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fizikohemicar 26th January 2013 01:17 PM

Help with PSU for guitar amp!
I'm going to make 5W guitar amp (AX84 Hi octane) ,but I need help with PSU...
I have transformer that have 270-0-270 secondary,and i wolud like to use EZ80 rectifier tube (transformer also haves 2x6.3V,one for rectifier tube,and other for all other tubes). I measured EZ80 output voltage without load and it was 250 volts. Can first C filter (47uF) change voltage in any way? Voltages I need are 250V,240V and 235V.
Can anyone help me with construstig PSU?

Cassiel 26th January 2013 01:55 PM

With that transformer I think you would get around 325 volts with a CRC filter. There's a little program to help you with PSU calculations:


Enzo 26th January 2013 02:28 PM

If you are measuring that without a filter cap, yes, it will go much higher when the cap is added.

Trout 26th January 2013 02:49 PM

That EZ80 does have a pretty high voltage drop if I am reading this spec sheet correctly.

2X275 transformer should output 285V DC?

fizikohemicar 26th January 2013 08:17 PM

I used PSUD2 and C1 voltage is ~360V... Ok,I will use some RC filters to get voltages I need...

Tesla88 29th January 2013 09:19 AM

I will trust in datasheet , with 270Vac you will get about 280Vdc , i won't care about power section , only bias the power tube correctly , you will get a little more power from the amp running at 280-300V instead of 250V ...
Just increase the resistors in the RC network to supply the preamp stages , if you want to sound the same as 250V .
I won't care at all , cause IMHO 12AX7 in preamp sound really well and warm with b+ at 300V , expecially for clean and crunch suounds...

JMFahey 29th January 2013 10:34 AM


with 270Vac you will get about 280Vdc
Sorry but no, you will get almost 380V if SS rectified (1N4007) , consider "losing" 30 to 60V tops (depending on current) using a tube rectifier (EZ80), I think your transformer is too high for what you are building.
RC networks can easily lower excess voltage to whatever is needed for a preamp, because of lower current consumption, but for a power amp it's more complex.

Tesla88 29th January 2013 11:05 AM

I was talking about tube rectifer , EZ80 , the datasheet is linked in Trout reply....for sure if you go SS you get about Vdc=Vac*1,414 ...

Despite transformer rated power is fine , i won't care for little extra voltage , the power section for AX84 is based on EL84 , so no matter if voltage is 250 or 300V , just adjust bias , if you stay under max plate dissipation all is fine.

For preamp section , 12ax7 run well at 250V as 300V ... IMHO 300V sound better for clean , a 250V is ok for metal / rock compressed sound as ADA MP1 where preamp runs at 200Vdc

If you go solid state , you have more than 350Vdc ... don't know if the output xformer can stand this voltages , also not all EL84 are happy at more than 350V , but hey you can go with tube rectifier! :D

JMFahey 29th January 2013 03:29 PM


I was talking about tube rectifer , EZ80 , the datasheet is linked in Trout reply....for sure if you go SS you get about Vdc=Vac*1,414 ...
Sorry, but *ALL* rectifiers charge to 1.4 x VAC , be they tube or SS !!!
All rectifiers have a certain voltage drop or loss, but it does not depend on rectified voltage.
It certainly does not take 40% of peak voltage !!!

fizikohemicar 29th January 2013 05:15 PM

I have two 12au7's,but i will try to find at least 1 12ax7 for first two gain stages... EZ80 have been used on old tube radio,with more tubes than i will use in this amp. So,PSUD2 was correct,and i get ~360V at output,and i allready calculaed resistor for voltage drops,and it will be 280V for EL84,270V for 3rd gain stage and cathode follower and 260V for first two gain stages... OT is also from the same radio,and it was working on first voltage after EZ80...

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