Hum in homebuilt amp - 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 27th June 2012, 06:40 PM   #1
zoi is offline zoi
diyAudio Member
 
Join Date: May 2012
Default Hum in homebuilt amp

Hi all,
First-off, I apologize if this is an inappropriate question. I am new to building amps (working on my first build).

I set out to build this amp and have just finished wiring it. 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. Does this mean the bias voltage is clean?

Also, I have tried adding another parallel capacitor to the first in the filter and it doesn't change the hum. This is another indication to me that the filtering is not the problem.

I used good wire and was very careful to check all of the solder joints and connections.

I don't have an oscilloscope, so I am wondering if there is anything I can do to try to diagnose the problem.

Any help would be much appreciated, as I don't even know where to begin.

Thanks a ton
  Reply With Quote
Old 27th June 2012, 07:07 PM   #2
Zibi is offline Zibi  Poland
diyAudio Member
 
Join Date: Jan 2006
Location: Bialystok
Excessive Hum
http://www.guitarnuts.com/technical/...nericnoise.php
http://www.freewebs.com/valvewizard/heater.html
http://www.freewebs.com/valvewizard2/Grounding.html

Last edited by Zibi; 27th June 2012 at 07:15 PM.
  Reply With Quote
Old 27th June 2012, 07:18 PM   #3
diyAudio Member
 
Frank Berry's Avatar
 
Join Date: Sep 2003
Location: Midland, Michigan
One end of the output transformer secondary should be grounded. It's not. Without the ground connection, there is no negative feedback.
As for the hum, you'll need to check the grounding within the amplifier. It may be a ground loop.
__________________
Frank
  Reply With Quote
Old 27th June 2012, 08:02 PM   #4
zoi is offline zoi
diyAudio Member
 
Join Date: May 2012
Thanks so much for your replies.

To eliminate the possibility of heater noise, I disconnected the AC supply and powered the heaters with a 6V battery pack. The hum didn't change.

I grounded one side of the output transformer secondary (tried both sides) and that didn't change it either.

The wiring matches the diagram, and all ground points are at the same potential. Could there be another ground missing somewhere?
  Reply With Quote
Old 27th June 2012, 08:05 PM   #5
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
C3 is shorting the feedback above about 100Hz so should be removed, unless you want a very bass-light sound.

Are the grounds wired exactly as shown in the diagram?

How are the heaters wired?

What is the purpose of VR2? It is within a feedback network (or it would be if C3 is removed) so will have little effect.

Is this some sort of guitar amp?
  Reply With Quote
Old 27th June 2012, 08:32 PM   #6
zoi is offline zoi
diyAudio Member
 
Join Date: May 2012
I grounded one side of the output secondary (tried each side) and the hum didn't change.

The heaters are in parallel across the 6v supply from the power supply transformer. I eliminated the possibly of heater noise by disconnecting the AC heater feed and replacing it with a battery pack. No change.

The amp is designed to be a guitar amp. I assumed VR2 would be the volume control, with VR1 left in an optimal position or replaced with resistors, but maybe that should be the other way around (using VR1 as volume)?

The bass response is good as is, so I am hesitant to remove C3 but if there is a chance it would eliminate the noise I will do so.

The grounds are wired exactly as shown. None of the wire is shielded but all the runs are short and heavy gauge.
  Reply With Quote
Old 27th June 2012, 09:13 PM   #7
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
I assume you used twisted wiring for the heaters, spaced well away from the signal circuitry?

Signal wiring should be very short or shielded, including wires to the volume pots. By 'short' I mean about an inch or couple of cm. Ground wires are not shielded, as this would be meaningless!

Removing C3 will allow the feedback to work, if you also ground the correct side of the OPT secondary. This is the side which does not have the 33K feedback resistor. Alternatively, just remove the 33K as at present it does nothing. Up to you.
  Reply With Quote
Old 27th June 2012, 09:23 PM   #8
Bone is offline Bone  United Kingdom
diyAudio Member
 
Join Date: Jun 2004
Location: Dorset, UK
Short the input jack. If hum disappears then the hum is pickup into the first stage.
A shorting jack should be fitted.. Does it hum if an instrument is plugged in?
  Reply With Quote
Old 27th June 2012, 10:01 PM   #9
zoi is offline zoi
diyAudio Member
 
Join Date: May 2012
I used twisted wires for the heaters, but I don't think it is heater noise, since if i disconnect the heaters while the amp is running the hum continues (for 10 seconds or so until the tubes cool and there is no output)

Shorting the input jack doesn't change the hum.

Also, the schematic doesn't reflect this but I replaced the diodes with a single 6x4 tube as the power supply. Is anything additional needed to properly make that substitution? As of now, the circuit is the same aside from the diodes replaced.
  Reply With Quote
Old 27th June 2012, 10:55 PM   #10
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
How is the transformer secondary CT connected to ground and the negative end of C9 and then the other smoothing caps? The details here are important, as it is easy to inject charging pulses into the ground if you get it wrong.
  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 01:29 AM.


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