[Inkthinker2]

Hello! 

I am new to this forum.  I am remodeling a Philco 38-14 table top radio with improvements to eradicate the hum. Several tubes look decent but I do need to replace Types 6A7. 78 and 75. The other two 25Z5 and 43 (output) look fine. I've been online and cannot seem to find the Philco brand but since my 43 is an RCA, is it possible to use other brands? Perhaps someone has a chart that references other compatible tubes. 

Thanks! 
Meghan in Colorado

[morzh]

Any tube that says, say, 6A7 or 25Z5 or whatever is the same tube, Philco or not.

[Inkthinker2]

Some of the tubes were missing altogether. So 6A7 (tuner) and 78 ? with top connectors are easy enough but 75 is a misnomer. There might be new capacitors inside too so the extra overflow bulb might not be needed. I have the schematic and I will refurbish this with a new polarized plug so the chasis isn't a shock hazard. Fun project but I do need help from anyone who is willing to share tips.

[Radioroslyn]

Hi Meghan and welcome,

As Mike said as long as they have the proper numbers your OK. You could do a very cursory test by using an ohm meter by measuring the resistance between the to fat pins on the base of each tube. What that will tell you is if the heater inside the tube is good. The heater is the part that glows when the set is on. If the heater is bad then the tube is likewise. So you should see several ohms between the two pins.

GL with your project!

[Inkthinker2]

Thanks, Roslyn! I have found tube type 78 on Reverb but cannot locate type 75 yet. One site posted: Type 75 is the same as 6SQ7 and 6SQ7GT. So, I have a lot to learn about the sum of the tubes and the way certain tubes function. 

I have 25Z5 and 43 and will test them today. Next issue are the capacitors that appear to be updated in the chassis and what exactly it means to the tubes or the plug which I absolutely must upgrade for safety.

[morzh]

6SQ7 and 75 are NOT the same.

Also

>>but 75 is a misnomer. --- not sure understood that.
>>There might be new capacitors inside too so the extra overflow bulb might not be needed. - not sure understood that either about the "overflow bulb".

[RodB]

Try getting tubes at findatube.com. Bob has a good supply and service.

[Inkthinker2]

Thanks! I did find original Philco type 78 and 75 bulbs today and purchased them!  Just need to find the tuner bulb 6A7 and then I can move onto the electrical schematic. This is a good winter project. Looking forward to exploring the stations that I can pick up. Reminds me of the old victrola that I used to own in New Orleans.

[Inkthinker2]

Thanks! Assuming my 43 and 25Z5 are functioning, I've got all of the tubes. Now to compare the schematic and work on the electrical. There are definitely some newer components and the old plug wiring needs to be updated. It is soldered directly to a screw on the chassis. 

Here are some cautionary steps that I read on another forum: 

1. Do not power on your radio. You could cause damage if you power it on before replacing the capacitors. There are several flavors of capacitors but the ones that typically decay with age are the electrolytics (usually found in power supply and on audio output circuit) and the wax-paper coupling capacitors.
   
   
2. Find a service manual. Most are available online.
   
   
3. Replace all electrolytic and paper capacitors with new ones of similar value and same or better voltage rating.
   
   
4. Examine closely and replace anything that looks damaged, such as burned up resistors.
   
   
5. Test the radio using a dim bulb tester, which is a 60 to 100 watt lightbulb wired in series with your radio’s line voltage. If the bulb glows dim after a while then there are likely no shorts across the B+ lines within your radio. You might even hear a crackle or some signals coming through.
   
   
6. If it passed the dim bulb tests then try to power it up directly off the line. Most antique radios will work once the capacitors have been replaced.
   
   
7. If it does not work then signal trace through the circuit by injecting a signal at the IF or RF, replacing resistors or occasionally a tube where needed. Old resistors have a tendency to increase in value with age. Often a bad stage in a tube radio is due to a resistor that has increased in value to the point of biasing the tube into cut-off. Tubes are usually not what caused the radio to be put out of service. The quickest way to find bad resistors is to probe the tube pin voltages. There will be a tube pin voltage chart in your radio’s service manual given a specific volt meter impedance. Wherever you find a voltage out of tolerance (greater than 10%), then check the resistors connected to that pin or to other circuits connected to that pin.
   
   
8. If the stations are too weak then perform an alignment following the procedure in the service manual.
   
   
9. Turn on the radio. It should work!
   

[RodB]

Hi Meghan. That's pretty much the routine to follow. When you buy capacitors to replace the paper ones buy the mylar film caps at 630 volts. I get mine from Mouser. Watch the quantities for bulk savings. You can often buy 10 for the price of 5 or 6.

[Inkthinker2]

Just saw your enbedded comments, Rob. Not sure what the type 75 bulb does yet. It may go to the speaker. The "overflow" concept was one bulb in the string perhaps load balancing which means it can get pulled if you have different capacitors? Anyway, next step is to inventory my schematic and replace all the capacitors and resistors. I've not done a lot of soldering of axon capacitors but I have the tools and am willing to go slow to get this done right. My goal is to listen to some old music on New Year's Eve and then to get a ham license.

[rfeenstra]

Hi Meghan.  here is a rundown of  what each tube does.  The 25Z5 is a rectifier that converts the 120 volt AC input to DC voltage that the circuits can use.  The 6A7 is a dual purpose tube that is both a radio frequency amplifier and the superhetrodyne oscillator  to convert the incoming radio frequency to the intermediate frequency (IF frequency) of 470 khz.  The 78 tube amplifies the IF signal to a useable value.  The 75 tube then extracts the audio from the IF signal using the diode part of the tube.  That signal is then amplified in the second part of the tube (a triode).  The audio signal from that amplifier is fed to the 43 tube which amplifies the audio signal sufficiently to power the speaker through the output transformer.

Since all 5 tubes as well as the resistor marked 36 in the schematic are in series,  all must be in place.  If you pull any one tube, or one is burned out, the entire radio will be dead since none of the tubes will then light.  There is no "overflow concept" involved.

Good luck and have fun!

[morzh]

OK. I have to be a bit blunt here.

Megan, a piece of advice:
38-14 is not the best radio to start learning from. It is an AC/DC set. There is exact same radio in exact same case, 38-15 which is an AC only set that has a power transformer that makes it that much safer to work on. 38-14 has no transformer and so it puts you under the Mains potential when working on it.
So. I take it, you are not yet a very experienced electronics person, so: if you are serious about it, get at least an isolation transformer. Let alone the fact that if the radio works you will have to align it and for this you will be touching stuff on it.
Oh....and you will need some things to align it with, meaning you will have to buy some testing equipment. Of course there are shortcuts....but then they are shortcuts.

Best of luck.

[Paul Philco322]

Caution is the watchword for this hobby, even the experienced repair person can never cease vigilance regarding electricity.

Paul

[Inkthinker2]

Hello! I did put the project aside whilst collecting all of the bulbs (as a novice mistakenly does) whilst just now searching for the paper caps. I definitely do not want to get electrocuted, so I will have to investigate the isolation transformer in order to make it safe. If I cannot come around the corner of AC/DC radio electronic engineering 101 then I will revert to an AC only radio as suggested by Morzh.