Solid state rectifier for SE guitar amp (Vox AC4) - diyAudio
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Old 21st May 2010, 10:44 PM   #1
ddr is offline ddr  United States
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Default Solid state rectifier for SE guitar amp (Vox AC4)

Hi Folks,
I would like to build an amp similar to the 1960's Vox AC4. I would like to use a solid state rectifier instead of a tube. I would also like to use two power supplies, one for 250VAC and one for 6.3VAC (I have several Triad N68X power supplies on hand which should give me 250VAC if I run 125VAC into the secondary and use the primary for B+).

Can someone check my plan? It looks correct in Duncan Amp's power supply designer, but I have never done something like this before:
Click the image to open in full size.

Here is the original showing B+ of 270V:
Click the image to open in full size.

Thanks for your help,
Dan
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Old 21st May 2010, 11:18 PM   #2
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The only caveat is that the inrush current may be a bit much for the rectifier. Check the resistance of the winding and, if necessary, add a resistor to protect the diodes.
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Old 21st May 2010, 11:27 PM   #3
kevinkr is offline kevinkr  United States
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I think the triad isolation transformer will work just fine as a step up doing as you propose. You also have the option of making a Graetz bridge which is a tube/ss hybrid rectifier if you want to.. (I probably would)

Most solid state rectifiers have pretty healthy inrush ratings so a 2 - 4A 800PIV bridge would work well by itself or as the negative half of a Graetz bridge with a 6CA4 or 6V4..

Note that a solid state bridge rectifier may produce 5 % - 10% more voltage than the 6V4 rectifier shown, you can fix this by adjusting the value of the 1K resistor shown in your schematic
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Last edited by kevinkr; 21st May 2010 at 11:31 PM.
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Old 22nd May 2010, 02:13 AM   #4
ddr is offline ddr  United States
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Thanks! I will research inrush current and Graetz bridges before I get started. I appreciate you both looking at this for me.

Take it easy,
Dan
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Old 25th May 2010, 06:55 PM   #5
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You should use diodes with a higher voltage rating. Usually diodes for bridge rectifiers should be rated for 1,25 x Upp. For 250 V that is 1,25 x 1,41 x 250 = 440 V. The 1N5404 in your schematic are rated 400 V. 1N5405 would be the correct choice, if you build the rectifier bridge from single diodes.
Of course you could also buy a complete bridge rectifier, which make things easier, reduces the possibility of mistakes and usually also comes cheaper, if you don't go the hybrid rectifier path.

Quote:
Originally Posted by ddr View Post
I will research inrush current and Graetz bridges
Looking for Graetz bridges will turn up what looks like BR1 in your schematic. You will have more success looking for hybrid rectifiers as a special form of the Graetz bridge.
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Old 3rd June 2010, 01:04 PM   #6
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I've used 1N4007 as the diodes or better in all my tubeamps - no problem .... BUT you might consider improving the hum in the circuit by adding a resistor in series with the bridge value between 1R5 and 47R, 5W to 10W. This way you're removing all the spikes generated from the rectifier and you get a circuit that behaves more like the tuberectifier + you can use bigger capacitors. Tube rectifiers generally have a max. capacitance of 40uF as "load" and that is a major reason why they aren't bigger in old amps + the price of those bigger back then. Increasing the capacitors tightens the sound too - not what everyone wants in a Guitar Tube Amp, but many do, so try.

Also important - If you use EL84 (A GREAT little output Pentode), they sound their best at a max no signal voltage of 340V between anode and cathode. I know that many new amps from otherwise good brands are using much higher voltage in order to get more output power - it works but the sound gets harsh.
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Old 4th June 2010, 01:09 PM   #7
ddr is offline ddr  United States
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Thanks folks, I am going to place an order next week for caps and a filament transformer and get this built! Thanks again for all of the great advice, I am learning quite a bit by reading the forum.

Have a good one,
Dan
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