I recently acquired a Philco 38-690 console for $25.00 at a yard sale. The catch is that the amplifier chassis and 14" speaker are missing, and according to the previous owner, they disappeared many years ago. I plan to keep the set, I didn't buy it to re sell. The cabinet is in reasonable condition, with no veneer delaminating. There are a few scratches, and the finish is flaking off around the bottom of the cabinet. the worst problem is that something sitting on top of the cabinet became embedded in the finish. I realize that it will be nearly impossible to locate the original missing parts ( especially at anything approaching a reasonable price ), as these sets were not produced in great quantity. I plan to use a jukebox amplifier ( P-P 6L6's about 20W out ) that I already own for now-I have figured out how to adapt it to work and power the receiver chassis as well as work the voltage controlled bass boost. I will not modify the original receiver chassis in any way. I will fabricate the receptacles that mate with the plugs from the receiver. I have already fabricated the 10 pin receptacle. I used a 1.5" dia. nylon rod for the shell. A template was made, and the pin positions were transferred to the shell and drilled to accommodate the female pins. If I find an original amplifier chassis and speaker, all I have to do is remove the replacement amp and plug in the original. Another option would be to obtain a new chassis and build an amplifier that closely duplicates the original. Power transformers could come from other scrap Philco radios. This would be quite an ambitious undertaking, and the result would still not be exactly original. I don't know if it would be worth it.
Are the tweeters missing also? They don't really make much sound from my experience so no big loss sound-wise if gone.
The 38-116 woofer may be the same size. I never measured mine for diameter.
Hi Peteivan
Yes, the efforts would be worth your while, especially since you only have $25 invested in it. Congratulations!
You could possibly adapt a jukebox amp, but you'll also have to be able to supply needed voltages to the upper (tuner) chassis. The original used two 6L6G output tubes.
Bruce mentioned a 38-116 woofer as a replacement. One of these would be much easier to find than a 38-690 woofer, and yes, they are the same size. However, the 38-690's amp wants to see a woofer field of 4000 ohms. The 38-116 woofer's field coil only has a DC resistance of ~350 ohms. There are ways to get around that, of course.
As for the tweeters. If they are there...it is a great thing...and if they did not make much sound, it is because of a factory wiring error that had the tweeters wired out of phase, cancelling out their sound. Details, with a fix, here:
http://www.philcoradio.com/tech/tweeters.htm
The jukebox amp I have is out of a Rock-ola Fireball jukebox made in 1952. I plan on replacing the original woofer with a PM speaker. I plan to run a voltage divider series of resistors from the point that used to supply +300v to the speaker field coil. The jukebox speaker used a 5K field coil. The new voltage divider would load this point, tending to drop the voltage back down to what it would be if a speaker field coil were connected.This would supply all the positive voltages to the receiver chassis. There also some negative voltages required. The 6L6 output stage is already taken care of, as I am using an existing amplifier. This amp uses a separate transformer to supply 6.3 VAC for the amp filaments. There also 2 other windings on this transformer for the jukebox control circuitry. These aren't needed anymore for their original purpose. These 2 windings are (1) 24VAC, and (2) 35VAC. Essentially, the negative bias voltages to the receiver chassis are obtained from the negative 22-24 VDC bias to the 6L6 output stage on the original chassis. Either the 24VAC or 35VAC windings can be rectified and used to generate negative bias voltages through a voltage divider network. On the original amp chassis, the voltage divider network current seems to be about 40 MA. The service literature I have gave no voltage readings. I had to refer to 37-690 service literature which provided voltage readings ( Relevant to the receiver chassis ). I realize that the amplifier chassis is quite different as far as the output stage goes, but the receiver chassis is quite similar except for the DC controlled bass boost circuit. The original mid-range/tweeters are present,as are the 4 passive radiators. I was a senior microwave technician when I retired from DTE Energy, our local electric and gas utility. I also have extensive experience in the mobile radio field. I have been restoring antique radios for many years. Electronics was a hobby of mine since my elementary school days. I used to go out on trash day, and brought many fine radios home. Unfortunately, I didn't know how to fix some of these radios. They got torn down for the parts and hardware, some of which I still have today. My greatest regret was letting a roommate talk me out of moving an Atwater Kent Model 810 cabinet when I moved. (Excellent sound quality, 3 shortwave bands, and was a very sensitive 10 tube console ) This was around 1975-These things had no value then. I still have the working chassis, and am looking for a cabinet and speaker. As far as the tweeters go, I will drive them from the output of the jukebox amp with a 4.7uF cap in series. If the treble response is excessive, I will add a series resistor, or L pad control. The field coils could be driven from the voltages obtained from the amps 25 and 35 VAC windings.
