[Tubesforme]

I recently did a full restoration on a Philco model 20 chassis (Schematic). For the filter rebuild I used a 1uf @ 630V for the filter before the choke and a .15uf @ 630V for the final one off the choke. I used a 10uf @ 450V to replace the 1.5 audio filter, watching my polarity. The .13 across the choke, I replaced with a .1uf @ 630V. All of the bakelite blocks have been rebuilt with the proper value of NOS 630V capacitors. All of the resistors were way out of tolerance so they were replaced with NOS 1W carbon comp resistors. The interstage transformer had one half of the secondary open so it was replaced with a NOS Stancor A-53C. The radio came without a speaker so I purchased one of the same size, a Jensen D-16 with a 4K field coil and a NOS SP/PP output transformer strapped for the PP 71A's. All tubes have been tested in two different testers, one a standard emission type and the other a transconductance. The original 71A's and the 80 rectifier were replaced with good used tubes. The radio picks up all the stations with plenty of volume. PROBLEM:    When turned on the radio immediately starts to hum to where the volume has to be too loud for comfort. I have 120VAC input from the mains and the filaments are all well withing spec. With all tubes installed, The B+ off the output of the choke reads 295VDC. The B+ on the plates of the 71A's is 295VDC and their grid bias from either side of the filament reads -55V. The only way to kill the hum entirely is to pull the 227 AF amplifier. Since I have found another 227 that did lower the hum somewhat. I have checked and rechecked all of the wiring and had another person check it over and nothing is wired incorrectly. I have never had a radio stump me in my 52 years of repairing them. Any suggestions would be greatly appreciated. Incidentally I have changed out all of the tubes too and that didn't help.  Many Thanks

Another question is how do you keep the tunning capacitor from slightly slipping when tuning the stations in the middle of the dial or when the tuning capacitor is nearly full open? Do you take the tuning shaft out and stretch the spring to make it hold the dial tighter? The tuning dial has been thourghly cleaned and just a minimal amount of heavy grease applied to where the rotor shaft rides on the capacitor frame. This stopped the scratching noise when tuning stations.  Any ideas?

[klondike98]

Hi Tubes, Welcome to the Phorum!! I edited your post just to add the link to the schematic and then will let some of the more experienced folks offer their thoughts.

[Mondial]

The first problem is that your B+ voltage is way too high. If you used a 10 uF as the input filter cap, this would cause high B+ as the original filters were only 1 uF. The 71A's are only rated at 180 V max on the plate, so you are really pushing them way past their ratings.

This high B+ may not be causing your hum, but it should be addressed before you damage the tubes and possibly other parts.

The best cap to use for the input filter is a 1 uF film cap, which can take the high ripple current involved. An electrolytic will not last in this application. The power supply was designed for 1 uF and if you increase the capacitance, the DC output voltage will rise to excessive levels.

You can use a larger cap after the choke, as it will not affect the output so the 10 uf is ok here. The input cap is the one that is critical. Also the .13 uf cap should be as close as possible to the value, as it forms a tuned circuit with the choke to notch out the 120 Hz ripple. You can add a .03 uf across your existing .1 uf to get to .13 uF.

[Warren]

One of the problems could be using nos caps.. they go bad just with time not just if they are used. Might try using new caps. The nos caps wont last long anyway even if some are still good now being so old.

[Tubesforme]

Warren, All of the caps were just purchased from a very reputable supplier where I have always purchased my capacitors, since the demise of the local parts distributor. I never keep any quantity of capacitors on hand anymore. I only purchase them as I restore a radio. I agree that the B+ being too high will destroy a capacitor, but that isn't the case here. Thanks for the suggestion.

[Warren]

Yes, but if they are NOS caps then they are already old when you buy them and probably not reliable. You might check them to be sure. Your dealer could be reputable but NOS caps usually are not. It is usually recommended to not use NOS caps. Good Luck

[Mondial]

Tubes, you might try exchanging the 1.5 uF film cap you already have with the input 10 uf cap after the rectifier. Connect the 1.5 uF to the side of the choke connected to the 80 filament, the 10 uF to the other side of the choke as it is now, and the remaining 10 uF to the output of the field coil where you had the 1.5 uF. The last 10 uf would then be from the B+ side of the audio transformer primary to B-. This would give you additional filtering at the 27 audio amp stage, where you need it the most.

This should act to reduce the B+ voltage and bring it down close to the level it was originally. Then you can proceed to track down the hum problem.

