I am in the process of building a suitcase (old 40's/50's case I bought) guitar amp and was originally going to gut a guitar amp for it and then use old knobs until I remembered my dad had a working old tube AM radio on his balcony wasting away.
It is a floor model cabinet with a 1941 Philco AR-40 car radio in it. The cabinet has a drawer that opens from the front and my dad said there used to be a phonograph in there. There is also a 2 lead wire and round connector directed into there. I told him what I wanted it for and he gladly donated it for the project. The cabinet is trashed from the weather so I don't feel bad about not restoring it. I opened it up and there are 2 transformers. It has six tubes, 7A7 7B8 7A7 7B6 7C5, and 7Y4. I've done enough digging to know I want a radio with a phonograph input and 2 transformers but am unsure on the these tubes and if I can swap them out. The controls are remote cable type with the AM dial in between. The spacing is that of a car radio mount so it's not too big. There is only the on/off volume switch and the tuner knob but the volume knob has a push pull function for voice or music.
My thought is (based on other sites) is I can swap out the power cable to provide a chassis ground, solder in a 1/4 jack and then mount it in my case. I have the original 12" speaker also but it needs to be re-coned. I'm unsure If I will mount the remote control head on the top of the case and leave the radio in tact (if I can figure out how to switch between radio and phono) or pull the on/off volume cable out and use it separately and disable the radio. I would like to come up with some sort of tube pre-amp to use and then I could use the other cable also.
I'll try to post a few pics if I cab figure it out.
Any thoughts or suggestions would be great.
https://www.facebook.com/media/set/?set=a.10152898134276359.1073741837.575601358&type=1&l=e1898a0f00
It is a floor model cabinet with a 1941 Philco AR-40 car radio in it. The cabinet has a drawer that opens from the front and my dad said there used to be a phonograph in there. There is also a 2 lead wire and round connector directed into there. I told him what I wanted it for and he gladly donated it for the project. The cabinet is trashed from the weather so I don't feel bad about not restoring it. I opened it up and there are 2 transformers. It has six tubes, 7A7 7B8 7A7 7B6 7C5, and 7Y4. I've done enough digging to know I want a radio with a phonograph input and 2 transformers but am unsure on the these tubes and if I can swap them out. The controls are remote cable type with the AM dial in between. The spacing is that of a car radio mount so it's not too big. There is only the on/off volume switch and the tuner knob but the volume knob has a push pull function for voice or music.
My thought is (based on other sites) is I can swap out the power cable to provide a chassis ground, solder in a 1/4 jack and then mount it in my case. I have the original 12" speaker also but it needs to be re-coned. I'm unsure If I will mount the remote control head on the top of the case and leave the radio in tact (if I can figure out how to switch between radio and phono) or pull the on/off volume cable out and use it separately and disable the radio. I would like to come up with some sort of tube pre-amp to use and then I could use the other cable also.
I'll try to post a few pics if I cab figure it out.
Any thoughts or suggestions would be great.
https://www.facebook.com/media/set/?set=a.10152898134276359.1073741837.575601358&type=1&l=e1898a0f00
Relocated to Instruments and Amps per forum policy.Presumably the 2 transformers are power and audio output. You'd only use three of the tubes for a guitar amp; the rest are RF tubes for the radio.
If you swap the tubes, the best bet would be to go to octal and/or miniature sockets, replacing the loctal sockets and tubes, which are getting hard to come by. You'd also have to add a filament transformer if you use a 5Y3.
For the amount of conversion work you'd have to do, it would be easier to just build a Fender 5F1 Champ circuit with a 3 wire power cord into your case and feed a separate radio (possibly mounted in the case) into the input.
Several companies sell 5F1 kits, to simplify things.
If it already works, that's fine, but your challenge would be finding spare tubes.
Tube equivalents:
7A7 = 6SK7 octal or 6BA6 miniature
7B8 = 6A8 octal or 6BE6 miniature
7B6 = 6SQ7 octal or 6AV6 miniature - Fender used 6AV6 occasionally, the triode section is 1/2 of a 12AX7
7C5 = 6V6 octal or 6AQ5 miniature - again Fender Champ
7Y4 = 5Y3 octal or 6X4 miniature
If you swap the tubes, the best bet would be to go to octal and/or miniature sockets, replacing the loctal sockets and tubes, which are getting hard to come by. You'd also have to add a filament transformer if you use a 5Y3.
For the amount of conversion work you'd have to do, it would be easier to just build a Fender 5F1 Champ circuit with a 3 wire power cord into your case and feed a separate radio (possibly mounted in the case) into the input.
