Dropping rectifier filament voltage - 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 25th January 2009, 09:13 PM   #1
diyAudio Member
 
Join Date: Apr 2006
Location: copenhagen
Default Dropping rectifier filament voltage

Hi.

I am gathering some parts for an experimental tube linestage project. I have searched around for a pwr transformer with 4VCT winding for european direct heated rectifiers, but no luck. However, Hammond have a range of nice trannies with 5VCT tapping. Do any of you know if it would be OK to drop the voltage to 4VCT with resistors considering B+ is to be taken from the CT?

Best

Theo
  Reply With Quote
Old 25th January 2009, 09:48 PM   #2
diyAudio Member
 
Join Date: Oct 2004
Location: Maui, Hawai'i, USA
Sure, you can do that.  Be aware that you're going to burn 1V × Xamps (whatever the rectifier heater draws) in the resistance, so size accordingly and leave some space around the resistor(s).  When it gets up in this range, I'll usually split the resistance, just so I don't have blazing hot objects in the chassis.

Aloha,

Poinz
  Reply With Quote
Old 25th January 2009, 09:51 PM   #3
diyAudio Member
 
yagoolar's Avatar
 
Join Date: Oct 2004
Location: Mazowieckie, Jozefoslaw
Send a message via ICQ to yagoolar Send a message via Skype™ to yagoolar
Have you tried to order a custom made xformer? I used to do it in Poland and, think, it is cheaper than Hammond's. Write the specs and I will check with locals.
  Reply With Quote
Old 25th January 2009, 11:41 PM   #4
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
Theo,

IMO, you will do better by employing 4X Schottky diodes arranged in "yin/yang" pairs at each end of the 5 VAC winding. Instead of resistively dissipating power as waste heat, the forward drop of the diodes achieves your purpose.

Schottky diodes, with their "noiseless" operation, are essential to the tweak. PN junction diodes will introduce switching noise into the B+ rail.
__________________
Eli D.
  Reply With Quote
Old 26th January 2009, 03:46 PM   #5
diyAudio Member
 
Join Date: Oct 2004
Location: Maui, Hawai'i, USA
Watt's Law never sleeps, Eli.  If we drop Avolts at a draw of Bamps, we shall inevitably burn A×B watts.  Depending on the rectifier draw, there may be problems with a diode of high enough current rating, but otherwise it sounds like a cool idea to me, as long as the heat is taken care of, and if the voltage we need to drop is some multiple of the diode drop.  The voltage will automatically be correct, and we'll simply burn the amount of power needed to do it.  It usually takes me two or three tries to get it just where I want it with resistors.

Aloha,

Poinz
AudioTropic
  Reply With Quote
Old 27th January 2009, 12:35 AM   #6
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
Poinz,

The forward drop in Schottkys is "typically" 1/2 V. 2X "yin/yang" pairs achieve the OP's purpose. Do you really think there will be trouble if diodes rated for a continuous 5 A. forward current are employed? We are dealing with a 50% duty cycle. PIV is not much of a concern in this application. FWIW, 10 and 20 A. diodes are available, should that much "beef" be adviseable.
__________________
Eli D.
  Reply With Quote
Old 27th January 2009, 03:04 AM   #7
diyAudio Member
 
Join Date: Oct 2004
Location: Maui, Hawai'i, USA
I very much doubt that there will be a problem with 5A diodes, Eli. The gentleman has not stated what the heater (or filament) draw of his rectifier is, but I do not know of any power amp rectifiers that draw over 3A heater.  You're right that the 50% duty cycle is germane; a 1A diode can typically pass 30A for a few milliseconds.  It's a heat thing, same as always.  With a little more data about that rectifier, we could advise the gentleman rather better.

Aloha,

Poinz
AudioTropic
  Reply With Quote
Old 27th January 2009, 04:11 AM   #8
rdf is offline rdf  Canada
diyAudio Member
 
rdf's Avatar
 
Join Date: Jun 2004
Location: big smoke
Default Re: Dropping rectifier filament voltage

Quote:
Originally posted by Nightpuma
... Hammond have a range of nice trannies with 5VCT tapping....
Hi Theo. You may find the Hammonds run high, it's been my experience 100% of the time. Personally I'ld look at hanging a 5 watt 'white coffin' power resistor off either end of the winding, easily available in a wide range of values under 1 ohm. Bombproof, no danger of added switching noise, a minuscule increase in power supply impedance and a bit of cold filament current limiting; what's not to like for a less than a couple dollars?
__________________
Ears aren't microphones.
  Reply With Quote
Old 27th January 2009, 05:10 AM   #9
diyAudio Member
 
dsavitsk's Avatar
 
Join Date: Jan 2005
Location: Hartford
Quote:
Originally posted by Poindexter
but I do not know of any power amp rectifiers that draw over 3A heater.
Push Pull 2A3's draw 5A.
  Reply With Quote
Old 27th January 2009, 01:06 PM   #10
diyAudio Member
 
Join Date: Apr 2006
Location: copenhagen
Hi

Thank you all for the replies and advice. First of all, to clearify, the rectifier will be AZ1 that draws 1.1A. I have tried out some different rectifiers, and i really prefer direct heated soundwise. It has of course some other drawbacks, but do they sound good:-)
Resistors is easy and cheap to apply, so that is a safe choice.
Regarding the ying/yang schottkys, that sounds intersting, but what would the benifit be besides less heat? And what about the B+ of 300+V running through them?

Best

Theo
  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
Location of voltage-dropping power resistor hollowman Power Supplies 21 5th December 2008 08:13 PM
Dropping filament volts/ L-W ramblings 5u4 Tubes / Valves 3 1st November 2006 09:15 PM
dropping heater voltage jarthel Tubes / Valves 17 21st September 2006 04:50 AM
Why is my MOSFET dropping so much voltage? arnoldc Power Supplies 19 21st April 2006 07:52 AM


New To Site? Need Help?

All times are GMT. The time now is 03:23 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