Nostalgia Air: Those pesky negative-filtered power supplies

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Those pesky negative-filtered power supplies From Colorado Radio Collector's "The Flash!!"

George Gonzales

12/96

     Sure enough, just last night I found another instance of that problem in a Philco 40-1900 chassis. The problem? Bugs in tube-type radios with power supplies where the B- does not go to circuit ground.
     So it's perhaps a good time to review this kind of power supply circuit and its potential for problems.
     If you've been trained on repairing post-1955 electronics, you probably haven't been exposed to this kind of power supply. Somewhere around that time, power supplies on the better equipment started having separate B- bias supplies. Look at any good scope made since 1955 and you'll see a separate B- supply, usually regulated.
     Power supplies on consumer items generally got their bias by floating the cathodes up a bit with resistors. In either case, the main B+ supply has its B- tied firmly to circuit (and usually chassis) ground.
     This is a comforting feature -- you can tie your test equipment to chassis ground and have a safe, secure, reliable reference point.
     But if you look at radios made before then, there is a subtle but crucial difference -- known by many names, such as "negative-lead filtering" "depressed ground", and probably many other names.
     In this scheme, the B- is not tied to circuit ground -- there are one or more resistors and/or speaker field coil between B- and chassis.
     This design does several things at once: The resistor or field coil takes up most of the 120CPS ripple left by the first filter capacitor. it also drops 10 to 40 volts, providing a source of negative bias voltage for the audio output tubes (and sometimes other stages).
     The advantage is they get double-duty out of one resistor or field coil. If the resistor or coil was in the B+ path, it would have the same filtering effect, but would not provide a negative bias voltage. Negative-lead filtering saves the manufacturer the cost of a power resistor and also saves a watt or two of power.
     The downside is that the circuit is a bit trickier to work with and restore, especially for people trained on newer equipment that usually doesn't have this arrangement.
     There are plenty of pitfalls that the unwary repair person can fall into. I've fallen into at least two of these myself, and have seen the rest in old radios that I've worked on.

Pitfall #1 - Grounding the first capacitor can      If you replace the first electrolytic capacitor, you have to ensure that the replacement cap does not end up with a grounded can. The older caps had a separate (black) minus lead which is usually not connected to the can. When the minus lead is connected to the can, there are some fiber washers used to insulate the can from the chassis. Our natural tendency is to assume the capacitor can may be bolted right to the chassis, which is wrong, wrong, wrong, wrong, if negative-lead filtering is supposed to be going on.
     As an example of this problem, I have this Philco 40-1900 chassis, where an electrolytic was changed long ago. The repair person bolted in a nice. old-style one-hole screw-mount capacitor. But they forgot to put in a fiber washer on the top side of the chassis. As a result the B- lead was grounded, there was no negative bias, and the audio output tubes ran very hot and the radio had a weak and fuzzy sound.
     Luckily the field coil had enough resistance (and endurance) to limit the current so the power transformer and output tubes didn't burn up. The radio has been running this way for at least 30 years it seems, but this is an exceptional history. Many designs will not run at all or will quickly burn up the field coil, output tubes, or power transformer.

Pitfall #2 - Replacing both caps with a dual-section cap      We're tempted to replace two electrolytics with one dual-section cap. But every dual-section cap I've ever seen has both negative leads hooked together, in anticipation of being used in a more modern power supply. This again is wrong, wrong, wrong in a negative-lead filtering power supply circuit. If you unthinkingly put in a dual-section cap without thinking too much, you'll end up tying both negative leads of the old caps to the common negative lead. This forms a dead short across the old resistor or field coil.
     I found this fault on another Philco radio. It turns out there was enough residual magnetism, even with a shorted-out field coil, so the radio would play, but weakly. So watch out for this. You can't effectively use a multiple-section, common negative lead capacitor with negative-lead filtering.
     In most Philco's, there were two separate capacitor cans, so I just mount two multi-section capacitors, with cases insulated from ground, and just use one section of each capacitor. If there is room under the chassis, you could also leave the old capacitors in place and mount small lead-mounted capacitors under the chassis.

Pitfall #3 - Goofy biasing and filtering "fixes"      On some radios, the B- had too much ripple on it to be directly used for bias. So there sometimes is another stage of RC filtering between the B- supply and the tube grids. If someone has messed-up the B- supply, they often had to mess up the RC filter stage to try to "fix" the problem. Look for extra electrolytic caps in the circuit that you don't see on the schematic. These are often attempts to fix the bias and hum problems caused by the first problem.
     An example of this was a small Crosley, where I saw the usual pitfall #1, followed by an extra electrolytic capacitor added from B- to ground, in an attempt to kill the extra hum generated by problem #1.
     It's not unusual to see all kinds of "fixes" in the bias circuit to compensate for the basic problem. They may have added a cathode resistor to the output tube or tubes to keep them from glowing too much. Or they changed some of the grid voltage divider resistors in an attempt to get the bias voltages nearer to reality.
     There's one good clue for this problem: If you fix the first B- problem and then the radio plays worse, keep looking, there's probably some other fix to undo. The only up-side is that this part of the repair is cheap and easy -- just removing a few extra resistors and caps that were cobbled on to get the radio to play.

Pitfall #4 - Hidden damage      If the radio didn't completely burn up due to the lack of bias, there may be some parts that have suffered a lot of stress. Check the output tubes for emission and gas. Check the field coil for a burnt smell or diminished resistance (a sign of shorts). Check the power transformer for a burnt smell or dripping wax. Weak or bad tubes can be replaced. Overheated field coils and power transformers may continue to work well for decades, or may fail at any time. Since these are difficult and expensive to replace, use your best judgment on these.
     Now. enjoy your radio as it was intended to play. - George.