I'm working on a very nice (though dusty and totally cruddy)old Howard 220/270 that came in with a very loud scary rattling buzzz and an 85 tube where the schematic indicates an 75 ought to be. Not knowing any better my first assumption was that "perhaps" it's an acceptable substitute. So I went on checking things. I also see a broken/disconected wire from the 2nd filter cap in the power supply up to ( or it appears to be) disconnected from one of the several contacts at the speaker.
i found the loud buzz was from the power transformer suspiciously only snugly held down with one bolt and the other bolt almost falling out. So it was rattling but tighteng the bolts made it silent.. and no heat.

Well the first question is can an 85 tube ever work in here? Even though they are both "similar type" tubes I cannot find anything to support it as a substitute for the 75.
The B+ at the power supply was fine 280v at the 1st cap and 180 at the other side of the field coil.

There was just a very faint signal with ther vol at max.
I measured 280v at the plate of the 41 but only 20v on the plate of the 85 !!
What could cause that? Max current draw through the 85 sucking down the B+?
I puuled the 85 and left it out. Sure enough the B+ went right up to 280v...
So I put a 75 tube in there and sure enough.. NOW the plates of the 75 went right up to about 140-150v and the 42 is right at around 180v as the schematic shows.
AND.. after cleaning the vol control abit it now is making nice almost normal music... with the long wire antenna stretched out there a bit (8-10ft).
I haven't recapped anything yet... so that's

Now the main question is just that 180v loose wire from the power supply. If I connect it to the point at the speaker that it appears to have come off of... there is no difference in sound... on or off that point.
So what is this wire?
And why is the 3rd winding shown as it is in the schematic?.. and not shown connected to anything else except itself ( internally? in series with the voice coil)??

By the way... that 85 tube ..all sections tested good on my Precision 10-20 tube tester

Hi Peter

A 75 & 85 tube are interchangeable in most radios. The 75 has higher gain but draws less current. Since you have a high value plate resistor and 85 tube it draw more current, lowering voltage.

Here is the schematic for others to view:

http://www.nostalgiaair.org/PagesByModel/218/M0009218.pdf

Not sure what you mean about a 180 volt wire? Could someone have added and then removed electrolytic filter caps? They could have left a wire?

Norm

:I'm working on a very nice (though dusty and totally cruddy)old Howard 220/270 that came in with a very loud scary rattling buzzz and an 85 tube where the schematic indicates an 75 ought to be. Not knowing any better my first assumption was that "perhaps" it's an acceptable substitute. So I went on checking things. I also see a broken/disconected wire from the 2nd filter cap in the power supply up to ( or it appears to be) disconnected from one of the several contacts at the speaker.
:i found the loud buzz was from the power transformer suspiciously only snugly held down with one bolt and the other bolt almost falling out. So it was rattling but tighteng the bolts made it silent.. and no heat.
:
:Well the first question is can an 85 tube ever work in here? Even though they are both "similar type" tubes I cannot find anything to support it as a substitute for the 75.
:The B+ at the power supply was fine 280v at the 1st cap and 180 at the other side of the field coil.
:
:There was just a very faint signal with ther vol at max.
:I measured 280v at the plate of the 41 but only 20v on the plate of the 85 !!
:What could cause that? Max current draw through the 85 sucking down the B+?
:I puuled the 85 and left it out. Sure enough the B+ went right up to 280v...
:So I put a 75 tube in there and sure enough.. NOW the plates of the 75 went right up to about 140-150v and the 42 is right at around 180v as the schematic shows.
:AND.. after cleaning the vol control abit it now is making nice almost normal music... with the long wire antenna stretched out there a bit (8-10ft).
:I haven't recapped anything yet... so that's
:
:Now the main question is just that 180v loose wire from the power supply. If I connect it to the point at the speaker that it appears to have come off of... there is no difference in sound... on or off that point.
:So what is this wire?
:And why is the 3rd winding shown as it is in the schematic?.. and not shown connected to anything else except itself ( internally? in series with the voice coil)??
:

Thanks Norm:
I just tried putting that 85 tube back in there and yes the audio dies right away again and it's plate drops to 20vdc.
On....that wire near the humbuck...
Sorry if I didn't quite make myself clear about this wire... lol
I called it a 180v wire but only because it comes up off the second filter cap where the voltage is 180vdc at that point.
Anyway it goes straight up along side of the speaker and looks like it broke off from a contact point there.

The output side of the audio transformer has one lead that goes directly to a contact point where it is soldered to the voice coil of the speaker... okay

The other lead on the secondary of the audio transformer is soldered to a contact point that joins to one lead of the humbuck coil and then the other lead of the humbuck coil is soldered to a contactpoint where it is joined up with to the other lead of the speaker voice coil.

