I'm recapping this Remler "early" model (bottom of page schematic)
http://www.nostalgiaair.org/PagesByModel/419/M0016419.pdf
On the 6f6 audio output tube I found the top resistor of two grid bias resistor-divider that was supposed to be connected to the grid.. disconnected ...because the resistor fell apart... yet the radio sounds perfect... and when I do replace the proper resistors and the grid bias reads the proper -19v... there is still no difference that I can notice by ear.
Why doesn't this 6f6 seem to care about bias on the grid?

Hi Peter,

Short answer is I don't know. Assume you are referring resistor 4. You would have no negative bias on the 1st audio either and the B+ would have been pulled down a good bit as well. Imagine the power transformer was getting a bit toasty too. Always like to make basic voltage checks especially the final audio stage to insure the radio is healthy.

Richard

No Richard.. I refer to bias resistor 17.. for the grid bias on the 6F6 final audio tube.

:Hi Peter,
:
:Short answer is I don't know. Assume you are referring resistor 4. You would have no negative bias on the 1st audio either and the B+ would have been pulled down a good bit as well. Imagine the power transformer was getting a bit toasty too. Always like to make basic voltage checks especially the final audio stage to insure the radio is healthy.
:
:Richard

Have you recapped the radio yet? If not, what could be happening is the grid could be biasing itself when, left wide open, with the grid leak method. Now, when a grid is left wide open, it will eventually bias itself to cut-off. However, if you didn't replace the capacitors yet, and the coupling capacitor to this grid is leaky by just the right amount, the positive voltage it leaks might be just enough to compensate, and keep the grid negative by just the right amount.

T.

:Have you recapped the radio yet? If not, what could be happening is the grid could be biasing itself when, left wide open, with the grid leak method. Now, when a grid is left wide open, it will eventually bias itself to cut-off. However, if you didn't replace the capacitors yet, and the coupling capacitor to this grid is leaky by just the right amount, the positive voltage it leaks might be just enough to compensate, and keep the grid negative by just the right amount.
:
:T.

Thanks Thomas:
Well no, I have not recapped it yet..(just replaced those two (r17 & r18) 250k ohm bias resistors now) .. however someone before me DID replace (c32) that .02 coupling cap.
It's a nice new orange drop.. so I doubt it would be leaking.

follow up.

I just removed that bias resistor agin to test and found that the bias voltage goes from -19vdc to -.7vdc

And I notice that commensurate with that the 250v B+ drops to 175vdc.
So I guess when that bias becomes so much more positive it must be slamming the 6F6 toward saturation drawing tons of current and pulling the B+ way down to 175v ... However... as I said initially... the radio still sounds just "peachy" and beautiful.

And as soon as I touch that R17 bias resistor back to the grid where it belongs..the B+ jumps right back up to 250vdc as well and the bias voltage returns to -19v as it should.

In any case it seems that that 6F6 tube could be running away with itself like that forever and until the current cooked the power tranny or the output tranny or something... the listener would not be the wiser until the smoke alarm went off, I guess....lol.

Peter

With no leakage on the coupling cap negative bias can be generated by an audio signal. The audio signal can't go positive since positive causes the grid to draw current. The input signal will cause the cap to charge in negative direction.

With audio turned down the grid voltage should be near zero.

Norm

:follow up.
:
:I just removed that bias resistor agin to test and found that the bias voltage goes from -19vdc to -.7vdc
:
:And I notice that commensurate with that the 250v B+ drops to 175vdc.
:So I guess when that bias becomes so much more positive it must be slamming the 6F6 toward saturation drawing tons of current and pulling the B+ way down to 175v ... However... as I said initially... the radio still sounds just "peachy" and beautiful.
:
:And as soon as I touch that R17 bias resistor back to the grid where it belongs..the B+ jumps right back up to 250vdc as well and the bias voltage returns to -19v as it should.
:
:In any case it seems that that 6F6 tube could be running away with itself like that forever and until the current cooked the power tranny or the output tranny or something... the listener would not be the wiser until the smoke alarm went off, I guess....lol.
:
:

Yes Norm,
without the bias resistor properly connected to the grid... the grid sits at about -.7v when the volume control is very low or min.

.. but this is such a "minimum" negative bias...(almost positive condition) and that must be turning on the 6F6 a lot... and that must be what's pulling down the B+ from 250v to 175vdc at that time... right?

But regardless... the radio sounds great.

I just thought that without the proper bias I'd be able to detect something was wrong by funny sounding audio or something... y'know?

Peter

Your output tube is new or in very good condition. After some time grid leakage will show up. The grid resistor overcomes this leakage.

With less negative grid bias the tube draws more plate & screen current. This loads down B+ voltage. Best to operate with higher grid bias. Tube will last longer and power transformer won't be as warm.

Norm

:Yes Norm,
:without the bias resistor properly connected to the grid... the grid sits at about -.7v when the volume control is very low or min.
:
:.. but this is such a "minimum" negative bias...(almost positive condition) and that must be turning on the 6F6 a lot... and that must be what's pulling down the B+ from 250v to 175vdc at that time... right?
:
:But regardless... the radio sounds great.
:
:I just thought that without the proper bias I'd be able to detect something was wrong by funny sounding audio or something... y'know?
:
:

:Peter
:
: Your output tube is new or in very good condition. After some time grid leakage will show up. The grid resistor overcomes this leakage.
:
: With less negative grid bias the tube draws more plate & screen current. This loads down B+ voltage. Best to operate with higher grid bias. Tube will last longer and power transformer won't be as warm.
:
:Norm
:

Yes Norm certainly...
I didn't intend to ever run it without the proper grid bias.
My question was... about.. when the grid bias resistor broke and wasn't connected... why didn't I seem to hear any ill effects from it?
I thought that without the proper grid bias (class A) at quiescence ...that the tube would no longer be operating in the center of it's linear range.
And since without that grid resistor..it's biased so close to zero volts ....turning on hard at idle.. then why doesn't the audio seem to sound clipped ( since it must be going into saturation) when the volume is higher or normal?

::Peter
::
:: Your output tube is new or in very good condition. After some time grid leakage will show up. The grid resistor overcomes this leakage.
::
:: With less negative grid bias the tube draws more plate & screen current. This loads down B+ voltage. Best to operate with higher grid bias. Tube will last longer and power transformer won't be as warm.
::
::Norm
::
:
:
:Yes Norm certainly...
:I didn't intend to ever run it without the proper grid bias.
:My question was... about.. when the grid bias resistor broke and wasn't connected... why didn't I seem to hear any ill effects from it?
:I thought that without the proper grid bias (class A) at quiescence ...that the tube would no longer be operating in the center of it's linear range.
:And since without that grid resistor..it's biased so close to zero volts ....turning on hard at idle.. then why doesn't the audio seem to sound clipped ( since it must be going into saturation) when the volume is higher or normal?
:

I wish I could tell you why, but I sure agree with you, the audio ought to stink after a couple of minutes. I thought about the coupling cap, but an orange drop almost gets a free pass. My professional answer is "hell if I know".
Lewis

Peter, Take a look at the tube characteristics graph in the pdf link below. Appears the 6F6 tube will have linear current over a wide range of grid bias and plate voltages. Current is what any transformer responds to. Of course still want the 6F6 to operate with high negative bias to minimize load current on the power transformer.

Richard

http://www.mif.pg.gda.pl/homepages/frank/sheets/049/6/6F6.pdf