Troubleshooting hum in 12b4 preamp.. finally almost done!! - Page 6 - 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 29th October 2010, 12:37 AM   #51
240z4u is offline 240z4u  United States
diyAudio Member
 
Join Date: Dec 2004
Location: Madison WI
Send a message via AIM to 240z4u
Alright, well I went ahead and grounded the center-tap which seemed to make the hum worse. I am going to try to figure out what rating of resistor to use so I can drop the heater voltage 2v, from 14 to 12.

Evan
__________________
Yeah... it seemed like a good idea at the time.
  Reply With Quote
Old 29th October 2010, 04:29 AM   #52
diyAudio Member
 
Join Date: Aug 2009
Location: Sacramento
Quote:
Originally Posted by 240z4u View Post
Alright, well I went ahead and grounded the center-tap which seemed to make the hum worse. I am going to try to figure out what rating of resistor to use so I can drop the heater voltage 2v, from 14 to 12.

Evan
Hello,
If you grounded Pin #3 and the hum got louder that raises my suspicions.
I looked sorta through the thread and did not read if the heaters are DC or regulated DC. My feeling is that a line stage should have well regulated DC heaters. Plus the heater should be at an elevated voltage with a bypass capacitor to ground. By elevated voltage I mean set up a voltage divider between B+ and ground made up of say a 300K resistor and a 100K the negative side of regulated heater DC then ties to the node between the resistors the bypass capacitor ties to the same node and ground. The heaters will be quiet!
DT
All just for fun!
  Reply With Quote
Old 29th October 2010, 10:22 AM   #53
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
A line stage should not really need DC heaters, unless the valves have h-k leakage or the wiring is poor. In the olden days they used AC heaters for tape and phono preamps without any serious trouble.
  Reply With Quote
Old 29th October 2010, 01:37 PM   #54
240z4u is offline 240z4u  United States
diyAudio Member
 
Join Date: Dec 2004
Location: Madison WI
Send a message via AIM to 240z4u
AC heaters are what is being employed here. As a rookie, I can only assume DC would be better. The guys telling me AC is fine are people that know way more than me, and I can only hope to have that knowledge one day.

That being said, 5 ohms of resistance dropped my heater voltage to 11.85 (or something close to that). I had two 5 watt 10 ohm resistors, wired em' up in parallel and we were in business. I ended up using trial and error honestly.

The good news? There is no heater buzz until I am up into a very high area of gain. So high, I doubt the preamp ever gets turned up that high except for looking for noise.

I still have a low 60 cycle hum. I haven't ruled out ground loops, and the dynaco MKIIIs I have hum just a hair anyway.. which may be accentuating it. I have the center tap grounded now, along with everything else. I find if I don't connect the star ground to the top plate I get massive hum. If I could even cut the hum in half on this preamp, it would likely be fine.

I haven't ruled out the hum-dinger pot idea. I hate to ask this because I know you guys hate spoonfeeding but Ill go ahead anyway.

1) What wattage pot would be appropriate? Judging by how warm the resistors get that are dropping the voltage 2v I assumed 2 watt would be ok.

2) What resistance pot should I be using? 5 ohm? 50 ohm? 5k ohm?

3) Can I run both tubes through the same humdinger pot? Connect both heater CT's to the same terminal on the pot and a single ground from the pot to the star ground is what I was thinking.

Thanks again for all your help guys. Even though this is a basic preamp, it is very hard to diagnose and think logically with such little working knowledge about layout and wiring etc....

Evan
__________________
Yeah... it seemed like a good idea at the time.
  Reply With Quote
Old 29th October 2010, 02:29 PM   #55
diyAudio Member
 
Join Date: Aug 2009
Location: Sacramento
Quote:
Originally Posted by DF96 View Post
A line stage should not really need DC heaters, unless the valves have h-k leakage or the wiring is poor. In the olden days they used AC heaters for tape and phono preamps without any serious trouble.
I agree that AC heaters can be relatively quiet. The valve data sheets do say AC or DC. As you say the wiring is key to keeping the AC out of the circuit. Even using DC heaters I twist the conductors tight and run them tight to the metal case.
DT
All just for fun!
  Reply With Quote
Old 29th October 2010, 02:35 PM   #56
Doz is offline Doz  United Kingdom
diyAudio Member
 
Doz's Avatar
 
Join Date: Jul 2010
Location: Sat Down
<QUOTE> 3) Can I run both tubes through the same humdinger pot? Connect both heater CT's to the same terminal on the pot and a single ground from the pot to the star ground is what I was thinking. </QUOTE>

If you've grounded your centre-tap, there's no need for a hum-dinger. Where did you put your 5 ohms? On one side of the winding ? Or both? If you've put it on one side , you will have unbalanced the heater supply, and your centre tap ground will not be as sucessful at killing the hum. Stick 2x 2.5 ohms from each "hot"....

BTW My line stage has AC heaters , and there's no hum....
  Reply With Quote
Old 29th October 2010, 02:39 PM   #57
240z4u is offline 240z4u  United States
diyAudio Member
 
Join Date: Dec 2004
Location: Madison WI
Send a message via AIM to 240z4u
Doz, I did actually inadvertantly unbalance the heater load. Ill try to get some resistors after work so I can place them on each leg.

Thanks!
__________________
Yeah... it seemed like a good idea at the time.
  Reply With Quote
Old 29th October 2010, 06:18 PM   #58
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
It is one humdinger (or winding centre-tap) per heater winding. If you use two, or one of each, then you will inject hum into your ground.

With DC heaters it is not necessary to twist heater wiring or put it near the chassis, unless it is poorly smoothed.
  Reply With Quote
Old 29th October 2010, 06:43 PM   #59
240z4u is offline 240z4u  United States
diyAudio Member
 
Join Date: Dec 2004
Location: Madison WI
Send a message via AIM to 240z4u
Aw man, I feel kinda dumb here

DF; you are indicating that I need a seperate pot for each heater's center tap run to ground in the humdinger scenario right?

By heater winding, you mean the heater winding in each tube right? Or was I only supposed to ground the CT from one of the tubes and not both?

Thanks for your patience.
__________________
Yeah... it seemed like a good idea at the time.
  Reply With Quote
Old 29th October 2010, 06:55 PM   #60
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
No, winding=transformer secondary.

I forgot about heater centre-taps. So, a given heater circuit has one transformer winding and one or more valve heaters (usually wired in parallel). Every heater circuit has to have a reference voltage, which can either be ground or something else. The reference voltage can be simply connected to one side of the heater circuit, but for lower hum it is better to connect to a centre-tap. A centre-tap can be on the heater winding, a valve heater, the connection of a pair of resistors, or the slider of a humdinger. Each heater circuit should be connected to the reference voltage at one place only. A heater centre-tap is not usually a good place to do it as you lose the centre-tap if the valve is removed for testing.
  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
12B4 Preamp philipbarrett Tubes / Valves 126 16th November 2012 09:17 AM
WTB: 12B4 Tubed Preamp. Zekk Swap Meet 0 18th March 2009 03:17 PM
12B4 Battery Bias from (Preamp: Foreplay or 5687 or 12B4 ) DougL Tubes / Valves 3 2nd August 2004 06:06 PM


New To Site? Need Help?

All times are GMT. The time now is 02:37 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