heater voltage: why ac, and not dc? - 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 3rd October 2005, 01:53 PM   #1
alex278 is offline alex278  Netherlands
diyAudio Member
 
Join Date: Sep 2005
Default heater voltage: why ac, and not dc?

I wonder why in most circuits the 6.3V heater supply is fed with AC. The additional component count for a DC heater isn't that much, and it would save a lot of problems with hum. The only problem might be that if you go all the way for a regulated power supply, you end up with an odd voltage like 8.88V DC. But you could also work with an unregulated, but heavily filtered heater power supply; you can't get more than sqrt(2)*6.3V from rectifier.

Plus, when you're at it, you could also make a 'slow start' heater, which doesn't kick in directly with full voltage, which would prolong the lifetime of your tubes.

Correct?
  Reply With Quote
Old 3rd October 2005, 02:15 PM   #2
diyAudio Member
 
Ouroboros's Avatar
 
Join Date: Dec 2003
Location: Nottingham UK
You do sometimes get dc heaters used for DHTs, but the problem is that the dc heater voltage adds or subtracts from the grid bias, so you will get more emission from one side of the heater than the other.

I've not tried dc heaters on indirectly heated valves because if the heater wiring is twisted, and the feed centre-tapped, I've never found hum to be too much of a problem.
  Reply With Quote
Old 3rd October 2005, 02:23 PM   #3
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
You are right, a DC heater will eliminate hum from the filament circuit. The filament circuit is not the only source of hum in an audio amplifier. In fact for most power amplifiers it is pretty far down the list of hum sources, so many designers just take the simple approach and use AC filaments.

DC heaters are an advantage in preamplifiers where the signals are much smaller compared to the hum voltages. In a quiet phono stage DC heaters are almost mandantory.

The other area of current controversy is the use of DC heater voltage on directly heated tubes. Here there is no doubt that DC filaments are quieter, but there are disadvantages. When the filament is powered by DC there is a voltage gradient along the filament wire. In other words, one end of the filament is at a higher DC potential than the other. This means that the grid to filament (cathode) bias is higher at one end of the filament than at the other end. Therefore one end of the filament will conduct a greater share of the plate current than the other end. Operating the filament on AC will average the emission out over the length of the filament wire. Some claim that DC operation causes shorter tube life, and degraded sound.

From a theoretical standpoint it would seem that this effect would be worse on tubes that run on relatively high filament voltages, and have a high Mu, requiring a low bias voltage. In this case the filament gradient is a large percentage of the bias so that one end of the filament will be responsible for almost none of the plate current. The 811A or the 211 operating at low plate voltages (bias voltage near zero) comes to mind. Tubes like the 2A3 have a 2.5 volt filament and need -50 volts of bias, so this effect really doesn't matter.


I find that DC filaments are an advantage, and use them in most of my designs. There are some tubes, like the 833A, that just require it. The filament current is 10 amps, the magnetic field created by the 10 amp wiring gets into everything. You can hear hum in the speakers as soon as the filament transformer is energized, without plate or bias voltage!

There are a few designers that claim that tubes will last longer, and sound better when fed by a constant current source, rather than a constant voltage. I haven't been able to verify this, however a constant current source will by its very nature, be a slow turn on circuit, so tube life should increase.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 3rd October 2005, 04:48 PM   #4
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
The definition of RMS is that it is the AC that gives the same heating power as the same voltage of DC. Thus, a 6.3V heater wants 6.3V AC RMS or 6.3V DC. The "designers" who slap a bridge rectifier followed by a 47,000uF capacitor onto a 6.3V winding and then feed the result to an EF86 would do well to note this fact.
__________________
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
  Reply With Quote
Old 3rd October 2005, 05:20 PM   #5
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
I also find it's harder to get rid of the 120Hz noise plus transients than just 60 hum. I just bias the heater string about 30V about the cathodes. Proper lead dress really helps also!

If you want to run DC, then R-C-R-C can bleed voltage and tame the spikes. You can then bias this up in voltage. DHT's were really designed for AC heaters as noted above.

-Chris
  Reply With Quote
Old 3rd October 2005, 07:47 PM   #6
diyAudio Member
 
aletheian's Avatar
 
Join Date: Apr 2004
Location: Philly
I may be insane, but I don't like the way DC heaters sound. i put a little switching circuit in an amp to test the difference, and there was one... they didn't sound as "juicy" to use a technical term ;-)
I have had better luck with noise running an elevated reference voltage, like in the above post anyway, except in one really cramped layout where I did the ghetto, unregulated, 6.3v to bridge to large cap to .1r wirewound route, which gave me about 6.2v in that particular circuit.
  Reply With Quote
Old 4th October 2005, 05:42 PM   #7
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
Just an observation. In my line stage ECC88 preamp I am using AC heaters and I have absolutely no hum from it. I am also using all AC in my 807 parafeed amp which uses an ECC88 followed by a ECC81 into the 807 with a KT88 CCS load, again I am getting a tiny amount of hum. I have very sensitive tannoy speakers.
Then again I have a 5687 preamp which needed DC to tame the hum.
My default position is AC first and only use DC if absolutely essential. As said earlier the heater supply is low on the list of possable hum sources.

Shoog
  Reply With Quote
Old 6th October 2005, 12:44 AM   #8
diyAudio Member
 
Join Date: May 2005
Location: Pretoria, South Africa
There you already have a totally interesting collection of thougts!

I would agree with Tubelab.com (post #3) that by and large filaments are not the greatest source of hum (transformer radiation is often worse!), at least not in my experience over dozens of tube amplifiers. You must not wire stupidly, of course. Often an arrangement where you earth through the slider of a small pot (few 100-ohm) will give you the best adjustment by trial-and-error - it is not always exactly in the centre.

I must stress that in my experience raw rectification-and-smoothing of the available 6V can give more problems than it solves. The resultant capacitor charging peaks are often sharper than sine wave sides, and in the end generate more noise than you would have had with AC. Unless you can smooth properly, don't go this route.

For pre-amps where one can serie the 12V 150 mA heaters it is a different matter - if you have the odd 50V available. If it is not blasphemy to you to use a semiconductor regulator (are you looking for results or appearance? - diodes are also semi-conductors) then some of the LM.....s available will yield excellent heater DC. I have used this for very low inputs, but the necessary voltage must of coarse be available.
  Reply With Quote
Old 6th October 2005, 04:09 AM   #9
diyAudio Member
 
Robski666's Avatar
 
Join Date: Sep 2005
Hi All,

Not sure but I think it's all been covered, I guess 6.3V AC become the "standard" and when valves ruled no manufacture was going to try to rectrify the heater supply and feed the filaments with DC not practical.

I like the idea of constant current for the heavier filament current and 10amps is only a small load for some of the high power transmitter tubes where inrush current can be a 100+ amps and causes stress on the tube structures, this is usually protected by filament supply impedence but is a cause of failure of these tubes.

Just a point, filament voltage needs to be fairly accurate for optimum tube life and function, so if you want to run AC filaments from DC for low level stages regulate it! and as the other posts indicate magnetic induction would likely be your bigest problem noise source in the higher filament current tube circuits. for higher current filaments use balancing pots accross the filaments to ground.

One final point DC filaments were usually folded not twisted (the few examples I can remember) I guess in an attempt to equalize the cathode emission as indicated.
  Reply With Quote
Old 6th October 2005, 04:46 AM   #10
wa2ise is offline wa2ise  United States
diyAudio Member
 
wa2ise's Avatar
 
Join Date: Apr 2005
Location: NJ
Quote:
Originally posted by tubelab.com


The other area of current controversy is the use of DC heater voltage on directly heated tubes. Here there is no doubt that DC filaments are quieter, but there are disadvantages. When the filament is powered by DC there is a voltage gradient along the filament wire. In other words, one end of the filament is at a higher DC potential than the other. This means that the grid to filament (cathode) bias is higher at one end of the filament than at the other end. Therefore one end of the filament will conduct a greater share of the plate current than the other end. Operating the filament on AC will average the emission out over the length of the filament wire. Some claim that DC operation causes shorter tube life, and degraded sound.

Some DHT tubes were designed to expect DC of a specific polarity on the filament. That the grid was designed to accomidate the differing biases along the length of the filament. Tubes like 1U4, 1U5, 3S4 and such small portable battery radio tubes were made like this. Don't know if any power tubes you'd use in an audio amp feature this, though.
  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
DC Heater Voltage Bengali Tubes / Valves 9 2nd February 2009 04:19 AM
heater voltage Jaap Tubes / Valves 3 18th November 2008 11:39 PM
Too low heater voltage? jkeny Tubes / Valves 6 26th August 2008 12:08 AM
Need Help Setting Heater Voltage cgrums Tubes / Valves 10 25th January 2008 05:40 PM
DHT heater voltage zobsky Tubes / Valves 23 9th September 2006 03:55 PM


New To Site? Need Help?

All times are GMT. The time now is 05:13 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