Several companies sell 5F1 kits, to simplify things.
If it already works, that's fine, but your challenge would be finding spare tubes.
Tube equivalents:
7A7 = 6SK7 octal or 6BA6 miniature
7B8 = 6A8 octal or 6BE6 miniature
7B6 = 6SQ7 octal or 6AV6 miniature - Fender used 6AV6 occasionally, the triode section is 1/2 of a 12AX7
7C5 = 6V6 octal or 6AQ5 miniature - again Fender Champ
7Y4 = 5Y3 octal or 6X4 miniature
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Hmmm… really appreciate the input. Knowing which tubes are which really helps. I do have a good schematic and local friends who speak that language. I am good at the mechanical tasks associated with building and repair of electronics as long as I have clear direction. With that said, this is my first attempt of doing this type of conversion. I’ve done this type of thing with non-tube amps that I been given or picked up for next to nothing but that was just a matter of moving the chassis and speaker from one box to another.
The allure of using this for an amp (again the cabinet is trashed or else I wouldn’t consider it) is that I want to have an old original amp conversion to use with my ’62 Harmony Hollywood electric/acoustic archtop guitar that I have been restoring. I’m not looking to buy a kit to build (I do want to do that someday though) or chop this up. What I was hoping is to make sure all the components are good and replace only those that are bad (or will likely fail in the near future), add a 3 prong power cable and ground the chassis, disable the radio (is it as simple as removing the tubes?) and convert the phono input and speaker out to ¼” jacks. I do need to know how to make the sure the phono input wiring is active. I couldn’t find anthing in the way of a switch and there is only 2 leads so I am assuming there wan’t a remote switch on the turntable that was there. My dad doesn’t remember either. I don’t want to change out any of the tube sockets as I want to keep it original but was wondering, using the existing components what kind of sound would I get from these tubes and are there replacements that will work in these sockets. I am prepared to buy and store extra tubes. I have read on other sites that there have been conversions with working tube radio/phonographs where they have only taken the radio tubes out, used an adapter (banana plug or RCA) to ¼ and they really dig the sound.
After really looking at the radio and enclosure, I think rather than take the components out and put them in the old case I bought, I may just convert this to an amp head. It looks really cool and think it would really look cool sitting on an old cabinet. I may even have the original speaker re-coned and use my old suitcase as a dedicated speaker cabinet. There would be room in the bottom to secure the head and carry it around when I wanna look cool.
As far as I can tell, the remote control is just an on/off switch with a hot going into the housing to the on/off switch and then another running from the switch back to the housing, a volume control knob, the radio dial knob and the sliding pointer for the radio. The cables are just extension that plug onto the pots on the side of the radio. If I pull one of the white knobs off the remote and plug it onto the radio volume pot and install an on/off switch I should be able to pack up and shelve the remote in the shop.
Please correct me and point me in the right direction if I am off base but can I do this?
The allure of using this for an amp (again the cabinet is trashed or else I wouldn’t consider it) is that I want to have an old original amp conversion to use with my ’62 Harmony Hollywood electric/acoustic archtop guitar that I have been restoring. I’m not looking to buy a kit to build (I do want to do that someday though) or chop this up. What I was hoping is to make sure all the components are good and replace only those that are bad (or will likely fail in the near future), add a 3 prong power cable and ground the chassis, disable the radio (is it as simple as removing the tubes?) and convert the phono input and speaker out to ¼” jacks. I do need to know how to make the sure the phono input wiring is active. I couldn’t find anthing in the way of a switch and there is only 2 leads so I am assuming there wan’t a remote switch on the turntable that was there. My dad doesn’t remember either. I don’t want to change out any of the tube sockets as I want to keep it original but was wondering, using the existing components what kind of sound would I get from these tubes and are there replacements that will work in these sockets. I am prepared to buy and store extra tubes. I have read on other sites that there have been conversions with working tube radio/phonographs where they have only taken the radio tubes out, used an adapter (banana plug or RCA) to ¼ and they really dig the sound.
After really looking at the radio and enclosure, I think rather than take the components out and put them in the old case I bought, I may just convert this to an amp head. It looks really cool and think it would really look cool sitting on an old cabinet. I may even have the original speaker re-coned and use my old suitcase as a dedicated speaker cabinet. There would be room in the bottom to secure the head and carry it around when I wanna look cool.
As far as I can tell, the remote control is just an on/off switch with a hot going into the housing to the on/off switch and then another running from the switch back to the housing, a volume control knob, the radio dial knob and the sliding pointer for the radio. The cables are just extension that plug onto the pots on the side of the radio. If I pull one of the white knobs off the remote and plug it onto the radio volume pot and install an on/off switch I should be able to pack up and shelve the remote in the shop.
