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 Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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diyAudio Member

Join Date: Jun 2003
Frank,

Quote:
 lso keep in mind that volpots really don't like DC on their wiper so either a switched attenuator or coupling cap are the safer solutions.
Where is the DC coming from in this case?

What would be appropriate in value for the input cap?
__________________
Dont take life too seriously, you will never get out alive!

diyAudio Senior Member

Join Date: Aug 2002
Location: Belgium
Hi,

Quote:
 Where is the DC coming from in this case?
The grid is biased negative and tubes can and do fail occasionally.

Into a 1M impedance 0.100µF is already more than big enough...
I think I used 0.047µF for my CF.

Here's the equation:

f = 1/(2*pi*R*C)

Where f is the lowest frequency you want to pass, R is the input impedance, 1Meg in this case. C is what you're trying to resolve.

Cheers,
__________________
Frank

 11th April 2004, 09:15 PM #13 diyAudio Member   Join Date: Jun 2003 Location: Reading MA Thanks Frank, I'll give it a try, as I have a few 0.1uF Hovlands in the parts bin. I plan to use a stepped attenuator in the final version. How did you calculate the input impedance in this case? Thanks for the lessons. Bryan __________________ Dont take life too seriously, you will never get out alive!
diyAudio Senior Member

Join Date: Aug 2002
Location: Belgium
Hi,

Quote:
 How did you calculate the input impedance in this case?
There's nothing to calculate...You picked a 1 Meg Ohm gridleak resistor and that's what the cap sees.

So here Rin = Rg

Cheers,
__________________
Frank

 12th April 2004, 03:45 PM #15 diyAudio Member   Join Date: Jun 2003 Location: Reading MA So, it the tube were to fail, would it be an instantaneous event, or would small amounts of DC appear periodically with time? I take it the cap is to protect the pot, but if I were using a stepped attenuator, this would then be unnecessary, correct? Thanks, Bryan __________________ Dont take life too seriously, you will never get out alive!
diyAudio Senior Member

Join Date: Aug 2002
Location: Belgium
Hi,

Quote:
 So, it the tube were to fail, would it be an instantaneous event, or would small amounts of DC appear periodically with time?
Most of the time it's a work or fail situation.
It doesn't normally happen but better safe than sorry.

Quote:
 I take it the cap is to protect the pot, but if I were using a stepped attenuator, this would then be unnecessary, correct?
Correct.

Cheers,
__________________
Frank

 12th April 2004, 04:38 PM #17 diyAudio Member     Join Date: Mar 2002 Location: Aveiro-Portugal Hi Bryan The 1 MOhms (Safety) grid leak, as in this circuit the efectif grid leak is the 100 K pot and the 1M is only for safety in case of bad contact in the pot. This 1 M Ohms grid leak is best connected in the left side of the 1K Ohms grid stopper...as in that case only one connection is done in the grid input pin (the 1k resistor). __________________ Jorge
 12th April 2004, 11:14 PM #18 diyAudio Member   Join Date: Jun 2003 Location: Reading MA Hey guys, I have a quick question...... With respect to the 12B4A, do I need 2 grid leak resistors, as the grid has two connections. Currently, I have 2 1K grid stopper resistors, one connected to each pin, and then joined together at one point for the input. I have one grid leak resistor, coming from only one of the grid connections. I hope this question makes sense, I will touch up the schematic to include this later tonight when I get home..... Thanks, Bryan __________________ Dont take life too seriously, you will never get out alive!
diyAudio Senior Member

Join Date: Aug 2002
Location: Belgium
Hi,

Quote:
 With respect to the 12B4A, do I need 2 grid leak resistors, as the grid has two connections.
Just one, the pot, is fine.

Quote:
 Currently, I have 2 1K grid stopper resistors, one connected to each pin, and then joined together at one point for the input.
That's fine.

Cheers,
__________________
Frank

 13th April 2004, 07:46 PM #20 diyAudio Member     Join Date: Jan 2004 Location: Budapest, Hungary, Europe Don't use more capacitors after rectifying ! 4 x 3700uF -> 1 x 100000uF (!) with corresponding coil protecting the rectifier diodes from too high current when you turn on the device performs extremely well (enormous reserve for ultra-fast and strong performance-peaks). Not a lucky idea using more caps, they're all different. The smallest capacitance will get the highest voltage, the biggest the lowest voltage. You can compensate this by parrallel-connected resistors, but it's still not the best you can get. (The capacitors will have the same Q where Q = C x U). So try to use one huge cap there, be sure of it's high quality.

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