-air....it is hard to get those kinds of joins to connect well.

You might wanna search out some gut shots of the amp you are trying to copy and base your layout on one of them...at least when you ask for help folks might be able to easier see where an error might have occured.
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Of cabbages--and kings--"
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Re: Tube amp noise diagnosing help!

Postby LUC.key on Sat Dec 28, 2013 6:53 pm

Welcome! It may sound harsh, but Papa Dog is right... it's a bit of mess inside your amp. It might work "on-the-table" as you tinker with your amp, but thats not how it should look like when you finish it in the chassis. Also those insulate tapes will fall off after a time and heat cycles... You should take your time and correct the lead dress, follow the layout you're actually going to copy... otherwise any troubleshooting it is kind of difficult.
Problem might be in the lead dress - some signal wire is prone to catch noise from other sources? What do you actually call a noise? Is that a hum (like 50-60Hz) or static hiss? Carbon comp resistors can "create" hiss noise when used in certain positions.
we do all learn :cheers:
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Re: Tube amp noise diagnosing help!

Postby Unit_1 on Sat Dec 28, 2013 8:49 pm

msca8h wrote:
Unit_1 wrote:i've also seen people shield the output from the first stage and the NFB

you can also try reflowing the solders just to be sure there's no cold solders - gently tug on the wire to be sure there's good contact before the solder cools

happens to the best of us!


I'll check the joints.
BTW the volume control attenuates the noise so I don't think I should shield the rest.
And VOX ACs don't have NFBs!
it's a big part of their natural sound


this is *exactly* the symptom of run away gating - the more volume the more noise and then at max volume the volume is actually decreased severely - not the kind of thing you learn in electronics school.

i had a friend that had a master's degree and was an audio/video tech for paramount studios (god rest his soul) and we both scratched our heads over this for months until he talked to a guy that was fleetwood mac's on-tour-tech for a few years and he pointed us in the "use a chopstick to move wires around while hooked up to an oscilloscope" direction - took Bob two months to ask, he was like that...

sure enough we hooked up a massive 8 ohm 1000 watt resistor to the speaker jack for a dummy load so we wouldn't fry our ears, hooked up the scope, hooked up a signal generator, turned it to max and when we moved that one wire watched in AMAZEMENT as the squiggly box full of lines turned into a perfect sine wave. "There's your ringer Al" said Bob. then we moved it back and forth a few times just to watch it go back and forth from messed up to perfect.

many a beer was spilled in celebration that evening!!!! we called it quits when we noticed the sun was starting to rise. 8) "Oh poop Bob, the sun's starting to rise" "Oh poop Al, i still have to drive home!"

on the 5e3's i had with this problem i solved it by doing this, curling the output from v2 around a pencil before soldering it up prevents the coronal discharge into v1 - see the curly blue wire center of photo. through moving this little wire around, and testing the sound with a guitar, i've found this is the perfect position to get really super clean clean's while having a nice chunk when dirty. you can actually control the frequency response with this little bugger!!!!

i also agree with what IS the sound? popping? scratching? humming? buzzing? this will help to narrow the possibilities. even more helpful would be a youtube video where you demonstrate the amp. worth 1000 words squared! even if it's low quality video sound it should help tremendously.

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The ability to play/make music is a gift that not everyone gets. Those of us who have it should use it.

Asimov:Individual science fiction stories may seem as trivial as ever to the blinder critics and philosophers of today — but the core of science fiction, its essence, the concept around which it revolves, has become crucial to our salvation if we are to be saved at all.
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Re: Tube amp noise diagnosing help!

Postby msca8h on Tue Dec 31, 2013 5:53 am

Thanks asimiv, I'll try to post a videos soon.

Please stay here


And thanks for everybody paying attention here!
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Re: Tube amp noise diagnosing help!

Postby msca8h on Tue Dec 31, 2013 7:40 am

http://youtu.be/Y5_gMDBojlA

Here's the demo video

Sorry for my English :oops:



Hope you can get some ideas!
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Re: Tube amp noise diagnosing help!

Postby LUC.key on Tue Dec 31, 2013 10:08 am

What I can hear in your vid is both hum and oscilation - with only possible reason - poor lead dress. Google a component layout for your amp and follow it. I see a lot of wires hanging in the air, prolonged leads of wires, that are already too long. You want to keep it as short as possible to prevent oscilations and hum inducing. Keep the signal wires away from power wires (filament, AC), use shielded cable...
Your amp looks good on the outside, lets make it look good in the inside - then youre be fine ;-)
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