[rfeenstra]

I'm not sure how to calculate the best value for the inductor in a pi filter arrangement. Not sure that the .05 has a lot to do with it but I'm not fully knowledgeable in these things. I think it easiest to just measure it! I will do that for you.

[rfeenstra]

I tried to measure the choke. It has a DC resistance of 1850 ohms. My inductance meter goes up to 20 Henry but it would not measure it. So I assume it is over that.

[aumamps@gmail.com]

What voltages are you getting?
Looks like a 380RMS PT and a 3.6k OT

[EdHolland]

Could you put a decent sized capacitor across the choke and look at the resonant frequency that results from the LC circuit? If I've calculated correctl, for example, 1 uF should yield about 35 Hz, for a perfect choke of 20H.

Just a thought...

[aumamps@gmail.com]

My meter only goes up to 20H as well and I have no reading. I'll have to pull out the oscilloscope

[morzh]

I doubt you need to know the exact value of the inductor.
Suffices it to know, it is big and in 10s of H.

0.05uF cap is there to try to compensate for the ripple voltage.
The idea is, I think, that if there is a ripple via the inductance, and the current lags the voltage by 90 degrees, then that ripple could be nullified by parallelling a capacitor that would provide the same size ripple current which leads the same voltage by 90 degrees. The resulting phase shift between the AC component of the currents (ripple current via inductance and the current via the cap) is 180 degrees.
Of course the shape of the current through the inductor is not exactly the same as it is via the cap. So they will not totally compensate each other.

Picking the cap depends on the load. With the load current of a certain size the ripple voltage is Iripple*Rload.
You could watch this on a scope.
Then knowing the voltage you could keep inserting the caps until you see the optimal reduiction.

Or if you want to be a bit more scientific and calculating, measure both load current and the load voltage, calculate the impedance, and solve for the capacirtive impedance so the divider between the cap and the load gives you the same amplitude of the voltage as the ripple AC component given the AC ripple before the choke.
I would do the former as it is easy enough.

[rfeenstra]

I didn't check voltages while I had the bottom cover off. If you get a similar DC reading you probably can assume the choke is ok. Is that what you are trying to determine? My amp runs pretty much hum free. with the caps I put in it (about the same as original.)

[aumamps@gmail.com]

I was curious about plate voltages, not necessarily in regards to the choke.

[rfeenstra]

On which tubes? It'll be a while before I can take it apart again.

[captainclock1988]

I unfortunately do not have the service manual, or schematic for the Newcomb Pathfinder Amp, as that was in the original owners manual that came with that unit and that unfortunately went with the unit to the current owner of the unit when I sold it to him a couple of years ago.

I do think maybe I could look it up in Riders or Beitmans or perhaps it was covered in Sam's?

EDIT:

I found the Schematic I had from the Newcomb Amplifier I had, it was on another audio forum I used to frequent.

See the picture below.

   

[rfeenstra]

Thank you!

[aumamps@gmail.com]

That is really high voltage for 6L6

[Radioroslyn]

7027A's they have a little higher ratings than the 6L6G's.
Better pray the bias supply doesn't give up or you can
kiss the opt goodby.

[rfeenstra]

I measured some of the voltages on my amp. Output from the rectifier is just over 400 and that feeds the center tap of the OT. The screens are at 295. I couldn't measure the plates as the amp goes into convulsions when I put the meter on the plates! The self bias is as 22 volts.

[morzh]

Do you have a modern meter or an old type?