[wil]

I'm trying to get a 37-650 up and running. I've replaced most of the resistors and capacitors and checked tubes. When I turn it on I : get a 'buzzing' sound and I notice that the 6F6 output tube gets hotter than normal and has what appears to be a bluish haze inside. I've switched the tube with another...same thing. Any ideas?
Bill

[Ron Ramirez]

You say you've replaced most of the capacitors and resistors...have you replaced the electrolytic capacitors? These are usually the culprit when the complaint is loud humming or buzzing.

Other possibilities: bad ground connection(s), heater-cathode short in a tube, poor solder connections, miswiring/a component mistakenly soldered to the wrong place.

The 6F6 tubes (the 37-650 uses two of these) will run hot; too hot to touch in normal operation. As for the blue glow inside, if the blue appears to be just inside the glass, than the tube is probably OK. But if the glow in inside the tube elements and more purple than blue, then the tube is definitely bad.

[wil]

Thanks Ron. I have replaced the electrolytic capacitors. I'm not able to get at paper caps buried in the tuner section. I've re-checked connections and components and everything looks to be where it should. There was a 'jumper resistor' that ran between the 2 6F6 tubes that wasn't on the schematic so I removed it. I've got to replace another resistor (spec is 9k 2w...reading at 14k). Of the 2 6F6 tubes, only 1 will glow blue (maybe more violet after looking closer). When I switch the 2 tubes, the other will go blue. It appears to be more about the socket? I tested both tubes and both are weak-marginal at best. Could bad tubes create the buzzing (definitely more buzz than hum)? I'm not sure this means anything but when I pull the tube from the location that glows and turn the radio on...silence. Replace the tube...buzz.
Bill

[Ron Ramirez]

Ah-HA! You say you only get silence if you pull one of the 6F6 tubes? That means your audio output transformer is likely bad, allowing B+ to reach only one of the 6F6G tubes.

A quick resistance check with a multimeter (with the power off and the set unplugged) will confirm this.

[philcoradio1234]

ron what does it mean when the tubes glow blue/violet? i have an old organ and the organ makes a huge buzz but because two of audio output tubes if the one is removed it will still work.

[Ron Ramirez]

As for the blue glow inside, if the blue appears to be just inside the glass, than the tube is probably OK. But if the glow is inside the tube elements and more purple than blue, then the tube is definitely bad.

Additionally: The bluish-pinkish-purple glow inside tube elements is caused by the tube becoming "gassy" (air has leaked inside, rendering the tube useless). A blue glow just inside the glass, not inside the tube elements, is not anything to worry about. This blue glow just inside the glass usually shows up only in audio output tubes.

[wil]

Ron,
I'm assuming the audio output transformer and the power transformer are one and the same. I'm not sure where to check so I checked the wire coming directly from the transformer to the socket in question. Zero resistance to ground. Did I check the right stuff and what should the reading be?

[Ron Ramirez]

No, the two are not the same.

The audio output transformer is mounted on the speaker frame. Hang on...

(goes downstairs to retrieve notebook of Philco schematics)

(returns and looks inside)

You should be reading around 700 ohms between each plate of the two 6F6G tubes with the speaker connected to the radio. If not (and I suspect you will see infinite resistance, based on your earlier comments), one half of that audio output transformer is bad.

[wil]

Ron
Really appreciate you hanging in there with me on this. I see the transformer on the speaker but being the novice I am I'm not clear on exactly where/how I should be measuring per your instructions. Also, tubes should be in their sockets when I measure...correct?

[Ron Ramirez]

With the radio unplugged and the speaker connected to the radio, measure resistance between pin 3 of one 6F6G tube and pin 3 of the other 6F6G tube.

Don't know which pin is pin 3? Look here:
http://www.nj7p.org/Tube4.php?tube=6F6G
Keep in mind that the pin layout shown in the link above are the pins as they look from underneath the radio (pin view).

For this measurement, it does not matter if the tubes are in their sockets or not.

[wil]

Checked pin 3 to pin 3 and got a reading of 670. Back to the drawing board...

[Ron Ramirez]

AAAARRRRRGGGGGHHHHH!!!!!

D**n it, that's what I get for having too many things going on at once...

Wil (Bill), I gave you incorrect information. I apologize.

I completely forgot that the 37-650 uses a "trick" audio circuit. To avoid using an audio interstage transformer, audio is fed from the 1st audio tube (6K5G) to the control grid of one of the 6F6G output tubes. Then, to achieve the phase inversion necessary in a push-pull audio output circuit, an out of phase signal is taken from this same 6F6G and fed to the control grid of the other 6F6G tube.

So, if you happened to remove the first 6F6G, you would have no sound. But if you pulled the other 6F6G while leaving the first one in place, you would still hear sound.

Therefore my original advice applies, which was:

Other possibilities: bad ground connection(s), heater-cathode short in a tube, poor solder connections, miswiring/a component mistakenly soldered to the wrong place.

When you replaced the electrolytics, did you connect the negative leads of (57) and (59A) to B- (center tap of the power transformer, and the ungrounded end of resistor (58) )?

Electrolytic (59) is drawn backwards in the schematic, also. The positive lead of this electrolytic (10 uF, 50 volts) should be connected to ground (chassis). The negative lead goes to B-. In other words, the opposite of how it is shown in the schematic.

Try those thoughts, take two aspirin and call me in the morning. I've got a migraine already.

[wil]

Ron
No problem...I learned about pin location. It must take a 'trick' to get this thing going. Checked the electrolytic's again. Negative of 57 and both 59's all connected to the ungrounded end 58. (by ungrounded I get a reading of .143 to grnd from that terminal...is that ok?) Positive of 59 (10 ohm) goes to 55 and 62 at ground. I'll recheck circuits and tubes. Is one one tube more prone to a short than another? Decided to have a beer instead of aspirin.

[Ron Ramirez]

Another thing to check for that I forgot to mention: poor ground connections. Philco often connected wires to chassis ground through a riveted connection, and corrosion can build up in the chassis hole where the rivet is. You may not see the corrosion but it is there, making ground connections go intermittent or bad altogether.

By the schematic, you should measure 138 ohms across resistor 58 (between B- and ground). If you are reading 143 ohms (not 0.143 ohm), that is close enough.

You should not read more than 138 (143?) ohms across electrolytic 59...the resistance of resistor 58.

[wil]

Took a break from the radio...one last question. In looking at the schematic I've got 2 views. There's an overhead view (from the bottom) that only shows the positioning and the schematic number of components (resistors, caps etc). It shows #46 330K resistor connected to pin 4 of right side 6F6 and #52 35K connected to pin 5 which is the way I have them. However when I look at the schematic (and I'm not the best at reading them) it appears to be the opposite? The other tube connections look like this: #47 and 53 to pin 5 of left side 6F6 (47 connected to pin 4 of right 6F6). If this isn't the issue then I've dead ended.