Please correct me and point me in the right direction if I am off base but can I do this?
Sorry for the delay Boobtube. It is the same schematic that I have and appears to be the correct one.
So a little update. I pulled it apart and everything seems to look fine. No burned up parts though the soldering to convert it to home use is rather sloppy. The original speaker cone is tore up pretty bad but I went ahead and checked all the wiring for frayed areas and repaired any I found. I went ahead and plugged it in and let the tubes heat up and low and behold a local am station started playing. Hardly any noise too.
This got me excited so I did some digging on this site and then tapped into the volume pot for a guitar input, turned the radio dial away from the station, plugged my guitar in (after checking several times for any voltage) and the guitar was playing through the speaker.
I then tapped into the leads that go into the cone and wired it to one of my guitar speaker cabs. What I ended up with was a sweet clear tone. The remote has a volume switch that is also the on/off switch. When you push it a round tone control switches between 3 setting. Sweet. I only played for a few minutes in case any of the paper caps were ready to blow.
So this shores up what I want to do to keep it intact and use it as a guitar amp.
• Replace all the paper caps, etc…
• Replace the tubes (not sure the condition of these but having spares isn’t such a bad thing).
• Install a power switch and fuse,
• Disable the radio,
• Install an input ¼ jack,
• Install a 3 way switch to maintain the cool tone control,
• Mount a speaker output jack. From what I have read, I need to wire a choke to use a PM speaker cab. My speaker is electromagnetic but does not have a transformer on it.
Where I need the help is disabling the radio and adding the choke for the speaker. Is disabling the radio as simple as taking out the tubes? That would be cool so I could use it again as a radio if I wanted. If so which tubes do I take out?
For the speaker. I have 4 wires coming from the radio. I am assuming 2 are there to supply power to the magnet. What do I have to do to wire it to a speaker output jack?
Also, is there anything I can do to get a little more volume?
I appreciate any help. Thanks….
So a little update. I pulled it apart and everything seems to look fine. No burned up parts though the soldering to convert it to home use is rather sloppy. The original speaker cone is tore up pretty bad but I went ahead and checked all the wiring for frayed areas and repaired any I found. I went ahead and plugged it in and let the tubes heat up and low and behold a local am station started playing. Hardly any noise too.
This got me excited so I did some digging on this site and then tapped into the volume pot for a guitar input, turned the radio dial away from the station, plugged my guitar in (after checking several times for any voltage) and the guitar was playing through the speaker.
I then tapped into the leads that go into the cone and wired it to one of my guitar speaker cabs. What I ended up with was a sweet clear tone. The remote has a volume switch that is also the on/off switch. When you push it a round tone control switches between 3 setting. Sweet. I only played for a few minutes in case any of the paper caps were ready to blow.
So this shores up what I want to do to keep it intact and use it as a guitar amp.
• Replace all the paper caps, etc…
• Replace the tubes (not sure the condition of these but having spares isn’t such a bad thing).
• Install a power switch and fuse,
• Disable the radio,
• Install an input ¼ jack,
• Install a 3 way switch to maintain the cool tone control,
• Mount a speaker output jack. From what I have read, I need to wire a choke to use a PM speaker cab. My speaker is electromagnetic but does not have a transformer on it.
Where I need the help is disabling the radio and adding the choke for the speaker. Is disabling the radio as simple as taking out the tubes? That would be cool so I could use it again as a radio if I wanted. If so which tubes do I take out?
For the speaker. I have 4 wires coming from the radio. I am assuming 2 are there to supply power to the magnet. What do I have to do to wire it to a speaker output jack?
Also, is there anything I can do to get a little more volume?
I appreciate any help. Thanks….
Don't forget to add a 3 wire power cord and ground the green wire to the chassis near where it enters the chassis, unless it already has one.
You don't have to add a choke to replace the speaker field coil, in this case. The field coil is wired across the 12 VDC input to the vibrator. You'd just wire the two audio output wires to your speaker jack.
You might check and/or replace the power supply filter caps if they are dried out. You can wire new caps under the chassis to the old caps' terminals if you don't want to put a spendy new can in there. The third section is the cathode bypass cap for the output tube, so it will have a lower voltage rating.
You could pull just the IF stage 7A7 to disable the radio, or wire an SPDT switch to the top of the volume pot to select either the radio or the input TS jack.
I'm assuming there's a separate 12 volt power supply in the cabinet to run the radio. Is this true?
