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 22nd December 2009, 06:07 PM   #1
diyAudio Member
 
Join Date: Dec 2003
Location: Midwest
Default Hum with EL84 Push-Pull

My EL84 has a bit of "hum", or more accurately, "120Hz noise".

The 300V B+ power supply uses a split-secondary Edcor power transformer with full wave rectification (two diodes). The B+ is made henceforth:

1. Transformer
2. 200mA fuse
3. Full wave diode rectification
4. 100 Ohm resistor (to limit peak transformer current)
5. 2.2uF Film capacitor
6. 470uF 450V electrolytic capacitor
7. MOSFET voltage regulator
8. 470uF 450V electrolytic capacitor
9. 8mH toroidal choke (damps high frequency components)
10. 2 x 12uF Film capacitor

The filament uses a 12.6V winding on the same power transformer. I have used both AC and DC on the filaments and get the same problem. The DC filter looks like this:

1. Transformer (12.6V secondary)
2. Full wave bridge
3. 0.1uF at the bridge
4. 1 Ohm resistor (adjusted as necessary to control voltage)
5. 10,000uF 25V
6. 5mH Toroidal choke (2.5A maximum current)
7. 10,000uF 25V
8. 0.1uF at the output

I can bypass the entire DC electronics and wire the filament straight to AC.

I use 12.6V across two EL84 filaments in series, consuming 0.76A. The filament reference is tapped between the two EL84 (thereby referencing the filament halfway).

Using my Fostex (high efficiency) speaker, I can hear hum. Not smooth AC hum, but it sounds like 120Hz "spikes" ..tic..tic..tic..tic at 120Hz.

The B+ is squeaky clean.

My first suspension is the high-frequency diode commutation from the B+ rectification is "jumping" across the B+ winding to the filament winding inside of the power transformer.

Any ideas?
  Reply With Quote
Old 22nd December 2009, 07:22 PM   #2
dhaen is offline dhaen  Europe
diyAudio Moderator Emeritus
 
dhaen's Avatar
 
Join Date: Jun 2002
Location: U.K.
Why not go with your intuition and try an AC 6.3v supply from another (separate) transformer? With twisted wiring and a centre ground, you should not hear hum even with high efficiency speakers.
BTW You have grounded one side of your DC heaters..,?
  Reply With Quote
Old 22nd December 2009, 09:33 PM   #3
diyAudio Member
 
Join Date: Dec 2003
Location: Midwest
I rewired the power transformer, cleaned up the layout, and removed the Variac from the circuit (used for initial testing). The sound cleaned up a lot.

I can still hear - very faintly - the buzz. I am convinced the diode commutation noise is jumping across the B+ and Filament windings on the transformers (see below). I could use a separate transformer to verify this theory, but the noise is now so small it can only be heard very close to the speaker.

Another solution is to use snubber capacitors across the diodes to suppress commutation noise.

Quote:
Ringing occurs in AC-DC supplies when the diode is just turning off. Using the 1A, 12.6VAC transformer of the previous example we had a natural ringing frequency of 560kHz. This is much higher than the audio band. However, it is modulated by the power line frequency. A half-wave bridge will "buzz" at the line frequency and a full-wave bridge at twice the line frequency. The ringing frequency acts as a carrier to couple with other circuits.
The heater circuit (either AC or DC) is center referenced - the heaters are grounded between the two EL84 output valves.

Last edited by Kashmire; 22nd December 2009 at 09:38 PM.
  Reply With Quote
Old 22nd December 2009, 10:12 PM   #4
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Quote:
Originally Posted by Kashmire View Post
My EL84 has a bit of "hum", or more accurately, "120Hz noise".

My first suspension is the high-frequency diode commutation from the B+ rectification is "jumping" across the B+ winding to the filament winding inside of the power transformer.

