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16th March 2012, 12:43 PM  #1 
diyAudio Member
Join Date: Mar 2012

calculating PS resistor values
Hi all
I am designing my first PS to suit a valve amp. I have been searching the forums for information on calculating the resistor values I need to drop the voltage to suit the circuit I am building. http://www.ax84.com/static/p1/AX84_P1_101004.pdf I have a 250VAC 120mA Transformer. I have found a similar circuit that has a 250VAC 150mA Transformer. The output after the bridge rectifier is 320V. I need to drop this to 266v. Is there any good resources for working out the resistors that I need or for designing power supplies for valve circuits in general? Thanks in advance! 
16th March 2012, 01:51 PM  #2 
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16th March 2012, 02:05 PM  #3 
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Join Date: Jan 2010
Location: Titusville, Fl.

From the schematic that you posted you can calculate the current that is drawn by the circuit. You have your B+(266V) dropping to your B+1(261V) across R2 a 100 ohm resistor which gives you a current of (266261)/100 = 50ma. 5V drop 100 ohms. Now if you start at a B+ of 320V and you need to drop it to B+1 at 261V you would need to drop 59V across R2. To calculate the new value forR2 you divide 59V(the voltage drop you need across R2) by 0.05A (the 50ma current draw of the circuit) this gives you a value of 1180 ohms. If you use a 1.2K resistor (standard value) this will reduce your B+1 voltage to 260v. Close enough. The 1.2K resistor dropping 60V across it will dissipate 3W (E*I)( 60*.05). So you should use a 1.2K 5W resistor for the new value of R2.
If you wanted to get almost exactly 1180 ohms you could add a 68K 1/4W resistor in parallel to R2(1200 ohms) for a value of 1179.191 ohms. Last edited by RJM1; 16th March 2012 at 02:13 PM. 
16th March 2012, 02:18 PM  #4 
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Join Date: Mar 2012

of course! thanks a lot for that. that is a lot simpler than what I was currently calculating (looking at valve data sheets). so the voltage drop across the first resistor to get to the desired B+ voltage gives you the current, and then once you know that, calculating the resistance for the desired voltage is easy.
I'm having one of those forehand slapping moments right now... but then again, it's easy when you know what you're doing right? Thanks again for you help Regards Mike 
16th March 2012, 02:25 PM  #5 
diyAudio Member
Join Date: May 2007

What is the onload voltage? This is what you need to calculate from, not the offload voltage. There is not much difference between a 120mA and 150mA transformer, so you might need no change at all. The mains voltage will probably vary more than this.

16th March 2012, 02:49 PM  #6 
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Join Date: Mar 2012

yes you are right DF96, but if you look closer at the ax84 link I posted above you will notice the transformer is a hammond 269ex (1800180).
I am using another PS circuit as I have a transformer with a 0250v AC 120mA output and I want this to work with the ax84 P1 project  hence the reason to change resistor values. Thanks Mike 
16th March 2012, 03:11 PM  #7 
diyAudio Member
Join Date: May 2007

Sorry, you confused me by talking about a 250V 150mA circuit. A 250V secondary is simply wrong for a circuit designed for a 180V secondary. Either buy the right transformer or redesign the circuit. A big voltage drop through a resistor will change the circuit behaviour unless the current draw is independent of signal  unlikely.

16th March 2012, 03:31 PM  #8 
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Join Date: Mar 2012

good point  I do realise that would cause a lot of heat across the R2 resistor which isn't the most efficient way of going about things.
do you mean redesign the power supply circuit, or the whole circuit? I guess I can always stash the transformer and get another one if I really want to build the ax84 circuit. 
16th March 2012, 03:38 PM  #9 
diyAudio Member
Join Date: May 2007

Either. The simplest way to redo the PSU with your transformer would be to have a choke input arrangement. That would give you about 220V  a bit low so you might have to reduce a few resistor values. Alternatively change the amp to run on a higher voltage. In either case you end up with a different unit with different behaviour.
To replicate a circuit, you need to replicate the circuit! 
16th March 2012, 04:04 PM  #10 
diyAudio Member
Join Date: Mar 2012

true true. I should explain that I have 4 commercial valve amps (fenders/marshall/mesa/peavey) that are all fantastic amps, but with a young family are simply too loud to push to that sweet spot without threats of eviction. Some of my amps I cannot turn above 1 :
I was looking at the ax84 circuit simply as it is well documented and they seem to have a good forum. having said all this, I completely understand where you're coming from  if you want it to sound like something, you don't change central parts of the circuit and expect it to sound the same. To that end, this will be a work in progress, and I am looking to experiment a little with the preamp section once I get the initial build finished and have some fun building it. I was considering a 5f1 clone with 6v6 but I couldn't find a welldocumented circuit with a SS rectifier. I would like to build an amp with either 6v6 or el84 tubes. My OT is 5k impedance and these two tube types seem to be a good fit. I have been looking through this site as there seems to be a lot of people in my situation. If you know of anything that might fit the bill let me know. The power transformer and OT are meant to be for a 5f1 clone (would need to be a SS rectifier or nonCT valve rectifier) Thanks 
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