Big B+ voltage drop using diode rectifier.
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vingenz
diyAudio Member

Join Date: Dec 2010
Big B+ voltage drop using diode rectifier.

Hello folks!
I've built an amp with 2x6l6gc P-P AB1 in the power section. There is problem that I can't solve: after full wave rectifier, B+ is 460VDC (372VAC on secondary) at idle, and 395VDC (350VAC on sec.) at full power.
Power transformer specs are: 250mA; 372-0-372VAC.
6l6 are biased (fixed bias) at 35mA each. Diodes are 1n5408 (1000v, 3A)
Well, i need more clean headroom and I can't understand why it's sagging too much . Suggestion? Tricks?
Thanks!
Attached Images
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 12th November 2012, 03:31 PM #2 DF96   diyAudio Member   Join Date: May 2007 Remember that transformer specs assume a resistive load. A rectifier/cap load will impose a peaky load so voltage drop will be greater. The figures you give don't seem quite right. A 372V AC secondary should give an off-load DC of around 520V, but you are getting about 10% less. It may be that the AC waveform has the peaks flattened so the ratio between peak and RMS is not the 1.414 for a pure sine wave. Take the 350VAC under load, knock off 10% then multiply by 1.4 to get 441V. If the DC current is 200mA then the ripple will be about 33V so we are down to 408V. It is a feature of power supplies that they usually droop more than expected.
vingenz
diyAudio Member

Join Date: Dec 2010
Quote:
 Originally Posted by DF96 It may be that the AC waveform has the peaks flattened

 12th November 2012, 05:13 PM #4 DF96   diyAudio Member   Join Date: May 2007 A high resistance joint in your supply wiring, or neighbours drawing heavy current into conventional PSUs can do this.
 12th November 2012, 06:04 PM #5 Yvesm   diyAudio Member     Join Date: Jul 2004 Location: Ardeche Could you measure the resistance of the primary and secondary windings of your transfo ? If it was designed for use with tube rectifier, secondary resistance if probably too high. Yves.
 12th November 2012, 07:23 PM #6 tomchr   diyAudio Member     Join Date: Feb 2009 Location: Calgary I'm guessing the sag is caused by the increased supply ripple. I suggest increasing the reservoir cap. A quick calculation estimates the ripple voltage to be about 40 V at max load; 5.8 V at idle. Increasing the reservoir cap to, say, 470 uF would give you 5.3 V ripple at full load... For a better understanding, I suggest running a sim in LTspice or PSUD II. From physics: C = Q/V; Q = i*t --> V = (i*t)/C; t = 1/(2*f) for a full-wave rectifier. --> V = i/(C*2f) C = capacitance in Farad; Q = charge in Coulomb; V = peak-peak ripple voltage in Volt; t = time in seconds; f = line frequency in Hertz; i = load current in Ampere. Above calculation assumes zero resistive losses, neglects the diode forward drop, and assumes a zero conduction angle. So take it as a worst case ripple estimate. For a more exact value, run a sim. ~Tom __________________ Modulus-86: 40W/8Ω @ 0.000061% THD. HP-1: 3W/20Ω, 450mW/300Ω @ 0.000032% THD. DG300B and Other Tube Circuits. Neurochrome : : Audio - www.neurochrome.com - Engineering : : Done : : Right Last edited by tomchr; 12th November 2012 at 07:27 PM.
 12th November 2012, 09:10 PM #7 Mickeystan   diyAudio Member   Join Date: Jan 2008 Location: Canby, Oregon I agree with Tom's suggestion of excessive sag due to too small a reservoir cap. Try Tom's suggestion and you will see a marked increase in the B+ Mickeystan
 13th November 2012, 12:27 AM #8 kevinkr   diyAudio Moderator     Join Date: Sep 2004 Location: Boston, Massachusetts Excessive winding DCR? __________________ "To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
 13th November 2012, 08:30 AM #9 vingenz   diyAudio Member   Join Date: Dec 2010 But if i use a bigger capacitor value, more current is need to recharge the cap [V = i/(C*2f)] and so more voltage drop. Sag is not on secondary winding (only few volts), but after the diodes/filter cap. Last edited by vingenz; 13th November 2012 at 08:33 AM.
bayermar
diyAudio Member

Join Date: Oct 2007
Location: Tuebingen
Quote:
 Originally Posted by vingenz Sag is not on secondary winding (only few volts), but after the diodes/filter cap.
That is exactly what you would expect if the reservoir cap is too small.
It's quite commonly used in guitar amps where some designers use that on purpose to create PS sag.

What is coming after the first cap? Choke or resistor?
I can't imagine this is directly connecting to the OPT center tap?!?
Try to sim it in PSUDII once you know the winding resistance of the PT (as tomchr suggested), it's an eye-opener.

Cheers,
Martin

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