Now the wire I'm confused about comes up from the 2nd filter cap ( with 180vdc on it) and seems like it broke off of the contact point leading into the humbuck coil.
But this doesn't seem to make any sense.
It is positioned right next to and looks like it broke off from the solder point where the secondary of the audio transformer connects to the humbuck coil.
The schematic shows no such wire.
...and re-connecting it there has no effect eitherway on the sound as far as I can tell.
I just double-re-checked and sure enough I can see a few tiny broken strands at that solder joint that appears that this wire broke off from and it is positioned right there perfectly too.
But I still don't see why. Is there any good reason to ever connect the 180vdc from the powersupply to the secondary of an audio transformer ever?? if it has a humbuck coil in there especially?.

Hi Peter

If your tubes have plate & screen voltage this wire isn't needed. Pin #2 is plate & #3 screen on tubes.

The hum bucking coil is in series with your voice coil and doesn't connect to voltage. If you are connecting it to this circuit nothing will happen. Secondary of the output transformer, hum bucking coil and voice coil are all in series. All this isolated from everything else in the radio.

It's not unusual for additional filter caps to be added to a B+ line. This could have happened in the past and the wire not been removed.

Have you aligned the radio? Just peaking up IF Transformer adjustments may bring up volume.

Norm

:Thanks Norm:
:I just tried putting that 85 tube back in there and yes the audio dies right away again and it's plate drops to 20vdc.
:On....that wire near the humbuck...
:Sorry if I didn't quite make myself clear about this wire... lol
:I called it a 180v wire but only because it comes up off the second filter cap where the voltage is 180vdc at that point.
:Anyway it goes straight up along side of the speaker and looks like it broke off from a contact point there.
:
:The output side of the audio transformer has one lead that goes directly to a contact point where it is soldered to the voice coil of the speaker... okay
:
:The other lead on the secondary of the audio transformer is soldered to a contact point that joins to one lead of the humbuck coil and then the other lead of the humbuck coil is soldered to a contactpoint where it is joined up with to the other lead of the speaker voice coil.
:
:Now the wire I'm confused about comes up from the 2nd filter cap ( with 180vdc on it) and seems like it broke off of the contact point leading into the humbuck coil.
:But this doesn't seem to make any sense.
:It is positioned right next to and looks like it broke off from the solder point where the secondary of the audio transformer connects to the humbuck coil.
:The schematic shows no such wire.
:...and re-connecting it there has no effect eitherway on the sound as far as I can tell.
:I just double-re-checked and sure enough I can see a few tiny broken strands at that solder joint that appears that this wire broke off from and it is positioned right there perfectly too.
:But I still don't see why. Is there any good reason to ever connect the 180vdc from the powersupply to the secondary of an audio transformer ever?? if it has a humbuck coil in there especially?.

Thnk you Norm..
Actually now the volume is reasonably good already Norm ...now that the 75 tube is in as its supposed to be.

I haven't otherwise changed or replaced a thing yet. Not even the dual 10uf/350v filter-cap canister. But I'm about to start the re-capping everything now.
I'll re-align everything after re-capping.

Mechanically...So far I have just removed the dial face and mounting brackets and dial cord etc in preparation for cleaning and easier access as I try to clean up the entire chassis.

But that wire I was questioning puzzled me mostly because it extends up thrpugh a hole to the above chassis area and near to the audio transformer.... ....hummm.

AHhah... I've got it now... lol
upon closer examination and seeing other people's sloppy work.. I now understand what that wire was for.
Apparently the audio transformer was once replaced! I can see now...how the new one was soldered to the speaker where the old one was removed.
The new input leads were sloppily "tack" soldered on. And one "tacked-on" lead comes right from the 2nd filter cap where the mystery wire is still soldered. .... BUT that mystery wire was just the old lead that was never properly removed ... just snipped off right at that solder lug near the humbuck coil... but so close to a solder lug..that it looked like it came off there... lol
So that's it... the old audio output lead..sloppy job!
confuses my poor little mind... lol

Check your resistors for drifting, too. I have never tried exchanging 75 and 85 tubes with eachother, so I don't know what results to expect. If the plate resistor is drifted, though, it can be loaded down easily. Also, you won't know for sure how this set will perform with a #85 tube until you replace all leaky condensers. Leaky condensers can load down the plate resistor. A leaky resistor in the grid circuit can also cause the tube to conduct abnormally. The #85 tube may conduct more easily with any given grid signal, causing a severe drop in plate voltage.

....These are all just things to think about for now...check all of those condensers and resistors. Then, just for the heck of it, once you get the radio working perfectly, you can try out that #85 tube again.