The 38-116 speaker to replace the 38-690 woofer is a good suggestion. The extra control transformer windings could supply the field coil current. A voltage doubler circuit could be used if necessary, and the 25 and 35VAC windings on the jukebox chassis could be placed in series.
I don't recommend replacing the field coil in such a large set with a resistor network, the simple reason is current draw. A filter choke/field coil resists changes in voltage and blocks AC ripple but allows DC to pass, so it acts like a resistor to AC but more like a length of wire to DC. It is also much more efficient at eliminating ripple then a resistor, a 10 Henry choke is ten time more efficient at eliminating ripple then a 400 ohm resistor even if the DC resistance is the same. The problem with using a resistor is that the voltage drop is the same whether AC or DC passes across it, and the voltage drop increases with current. So if you want to replace the field coil with a PM speaker use an iron filter choke or a solid state substitute, it's a top end radio not a cheap AA5, on a 38-690 it's worth the effort.
Regards
Arran
Arran:
You are quite correct in what you are saying if the field coil was wired as part of a Pi filter. However, since the original amplifier chassis is missing, I am adapting a jukebox amplifier that ran a 5K field coil from a + 300 V source with the other lead going to ground. ( 60 ma current draw ) The amp already uses filter resistors. I plan on using an additional filter resistor and capacitor to feed the voltage divider. This will keep hum out of the receiver, The voltage divider will load the +300V supply to keep that supply point at near +300V.
On another project, the restoration of a 1942 Freed-Eisman console, I redesigned the amp chassis when the field coil failed on its 15" woofer. In the original circuit, the whole receivers current draw was fed from the rectifier cathode through the 2500 ohm field coil. The output tubes were P-P 6Y6's. The tube manual stated that 6Y6's were designed to work at low plate voltages. No wonder-after the voltage drop through the field coil, there wasn't much left. That was a very poor design. To replace the field coil in the existing circuit with a resistor was impractical, as the resistor would dissipate a large amount of power. I replaced the output tubes with 6L6's with the plate supply coming directly off the rectifier cathodes. A suitable value filter resistor was chosen to provide B+ to the rest of the radio to supply the same voltage as before. The wattage required now was far less, as the output tube current no longer passed through this resistor. This is common practice in commercially made equipment, and the radio didn't hum after this mod. With your ear next to the speaker, hum was almost inaudible. The tone control circuits were also modified-the bass and treble controls had very little effect. The treble control was a pot wired in series with the tweeter, and the bass control was a pot in series with the volume loudness compensation network. This was replaced with a tone control circuit (12AX7 on an add-on sub-chassis) from the RCA tube manual. Even with the much increased bass boost, no hum could be heard. The set never sounded so good.
Yesterday, I fabricated a shell for the 8 pin female receptacle. The shell came out fine, but the idea of using brass tubing to make the female pins didn't work too well. the brass is not springy enough. I went on EBAY to look for a socket from which I might salvage pins that I could place in the shell I made. I found a socket that fit a 2A3 tube, with 2 sizes of female receptacle pins. On their socket design, the pins are held into the shell by a nut on each pin. It would be easy to remove the nut, and transplant the pins into my shell. Today, I vacuumed off the dirt on the receiver chassis while using a small brush to sweep dirt towards the vacuum cleaner nozzle. I then applied a cleaning solution to the RX chassis, while keeping moisture away from critical components. The chassis will be allowed to dry for a long time. I applied a light coating of Krylon silver paint to the tube shield cans, as some of them had some rust. The paint was advertized as duplicating a plated finish, and it looks good. I refinished all tube shields, so they would all look the same. I am missing a couple tube shields-I have an old Philco farm radio that runs on 6 Volts that I restored many years ago. For now, I will use tube shields from this radio-they are the same. I will replace them later, when I find them.
I have located replacements for the missing tube shields. I found that I had a couple scrap Philco chassis that had the correct shields. These were painted with the same silver paint as the others. I pulled all the tubes, and cleaned off the dirt with a damp rag. ( Water only-no detergents or soap to avoid washing away the the numbers on the glass. ) 3 tubes are missing. I tested all the tubes I have.(TV-7D/U Tester) All are good except the diode sections on the 6B8 and 6R7 tubes. These read quite a bit low. I wonder if the tube tester is accurate as the pentode and triode sections show well above the specified reading. I own a Hickok tube tester, and will retest these tubes on it. The top dial light socket was broken where it attached to the bracket that held it. This was repaired with a piece of brass tubing that was completely flattened at one end, and the rest of it was flattened only enough to allow it to slide onto the mounting bracket. A hole was drilled at the completely flattened end, and a hole was drilled in what was left of the sockets broken off part. There is a small dimple where I center punched and drilled a hole. The completely flattened tubing end was attached with a 4-40 screw. At the transition between completely flattened, and partially flattened tubing, it was bent 90 degrees. The partially flattened end now slid onto the original bracket. The amount of flattening is adjusted to give a snug fit.
