B- voltage?

RE: B- voltage?, posted on May 19, 2016 at 08:40:15
Tre' Industry Professional Posts: 17296 Location: So. Cal. Joined: February 9, 2002	I assume you mean the C- voltage for fixed bias. With 365vdc plate to cathode and a plate load of 3500 ohms you want 70ma. of idle current. A look at the plate curves shows 70ma. at 365vdc is achieved with about -75 volts at the grid. You want this voltage to be adjustable. Tre' Have Fun and Enjoy the Music "Still Working the Problem"

RE: B- voltage?, posted on May 19, 2016 at 11:18:01
Anthony.R.Guarnera@outlook.com Audiophile Posts: 18 Location: So Cal. Joined: March 16, 2016	Thank you for that info. it was helpful. But that is not what I meant. The power supply has a Positive rail, and a negative rail. With my positive supply set to 365 volts, my negative supply is -280 volts. On another forum some say that is O.K. with 300B tubes. And some say to lower it to -150 volts. I can't seem to get an answer one way or the other. Thank you all for your help.

RE: B- voltage?, posted on May 19, 2016 at 13:03:46

Tre'
Industry Professional

Posts: 17296
Location: So. Cal.
Joined: February 9, 2002

Can you use just the positive rail for the plate voltage of the 300b?

365vdc is plenty of voltage for a 300b.

You could use the negative rail for the bias voltage for the 300b through a voltage divider.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

RE: B- voltage?, posted on May 21, 2016 at 23:15:47

Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005

The only way you can do what you are saying is to use center tapped power trans like a 350-0-350 and then use one of the 350 taps for the pos rail and the 350 tap for a negative rail.You would use the appropriate dropping resistor(s) to get the minus rail down to where you want it..I just used the 350-0-350 as an example..You would probably need a lower voltage by the time you rectify and filter it.
Why do you need a negative rail in the power supply for a single ended 300b amp? The bias you can derive from almost anywhere such as a small torroid or tapping it off another part of the DC supply with some isolation.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

RE: B- voltage?, posted on May 22, 2016 at 12:10:13

hifipaul
Audiophile

Posts: 735
Location: NY
Joined: December 22, 2008

When building a C- supply for a SE amp, Use a separate xfmr of about 65vac. Bridge rectify it using soft recovery diodes (schottky diodes are good too). Using a corcom before the bridge is, also, a good idea. All rectification noise will be heard, thus C- needs to be as clean as possible. After the rectifier bridge a C-L-C-R-C-pot-R-gnd filter will work well. The caps need to be bypassed with film caps to shunt noise to gnd. You will need to calculate the R values on your own.

RE: B- voltage?, posted on May 22, 2016 at 20:51:26

Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005

When building a C- supply for a SE amp, Use a separate xfmr of about 65vac. Bridge rectify it using soft recovery diodes (schottky diodes are good too)"

That's an excellent suggestion for a PP amp as well.When the HK Citation 2 amp loses its bias winding in the power trans,we just put in a small torroid and series connect the windings and then bridge rectify it with soft recovery diodes and it makes a nice difference,so much that I do this to all CIT 2s whether the bias tap is open or not.

"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

RE: B- voltage?, posted on June 9, 2016 at 09:55:07

PakProtector
Audiophile

Posts: 12363
Joined: May 14, 2002

So, given you'll need around -75, you want the supply capable of say, -85V...a doubler with ~30VAC and a few hundred mA should suffice. might as well put a wee inductor in there as you build a Pi filter. 50V caps on the first stage, and 100V for the second. Nichicon Muse or Elna Silmic ii should be quite adequate...:)
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

SET Asylum

B- voltage?

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