Hum Pot Question - 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 12th December 2005, 03:01 AM   #1
Trout is offline Trout  United States
diyAudio Member
 
Trout's Avatar
 
Join Date: Nov 2004
Location: Midwest Madman
Default Hum Pot Question

Hi,
The PT I am using in my latest project just happens to NOT have a CT on the 6.3V winding. So, The original Amp schematic I am building happens to show grounding 1 leg if the 6.3V.

The Next newer year shows having a CT, I thought It might be a good idea to put in a hum balance pot rather than mearly ground the single leg of the 6.3V.

My question is, Does it really matter where in the chain it is located?

The easiest drawing in my chassis is item Number 3 in this
Picture

The transformer donor unit had the pot located after the 6L6;s in the chain.
Gene
  Reply With Quote
Old 12th December 2005, 04:10 PM   #2
diyAudio Member
 
Join Date: Mar 2004
Location: aberdeen, uk
Send a message via AIM to martinab2 Send a message via MSN to martinab2 Send a message via Yahoo to martinab2
Any of the arrangements in your diagram should work ok with indirectly heated tubes. I have never found the location of the pot to be critical.
The purpose of the balance pot is to provide an adjustable ground reference for the heaters. In theory there should be a setting which produces lowest levels of 100/120 Hz buzz.
In practice, I can't hear any difference! Another option is to create a non-adjustable centre tap by connecting an 100 ohm resistor from each side of the heater supply to ground.
  Reply With Quote
Old 12th December 2005, 04:41 PM   #3
Trout is offline Trout  United States
diyAudio Member
 
Trout's Avatar
 
Join Date: Nov 2004
Location: Midwest Madman
Quote:
Originally posted by martinab2
Any of the arrangements in your diagram should work ok with indirectly heated tubes. I have never found the location of the pot to be critical.
The purpose of the balance pot is to provide an adjustable ground reference for the heaters. In theory there should be a setting which produces lowest levels of 100/120 Hz buzz.
In practice, I can't hear any difference! Another option is to create a non-adjustable centre tap by connecting an 100 ohm resistor from each side of the heater supply to ground.


OK Understood,

I have seen it done so many different ways that I felt it was not especially critical, But, Better to ask those who know rather than de-solder later.
Gene
  Reply With Quote
Old 13th December 2005, 01:24 AM   #4
diyAudio Member
 
Join Date: May 2005
Location: Pretoria, South Africa
Also from me a small contribution: I have very occasionally found that a pot gave a better hum null, especially where high-gain circuits (pre-amplifiers) were involved. But this was infrequent.

If cost is a factor you could always use a wire-wound resistor with a slider instead of a pot. If you have a supply of components, try this first and if no detectable difference, use fixed resistors or simply earth one side.
  Reply With Quote
Old 13th December 2005, 04:27 AM   #5
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
And keep in mind that for Leo Fender, "good enough" was the operating manufacturing philosophy. I even have a service bulletin from them regarding hum in the Champ. It tells us not to bother trying to get rid of the background hum, it is normal.

Grounding one side of the heaters is probably better than no hum cancelling, and a lot cheaper. SOme models have a hum null pot, but mostly the two 100 ohm resistors is what they used.
  Reply With Quote
Old 13th December 2005, 04:53 AM   #6
Trout is offline Trout  United States
diyAudio Member
 
Trout's Avatar
 
Join Date: Nov 2004
Location: Midwest Madman
So If I ground 1 leg of the 6.3V, How can I determine which leg to ground? or Does it matter. The Color code on the 6.3V is 1-green, 1 black. All the CT units I built never had audible hum issues but this is the first time I have used a PT without a CT.
Gene

Oh BTW this is on a modded Pro 5E5 using 6L6's on output.
  Reply With Quote
Old 13th December 2005, 08:44 AM   #7
diyAudio Member
 
Join Date: Mar 2004
Location: aberdeen, uk
Send a message via AIM to martinab2 Send a message via MSN to martinab2 Send a message via Yahoo to martinab2
I doesn't matter which leg you ground. Remember that the heaters are insulated from the cathodes (up to the maximum heater to cathode voltage specified for that type of tube). By referencing the heater supply to a fixed voltage (in this case ground) we are just preventing it from acting as an aerial for hum and noise.
With directly heated tubes such as 2a3 etc, the heater is the cathode and the heater supply is part of the signal path. In this case you do need to use an adjustable hum cancelling pot or DC heaters supplies as hum is much more likely to be audible.
Luckily you don't have to worry about any of this in your current project.
  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
newbie with input tran. question and wiring question imo Solid State 0 18th January 2006 10:10 PM
Capacitor Question /question on cap TunaFish Tubes / Valves 14 12th January 2004 02:23 AM
PS question - tformer question actually breguetphile Chip Amps 2 30th May 2003 01:29 PM
old speaker question, bullet midrange question & link to speaker project wallijonn Multi-Way 10 5th November 2002 06:03 AM


New To Site? Need Help?

All times are GMT. The time now is 07:03 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