Your 6B8 and 6R7 are probably OK. It is normal for the diode section to read low. Most testers have a separate meter scale for the diode test.
Steve
I acquired a NOS 6B8 and 6R7. The diode sections on the new tubes tested much higher (read about 40 on tube tester, VS 11-15 for old tubes). It would not surprise me if the old tubes would work in the circuit. I have now finished fabricating the 2 sockets that duplicate the sockets on the missing amp that the receiver chassis plugs into. 1.5" Dia nylon rod was used to fabricate the shells, the 10 pin female contacts are the same as what are used for a PATA computer hard drive power connector. Receptacle pins for the other socket came from 3-4 pin sockets that would fit a 2A3 tube. All items were found on Ebay. The 4 pin socket pins were held into the original shell by a nut. Remove the nut, and the pins come out, and can be inserted into the new shell after wire leads are soldered on. I have fabricated a voltage divider resistor board that will power the receiver chassis from the jukebox amplifier. I have re-capped the amp, and removed all components that were used in the jukebox control circuitry. 2 Hammond 6.3 V 4A filament transformers were added to provide the separate F1 and F2 filament feeds as on the original amplifier. I don't know that is necessary to have 2 separate feeds, but the designers must have had a reason for doing this. Perhaps it was just the ratings of the transformers they used, or perhaps it was an attempt to avoid feedback through the heater circuits. I didn't use the jukebox filament source because I wanted to avoid the possibility of overloading the amplifier filament transformer.
I have been sidetracked from this project by several other projects. I restored a military R390A/UUR communications receiver for a customer. Also restored a Fisher model 500 mono tube type receiver, and a 1931 Dewald radio, including cabinet restoration. I also put together an amplified antenna kit sold by Ramsey. (For the AM broadcast band) It uses a shielded ferrite loop. The amp consists of a MPF 102 JFET feeding a 2N3904 emitter follower. I tried it on the R390A I was working on, tuned in a weak station at 1600 Khz. With a 30' wire streched out in the basement for an antenna, the station was barely readable-the signal being covered by noise from the CFL bulbs in the house. With the active antenna in the circuit, the interference almost completely ceased, the signal level jumped up by 20 Db, and the station was completely readable. I didn't like the cheap RCA jacks supplied with the kit. I used the plastic cabinet back on the control/power supply unit as a template to make a back made from brass on which I placed chassis mount BNC connectors at the same location where the RCA phono connectors were. (Don't forget to ground the brass plate to where one of the RCA connectors would have been grounded on the circuit board) The F connector on the remotely mounted antenna/amplifier unit was also replaced with a BNC connector. I didn't use the aluminum foil they suggested for the antenna shield. Thin copper sheeting is far easier to use, as it can be easily soldered, and is available at any hobby shop. (as are brass sheets in various thicknesses)
I plan to get back to the Philco 38-690 project soon. For the most part, the jukebox amp has been modified to work with the receiver chassis. I duplicated the Philco loudness compensation circuit on the jukebox amp chassis. This will probably require some tweaking of component values. The receiver/Amp combination is now ready for testing.
The manual for this amplified antenna kit can be downloaded from the
Ramsey website. It explains the theory of operation, but does not give you a schematic for obvious reasons.
Wow.. I have a pair of 38-690 chassis, and I would NOT want to try to duplicate all the connections between the tuner and amp chassis'. I still have some work to do on the tuner, it has some open coils on the bandswitch. I sent the antenna section off to Neil Sutcliffe to be repaired, but there is/are a couple of coil(s) in the oscillator section that are bad in the high bands. Other than that, the tuner is super neat.. the variable IF is awesome and the audio overall is really nice, and that's without a cabinet.
I had no choice but to adapt an existing amp to the tuner chassis. Finding an original amp is almost impossible. (Certainly not at a reasonable price) This will get the radio in working condition. The jukebox amp I chose was a good choice as there is lots of room on and under the chassis to add the necessary extra components. As I mentioned in a previous posting, I might eventually build a copy of the original amp if I can find some scrap Philco radios from which to salvage the transformers. In any case, the connectors that connect to the tuner must be fabricated, as originals are unavailable. The only alternative would be to cut off the existing tuner connectors, and use modern connectors. This is unacceptable, as the originality of the radio would be further degraded. If I acquired an original amp, I would be unable to just plug it in and make it work. One never knows what one might find at swap meets and yard sales-I never expected to find this radio at a yard sale for $25.00! Fortunately, I recognized it for what it is, as I have read about, and have seen pictures of this radio before.