The tubes and especially the vibrator will take some time to find, so you definitely want to collect some spares.
You don't have to add a choke to replace the speaker field coil, in this case. The field coil is wired across the 12 VDC input to the vibrator. You'd just wire the two audio output wires to your speaker jack.
You might check and/or replace the power supply filter caps if they are dried out. You can wire new caps under the chassis to the old caps' terminals if you don't want to put a spendy new can in there. The third section is the cathode bypass cap for the output tube, so it will have a lower voltage rating.
You could pull just the IF stage 7A7 to disable the radio, or wire an SPDT switch to the top of the volume pot to select either the radio or the input TS jack.
I'm assuming there's a separate 12 volt power supply in the cabinet to run the radio. Is this true?
The tubes and especially the vibrator will take some time to find, so you definitely want to collect some spares.
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You're really one preamp stage short of a guitar amp. Pulling all three radio tubes would give you a little more power, but perhaps inaudibly so.
I appreciate the info. I am lost on the theory side but can build a watch if I have direction...
I forgot to add that I am adding a grounded power cable. Currently one side of the 2 wire cable runs to the on/off switch and the other to what I only know as a junction post where at least 2 (maybe more) leads are soldered to it. One of the leads runs into the transformer. Do I run the common or hot through the switch? I'm at work or I would take a picture of the bottom.
Can you tell me which wires are the audio output wires? Are they at the plug? I tried all combinations at the plug with no luck.
I’m not completely sure how they converted it but I they installed a transformer so it could be mounted in the cabinet. I believe it may be 6 volt though.
If I buy a preamp stomp bob would that fix my preamp issue? I am assuming that would make the gain better.
I forgot to add that I am adding a grounded power cable. Currently one side of the 2 wire cable runs to the on/off switch and the other to what I only know as a junction post where at least 2 (maybe more) leads are soldered to it. One of the leads runs into the transformer. Do I run the common or hot through the switch? I'm at work or I would take a picture of the bottom.
Can you tell me which wires are the audio output wires? Are they at the plug? I tried all combinations at the plug with no luck.
I’m not completely sure how they converted it but I they installed a transformer so it could be mounted in the cabinet. I believe it may be 6 volt though.
If I buy a preamp stomp bob would that fix my preamp issue? I am assuming that would make the gain better.
I agree with okcrum about adding the preamp stage. The pedal will increase your signal but you need more gain in the amp in spite of this. Is the amp loud enough the way it is right now? If not, the extra gain stage is needed. The 7C5 power tube only gives 4-5 watts so you might want all the front end help you can get. If you tapped into the volume then you are only getting one preamp stage before the power tube.
Which audio output wires are you asking about? You said you tapped off the speaker to your guitar amp cab. Just follow those back to the amp? Not sure about the plug you ask about. Maybe post some pics.I must admit I was one day away from deleting the schematic because of no response, so I need to re-visit it more now to refresh my memory of how it works. Those old circuits are very different then the newer ones.
Which audio output wires are you asking about? You said you tapped off the speaker to your guitar amp cab. Just follow those back to the amp? Not sure about the plug you ask about. Maybe post some pics.I must admit I was one day away from deleting the schematic because of no response, so I need to re-visit it more now to refresh my memory of how it works. Those old circuits are very different then the newer ones.
Currently, the amp is loud enough to hear and enjoy but I would say that it is not loud enough to sing over which is my goal. I would say it is about 5 amps or less. I’ve had a 10 amp practice amp which would be way louder than this. If I use it at a gig, I can always mic it. I know nothing about tube amps and I appreciate the help. My goal is to get this running by Saturday. It is my 50th birthday and I am having a big party and I’ve started a country band that will be playing. My dad is a longtime county musician and he will be there to see me play country for the first time. He was also a big “use and convert old equipment cause we’re poor” guy so he will love this. The more I get into this the more excited I get and the more I want to do. I can see how this can be an addiction. I am the type of guy that needs to know how things work but for now, I just want to get it working. I have questions that are probably simple to you guys but seem to be way out of my reach.
How hot should the tubes be getting? I can touch and hold them without any discomfort. I am wondering if they may be weak.
Is there a better location to tap into for the input? Is there more than one pre-amp stage?
For the speaker, I tapped into the wires that were coming from the coil to the cone. When I tried to tap into the wires going into the coil, I go nowhere. I’ve added pictures to a Facebook folder I opened to the public. Please let me know if you can’t open them.
https://www.facebook.com/media/set/?set=a.10152918258986359.1073741838.575601358&type=1&l=7b925d02e3
One of the pictures shows the power supply conversion they did. I am not sure what the vibrator does but reading the posts that are out there, I am assuming that is the power supply? I think in my case, they took it out and replaced it with a transformer.
