Philco Model 20 Filter Condenser Block

11/11/2006 7:54:40 AMGary W. Prutchick

I am rebuilding the filter condenser block on a Philco Model 20.

The block contains 4 capacitors, 1uf, 1uf, 1.5uf and .13uf. I plan on replacing these caps as follows:
3.3 uf for the first 1uf
3.3 uf for the second 1uf
3.3 uf for the 1.5uf
and .2uf for the .13uf

The 3.3uf are 450V electrolytics with negative connected to terminal 1. The .2uf is a 600V cap.

Does anybody see a problem with this plan?

Gary

11/11/2006 10:13:42 AMDoug Criner

Gary, I would avoid jacking up the cap values so much above the original design. That could increase the wear and tear on the rectifier tube.

:I am rebuilding the filter condenser block on a Philco Model 20.
:
:The block contains 4 capacitors, 1uf, 1uf, 1.5uf and .13uf. I plan on replacing these caps as follows:
:3.3 uf for the first 1uf
:3.3 uf for the second 1uf
:3.3 uf for the 1.5uf
:and .2uf for the .13uf
:
:The 3.3uf are 450V electrolytics with negative connected to terminal 1. The .2uf is a 600V cap.
:
:Does anybody see a problem with this plan?
:
:Gary

11/11/2006 10:35:29 AMNorm Leal

Hi Gary

Agree with Gary, you don't want to go too high in value but 3.3 mf isn't very much capacity. The problem comes about when the first filter cap after the rectifier is very large. This will increase B+ voltage and surge current through the rectifier.

It was common to use 10 mf in later radios. Be sure to use 450 volt rated caps.

Remove but don't replace the .13 mf cap. This is part of a tuned circuit along with the choke that reduces hum. You don't need it with higher values filter caps. If the value isn't exact hum will be increased.

Here is a nice Philco 20 schematic:

http://www.philcoradio.com/tech/images/20b.jpg

Norm

:Gary, I would avoid jacking up the cap values so much above the original design. That could increase the wear and tear on the rectifier tube.
:
::I am rebuilding the filter condenser block on a Philco Model 20.
::
::The block contains 4 capacitors, 1uf, 1uf, 1.5uf and .13uf. I plan on replacing these caps as follows:
::3.3 uf for the first 1uf
::3.3 uf for the second 1uf
::3.3 uf for the 1.5uf
::and .2uf for the .13uf
::
::The 3.3uf are 450V electrolytics with negative connected to terminal 1. The .2uf is a 600V cap.
::
::Does anybody see a problem with this plan?
::
::Gary

11/11/2006 5:27:33 PMGary W. Prutchick

Thanks for the help,and the schematic.

Another question,

What is the reason for the center tap on filament winding A and C?

Gary

:I am rebuilding the filter condenser block on a Philco Model 20.
:
:The block contains 4 capacitors, 1uf, 1uf, 1.5uf and .13uf. I plan on replacing these caps as follows:
:3.3 uf for the first 1uf
:3.3 uf for the second 1uf
:3.3 uf for the 1.5uf
:and .2uf for the .13uf
:
:The 3.3uf are 450V electrolytics with negative connected to terminal 1. The .2uf is a 600V cap.
:
:Does anybody see a problem with this plan?
:
:Gary

11/11/2006 6:59:28 PMDoug Criner

Hi, Gary.

For some strange reason, I'm unable to pull up the schematic.

But, many heater windings have a grounded CT, often through a cap.

Here are my theories, subject to correction by our resident gurus:

If the heater winding weren't referenced to ground, it would be unconnected to anything besides the heaters themselves. The ckt could act like an antenna, and capacitively coupled to other ckts in the radio, with various voltages impressed on it. These could show up on the audio.

OK, why not just connect one leg of the heater winding to ground rather than a CT? This would double the max heater voltage swing to ground, and could increase hum. My theory.

:Thanks for the help,and the schematic.
:
:Another question,
:
:What is the reason for the center tap on filament winding A and C?
:
:Gary
:

11/11/2006 7:02:35 PMDoug Criner

Here's a suggestion from my Uncle Earl (Madman Muntz): lift the heater winding CT connection, and see what happens. It won't hurt your radio.

:Hi, Gary.
:
:For some strange reason, I'm unable to pull up the schematic.
:
:But, many heater windings have a grounded CT, often through a cap.
:
:Here are my theories, subject to correction by our resident gurus:
:
:If the heater winding weren't referenced to ground, it would be unconnected to anything besides the heaters themselves. The ckt could act like an antenna, and capacitively coupled to other ckts in the radio, with various voltages impressed on it. These could show up on the audio.
:
:OK, why not just connect one leg of the heater winding to ground rather than a CT? This would double the max heater voltage swing to ground, and could increase hum. My theory.
:

11/11/2006 11:36:55 PMNorm Leal

Hi Gary

Center tap on windings are there to reduce hum. In the case of "C" it powers 71A filament type tubes. Since they operate with AC on the filament center tap needs to be the reference. Sometimes resistors were used to make a center tap. Other times radios used a pot to balance hum.

Doug, have you tried to pull up the schematic again?

Norm

:Hi, Gary.
:
:For some strange reason, I'm unable to pull up the schematic.
:
:But, many heater windings have a grounded CT, often through a cap.
:
:Here are my theories, subject to correction by our resident gurus:
:
:If the heater winding weren't referenced to ground, it would be unconnected to anything besides the heaters themselves. The ckt could act like an antenna, and capacitively coupled to other ckts in the radio, with various voltages impressed on it. These could show up on the audio.
:
:OK, why not just connect one leg of the heater winding to ground rather than a CT? This would double the max heater voltage swing to ground, and could increase hum. My theory.
:
::Thanks for the help,and the schematic.
::
::Another question,
::
::What is the reason for the center tap on filament winding A and C?
::
::Gary
::
:
:
: