Adding VR tube to FP II ???

[kuramura]

Hi
I've posted the same question on AA forum but fut few folks suggested that I might get more help here.
I have FP II with anticipation upgrade and many other modifications that is combined with Paramours in the same chassis and this combo sounds dazzling through my lcd-2 headphones. I have diy fostex fe 207 based speakers but I use headphones for 95% of time.  I have been wondering if I could gain some sonic benefit from building new power supply for B+ 157vdc Foreplay section using VR tube. I've purchased 140vAC 100mA transformer that I plan to use for this task.
I'm not sure what tube I should use and how exactly to use it, OC3 OA2 etc. Any help would be appreciated. 

 

[Paul Joppa]

The 0A2 and the 0D3 are both 150v gas regulator tubes, 7-pin mini and octal respectively.

You must provide the tube with a minimum 5mA more than the preamp circuit consumes (maximum 30mA or 40mA depending on the tube). I'm sorry I don't have the Anticipation manual at hand, but IIRC the preamp needs less than 10mA. You can do this with a high voltage supply and a dropping resistor, or with a current source. The original power transformer can't supply enough current, which is why this mod never appeared.

Since you have a 140v transformer, a fullwave bridge might give you 175v after the filter section. That leaves 25 volts to drop, but if the power line voltage changes 10% you'll have only 7.5v, or 42.5v. That's too much variation to get by with a simple resistor. So there are two practical answers:

1) Use a voltage doubler to get around 350v. Then drop 200v in a resistor - let's say 10K ohms which will give 20mA, half to the regulator tube and half to the preamp circuit. This is the circuit used in Foreplay III. The resistor will need to be rated at least 10 watts and will get hot!

2) Use a fullwave bridge to get 175v, and use a current source to supply 20mA to the regulator and preamp. This is how we do it in the Extended upgrade, except it's a hybrid shunt regulator instead of a gas reg tube (and it's dual mono).

Hope that helps.  :^)

 

[xcortes]

I FINALLY get it! The current through the dropping resistor is defined by the voltage difference and the resistor resistance. You just make sure the current is within the right parameters for the regulator. Right?

Thanks!

 

[Paul Joppa]

haha - bingo, Xavier!

 

[kuramura]

Thank you Paul for great explanation, I will try to attach a pic of schematic that I made up, would you be kind enough and tell me if it would work and made some corrections if necessary? I'm also not sure if both channels are connected to the output of the regulator or is there a way to separate it in order to create "dual mono" or something that would allow for better channel  separation? Also, I have a box of asc 60uf 330vac pp film capacitors that's why I used those in schematic, however,  I think I read somewhere that right after diodes caps used should be rated for higher voltage?
Thank you, 

 

[Paul Joppa]

The circuit looks good to me.

See if you can find a DC voltage rating for the caps you are using. You might find a manufacturer's data sheet online, for example. Caps are funny about their ratings, and you can't always deduce the safe DC from an AC rating, or vice-versa. The first (doubler) caps see half the voltage and are probably just fine.

 

[Grainger49]

DO throw a couple of resistors across the first two caps in series.  Parallel a resistor, about 1/2 M ohm (Bottlehead uses 479k), across each cap.  This evens out the voltage and acts as a bleeder when you turn it off.  That is much safer.

 

[Paul Joppa]

Actually, the first two caps are the voltage doubler, so they are not subject to voltage unbalance - but a bleeder is always recommended.

 

[kuramura]

Thank you Paul and Grainger49, I will do as advised. I have parts ordered and will be assembling prototype by weekend.