2A3 monoblock power supply

[tenderland]

2a3 mono block powersupply

Well IM ready to build but before I spend $500.00 for a pair of plitrin output xformers I have one more question. The desiagn attached is the stereo version I assume If i leave the choke and capacitors the same the b + will be higher in a mono design. Is this a concern?

Thanks Jeff

 

[arnoldc]

You can leave the caps and choke as they are but you need a lower HV secondary from your power transformer as you'll have only half the current draw now.

 

[SHiFTY]

It might be a little higher, so maybe a slightly lower voltage transformer would be a good idea. You can always use a 5U4 or 5Y3 rectifier instead, they have a higher voltage drop than a GZ37 or GZ34.

Otherwise, increase the cathode resistors on the output tubes a little, or get sovteks and run them HOT!

 

[arnoldc]

Otherwise, increase the cathode resistors on the output tubes a little, or get sovteks and run them HOT!

Here goes I was expecting somebody would say that

 

[tenderland]

Big Question

Well I gues the question is how do you know what voltage xformer to use , is there a way to estimate the correct value , and thank you for the replys

 

[arnoldc]

Hi tenderland,

For a single channel (mono) we want ~300V at 65mA. Hammond make a 275-0-275 transformer - 270EX (but you'll need a filament transformer for your 2A3, I used 166L2 on mine). Using that transformer, and 5U4-G rectifier (GZ37 is a bit hard to find), 10uF Solen -> 100mA Hammond choke -> 100uF Nichicon Muse, we will get the 300V we want.

You can download PSUD from Duncan Amplification website to verify.

Cheers!

 

[tenderland]

wow

Thanks Arnoldc , very helpfull , was looking at the Plitron powerxformers and couldnt even come close I do like the plitron chookes though

plitron chokes

I will download PSUD and check it out , thanks again , big help

 

[tenderland]

PSU II design question

I am trying to model the attached scheme only in a mono configuration. I am using PSU and for some reason it seems the output voltage is not correct.

power xformer = Hammond 270 ex 275 volt ct @ 125 ma
Im not sure what to use for source resistance i tried 2.2k ohm

im using a Full wave bridge using a 5u4-g

L1 = 10 H @ 50 ohms

C1 = 1000uf wkz Black gate ESR = 1/10 ohms

R1 i used 5 k ohm

I attached the origainal print "stereo version"

Help

 

[arnoldc]

Hi Jeff,

Do not use a resistive load. Change it to constant current and put in 65mA (the 2A3 in the diagram already draws around 62mA).

Also when modelling for tube rectifiers, make sure that you don't exceed the maximum value for capacior input design. THe limit is on the tube rectifier manual.

Cheers!

ps.

What I suggested will include CLC

10uF -> 100H -> 100uF

 

[tenderland]

psu

Well looks like there is no CLC option for filter , The sorce resistence you can use the source calculator but need the primary and secondary dc resistence of the trany , I emailed Hammond 2.2k doesnt provide good results

Thanks
J

 

[arnoldc]

Insert a C before the LC

 

[tenderland]

oops

doh

 

[Tom Bavis]

For a transformer of that size, I'd estimate 200 Ohms.

 

[tenderland]

huray

Well I think I finaly have it modeled properly and it does level of to a ss voltage of about 300 v . the original was 350 V. Im assuming its not a problem to run them with less B+ but what is the effect of this if any ?


Thanks agian , much appreciated

j

 

[tenderland]

PLate curves

Well the stereo ver has 245 V on the plate of the 2a3 I assume the the mon as described will have about 300 , after looking at the plate curves this is a huge difference in plate current . Im just spooked out about using sucj a lower plate voltage , after doing a search seems that lower plate voltage increases distortion. ANy input ?


Thanks J

 

[arnoldc]

Why are we modelling a PSU for mono and you mentioned Stereo?

If you made a PSU for mono and use it for stereo then it's not obviously going to give you the right voltages. There will be twice the current draw.

What you wanted is 250Vp which is the voltage you measure on the plate of the 2A3, minus the voltage you measure on the cathode of the 2A3. Example, if you have 300V on the 2A3 plate, and 50V on the 2A3 cathode, that gives you 250Vp and that's perfect. If you have 50V on the cathode and are using 880 ohm cathode resistor, then your current is around 56mA.

 

[tenderland]

Vp

The Vp in the stereo design is 345 voltage on plate - 54b v cathode for 291 V
when i modeled psu I used only 65ma constant current source


I think i may be strugling with some basic principles here but ill get it Thanks j

 

[arnoldc]

yes, joseph esmilla mentioned that in his article. 291V will be too much for single-plate 2A3s but will not harm a Sovtek.

ps.

65mA is for a *single* channel.

 

[tenderland]

Cool

Thanks again arnodc , I am a bit green but learning , im ready to start buying parts. Heres my first amp , the stereo ver

 

[arnoldc]

Nice

Just stick to this forum and you'll learn a lot, like I do.