Let me ask a crazy question for a possible future mod; If I choose to remove the radio components, could I use the existing sockets, tubes and empty space to add a tube preamp?
How hot should the tubes be getting? I can touch and hold them without any discomfort. I am wondering if they may be weak.
Is there a better location to tap into for the input? Is there more than one pre-amp stage?
For the speaker, I tapped into the wires that were coming from the coil to the cone. When I tried to tap into the wires going into the coil, I go nowhere. I’ve added pictures to a Facebook folder I opened to the public. Please let me know if you can’t open them.
https://www.facebook.com/media/set/?set=a.10152918258986359.1073741838.575601358&type=1&l=7b925d02e3
One of the pictures shows the power supply conversion they did. I am not sure what the vibrator does but reading the posts that are out there, I am assuming that is the power supply? I think in my case, they took it out and replaced it with a transformer.
Let me ask a crazy question for a possible future mod; If I choose to remove the radio components, could I use the existing sockets, tubes and empty space to add a tube preamp?
An externally hosted image should be here but it was not working when we last tested it.
I really hate to say this but I need to speak the truth as I see it. If you know nothing about tube amps, this is not the project to learn on. Because it is a radio, it was meant to handle RF, not audio frequencies. From what I know, it only deals with audio frequencies at the point you tapped in and after. This tube is the detector where the audio is pulled from the IF frequency part of the circuit. If you tap into the tube before this, you will be in the part of the circuit that handles the intermediate frequency, several thousand or hundred thousand Hz.
The other part of the equation is that I only have a basic understanding of radio. I know audio frequency amps, so I will only be of limited help.
You need to test the tubes, test the caps and more than likely replace many of them, provide a grounded power supply cord, add a fuse, figure out the speaker arrangement and add another gain stage. So you need to design a different tube stage before the one you tapped into and figure out how to couple them together. Right now, it is done with a transformer meant to handle IF. The pics on facebook only give me a partial picture of things and I can't get your full size image at the end to open.
I hate to be the bearer of bad news. I hope okcrum can be of more help. Or someone else. I'll continue to look at the schematic, but this is a project for later when you learn something about tube amps.
The other part of the equation is that I only have a basic understanding of radio. I know audio frequency amps, so I will only be of limited help.
You need to test the tubes, test the caps and more than likely replace many of them, provide a grounded power supply cord, add a fuse, figure out the speaker arrangement and add another gain stage. So you need to design a different tube stage before the one you tapped into and figure out how to couple them together. Right now, it is done with a transformer meant to handle IF. The pics on facebook only give me a partial picture of things and I can't get your full size image at the end to open.
I hate to be the bearer of bad news. I hope okcrum can be of more help. Or someone else. I'll continue to look at the schematic, but this is a project for later when you learn something about tube amps.
OK, let me try to answer your questions from last to first.
Yes, given enough gumption and some knowledge, you could repurpose the loctal sockets by gutting the radio receiver parts, substituting audio tubes in where the radio tubes (that work) are now, and rewire the gutted circuits with audio preamp parts. Then you would have an oddball preamp circuit with rare (translate expensive/hard to come by) tubes that might work, until maybe some year a tube went out... IMHO, you would be better off leaving the radio end how it is, removing the tubes if you're not going to use the radio. For the immediate future, I would try to boost the signal coming in from where you're at now. A boost or overdrive pedal may get you as loud as you want, as well as give you some tone or voicing control. A mini tube preamp, such as the 12A*7 series (*=U or T or X, in increasing order of gain), could be built into the chassis for $20 or so, that would give you as much gain as you would ever need.
Also, you could rig in a little solid state "booster" of some sort between the guitar and volume pot to get more signal. This would be even less expensive, but then you wouldn't have an "all tube" audio path. A TL072 op-amp chip would do the same job as the 12A*7 tube by raising the signal level feeding what you have. Kind of like a pedal inside the box that you just set once and forget about. Super simple for a quick and permanent mod.
Before you sink too much time or money into this rig, I would definitely suggest having it checked out by a competent tech with experience with antique radios. If you know any ham radio geezers, or know someone who knows an old amateur or commercial radio operator, I would take it to one of them any day over a modern repair shop. These guys absolutely will love to see this unit still working after so many years, and can tell you all you would ever want to know about what you have. The right guy might even help you tinker with the unit, so you don't blow it up trying to mod it. Most of them will not tell you it's garbage, even if it is. Kind of like classic motorcycles in general.
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