Any ideas?
I had a similar problem on a pre amp I designed and I had to slug the diodes with a low value capacitor to stop the switching glitches. I used a CRC power supply for B+
__________________
http://www.murtonpikesystems.co.uk PCBCAD50 pcb design software.
  Reply With Quote
Old 22nd December 2009, 10:44 PM   #5
diyAudio Member
 
Join Date: Dec 2003
Location: Midwest
I built an Aikido for a preamp, and used an 6CA4. My most recent EL84 push-pull is the first time I've used solid-state diodes. What a mess!

Pictures attached of the power supply to my Aikido preamp.

I'll install the snubbers tonight, and report on findings.
Attached Images
File Type: jpg PSU 6CA7 Bright_small.jpg (142.0 KB, 299 views)
File Type: jpg PSU Assembled_small.jpg (137.8 KB, 287 views)
  Reply With Quote
Old 23rd December 2009, 03:31 AM   #6
diyAudio Member
 
Join Date: Dec 2003
Location: Midwest
Tweaked the layout a little ...

(quote from Snubber Networks)

Quote:
Judicious attention to wiring, loop area, components, and shielding will reduce the ability of the oscillation to couple with other circuits. However, this only attacks the symptoms, not the cause. This is where snubbers come in.
... and installed some snubber capacitors on both the filament and B+ solid-state rectifier diodes. I used 0.068uF 50V on the filament diodes and 0.01uF 1200V on the B+ diodes. I'll post pictures tomorrow. The best way would be a 110 Ohm resistor in series with 0.068uF, but I was lazy and omitted the resistor.

The combination of snubber capacitors and improved layout reduced the 120Hz buzz to about the same level as the endemic "hiss" of the amplifier. It is only audible in a quiet room (no HVAC operating) on highly efficient speakers (Fostex) with my ear near (<0.5m) to the speaker.

On inefficient speakers (89dB), the "hiss" is louder than the "buzz".

For all the trouble, I think I prefer vacuum rectification!

Last edited by Kashmire; 23rd December 2009 at 03:34 AM.
  Reply With Quote
Old 24th December 2009, 09:55 AM   #7
Ian444 is offline Ian444  Australia
diyAudio Member
 
Join Date: Nov 2008
Location: Brisbane QLD
The heater circuits, do they have a reference to ground.
  Reply With Quote
Old 24th December 2009, 02:58 PM   #8
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Quote:
Originally Posted by Ian444 View Post
The heater circuits, do they have a reference to ground.
OP says "The heater circuit (either AC or DC) is center referenced - the heaters are grounded between the two EL84 output valves."

So yes...
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 24th December 2009, 05:20 PM   #9
Arnulf is offline Arnulf  Europe
diyAudio Member
 
Join Date: Feb 2009
Get rid of 80% of your PSU capacitance (#6 on your checklist, the capacitor prior to the voltage regulator) and you'll mostlikely get rid of said buzz, if it is indeed caused by the diodes.

While you are at it, there is hardly any point in using 470 uF behind the regulator as well, even though it doesn't burden the transformer directly (except at power on).
  Reply With Quote
Old 24th December 2009, 08:09 PM   #10
diyAudio Member
 
SpreadSpectrum's Avatar
 
Join Date: Nov 2007
Quote:
Originally Posted by Kashmire View Post
The best way would be a 110 Ohm resistor in series with 0.068uF, but I was lazy and omitted the resistor.
You stopped just short of getting a really good result. You have only reduced frequency and amplitude of the ringing. Why not kill it completely?
  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
EL84 push pull slideman82 Tubes / Valves 4 19th June 2009 03:40 PM
Will this work for an EL84 push pull? zobsky Tubes / Valves 5 10th October 2006 07:42 PM
modify parallel push-pull EL84 to single push-pull chungtat Tubes / Valves 12 3rd November 2005 11:25 PM
EL84 push pull idea mark_titano Tubes / Valves 20 13th June 2005 02:51 PM
EL84 Push Pull amp. hacknet Tubes / Valves 8 23rd September 2004 05:53 AM


New To Site? Need Help?

All times are GMT. The time now is 08:40 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