[radiola]

I like this 1940 wooden set, however, when I received it, I noticed that the speaker and field coil were removed. The original coil wad 1700 ohms, which I replaced with a 2k resistor, and am trying to go with a pm speaker. I get reception, but it is very low. I wonder if by upping it by 300 ohms, I overdid it, taking away the set's power. Do you think the 2k, 10w resistor is too much? Couldn't find a 1700 ohm resistor. Any assistance will be appreciated!!!

[TA Forbes]

A 10W 2Kohm resistor should be adequate.

The field coil is part of the filtering system on this set, along with caps 48 and 49. Using a resistor will replace the field coil for resistance, but will not have any filtering qualities.

[radiola]

Are you saying that replacing the field coil with a resistor will not be sufficient to run the set, that there will be no filtering?

[codefox1]

No, just that there would be need to increase the filter capacitor value after the first resistor to minimize hum. Long ago chokes were cheaper than filter capacitors, and permanent magnets did not exist. What we want is a smooth DC curent for the set. So the smoothing capacitors must be a little larger, first one not more than 20% of original, second one not more than 30% of original nominal value. OK and preferred to use much higher voltage values. Observe polarity!

[TA Forbes]

Good advice, Codefox!!

[jerryhawthorne]

All good ideas above but mostly to address hum. If you have low volume, I would start looking at other things. Have you replaced the old caps and checked the resistor values? Also have you checked the tubes? After that is done, if the sound level is low, may be time to check the IF alignment. Also, do you have a good antenna hook up? So many things that can affect volume level and I doubt it has anything to do with your changing out of the field coil with a resistor. Check voltages on the tubes. Open antenna coil? Good luck.

Jerry

One other question after thinking about it a bit. Do you have the original output transformer on the radio?

[Arran]

On a smaller AC set like this you can use the Pi filter trick with a wire wound resistor, however I would not recommend it for a higher tube count set with 8 or more tubes. The reason being is that in a larger tube count radio the B+ current draw will be higher so if you use a resistor the filter caps will discharge more resulting in a lower B+ voltage.
Regards
Arran

[jerryhawthorne]

Radiola, are you out there? Hope you have not given up! I really doubt through all the comments above that your problem is in your conversion to a PM speaker, assuming that the speaker is about the same impedance of the original. Even off would not give very low volume out. Check your voltages on the B+ line at the before and after your added resistor. Should be an easy check and report back. We need to get the volume level up. Give a try to touch a metal object to the center tap of the volume control with the volume on high. You should get a nice loud hum. If you have adequate HV this will sound loud. Then time to do a little more testing. I'm not the brightest on this but quit chasing your tail on the modification at this time. Get good gain and loud stations and if you can hear hum, then time to perhaps up a couple of filter caps.

Regards, Jerry

[Arran]

The problem with using a pi filter as opposed to a LC filter is that the resistor behaves the same way to AC as it does to DC current, also the higher the current load on the B+ the greater the voltage drop. With a filter choke the choke will pass DC current easily but resists AC current or any voltage fluctuations. For example a 10 henry choke with a DC resistance of 150 ohms blocks acts the same way as a 3700 resistor does to AC ripple, so it will filter better then a 500 ohm resistor but not cut the DC voltage anymore then a 150 ohm resistor under load. In something like a five tube AC/DC set it doesn't make as much difference but in something like a 12 tube console, with a push pull power output stage, it definitely will and should not be considered as a permanent repair.
As to why your volume may be low I would check the plate resistor on the 1st audio tube, these can go open or increase greatly in value cutting down on, or cutting off, the plate voltage going to that tube. It could also be as something as simple as needing an IF alignment, I can't tell you how many sets I have run into where someone fiddled with the IF trimmers and messed it up.
Regards
Arran

[jerryhawthorne]

Nice explanation of the differences of resistance vs. inductance on a power supply Arran. Clears up a few things for me. Still waiting to hear from the OP and find out what his voltages are and if his amp is putting out some nice hum off the volume control. Being a dummy, I like to start from the basics but like to learn more.
Regards, Jerry

PS: This is a 6 tube radio

[TA Forbes]

Excellent response!!

[jerryhawthorne]

Well, perhaps he just gave up. I hope not.
Jerry

[TA Forbes]

I meant the one's you and Arran gave...

[jerryhawthorne]

Arron's was great about understanding the difference between inductive and resistive in a Pi network. I learned a lot. Codefox hit the nail on the head for those of us than are not quite that sharp in electronics. Like me.
I would just like to hear from to OP and see where he is at. I still don't believe that his problem is changing the speaker out for a PM. I have done it when needed and all works well with a little increase in the caps. Hate to not get a response from the original poster. Guess I'm just impatient.
Jerry

[codefox1]

Hey, I'm not smart. Most of what I know I learned from grouchy old farts decades ago, an now that I am a grouchy old fart it's coming back to me. Can't remember what I had for lunch today, or whether or not I had lunch. Old time radio is kind of like Latin, not widely spoken as I know but widely used just because it has not been updated in centuries.

"Get a 5K sandy, neat it up in corner and tack on an extra 10 on each side right side up. and glue in this here 4 inch, and make it snappy!" C'mon I was like 15, and he was Radio God in the little shop where I went for a couple hours after school a couple of days a week for a year or two. Always got slipped a $10 with warning not to tell the boss. Well turns out he was the boss, and the other old fart was the employee. Lights out at just about all the little shops by the early to mid 60's, distributors a couple of years later, surplus stores a few years from then, and just about all gone. Just like old time cowboys.