Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kwm488 on January 29, 2013, 12:47:08 pm

Title: my marshall amp still have sound even i turn volume to zero
Post by: kwm488 on January 29, 2013, 12:47:08 pm

hi

i just build my last marshall superlead amp.
i find out my amp still have sound even i turn the volume to zero. channel 2 more serious than channel 1. i sure every ground is good contact. and the pots is no problem. because i change a new pots, problem can't fix.

do anyone can help me?

and i use follow in this amp

mercury transformer
PEC pots (it sound better than CTS)
RMG resistor (i use takman resistor before, sound suck.)
Sozo vintage cap
vintage filter cap
Lemco cap
kimber wire for the amp
Msolder for the amp soldering
Mullard 12ax7
Gold Lion KT77
Cliff jack
Belton tube socket
furutech power plug socket (it make sound better than standard socket)


(http://i23.photobucket.com/albums/b383/kwm488/2013-01-29230645_zps20db6eb0.jpg)

Title: Re: my marshall amp still have sound even i turn volume to zero
Post by: HotBluePlates on January 29, 2013, 08:43:57 pm

I think I see a black wire from the pot ground buss to the ground on the circuit board.

However, if you have sound when the volume control is on zero, you do not have a good ground for that control.

If it was wired correctly it would be working right now, whether you use expensive parts or cheap parts.

Title: Re: my marshall amp still have sound even i turn volume to zero
Post by: kwm488 on January 29, 2013, 09:50:51 pm

hi

i sure all the ground are connect. and this amp very very silence, no noise. just have volume even turn it to zero.
i will check the ground today again.

btw, will this amp have other problem? do  you mind to give me some idea?

because i use PEC pots, so i just connect the pots lug which need to ground to bare wire, and i use black wire to connect the bare wire to the chassis ground (which connect to filter cap ground also)

i remember some hifi people always suggest 1 point ground. but i don't know is it work on guitar amp?

thanks

Title: Re: my marshall amp still have sound even i turn volume to zero
Post by: HotBluePlates on January 29, 2013, 10:19:36 pm

This:

Quote from: kwm488 on January 29, 2013, 09:50:51 pm

i sure all the ground are connect.

does not equal this:

Quote from: kwm488 on January 29, 2013, 09:50:51 pm

just have volume even turn it to zero.

because if the grounds were good on the pots, there would not be sound at a zero volume setting.

Title: Re: my marshall amp still have sound even i turn volume to zero
Post by: G._Hoffman on January 29, 2013, 10:30:56 pm

Doug's grounding scheme.


http://www.el34world.com/charts/grounds.htm (http://www.el34world.com/charts/grounds.htm)


Though I find the sections in Merlin Blencowe's books more complete and easier to understand, and use his "system."  But then, I'm not very good at amps.



Gabriel

Title: Re: my marshall amp still have sound even i turn volume to zero
Post by: kwm488 on January 29, 2013, 10:39:05 pm

Hi

And i use cts pot before. I use bare wire solder to the back of the pots to be ground. But i also have this problem.

This problem is caused by the ground only ?
 if yes. I try to sand the ground to make the ground contact better today

Title: Re: my marshall amp still have sound even i turn volume to zero
Post by: plexi50 on January 30, 2013, 06:52:42 am

This problem is caused by the ground only ?


Take a jumper wire with alligator clips on it. Clip one end to the chassis and the other end to the volume pots left terminal facing the rear of the amp. Tell us what happens

Title: Re: my marshall amp still have sound even i turn volume to zero
Post by: kwm488 on January 30, 2013, 12:20:49 pm

hi

i find out more today. my friend superbass (vintage marshall 1969, but repair by me) also have this problem.
and i try to use jumper wire with alligator clips to connect the bare wire to the ground which connect transformer ground and power cord ground . but the problem still here. and i try to connect to the middle lug of the pots. still no good news. i will try to connect to the left of the pots later. your mean left lug, which is not connect the ground already, right? which connect to the 0.0022uf cap, right?
thanks

Title: Re: my marshall amp still have sound even i turn volume to zero
Post by: 12AX7 on January 30, 2013, 03:58:26 pm

I have a pre PI master in a JCM style amp and at zero there was no sound. Till recently that is, when i put a 120k resistor between the wiper and the PI input cap. (that mod to make the master's throw more linear tonally and volume wise) Now it also has some sound when turned 100% CCW.  I don't get why since the signal should still be grounded, and i know the ground for it is good.  Weird.

Maybe that tiny bit or resistance most pots have between the wiper and the lug isn't enough to allow an audible sound thru except under certain circumstances?

Title: Re: my marshall amp still have sound even i turn volume to zero
Post by: kwm488 on February 05, 2013, 12:23:31 pm

Hi
i try to short the left side of the pots lug to the ground. But my amp still have volume  Short the signal to ground should be no sound. But why????
Can anyone help?

Thanks

Title: Re: my marshall amp still have sound even i turn volume to zero
Post by: Geezer on February 05, 2013, 04:10:20 pm

If you short the signal to ground and still have volume, then there is something wired wrong earlier in the circuit that is bleeding signal to something after the volume pot....double check all the circuit, point by point until you find the culprit.

G