Hum in homebuilt amp - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 28th June 2012, 12:10 AM   #11
tim614 is offline tim614  United States
diyAudio Member
 
tim614's Avatar
 
Join Date: Jun 2008
what kind of grounding are you using?
star ground or ground bus?
can you take a pic of the wiring?
__________________
Tim
" perfect imperfection "
  Reply With Quote
Old 28th June 2012, 12:17 AM   #12
zoi is offline zoi
diyAudio Member
 
Join Date: May 2012
I laid out the circuit exactly how it is shown in the picture, with a ground wire wrapping around the outside that everything is connected to
  Reply With Quote
Old 28th June 2012, 01:19 AM   #13
zoi is offline zoi
diyAudio Member
 
Join Date: May 2012
Here is a picture. The green wire that wraps around the outside is the ground. The +/- from the power supply transformer and the b+ are routed far around the outside.
Attached Images
File Type: jpg photo-3.jpg (624.3 KB, 180 views)
  Reply With Quote
Old 28th June 2012, 01:39 AM   #14
diyAudio Member
 
Triodethom's Avatar
 
Join Date: Dec 2009
Location: Newnan GA
c1 and c3 look rather small try 100mfd for each . Is the power transformer ground and the curcuit ground at the same point ? Lift the earth ground off at that point if the hum stops then reverse the hot and neutral on the transformer and reconnect the earth ground . Let us know what that does.
  Reply With Quote
Old 28th June 2012, 02:29 AM   #15
tim614 is offline tim614  United States
diyAudio Member
 
tim614's Avatar
 
Join Date: Jun 2008
Are you breadboarding the amp right now?
One thing I do when I build an amp is to make sure everything is wire as neat as possible, that way you can trouble shoot things easily.
One thing that works for me is, I use a ground bus with the power supply ground at one end of the G-bus.
I agree with Triodethom about the lifting the earth ground from the signal ground.
__________________
Tim
" perfect imperfection "
  Reply With Quote
Old 28th June 2012, 02:58 AM   #16
gootee is offline gootee  United States
diyAudio Member
 
Join Date: Nov 2006
Location: Indiana
Blog Entries: 1
1. Any gap between the ground and signal paths will create a receiving antenna for hum.

2. Any gap between the wires in an AC-type circuit will create a transmitting antenna for hum.

3. Signal ground paths that share a wire or conductor with power supply grounds will inject hum into the signal paths they serve.



You have all three situations, pervasively. Below is my take on the easiest ways to fix them.


Referring to the schematic, remove the ground wire that goes all the way around the upper left, from between C6 and the ground symbol near R1.

Later, we will need to add a new connection from near that ground symbol to the new Star Ground point, which we'll define later.

[Edit: If you just want to try the one simplest thing first, then you can now connect a wire rom that ground symbol to the centertap's safety earth ground, and see if the hum has changed at all.]

But first, you need to close up any/all open area between the signal and ground paths for the input, as follows:

The ground and signal wires from the input jack must be tightly twisted together (at least four turns per inch), ALL the way to each end of the body of R1. And the point where R1 and the jack's ground connect needs to be right at the base of R4. And that whole assembly needs to hug right up against R2 and R4 and their leads, as it goes to the base of R4. R2 and R4 need to have very-minimal lead lengths connecting to the tube. C1 should have minimal lead length and should be as close as possible to R4 (touching all along their lengths, if possible, with no space between their leads anywhere).

ALTERNATIVELY, after the input jack's wires are twisted right up to each end of R1, you could think of the rest of the above as: taking the ground wire that's shown running between R1 and R4 and, instead, running it upward (on the schematic), right against R1, then rightward along and against R2 then downward along and against R4 and C1. And you could even implement it that way, if that would be easier or less disruptive of what you've already got. But you should also make all of the connections and leads as short as possible. And if there are any significant lead lengths left, the ground wire could be wrapped around them (twisted with them would be better), but it should at least be touching the leads and resistors, everywhere. Any gap between the ground and signal paths will create an antenna for hum.

Now do the same thing for the input signal/ground loops of the other two tubes: (Referring to the schematic: ) Take the ground from the previous cathode area's ground point up to the anode and then twist with or closely follow every wire, component, and lead, over to the next grid connection, and then back down along the cathode components to the ground point for that tube's input. Notice that the lines going up from ground to each pot will be gone, because the ground will already be up there, going to the left and right from each pot.

You should now have one compact point, for each tube, at the base of the cathode components, where the signal grounds all connect.

Those three points will all need to connect to the ground that is connected to the center tap of the power transformer. But we don't want to connect them to a point where the center tap's (and first smoothing cap's) large changing currents will induce noisy voltages across the impedance (inductance and resistance) of a shared ground conductor. Those voltages would make the entire signal ground conductor, everywhere upstream, have a noisy voltage on it, which would essentially be summed with all of the grid input voltages. So we need to define a separate "Star Ground" point, to try to avoid that.

