Output capacitor

[croccodillo]

Hi all,

I'm building, very very slowly, THIS tube amplifier.
I'm actually building the power supply (that will be a separate unit).
When the power supply will be ready I will begin to build the amplifier.
I'm collecting material, and I found a surplus stock of 2microfarad, 2000V paper in oil capacitors (VERY big) I would like to use as output capacitor.
So, my question is: how can I calculate the needed capacitance?
I was thinking to use 4 microfarad (as for the schematic): is it enough?
I'm not sure I will use any feedback, so forget it for the moment.

Thanks in advance,
Giovanni

 

[refference]

Hi ,

1 ) In my opinion, paper caps are not suitable for output coupling
due its high inductance
2) There is an error in the feedback connection ( +B is turning
back to the first cathode
3) The capacitor value is very dependent of the primary impedan-
ce and inductance
4 ) As a rule of thumb , you can use

C= 1.6 x 10e6 / f1 x Zp ( in uf )

If f1=20 Hz ( good enough ) and Zp= 5000 ohms
( hypothesis )
C= 16 uf you can use electrolytic caps

Regards

Carlos

 

[cerrem]

For good Phase response down to 20Hz you usually put your -3dB point around 3Hz to 6Hz range...
What is the transformer info??? Turns ratio, inductance, secondary termination load....ect..

Chris

 

[croccodillo]

Thanks for the answers.

Refference:
Yes, there is a mistake in my schematic, thanks a lot.
The feedback should be connected by a capacitor the the first cathode, not directy.

What about polypropilene motor capacitors? I have a lot of them.

I'm using toroidal transformers as OPT, I do not have too much characteristics: they have a niominal power of about 80VA, a turns ratio of about 25.5 (230/9V), and that's all.

Thanks again,
Giovanni

 

[Sheldon]

What about polypropilene motor capacitors? I have a lot of them.
Giovanni

Try em. If they are rated at about 330 - 400VAC, they should be ok (I assume they are motor run).


Sheldon

 

[refference]

Hi,

1 ) Polypropilene motor capacitors , are a better choice than paper
caps , you can try them , and as sheldon said , if they are
rated for 440 VAC or more , they are fine until 650 VDC ( +B )
2 ) With the turns ratio that you gave (230/9 VAC) , the Zp (
primary load ) will be 5200 ohms for a secondary load of 8
ohms , then C= 16 uf is OK !!
3 ) Don't forget the first cathode has an impedance of aprox.
47 K ohms ( before R6/R9 ), then you have to use a 2 uf cap ,
rated for +B x 1.5 Volts minimum . or try to connect the N.F.
to secondary winding ( you need to verify the phase of output
signal ( be careful !! )

Regards

Carlos

 

[jlsem]

1 ) In my opinion, paper caps are not suitable for output coupling

The caps are in series with the large primary inductance of the OPT so their tiny inductance is of no consequence. Experienced builders of parafeed power output amplifiers go to great lengths to avoid using electrolytic caps for good reason...

John

 

[rdf]

"The caps are in series with the large primary inductance of the OPT so their tiny inductance is of no consequence. "

Isn't inductance in any case more a function of physical construction than dielectric? Considering the linear length from solder to solder of, say, a high voltage Solen 47 uF axial polyprop vs. the close terminal spacing of the typical PIO radial of the same value it wouldn't suprise me to find the latter has a higher potential self-resonance frequency (i.e. lower inductance) in circuit.

 

[croccodillo]

Thanks to all for your answers.

I have a lot of 1 microfarad, 4 microfarads, 450Vac polypropilene motor run and flourescent lamps (used for PFC) capacitor...
I will use them for the input and the interconnection stage.
I just found some high voltage electrolytic capacitor (100 microfarad), used to start motors. I will try them as output capacitor too.

Ciao,
Giovanni