I have an early Philco 90 I am restoring. It has two 45 output tubes and I have a bad output transformer(chicago jefferson 2766J). Could some one please advise me as to where I could find a replacement for this. I have looked at several of the parts sites mentioned on this forum and in the forum posts and can't find any reference or source for this. It looks like Hammond doesn't make a suitable replacement.
Another question I have concerns the Philco 90 schematic which I downloaded from this web site. By the #47 part baloon just above the 27 oscillator tube is a capacitor connected to ground. No value is listed for this cap. What is it and where is it located? I would greatly appreciate any help anyone could provide. Thanks.
I think there's a mistake. The place where this cap goes to that coil should be connected to a C+ or similar voltage and there is no dot there in the sch.
Second, this cap becomes thus a decoupling capacitor and I think 0.05uF / 450V will do just fine.
I'll be darned, I did not notice that when I posted the schematic. It's like that in Rider's, too (unlabeled).
Just picked up this thread.
Will see what the original service bulletin shows.
I'd bet that this is an original typo from Philco.
Rider never re-drew schematics, that I'm aware of.
Also, maybe someone can eye-ball their early 90 and trace out the actual connection as a cross-check. I think my 90 lowboy is a Mid version chassis.
Chuck
The mistake is pretty obvious; when I looked at the cap it started making no sense within 15 seconds of me looking.
First, I want to thank those who have responded to my question about the unknown cap. Now I want to delve further into my inquiry about the bad output trans. and , hopefully solicit some responses on this.
I have been pondering this problem for several days and have come up with the following questions and observations:
1. The philco sch. says the plate current on each 45 tube is 30ma at no signal. From this value, I conclude that the radio is "fixed bias" and the max. signal plate current is approx. 138ma. This thinking comes from the 45 tube info. Is this correct?
2. Looking at the Hammond 125 series of output tr.'s, it looks like an impedance match could be made using the .7ohm speaker imp. to the input imp. necessary to drive the speaker. The tube info says 3200ohms(fixed bias, 5200ohms self bias) but the philco tech section info. on this site says 6700ohms output trans. imp. Which is correct? I realize that at max. volume that the plate current would overload the Hammond 125 series but wonder if volume levels are kept at normal listening levels if this would work? According to chart, the Philco output tr. has a pri. res. of 500ohms. I have measured a Hammond 125c at 275 ohms. Would this raise the plate voltage to an unacceptable level?
3. Surely some one has faced the bad output tr. problem before. What solutions are there?
Scott
Your DC resistance of the primary is not that important. Somewhat important, but what matters more is the impedance and it is a copmplex thing consisting of reflected voice coil impedance, parasitic inductances, DC resistances and it is also frequency-dependent.
Hi All;
That is way above my pay grade..
THANK YOU Marty
Scott, I am not sure where you are getting your tube data, but you need the characteristics under class A conditions, which is how the 45's operate in your radio. Average plate current only increases when in AB mode where the grids are actually driven positive.
In true class A, the average plate current remains constant, the instantaneous current of each tube varying from zero to twice the average value when driven to full output. So in your case, each tube draws an average of 30 mA, or 60 mA total for two tubes.
If Philco specifies a load resistance of 6700 ohms, then this is the value I would use as it is the one found to be optimum under the actual operating conditions of the tubes.
Thanks Mondial for your reply. I guess I was of the wrong impression that push-pull was always class B. Thanks for your clarification. Maybe you can give me your opinion as to whether a Hammond 125E output tr. would work for me in this Philco 90. I realize that the secondary tap chart only goes down to 1.5 ohms, but if you use the formula for reflected impedance(turns ratio squared times the .7 ohms speaker impedance) you can pick a secondary tap which comes close to the 6700 ohms required by the output tubes. My thought is that the chart with this tr. only goes down to 1.5 ohms just for convience and that a lower tap value could be calculated and would work. I have verified the turns ratio by applying low voltage on the secondary and measuring the primary voltage using a dmm for measurment. Any and all thoughts are appreciated.
Scott
Yes the Hammond should be fine, although I don't know if you really need one as big as the 125E. The PP '45 output stage only puts a max power of about 4 Watts, which is a lot less than the 15 W rating of the 125E. Perhaps more important is the physical size and what will match the existing mounting holes. I would check out the 125C or 125D.
As long as you can choose a turns ratio that will give the proper impedance transformation from 0.7 ohm to about 6700 ohms you will be fine. The exact match is not all that critical, and anything from about 5000 ohms to 9000 ohms should work fine.
Mondial, thanks for your reply. I have ordered a 125c output tr. I have used these before on other radios and have had good results.
I have another problem which has presented itself on this radio. The volume control is defective and is the type which has a carbon coating on the inside metal rim with another circular metal ring that presses against that by an insulated wiper arm. I have never seen anything like this but recall reading something somewhere about these. Seems like someone had a suggestion about fixing it. Maybe Mark Oppat does these. If anyone has a suggestion about how to repair it, please post a reply. Thanks.
Hi all: I have several more questions on my Philco 90 restoration. First I will say that the schematics for this radio are one of the worst I have seen and I have restored about 30 radios, mostly Zenith and Silvertone.
My radio is an early one with a cabinet date of April 27,1931 and a SN of 071645.
First question: Between part baloons # 4 and # 5 on the top line it shows them connected and both continuing to the grid of the 24 tube. My radio wiring shows no connection between these here. I don't believe the sch. is correct.
Second Question: Baloon 46 the bakelite block condenser is listed as part #5215 (.oo1mf). The installed part number on my radio is #3903J which is a .01mf condenser. Shoul I use the .001mf as listed for a replacement or the .01mf which was installed in the radio?
I would appreciate any help. Thanks.
Scott
I have the single output version of this radio. I agree that there seem to be a lot of errors or changes in the schematic (I've ordered Chuck Schwark's nice schematic sets, this should help me figure out some things). For instance, the can caps don't match what is in the parts list. Using the part numbers listed, and looking up the information on Chuck's site, the capacitor values are different than listed in the list next to that part number. Some of the bakelite blocks differ from what is listed in the parts list as well, but this may have been due to production run changes. The good news is, as I have replace various parts as listed on Chuck's site, the performance of the radio has become more and more phenomenal.