Thomas

Great idea Thomas... I will definately do that... thanks

Ok Thomas or any one elses thoughts:
(BTW... the general low-ish volume was cause by a broked hair wire on the antenna coil that I discovered quite by accident as I was re-dressing wires with a stylus and the volume burst up loudly intermittently...now re-solderd.. fine)
But as far as the 75 tube vs the 85...
I have everything replaced capacitor-wise and measured and changed all resistors in that area too.
With the 75 tube in..the radio plays beautifully and loudly 150v on the plate of the 75.. the 85 however is 1/2 the volume .. and the plate drops to 20 volts.
They both have about -1.16v on the grid cap measured w/respect to the cathode.

So what's the reason that the plate drops to 20v?

All I've found so far about the differences between 75/85 is:

"The 75 is similar to both the 55 and the 85, except that the triode sections of the 55 and 85 have a mu of 8.3, while that of the 75 has a mu of 100."

I can't imagine why the tube is dropping the voltage so low. Your plate resistor has a rather low resistance as far as they go. It's almost as though there's a short or leakage in the tube itself. You didn't wash the tubes in water, did you? I wish I had a #85 on hand that I could pop into my Philco 60. Hopefully someone else on here has some better ideas. I can't imagine that the tube is supposed to perform that way, but perhaps it is.

Thomas

No I haven't washed the tubes...

The 85 shows good on my tube tester as well.

But the fact that the 75 has 10 times the gain... and draws so much less plate current???
I don't understand exactly... but maybe this bias causes the 85 to draw 'mucho' current?

That could be. That could be why it has so much less gain, too. A tube that has more gain usually has a more sensitive control grid. That is, less voltage in either direction is needed to get a change in plate current in either direction. I think you hit the nail right on the head here. A tube with less gain will need a larger voltage on its grid in order to create the same signal at its plate that a tube with more gain would create. With that in mind, the control grid may have to be held much more negative in order to keep the plate voltage high (and to hold down distortion, since having the plate voltage at 20 doesn't give you much room to work with).

Ha! Looks like this time you answered my question!

Thomas

Take a look at the plate current and the plate resistance of the 75 vs 85 tube. The 75 has a plate resistance of 91000 ohms and the 85 has a plate resistance of 25000 ohms. Depending on configuration, the 85 will draw between 3.7 to 8 ma while the 75 will draw .8ma. Could this explain the voltage drop you are seeing.
MRO

Well there is a 250k plate resistor dropping 200 volts... that calculates to only .8ma right? So the 85 then is drawing .8ma
And the 75 has a plate of 150v or so it's only dropping about 70 volts... with the same 250k plate resistor that's .28ma

I don't fully understand what the correct voltages are supposed to be if the 85 is considered an acceptable substitute for the 75

Maybe this is just normal?

Hi Peter.

In the for what it may be worth department, all my tube substitution books (around 12 of them, Sams and other) there is no recommended sub for the 75 other than a 75S. No other recommended sub for the 85 other than an 85S.

Radiodoc

:Well there is a 250k plate resistor dropping 200 volts... that calculates to only .8ma right? So the 85 then is drawing .8ma
:And the 75 has a plate of 150v or so it's only dropping about 70 volts... with the same 250k plate resistor that's .28ma
:
:I don't fully understand what the correct voltages are supposed to be if the 85 is considered an acceptable substitute for the 75
:
:Maybe this is just normal?
:

Thanks Doc:
That's exactly what I had found, with my first quick search, and so indicated in the second paragraph of my first posting questioning the substitution in the first place.
But then Norm and Thomas sort of indicated that there is some common usage of the 85 as a substitute for the 75... but Norm also indicated why it would draw more juice.

If you again look at the tube ratings you will see that you are operating the 85 close to full conduction. At 250 volts on the plate (I know your radio is not operating at 250 volts this is from the tube ratings) the 85's grid voltage is supposed to be -20 volts. The 75's is supposed to be -2 volts. At those ratings the 85 should draw EIGHT ma and the 75 should draw POINT EIGHT ma. With your grid only slightly negative your 85 tube is at almost full conduction.
MRO

You are right mark.. see my answer to Doc above.
I was just chasing this down throughly as Thomas suggested..because he too has seen the two substituted...
Although it works ( after a fashion and in a pinch maybe) I just couldn't see why it would be viable alternative based on no documented evidence suggesting it and on my readings...
So I've just been poking around trying to make sense of it all.
So thanks to you all for you help.. but I guess we all now know that the answer is:
"NO" .. it's not an acceptable substitute... No matter how hard you want it to be... and even if you discover it in your radio.... pull it out like a loose tooth...lol
Thanks yoooooz guyz