02-25-2018, 02:46 PM
Jcassity;
I have a nice Variac Isolation transformer that I could use, but I have a decided dislike for AC/DC type chassis radios and seldom ever work on one. And I had a couple of AC/DC Admiral chassis that I used to repair one for a friend, but as soon as it was gone, I scrounged the two donor chassis for parts and threw the rest away. I do not intend to own any AC/DC radios.
You may want to go back to page 3 of this thread and compare the AC and DC voltages I measured at the 5Y3 rectifier socket both with and without the tube installed. Compare in particular the AC voltages coming out of the power transformer to the plates of the 5Y3 to what AC voltage your unit's power transformer is delivering to the same pins 4 and 6. If you see a much higher AC voltage there that would explain why you had to use a much larger power rated resistor in your radio. Keep in mind too that if the AC voltage is the same but the current capacity of the transformer secondary is greater, you have to dissipate much more power before you can get the downstream DC voltage to the desired level. Unfortunately we do not know the full specifications of the Philco original power transformer, but since this example still has its original transformer, it does at least give a fairly accurate set of voltage readings to compare against.
After much work I got a new Loctal socket installed for the 7C6 tube only to discover that I could not plug the tube into the socket. By that time I was so irritated that I decided to take it out and install an Octal socket and a 6SQ7 tube that has the same characteristics but just a slightly higher amplification factor. As I took out the Loctal socket I saw that I had managed to get solder to wick down into a couple of the contacts, thus making installation of the tube all but impossible. In any case I made the change to the 6SQ7 and Octal socket and this works very well. The set now comes on within 15 to 20 seconds or so. I noticed that although Philco had used a center-tapped filament secondary for the 7C6 and 6J5, they had not bothered to twist the filament leads going to those two tubes. Since filament wires radiate AC energy, I decided to twist together the filament wires coming from the power transformer to help eliminate radiated 60Hz energy around the two audio tubes. After getting the radio operating again, I do believe there is less residual 60Hz hum now.
As it turned out the Bozak preamp is just not usable in this radio. I tried two different magnetic cartridges and there is some sort of distortion when bass drums or bass notes are struck which makes a sputtering effect in the audio. No amount of tracking force or anti-skating adjustments with either cartridge made any difference in the distortion I was hearing. If there had been a difference between the two, one cartridge/needle assembly could have been bad, but instead both sounded exactly alike. So reluctantly I have ordered a Vaco/Varco TN4 ceramic cartridge and needle from eBay. The higher output signal will not require a preamp so the audio circuitry will be less complex. Fortunately the Bozak preamp only cost me about $15 and the Vaco/Varco cartridge is less than $18. I may eventually find a problem in the Bozak preamp, but it is not high on my project priorities. It uses two transistors in each channel. There may have been less of a chance of problems if an IC had been used. I have some 4858 Op Amp ICs I could use and may build up a preamp to take the place of the original Bozak circuitry.
Joe
I have a nice Variac Isolation transformer that I could use, but I have a decided dislike for AC/DC type chassis radios and seldom ever work on one. And I had a couple of AC/DC Admiral chassis that I used to repair one for a friend, but as soon as it was gone, I scrounged the two donor chassis for parts and threw the rest away. I do not intend to own any AC/DC radios.
You may want to go back to page 3 of this thread and compare the AC and DC voltages I measured at the 5Y3 rectifier socket both with and without the tube installed. Compare in particular the AC voltages coming out of the power transformer to the plates of the 5Y3 to what AC voltage your unit's power transformer is delivering to the same pins 4 and 6. If you see a much higher AC voltage there that would explain why you had to use a much larger power rated resistor in your radio. Keep in mind too that if the AC voltage is the same but the current capacity of the transformer secondary is greater, you have to dissipate much more power before you can get the downstream DC voltage to the desired level. Unfortunately we do not know the full specifications of the Philco original power transformer, but since this example still has its original transformer, it does at least give a fairly accurate set of voltage readings to compare against.
After much work I got a new Loctal socket installed for the 7C6 tube only to discover that I could not plug the tube into the socket. By that time I was so irritated that I decided to take it out and install an Octal socket and a 6SQ7 tube that has the same characteristics but just a slightly higher amplification factor. As I took out the Loctal socket I saw that I had managed to get solder to wick down into a couple of the contacts, thus making installation of the tube all but impossible. In any case I made the change to the 6SQ7 and Octal socket and this works very well. The set now comes on within 15 to 20 seconds or so. I noticed that although Philco had used a center-tapped filament secondary for the 7C6 and 6J5, they had not bothered to twist the filament leads going to those two tubes. Since filament wires radiate AC energy, I decided to twist together the filament wires coming from the power transformer to help eliminate radiated 60Hz energy around the two audio tubes. After getting the radio operating again, I do believe there is less residual 60Hz hum now.
As it turned out the Bozak preamp is just not usable in this radio. I tried two different magnetic cartridges and there is some sort of distortion when bass drums or bass notes are struck which makes a sputtering effect in the audio. No amount of tracking force or anti-skating adjustments with either cartridge made any difference in the distortion I was hearing. If there had been a difference between the two, one cartridge/needle assembly could have been bad, but instead both sounded exactly alike. So reluctantly I have ordered a Vaco/Varco TN4 ceramic cartridge and needle from eBay. The higher output signal will not require a preamp so the audio circuitry will be less complex. Fortunately the Bozak preamp only cost me about $15 and the Vaco/Varco cartridge is less than $18. I may eventually find a problem in the Bozak preamp, but it is not high on my project priorities. It uses two transistors in each channel. There may have been less of a chance of problems if an IC had been used. I have some 4858 Op Amp ICs I could use and may build up a preamp to take the place of the original Bozak circuitry.
Joe
