[retiredinprescott]

Hope someone here can help.
I have a Philco 42-395 with a bad power transformer. I found a power transformer in my parts collection that would give me the needed 6.3 fil Volts as well as the center tapped HV windings for the 7Z4 rectifier. BUT, the HV should be about 300-0-300 and this big transformer puts out 385-0-385. It also had 5V windings which I taped off.
I installed the transformer and brought the set up on a Variac (set is fully recapped). It plays fine but after a while the 7Z4 shorts out and shows a cathode short or a plate short or both on my tube tester when I re examine it. ; usually the next time I turn the set on. I've gone through three 7Z4 rectifier tubes and thought the smart thing to do now is ask for help.
Is the HV from this transformer the likely culprit? Is there a way to lower the HV to around 300-0-300 without also lowering the filament voltage which is right where it should be?

How dumb was I to try to use this transformer in place of the original? It's the closest thing I have with enough beef to put out about 4-5amps filament for the 9 loctal tubes...

Thanks for any help or insight.

Here is the schematic.
http://www.nostalgiaair.org/PagesByModel...013537.pdf

Mark

[Radioroslyn]

Hi Mark,
I think your problem is that the 7Z4 is rated at 650 p to p vac. what you could do is remove the input cap in the power supply. This will change it to a choke input supply. This will lower the output voltage The 7Z4 is rated at 900vac choke input @120ma. Up the value of the other filters to reduce the hum. You could try adding a small value cap in place of the large input cap. Maybe like a .1mfd 630v. I'd try to keep it lower than 1mfd.
Have fun with it!!!
Terry

[retiredinprescott]

Terry, Please excuse my lack of electronics expertise but are you saying I would replace the input cap (20uF, 500V at present) with a choke? If so, could you tell me what choke to use and where I might be able to purchase it? Thanks,
Mark

[Radioroslyn]

OK Take #89 and disconnect the + end which going over to the 7Z4. After disconnecting the + side Reconnect it to the chassis. The choke is already in the set. Try it and it will probably have a lot of hum. So double the value of the second filter cap.If you still have a lot of hum try adding a small cap in place of #89 like a .1mfd 630v.
Terry

[retiredinprescott]

OK Terry, if I understand you correctly (I'm looking at the schematic), I take the + side of filter cap 89 off of where it is connected to the cathode of the 7Z4 and connected it to chassis ground? The negative side of filter cap 89 remains connected to the candohm resistor ( 88 ) and the center tap of the HV windings?

The other two electrolytic caps (8uF, 500V each) can be increased to try to reduce hum.
Then, if needed, I can add a 0.1uF, 630V cap from the HV center tap to the cathode of the 7Z4 (which is how the filter cap 89 was attached originally).

Is that about correct? What purpose is the filter cap 89 (20uF, 500V) serving once I connect the pos side to chassis ground?
Again, thanks for holding my hand here...
Mark

[Texasrocker]

To bring down plate voltages at the rectifier in the past, I have lifted the ac power trans secondary HV centertap to ground, and added a wirewound resistor of appropriate ohms and watts ratings in series out to chassis ground has worked for me before. Ohms law is your friend in this situation. If the B+ is running too high , adding a bleeder resistor of needed value and wattage required across the 1st filter capacitor to ground helps also. I didn't take a close look at the schematic, I left my reader glasses in my workshop tonight. Please excuse me if I made a mistake here.

[Radioroslyn]

Yes that is correct. The purpose of 89 is to filter the - bias voltage that goes up to the grid of the 6k6 tube.
Terry