Using low voltage lamp in power supply
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EddieRich
diyAudio Member

Join Date: Jun 2011
Location: North of Boston
Using low voltage lamp in power supply

I'm designing a two channel tube amp using 60FX5's. I am using an isolation transformer and when rectified it gives me 161 VDC. I want to run the tubes with 140V on the tube and a cathode bias of 3V for a total of 143VDC. So, I need to drop 18V. The Q point of the circuit will draw 36mA plus 10mA from the screen per channel, for a total of 92mA.

If my math is correct, a 24V 3W bulb will drop 17.6V at 92mA.

So my question is, can I use a 24V 3W lamp to drop the voltage?
I know it's not a plain old resistor, so does this effect the circuit in any way?
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 20th October 2011, 01:31 AM #2 kevinkr   diyAudio Moderator     Join Date: Sep 2004 Location: Boston, Massachusetts It should be interesting as I expect that the filament's negative temperature coefficient might result result in a slightly more constant output voltage as a function of line and load variation. (Over a small range) It can't hurt to try it and if it doesn't work as well as expected you can just replace it with a resistor down the road.. The big question is how it deals with the capacitor inrush... __________________ "To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
 20th October 2011, 01:38 AM #3 Pano   diyAudio Moderator     Join Date: Oct 2004 Location: SW Florida The non linear filament might work as a compressor. Go for it Eddie and file a full report!
 20th October 2011, 01:42 AM #4 EddieRich   diyAudio Member     Join Date: Jun 2011 Location: North of Boston Good point, hadn't thought of in-rush. I'll give it a try and if I have problems I can always add a resistor between the bridge and first cap, and maybe use a smaller voltage bulb.
HollowState
diyAudio Member

Join Date: Aug 2006
Location: Taxland, New Jersey
Quote:
 Originally Posted by kevinkr It should be interesting as I expect that the filament's negative temperature coefficient might result in a slightly more constant output voltage as a function of line and load variation.
Actually modern metallic lamp filaments have a positive temperature coefficient. It's the old carbon filament predecessors that had a negative coefficient, but they're ancient history. IMO using a temperature sensitive variable resistor as a series element in the power supply is a bad idea. Regulation will suffer rather then be improved. If it was good, why is it that it's never commercially done?
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 20th October 2011, 03:22 AM #6 mah   diyAudio Member   Join Date: Sep 2008 Another solution: (If you are not familiar with safe electrical practise do not attempt this). Add a 115/12.6V, 150mA, isolating transformer in 'buck' connection. 1. Connect the 115V winding in parallel with the existing transformer 115V winding. 2. Connect one end of the existing transformer secondary winding to one end of the added transformer's 12.6V secondary winding. 3. Measure the voltage across the, now combined, secondary winding. If you measure about 102.4V you can connect the combined secondary winding to your circuit. If you measure 127.6V just swap the 12.6V secondary leads around and you should get around 102.4V. 4. Connect in the PS and measure DC out. Last edited by mah; 20th October 2011 at 03:43 AM.
kevinkr
diyAudio Moderator

Join Date: Sep 2004
Location: Boston, Massachusetts
Quote:
 Originally Posted by HollowState Actually modern metallic lamp filaments have a positive temperature coefficient. It's the old carbon filament predecessors that had a negative coefficient, but they're ancient history. IMO using a temperature sensitive variable resistor as a series element in the power supply is a bad idea. Regulation will suffer rather then be improved. If it was good, why is it that it's never commercially done?
DUH!!! You're absolutely right.. Have no idea what I was thinking of, well it was just before dinner.. Yeah, I'll go with that.. And I wasn't thinking of carbon filament lamps at that particular moment in time..

Might work great in a guitar amp!
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 20th October 2011, 01:14 PM #8 DF96   diyAudio Member   Join Date: May 2007 Having a PSU with a negative output impedance at subsonic frequencies might not work with some amps! Correction: ignore this post completely, I was thinking of NTC Last edited by DF96; 20th October 2011 at 01:16 PM. Reason: oops! I got it wrong too
EddieRich
diyAudio Member

Join Date: Jun 2011
Location: North of Boston
Quote:
 Originally Posted by mah Another solution: (If you are not familiar with safe electrical practise do not attempt this). Add a 115/12.6V, 150mA, isolating transformer in 'buck' connection.
A very good suggestion. If I could have found an iso-xfrmer with an additional 12.6V secondary I might have gone that route. I am familiar with safe electrical practice, doesn't mean I always follow it though . I have worked with low power lasers as a hobby and high voltage xray scanners in my career.

I'm building a small iPod dock, and was actually considering not using the iso transformer. This is one of those rare instances when I decided to be safe. When I bought my house 11 years ago, I ripped out all the old wiring and re-wired the house myself, so I am confident that my outlets are wired correctly.

Cost and space are an issue for this ( my first ) tube project, and I'm trying to use as few components as possible, especially transformers.

Last edited by EddieRich; 20th October 2011 at 03:01 PM.

 20th October 2011, 02:59 PM #10 Frank Berry   diyAudio Member     Join Date: Sep 2003 Location: Midland, Michigan You may be able to put a 24 volt zener in series with the output of the supply. That will give you a 24 volt drop. __________________ Frank

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