Changed caps. It works well, but no amplification. When volume is low, it plays fine--increase volume--distortion. Hooked up extension speaker but more of the same. Used a signal generator at grid of 42 power tube, but same low volume. I assume it is this stage. Tried changing tube. I was thinking about removing the wires of the output transformer to give it an ohm check. Any ideas?
You should check all resistors to make sure they are within 20% of rated specifications. Also check voltages - something may be a bit off.
Additionally, the 39-6 should have a 41 output tube, not a 42. The 42 draws more filament current than the 41, which may tax the little power transformer in the 39-6.
As Ron sez pay close attention to the HV at the plate and screen grid. If low try a new 84 tube. Also measure the grid bias by connecting the + meter to the cathode and the - to the control grid. Should see around -8v or so. Too low grid bias will cause distortion and cause the 41 to draw excessive plate current. I took a quick look at the schematic and see they added a little negative feedback around the 41. The cathode is tied to the high side of the speaker and receives it's ground though the output transformers secondary. This is done to improve the low frequency response and lower distortion.
Terry
I restored a 39-6CI [painted white] Philco, that had been hit by lighting,plays geat now .I have an extra chassie if you need something feel free to let me know. chassie at present is missing the tubes, power transformer and the antenna coil ,what I bought it for .,BILL
Thanks for all your replies.
Here are the voltages I took today on tube 41 Output: filaments, pins 1 & 6=12.70 v ; pin 2 (plate)=12.7 v ; pin 3 (Screen Grid)=180 v ;
pin 4 (grid #1)=1 v ; pin 5 (cathode and grid #3)=12.79 v.
Pin #4 in relation to pin #5 = -10.6.
Tube 75, grid cap to ground = -3.6
Resistors within 20%
Also, I did change the 84 rectifier tube with a known good one.
Bill, thanks for your offer!
Thanks all for your assistance !!!!
Something is very wrong here, pins 1 and 6 should read 6.3 volts, and of course the plate voltage should be over 100 volts (measured to B-.)
The voltage readings of the pins of the 41 tube were with the common lead of the voltmeter against the negative end of the first power cap, which is 10 uf, taking place of the original 12 uf. I got nothing when I measured them to chassis. If you care to look into the belly of the beast, you can find it here:
https://picasaweb.google.com/lh/photo/pX...directlink
Thanks, Mike.
As Codefox said, the plate voltage on the 41 is way too low.
I looked at the photo. Watch out - the wires coming out of the power transformer are rubber-covered wires, and they are dried out and crumbling!
You can find a schematic easily enough (nostalgia air for a no frills Riders, free This is a Canadian Philco, otherwise you;ss never find it, or Chuck Schwark who is a moderator of this forum can get you a very much better one at a very modest cost,) either of which will show you the voltages you should be seeing. I think you have a bad output transformer, arcing tube socket, or the absolute worst #41 tube I have ever come across. Let's see what others have to say, but I would vote for an intermittent or corroded connection between the output transformer and either the B+ or plate winding end. Ohm iur out and let us know
Anyway it's a nice set, so take your time, be careful, use a variac and continue to measure everything.
At this point I'll make the suggestion that you may want to build your self a piece of test equipment. It would consist of the speaker (pm type) an output transformer (P/P type) and a filter choke. With these parts you can build a dandy replacement speaker for any set that you may run up with. Can be use to test speakers and output transformers.
Terry
Agree that transformer is open. Not a good idea to run the set like this with the 41 tube in place can damage the screen grid as it will acting like the plate and is not design to dissipate alot of energy.
codefox1 Wrote:This is a Canadian Philco,
Model 39-6C is a USA Philco model.
I think you are all right. It was the output transformer. Used a bone-yard output transformer that measured 300 ohms across input wires. It's bigger which would definitely handle the watts. Can it be used? Thanks!
If it sounds good, enough volume, and you have enough room for it, it's probably OK. You could always use a "universal" replacement transformer to match the plate/voice coil impedence more closely to the tube and speaker at hand, but it's not critical.
You can carefully dissasemble the old output transformer, and you might get lucky and find a bad connection on the primary side that you can fix. Anyway nothing to lose by trying.