[Radioroslyn]

I thought I was being smart and put a 10mf @450v ecaps in my 20 but it made the bleeder/bias resistor get really HOT. As Modial sez replace the 10mf s with mylars of the proper values as per the schematic. The ecaps cannot take the ripple current .Sooner or later they will short.
Terry

[Tubesforme]

Tubes, you might try exchanging the 1.5 uF film cap you already have with the input 10 uf cap after the rectifier. Connect the 1.5 uF to the side of the choke connected to the 80 filament, the 10 uF to the other side of the choke as it is now, and the remaining 10 uF to the output of the field coil where you had the 1.5 uF. The last 10 uf would then be from the B+ side of the audio transformer primary to B-. This would give you additional filtering at the 27 audio amp stage, where you need it the most.

This should act to reduce the B+ voltage and bring it down close to the level it was originally. Then you can proceed to track down the hum problem.

This sounds like the best solution to me and would quickly let me know if the problem of loading is being caused by the first 10uf capacitor and I agree, it would add needed filtering to the audio amp. Other people have told me that they have always used 4.7uf caps for the first two filters and never had a problem with a model 20 doing that. Another old timer told me that the audio bias always needs more filtering than originally supplied by Philco. Your solution would verify this. Thanks

[Arran]

 I don't have a model 20 (as yet) to play with so I'm not familiar with the circuitry, however the most common reason I have encountered for a set to have excessive hum, even though new filter caps were installed, is that someone made an error in connecting the negative ends of the filter capacitors. What throws people off is that most sets built from about 1939 onward use what was known as cathode bias, sometimes called self biasing, in the audio output stages, most sets built between the late 1920s and 1938 used fixed bias in the audio output stages by using a large wire wound resistor, and sometimes the speaker field coil as well, in series with the center tap on the power transformer. So one filter cap will be connected directly to the center tap of the H.V winding, then there will be a wire wound resistor in series with the tap, the the next filter cap will follow that resistor, sometimes there will be another bias resistor and another filter cap following that with it's negative lead connected to the chassis ground. It's a long shot but there could also be a heater to cathode short in that 227 tube you pulled, have you tried substituting another?
Regards
Arran

[morzh]

One has to tell the hum from the noise reception and home wiring fom AC hum.

Also, 20 same as 70 or 90 is susceptible to the bad aluminum can shields connection to chassis, where the shield is floating rather than shielding.

All my baby grands, as well as other tube radios have at least a little hum, most of it being AC modulated RF, due to home wiring. A pocket transistor radio has the same but then it allows to orient it away from this noise. So is my 46-250 that has a loop type antenna, it also can be oriented out of it.

Grounding may reduce noise also.

Another thing to look for is bad grounding due to oxidized riveting when riveted contacts are used as grounding points.

[Tubesforme]

One has to tell the hum from the noise reception and home wiring fom AC hum.

Also, 20 same as 70 or 90 is susceptible to the bad aluminum can shields connection to chassis, where the shield is floating rather than shielding.

All my baby grands, as well as other tube radios have at least a little hum, most of it being AC modulated RF, due to home wiring.  A pocket transistor radio has the same but then it allows to orient it away from this noise. So is my 46-250 that has a loop type antenna, it also can be oriented out of it.

Grounding may reduce noise also.

Another thing to look for is bad grounding due to oxidized riveting when riveted contacts are used as grounding points.

The model 20 that I have does not use the aluminum caps. It has a single large square cap the contains only the original paper capacitors, potted, not the later electrolytic version. I have replaced both of the 1uf's with NOS 1UF @ 630VDC and replaced the .13 with a standard .1uf @ 630VDC and the 1.5 uf which is used to filter the bias supply I replaced with a 10uf @ 450VDC. That has lowered the hum major. The voltages are still too high on the plates of the 71A's. Yes, I have tried replacing all of the tubes except for the 80 rectifier. I have been told by many people and know from experience that in a large city, like I live in, all I need is an antenna consisting of about 20' of hook up wire. Thanks

[Mondial]

Did you replace the two .25 uF paper caps marked 14 on the schematic? These are in the long thin can under the chassis and provide additional filtering for the plate and screen of the 24 detector tube. 

If either one of these caps is open especially the plate supply filter, it will cause major hum in the audio.

[morzh]

I did not say "caps", I said "cans".

[Radioroslyn]

Mike is talking about the aluminum cans that are pressed into the chassis the cover the RF coils. In most cases you can ground the ant post to help to hear hum from the p/s. This will cut down the RF noise to a low level. Things have changed A LOT since 1930 a whole heck of a lot of more noise!!!!
Terry