[JimSirt]

My first post on this forum, other than a wanted ad, and I’m looking forward to soon posting the entire restoration of my Philco 66 (its actually a Canadian Philco 366A but same chassis). I have just finished the electrical part of the restoration. I removed the rectifier tube for the initial power up using a variac and iso and all seemed to be good except that I observed the secondary HT voltage getting too high once I passed 104V line in. At 120V, the voltage measured across rectifier pins 2 & 3 was 782V (391V from CT to each end of the secondary winding). I was expecting somewhere around 680V. I plugged the rectifier in and slowly powered up to 104V. All was good, and the radio came to life. I applied a 450 kHz signal into the antenna terminal through a .1uF cap and the sound came through loud and clear (alignment obviously required). The perceived problem I’m having is that when I increase the line voltage to 120V, all the plate voltages are what I believe far too high. I only ran the radio at 120V long enough to capture a few measurements as I was concerned about damaging the tubes. The specified plate voltage for the 6A7 tube in this radio with a line-in of 115V is 260V but at 120V in (our common line voltage), the plate measures 303V, far exceeding the specified maximum plate voltage for that tube. I’ll attempt to attach my voltage measurement results and I hope someone can let me know if they’ve seen this before, is it normal and if not any suggestions. I’m thinking I will need to somehow limit the line voltage, perhaps a ballast tube or something similar but if I do, the filament voltages will be a bit low. I thought about a power resistor in the B+ line after the rectifier but I’m not sure that’s a good choice. Any thoughts would be most appreciated.

Jim

6A7 G2-K voltage is high even at line-in voltage of 104 but is likely a different issue, haven't checked that yet.

[klondike98]

Today's line voltages are higher than they generally were when these radios were produced.  There are many discussions about possible solutions and I'm not the guy to write about those as I generally just plug the set in and go with it.  I know there are transformer solutions where you step the voltage down and the ISOTAP WP-25A isolation transformer I use on my bench actually has the ability to do that.  Others will be able to speak to their specific solutions.

[Ron Ramirez]

Another thing to consider: When the Philco 66 was new, the voltage readings were taken with 1000 ohm/volt meters. These loaded down the circuit under test, causing the voltages to appear lower than they actually were. Today's digital multimeters generally have an input impedance around 2 or 3 megohms if not better, and will cause your voltage readings to appear higher than the factory Philco specs even if you adjust your input line voltage to 115 or 110 volts.

[JimSirt]

Thank you for the reply gentlemen. I should have mentioned that I did consider the potential impact of meter impedance so I tested with a standard VOM, a VTVM and a Fluke Scopemeter and all three measured within a couple volts, and yes all of these meters are nowhere near 1000 ohms/volt. I'm still torn as to whether I should let it run as-is or use a solution to throttle it back.

Jim