Also, since the signal voltage for each tubes' input will be larger that that of the previous stage, and will produce larger ground-return currents, we might want to run the three signal ground points (we made one for each tube, remember?) back to the star ground using separate wires. So we could conceptually break the ground wire, between tube stages, probably between the pot ground and the previous cathode RC ground, and make three separate runs to the star ground. Or, only the first stage might need a separate ground run. Or maybe they can all stay connected together, which would amount to just connecting one tube's signal ground point to the star ground. (If separate runs are needed, I would tend to want to run them right alongside all of the other closely-spaced signal/ground paths we created, and keep them together all the way to the star ground point.

The star ground point will eventually connect to chassis ground, or the safety earth ground, possibly via a "disconnect network", which we won't get into yet.

Power and Power Grounds:

I would try to do the same thing we did above, but for the power and ground paths between the capacitors across the top of the schematic. Try to eliminate any space between the power and ground paths, there.

Also, the secondary wires and center tap wire/ground should be twisted or kept touching, everywhere possible, all the way to the first smoothing cap.

Also, the primary's AC input wires must be tightly twisted together, EVERYWHERE.

And keep all AC conductors and components away from everything else. And keep all power and power ground conductors and components away from everything else. And keep all signal and signal ground conductors and components away from everything else. If any of those ever even get close to each other, they should be at a right angle to each other.

Try that, for now. Every detail of it might be critical. Some of it might not be but you won't know until you try it.

Hopefully, someone else will know the optimal location for the star ground point, and how to route the rest.

Cheers,

Tom

Last edited by gootee; 28th June 2012 at 03:26 AM.
  Reply With Quote
Old 28th June 2012, 03:30 AM   #17
gootee is offline gootee  United States
diyAudio Member
 
Join Date: Nov 2006
Location: Indiana
Blog Entries: 1
Before doing anything else, you really should short the input jack, or try it with an instrument plugged in.
  Reply With Quote
Old 28th June 2012, 06:35 AM   #18
diyAudio Member
 
trobbins's Avatar
 
Join Date: Feb 2009
Location: Melbourne, Oz
Quote:
Originally Posted by zoi View Post
Hi all,
It works but there is a ton of 60Hz hum. The hum increases when i turn up either of the two potentiometers, but if the preamp one is all the way down and the other is all the way up there is no hum.
If you short the grid of the input valve V1A to the 0V end of R4, with both pots at max, then how is the hum?

As gootee indicates, normally the input signal socket has a shorting switch to ground the input when no guitar/lead is inserted.

Ciao, Tim
  Reply With Quote
Old 28th June 2012, 09:22 AM   #19
DF96 is online now DF96  England
diyAudio Member
 
Join Date: May 2007
The problem may simply be that you have lots of unscreened wiring hanging in mid-air near mains wiring with no metal chassis to provide electrostatic screening. Something which is not always realised is that the mere presence of nearby chunks of grounded metal reduce capacitive coupling of conductors even when not fully interposed between them. The plastic base may increase coupling as its dielectric constant will be greater than 1.

The picture appears to show the rectifier at the opposite end of the circuit from the mains transformer. This is asking for trouble.
  Reply With Quote
Old 29th June 2012, 12:55 AM   #20
gootee is offline gootee  United States
diyAudio Member
 
Join Date: Nov 2006
Location: Indiana
Blog Entries: 1
Quote:
Originally Posted by DF96 View Post
<snipped>
The picture appears to show the rectifier at the opposite end of the circuit from the mains transformer. This is asking for trouble.
DF96,

I meant to ask if that was what I was seeing! "Asking for trouble" is probably putting it too mildly.

But very often, for a first build, this is all to be expected, and should be a great opportunity for learning the importance of a few main concepts that should enable the building of more-or-less hum-free amps, from now on.

I just hope I didn't overwhelm him with too many pointy details. I seem to have been in overkill mode, again; probably inappropriate for a first-timer, unless he's an engineer or something.

Regards,

Tom
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Homebuilt variac SirByrd Equipment & Tools 21 2nd October 2011 04:13 AM
6 Channel Homebuilt amp svsommer Solid State 7 12th December 2010 07:39 AM
completely homebuilt subwoofer. breisch1986 Subwoofers 1 28th November 2006 09:38 AM
FS: Homebuilt "Aikido" 2A3 single ended triode amp jim_vt Swap Meet 11 21st January 2006 03:38 PM
Anyone interested in Homebuilt helicopters? ap0the0sis Everything Else 22 29th January 2003 11:14 PM


New To Site? Need Help?

All times are GMT. The time now is 12:40 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2