[DKinYORKpa]

DK, I replaced at ruinous expense two of those tubes that both measured between 8 and 10% on my cheap tube tester with NOS tubes. No real difference in performance. Of course they were "matched" for low emission!

So, is the temperature difference between them accounted for by their different emission quality?

[jerryhawthorne]

DK, not smart enough to answer that question with knowledge. I suspect some measurements on each tube to see how much current it is pulling would be helpful. Bias voltages can also change the heat inside a tube. Did you run them through a tube tester? Heating of a tube envelope has a couple (at least that I can think of) sources, heater output and emission probably plate voltage as well. How does the radio sound? Good volume, no distortion? If so I wouldn't worry about it much. But then again, what do I know? A testing of the tubes in a tester should give you a good idea of health, measuring some voltages and current draw could give you more. Good tubes with wrong voltages on them can cause the tubes to get real hot!

Jerry

[morzh]

Two sources: as you said, one is the heater output, and another is plate voltage times cathode-to-anode current, that is regular Joule heat.

[rocketeer]

[Image: http://s3.amazonaws.com/blixy-graphics/f...e-back.gif]

Larry

[Ron Ramirez]

The one 45 tests almost GOOD and the other one is down in the BAD. Does that explain why the one tube, while lit, is practically cold while the other one is hot?

Not necessarily. Check your audio output transformer. If one of the output tubes remains cold, I would suspect one half of the primary winding may be bad.

I'm not saying it couldn't be the tube because it certainly could be - but do check that audio output transformer before proceeding.

[DKinYORKpa]

About testing the output xformer.....does this not work with a DVOM?....when I measure from plate to plate of the 45s, which is all of the primary, I get nothing. When I measure from one plate to the choke I get nothing.

[Radioroslyn]

What you want to do is measure the voltage from each plate to the chassis. Both should like 200-250vdc. If only one does the output trans is bad. You could also measure from each grid to the chassis to be sure the driver is good too. Should see -25vdc or so.
Terry

[DKinYORKpa]

When I flipped the chassis over, I didn't re-plug in the speaker. So I get 145 ohms and 160 ohms now. I'll do the voltage measurements and report back.

[DKinYORKpa]

What you want to do is measure the voltage from each plate to the chassis. Both should like 200-250vdc.

At 115 VAC, I get 273 and 269 from the 45 plates. At full line voltage (121 VAC), I get 301 and 297. The Philco specs call for 230 VDC and a 45 is rated at 275 maximum. Why are my voltages so high? The grids are both -9. Why so low?........Unrelated, but with a ground wire connected, I was getting WBZ Boston, crystal clear from York, PA......another oddity: if you look at cap #11, it's supposed to go to the coupling caps on the 3rd and 4th RF transformers. It does not and doesn't appear to have ever. What am I missing here?

http://www.nostalgiaair.org/Resources/838/M0013838.htm

[morzh]

In the pushpull cascade when both halves of the primary exhibit similar (not equal) resistance to the Ohmmeter, this is good (albeit not 100% guaranteed) indication that the primary is OK. The inequality (about 10% or so) in the DC resistance is due to the fact that the second half is simply longer, being wound over a larger diameter of the first half, which in turn would on a slimmer bare bobbin.

As the real impedance (reactance) is determined by more or less the core size and the number of turns, the two reactances will be equal. The total impedance will be very close as the reactance will be order of magnitude larger than the DC resistance. And they are not added together but their squares are, and then the square root is taken.

Of course you can have a short turn which will not change (not necessarily) the DC resistance but will bring the reactance down to almost zero.
But first you have to establish you don't have a case of the open winding, and your ohmmeter is your best tool for that.

[jerryhawthorne]

DK, Mortzh explained the slight difference in resistance measurements for the output transformer's inputs. Your transformer looks to be fine. The slightly higher readings for the DC readings from each plate to ground on the 45s could well be due to measuring with a digital meter. Back in the old days when they measured these voltages and annotated them on schematics or tube data sheets, they were made with meters of the time, the good old analog VOMs. They loaded down the circuit a lot more than the modern digital meters and hence read lower voltages. I think your just fine. Not to panic. Now, could you please give us a summary of how the radio is working?
Jerry

[DKinYORKpa]

Now, could you please give us a summary of how the radio is working?

Flawlessly.......But, are those high plate voltages due to DVOM ?

[Ron Ramirez]

Jerry tried to explain, but I will try also.

The old-time meters used when these sets were new were rated at 1000 ohms per volt. These put a load on the circuit under test so that the readings were not really accurate, and displayed a voltage lower than it actually was due to the loading effect of the meter. But it was all they had in 1930, so they had to go with it.

Today's modern DMMs have input impedances in the megohm per volt range, which does not load down the circuit under test as did the old style 1000 ohm/volt meters, so the voltages read higher than published data of the time shows.

Factor in today's higher line voltages, and this is why your voltages are reading higher than they did in 1930.

If you had an older VOM with an input impedance of 1000 ohms/volt and used a Variac to reduce the line voltage going into your Philco 77 to 115 volts, and then measured the plate voltage of your 45 tubes with the old 1000 ohm/volt meter, you would get readings that are at - or close to - the published 1930 readings. These VOMs rated at 1000 ohms/volt were still being sold as late as the 1970s by such cheapie manufacturers as Calrad. By the 1970s, 20,000 ohms/volt had become the standard for VOMs. I remember seeing ads for these when I was a teenager and a beginner in this hobby. As I said, today's digital multimeters are rated at one - or several - megohms/volt.

[DKinYORKpa]

Factor in today's higher line voltages, and this is why your voltages are reading higher than they did in 1930.

OK, but, when I look in my RCA tube book and see 175VDC maximum
on a 45 plate, did they come up with that number on an old meter, as well?

And nobody took on this one:

if you look at cap #11, it's supposed to go to the coupling caps on the 3rd and 4th RF transformers. It does not and doesn't appear to have ever. What am I missing here?

http://www.nostalgiaair.org/Resources/838/M0013838.htm

[Radioroslyn]

#11 is the plate bypass cap for the two rf amps. It stop any rf signals from getting in to the hv. If you don't see it it may be inside the filter can.
Terry