Philco 38-4 questions

[darklife]

Just checked the speaker, now I know that is not the issue!
There is definitely feedback getting into the amplifier section from the RF section.
With volume all the way down, and even the dial disengaged (audio muted) I notice when turning the dial that I can hear RF feedback getting into the audio that sounds like hissing/squeeling. Changing the tone control effects this also. The problem seems to come and go at will.
Not sure how RF is getting back into the 6K5 tube but it must be. Tube shields are on by the way.
All capacitors check out fine so maybe it is just odd wire placement or something similar?
Stranger yet this only happens when the set is fed full line voltage. When I lower the voltage through a variac down to 100-110v the problem seems to goes away.

Edit: I tried a different 6A8G osc/det. tube that I had laying around and it seems to have cleared up the problem mentioned above. Not sure exactly why that is though.
Going to give it a few days of play and if no new problems pop up I am going to call it a done job since all seems to be working well and is playing nicely.
Thanks again everyone.

[Reece]

If you have 125VAC line voltage, wouldn't be a bad idea to install a bucking transformer. Radio Shack used to sell, may still sell, a filament transformer for 12.6 and 6.3 volts via a center tap. I'd hook it up for the full 12 volts drop on that set.

[darklife]

Originally Posted by dieseljeep

If you look up the information about bakelite block capacitors, some also have resistors inside, as well.

Yeah this Philco has none in the bakelite blocks.

I have been running the radio hard for a few hours now with good volume and notice there is still distortion when cranked up on a local station yet it sounds fine on distant stations at same volume.
I swear there is some feedback happening somewhere under the chassis. I would hate to give it back like this but the damn thing works perfect except at the most random time. All caps check out, resistors and so on.
Only thing I can figure is the higher AC line voltage used today makes the osc/mixer tube feedback more than originally expected.

I don't know at this point. The radio is playing so great but on strong stations I swear there is distortion on high volume that is not there on distant stations at same volume level with volume control compensated for weak stations.

Also switching the two 6F6 tubes around provides different volume/distortion levels even though both tubes are good. I tried a few different tubes but I am getting fustrated because the circuits itself does not seem consistant more so than the tubes themselves.

Sorry I am rambling.
The radio sounds perfect at regular volume but on strong stations and loud volume I hear distortion. I am thinking it should provide the volume or am I pushing the radio itself beyond the limits of audio amplification?

I got to keep in mind that the speaker will probably sound louder in the cabinet also. So maybe I am hearing distortion on loud volume because I am hearing it at higher volume than meant for w/o the cabinet to reinforce the amplification.

[Electronic M]

Could be an AVC issue. AVC or automatic volume control is meant to retard the amplification factor of the RF and IF stages on strong stations so that the volume of stronger stations better matches that of weak ones. It could be insufficiently retarding amplification on stronger stations resulting in signal overloading. If it bugs you then check the values of the resistors in the AVC feedback system (usually gets it's input from an extra detector diode that is not part of the audio signal chain), and those in the RF and IF system if any are outside of tolerance then it could affect AVC opperation....However it is good to know when to quit.

[Reece]

It will definitely sound better with the speaker in the cabinet as that will allow bass tones to come out. +1 on the possible AVC problem: local stations overloading somewhat.

[darklife]

Yeah thanks a ton guys. I am connecting the radio to a damn good outdoor 120' wire antenna plus a good set of radials so that alone may be overloading the AVC.
I guess at this point I will just send the radio back to its home and mount it back in its cabinet. Like you guys suggested and I thought, it will bring out the bass and loudness.
Being on this good of an antenna is probably overkill and more than the original AVC was designed to handle at the time.
With a 30' wire in the house it plays fine, it is just on the real long wire it distorts. All parts check out so that is not the problem.
This is the first antique radio of this age I have worked on so thanks for baring with me.
I am 29 and only have this much " " experience on tube electronics. I have built a few HF RF and AF tube amps but that is the extent of my knowledge.
This has been really fun to work on but I think it is time to give it back and let the owners enjoy it. Had her playing all night with Coast to Coast AM on multiple channels (thanks to syndication lol).
I have my limits, and this pushed it. I have a good part collection but this hit the replacement part limit, especially parts rated 400v DC+. I am damn lucky I had some old spare tubes from an old Philco I got as a kid that was junked.
Going to try and poke around the chassis with some bypass caps and see if that cleans up any residual RF bleed. After a few hours of play there is still RF getting back into the AF, though at almost unaudable levels, it is enough to cause distortion at fuller than loud audio.

[darklife]

One last thing.. I can not believe the fidelity had by this old radio. This seriously outperforms all the other SW/MW radios I have owned including modern units like my ratshack DX398 (ATS909) for sound reproduction and it somehow maintains good selectivity though the modern radio outperforms the antique for sensitivity.
I guess those big old coils w/o slugs really perform better for Q?

[DavGoodlin]

There are some bypass (one end to chassis ground) capacitors on the AVC bus. I beleive these may need to be checked to see if a different capacitance helps reduce feedback.

i've also seen where a capacitor across the audio output transformer needs its value changed to fully eliminate high-frequency feedback.

[darklife]

Well just found the final problem and now I am screwed.
The bad audio was coming from the transformer on the speaker. One of the 3 coil wires used for the push-pull tubes was bad so only one tube was driving the speaker. Oddly it did sound good for just running on one tube until turned up as described above.
So now I am stuck with a speaker that has no PP transformer
Does anyone have an idea where I could find a replacement transformer?
I feel so let down right now after having the radio partially working after all that work only to find the speaker transformer was partially bad.

[Reece]

Try Play Things of the Past. http://www.oldradioparts.com/index.html

[darklife]

Originally Posted by Reece

Try Play Things of the Past. http://www.oldradioparts.com/index.html

Thanks, looking at the site now.

I also had the idea to rewind the transformer but not sure how insane that would actually be. If only I could get the primary wire out w/o breaking it all up to see the full length.
Got the EI plates out and unwound the secondary but the primary is so small and fragile.
That may be a lost cause though.

Measurements show 3.2 ohm on the voice coil with the bucking coil. From a few minutes of research shows 10,000 ohms or so for the secondary P to P.
So That would be a 50:1 transformer that can handle 5 watts audio.
New is out unless it is cheap.

[maxhifi]

This is a pretty common spec, lacking more creative options bolt this to the side of the chassis and abandone the one on the speaker in place
http://www.tubesandmore.com/products/P-T125C

[rca2000]

You said it has P-P 6F6 amps. I would think ANY 6F6 P-P OPT would work, say a 15 watt power rating or so. With a 4 ohm or 3.2 ohm VC winding.

[rca2000]

Originally Posted by darklife

Thanks, looking at the site now.

I also had the idea to rewind the transformer but not sure how insane that would actually be. If only I could get the primary wire out w/o breaking it all up to see the full length.
Got the EI plates out and unwound the secondary but the primary is so small and fragile.
That may be a lost cause though.

Measurements show 3.2 ohm on the voice coil with the bucking coil. From a few minutes of research shows 10,000 ohms or so for the secondary P to P.
So That would be a 50:1 transformer that can handle 5 watts audio.
New is out unless it is cheap.

With P-P 6F6, running at maybe 300 volts on the plates----it will be MUCH higher power than 5 watts !! More like 12-15 watts. Like I said--a 15 watt tranny will work fine.

[rca2000]

This likely will work. 42 tubes are the same electrically as the 6F6 tube ::

http://www.ebay.com/itm/PHILCO-PUSH-...item4181414533