philly psu amp test

[mike hadley]

After rebuilding my Philly psu, I thought that it would be useful if I could test as a stand alone unit.

After checking of transformers, insulation wiring etc, the rectifier tubes were installed without the 6l6's, the 375v supply loaded, after the 30H choke or pin 3 on the speaker socket,

with three 240v 60watt bulbs in series and a 220 ohm resistor on the -28v rail the unit was powered up.

At 115v ac input there was around 370 volts across the bulbs and 30v on the 220 ohm resistor, the other supply through the 10H choke to the output tubes was close to 430v.

The ac supply was then disconnected, and the lamps removed.

The 6l6 tubes and speaker plugged in, or in my case my speaker adapter box.

I found that a load of 20 watts applied to the 250v dc supply and 220ohms on the -28v output, is pretty close to the power consumption of the tuner chassis, and the voltages from the supply were within a few volts as stated on the schematic.

Both of these loads are connected via the 7 pin radio supply socket,

To drive the audio a small mains transformer can be connected across the audio input transformer winding ends,

I used a 230v to 9v in reverse, but any small transformer would do,

this keeps any audio equipment isolated from the 250v dc rail,

DO NOT CONNECT THE TRANSFORMER BETWEEN THE CENTER TAP AND ONE END OF THE WINDING,

only across the ends, otherwise this will apply the 250v supply to the transformer input winding and burn the transformer out.

Please see diagram

This setup will allow the 6l6's to be driven and give some audio, don't expect hifi as there are some awful input impedance miss matches, but the chassis will play,

here is my chassis, upside down with the bottom cover removed to allow voltage measurements to be taken

this test proved to be very satisfactory, knowing that the supply and amp works is a great help when restoring the radio.

Mike

[Scott Seickel]

Your amp/PS has a bottom cover?  I have never seen one on these. 

Good job and it looks like your are having fun lighting up bulbs.  :-)

[David C. Poland]

Yes - the last of the 1941 power supplies (those having Jones plugs) seem to have bottom covers. For both the Philharmonic and for the Phantom power supplies.  Those amp chassis have small telltale holes in the center of the end aprons  to secure the bottom plate to the amp.

[Scott Seickel]

Thanks David.  That is interesting.  I guess we will never know why Scott decided to put covers on these amps at the very end. 

Also interesting is is something that Mike pointed out to me.  The rectifiers on these amps have the heaters tied to opposite pins on each rectifier.  So to clarify, pin 1 and pin 8 of different rectifiers are connected together to one side of the 5v supply and the same for the opposite side of the supply.  Instead of just tying heater pins 1 together and pins 8 together and connecting each to one side of the supply.  Mike's amp is like this, and I examined my late amp and it is the same along with 3 early 5Z3 amps that I looked at.  It clearly appears that this was done intentionally.  The reasoning eludes me, and frankly I don't think there is a valid one. It is not going to help with hum reduction.  

Also, on my late model amp, the center tap of the 6.3v filament supply is no longer grounded and Mike's set is the same.  It seems to be good practice to ground the center tap for hum reduction.  I can't imagine why that grounded center tap was eliminated.  

[mike hadley]

Scott

I found this, it may help,

https://dalmura.com.au/static/Hum%20article.pdf

it seems that there is more to this than meets the eye,

Mike

[Scott Seickel]

Hey Mike.

I skimmed through that paper and it looks like a solid paper on proper amp design in regards to hum.  There are several good books out there that cover this topic thoroughly.  The author doesn't cover the issue of rectifier filaments but rather good procedure for  amplifying tubes.  As I see it, there is no reason to give the rectifiers any special treatment because, after all, their output is pulsing DC from each tube.  Hum doesn't get any worse than that.  All tubes post filtering that are in the signal path should have proper care taken to reduce hum.  I have no idea why Scott did this and I doubt there is any validity to it.