Airgap for filament transformer ? - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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
Thread Tools Search this Thread
Old 27th April 2009, 03:12 PM   #1
diyAudio Member
Join Date: Oct 2007
Default Airgap for filament transformer ?

i have a SE amp with D3A interstage to 2A3. I use a separate filament transformer for the rectifier. 5V - 2A . First i have used 5R4GY, now GZ32 for slow start.
In use after 30 minutes, the filament transformer warms up a lot, near to hot. It is just the iron core. The coil is not hot . Dimensions of the EI-core: 60mm high x 55mm wide x 35mm deep.
The amp has 292V DC and 112mA DC current at the rectifier. I think the 112mA DC saturates the core and makes it hot. So i want to order a new filament transformer with a airgap like a SE output transformer. In this case airgapped for 115mA.
Is this a good idea, or am i wrong ? Thank you

  Reply With Quote
Old 27th April 2009, 05:52 PM   #2
diyAudio Member
HollowState's Avatar
Join Date: Aug 2006
Location: Taxland, New Jersey
Hi Johann,

Filament and/or power transformers do not use or require an air gap. If your transformer is running very warm or what seems too hot, it may be a normal temperature rise. It may also be that the unit in question is inexpensively made. Or it may just be under rated. I don't think you can even buy an air-gapped power transformer. Make sure you are using one of the correct rating for your power application as well as your mains frequency.
"The further a society drifts from the truth, the more it will hate those who speak it" ~George Orwell
  Reply With Quote
Old 27th April 2009, 06:01 PM   #3
diyAudio Member
Ty_Bower's Avatar
Join Date: Mar 2007
Location: Newark, DE
If you look at the data sheets for smoothing chokes from Triad Magnetics, they indicate they are "power supply filter chokes having a core with an air gap which prevents saturation at maximum current."
  Reply With Quote
Old 27th April 2009, 06:25 PM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Gone on holiday again. back soon.
are you rectifying and smoothing the transformer output?

The maximum draw from a 2A transformer is 1Adc, if you use a capacitor input filter.
At this value the transformer will run at it's maximum temperature.
It is usually recommended that continuous DC current be kept below 50% of the maximum.
That equates to <250mAdc.

What is your total filament load from this transformer?
regards Andrew T.
Sent from my desktop computer using a keyboard
  Reply With Quote
Old 27th April 2009, 06:55 PM   #5
tomchr is offline tomchr  Canada
diyAudio Member
tomchr's Avatar
Join Date: Feb 2009
Location: Calgary
I see you are in Germany, thus, would have 50 Hz mains frequency. Could it be that you are using a cheap transformer made for 60 Hz operation?

The cheap 60 Hz transformers have less iron in them as B field doesn't get as strong as for 50 Hz (16.6 ms period vs 20 ms for 50 Hz). The B field is the integral of the input voltage (i.e. the area under the sine wave curve). Thus, lower frequency --> higher B field = closer to transformer saturation.

You can use a 50 Hz transformer on 60 Hz mains but not the other way around. Many of the higher quality "60 Hz" transformers are really made for 50 Hz operation. That way they can be sold internationally.

As others say - you definitely don't want a power transformer with an air gap.

  Reply With Quote
Old 27th April 2009, 07:51 PM   #6
diyAudio Member
leadbelly's Avatar
Join Date: Dec 2002
Location: Calgary, Alberta
What everybody already said, plus a true NOS labeled GZ32 draws 2.3A, not 2.0A.

EDIT: Oops, that's not quite true, it depends on mfg. I think a Mullard is 2.3A.
The whole problem with the world is that fools and fanatics are always so certain of themselves, and wiser people so full of doubts. Bertrand Russell
  Reply With Quote
Old 28th April 2009, 02:54 AM   #7
diyAudio Member
Join Date: Oct 2007
I try to decribe it once again:
I use a separate EI-core power transformer for the filament of the rectifier. It is not cheap or underrated. It is oversized and from AE Europe in Holland.
I have tested the transformer with a 2.5 Ohm 30Watt wirewound resister. The transformer remained total cool after a hour test.
My Ultron GZ32 has the same cold DCR like my Brimar 5R4GY for filament. Philips datasheet for GZ32: filament 5V - 2A
The power transformer for the rectifier filament has a center tap. That have i used with the direct heated 5R4GY for the B+ voltage. The indirect heated GZ32 cathode is connected to one side of the filament. So for the GZ32 i have used that side of filament for B+ voltage.
The heat problem is the same with both rectifiers.
With the direct heated 5R4GY the filament is the cathode. When i take the 292VDC B+ from the center tap of the rectifier filament transformer, the 115mA current runs through the whole filament winding. I think 115mA current makes a lot stress for the iron core. And this saturation might be the cause for the hot iron core.
As i said i my first post, in use the iron core gets hot, but the coil remains cool.
Most people have the filament winding and high volt winding on the same big transformer. That big core might not saturate so much.
Sorry if i make it too complicate, i try to describe it with my school english.
My idea: the B+ DC voltage and current (from rectifier cathode) that runs through the filament winding, might saturate the core, if there is no airgap. Or am i wrong ? Thank you.

  Reply With Quote
Old 28th April 2009, 05:06 AM   #8
diyAudio Member
HollowState's Avatar
Join Date: Aug 2006
Location: Taxland, New Jersey
I understand exactly what you are describing. The transformer stays cool with a resistive load drawing 2 amps. But the core gets hot when used to power the rectifier filament and produce B+ voltage.

If you take the B+ voltage from the end of the winding (pin 5 on the GZ32) and not the CT, there should be no B+ current flowing through the secondary winding. It's potential will be raised to 292 volts, but that will be all that happens. Insulation rating is your only concern. The only closed loop circuit on the secondary is the tube filament.

Now, if the core gets hot when this condition is met, something is wrong with the transformer. (or your wiring) Unwanted DC flowing through the secondary could saturate the core. But in a properly wired and working circuit, that should not be happening. Do you know where the transformer was made? Was it made in China? It may have an unwanted internal leakage path.
"The further a society drifts from the truth, the more it will hate those who speak it" ~George Orwell
  Reply With Quote
Old 28th April 2009, 05:16 AM   #9
diyAudio Moderator Emeritus
ray_moth's Avatar
Join Date: Jan 2004
Location: Jakarta

Firstly, there is nothing wrong with your English.

Secondly, in the case of the indirectly heated rectifier, the rectified direct current will not run through the filament winding because you are taking it from the correct cathode pin.

Thirdly, in the case of the directly heated rectifier, although there will be DC flowing through the filament winding it will flow in opposing directions, from each end of the winding to the center tap. This will effectively cancel the DC magnetic field.

So, in either case, there will be no net DC flowing through the filament winding. You will not find power transformers with gapped cores, other than the tiny gap incorporated into Lundahl's C-cored power transformers and maybe 1 or 2 other examples, but that's to guard against DC in the incoming mains (to the primary).

For whatever reason, it would appear that the load on the secondary of your filament transformer is causing its core to overheat. This suggests that the transformer is underrated for the job you want it to do. Maybe that should not be the case, especially considering that you tested it with a 2.5 ohm resistor, but it does seem that way.
  Reply With Quote
Old 28th April 2009, 11:58 PM   #10
diyAudio Member
Join Date: Oct 2007

Thank you for the answers.

  Reply With Quote


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
Filament Transformer dejanm Tubes / Valves 11 26th December 2013 03:21 AM
Filament Transformer for 26 DHT korneluk Tubes / Valves 7 8th January 2008 07:25 PM
WTB: 6.3V Filament Transformer mrdon Swap Meet 1 20th March 2006 03:36 AM
Filament Transformer cantskienuf Tubes / Valves 1 24th June 2005 02:41 AM
filament transformer vs plate transformer contaxchen Tubes / Valves 10 14th January 2005 11:43 AM

New To Site? Need Help?

All times are GMT. The time now is 02:52 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.
Copyright ©1999-2016 diyAudio

Content Relevant URLs by vBSEO 3.3.2