Cathode bias amp variable resistor

[Colas LeGrippa]

Hello boys and gals !


All of my builds are cathode biased. I find the tone is  warmer, richer. But adjusting the bias is a hassle everytime I replace tubes or swap them for another type. I'd like to find a variable resistor from 200 to 600 ohm or so, 10 watts or so,  that I could use to bias my amps, or any circuit  that is designed specifically for that purpose.  I even tried a rotary switch with different resistors values but  it takes too much room inside the chassis (dont suggest me to put it outside....)
What could I do ?
Thanks


Colas

[sluckey]

https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=10+watt+500+ohm+rheostat&_sacat=0&LH_TitleDesc=0&_odkw=10+watt+rheostat&_osacat=0

[Colas LeGrippa]

You're my man.


Linda will be happy to see I am  happy.


Colas

[AmberB]

I would recommend that when you put the rheostat into the amp that you also put in a minimum cathode load resistor between the rheostat and ground, perhaps a 200 ohm.  You don't want to make the mistake of dialing the rheostat down to zero ohms while adjusting the bias, that would be rather bad for the output tube, or tubes.

[Latole]

Rheostat; follow AmberB advice.

You wrote no enough room for a rotary switch and resistor , That is my first choice, power rheostat with a single resistor ( AmberB) need some room too.

[PRR]

Use leverage. Use a 10k 1/4W pot in the grid feed. I know I have posted that plan many times.

[Colas LeGrippa]

I've never heard of that, PRR.  Wait a minute,  i'll ask Linda....




No, she's not aware of that method to bias a power tube either.


Now that you put me on a track, you havet to explain more to me...but  you said you've already posted something about it, here ? Lemme see....

[PRR]

.

[AmberB]

First time I've seen this myself, I had to save a copy of it for possible future use.

[kagliostro]

Thanks PRR

I must say I don't remember to have seen that configuration previously

There are explanations on how that sistem works ?

Can we consider it as  a form of Mixed Bias ?

And, more, is it a sistem that is doable also in SE amps ??

EDIT: Me stupid, Mixed Bias requires a negative voltage ......

Thanks

Franco

[Williamblake]

I think this is how it works, Franco is right. It changes the bias, you can reduce the negative grid to cathode potential with the voltage divider.

[Colas LeGrippa]

PRR, would'nt you be the keeper of old electronic knowledge ?
I can imagine you with a beard and a long dress, wearing sandals, living in a.stone built castle surrounded by thousands of dusty electronic theory books that you never open , each and everyone of these.books being recordedi in your brain cells....


PRR,  you oughta reveal us your real identity..
P stands for "PRIEST" ??? RR for what? 


Colas

[kagliostro]

PRR,  you oughta reveal us your real identity..

I know what you common mortals (as I'am) would like to know
But can't reveal, I swore to the high master not to reveal it 😉

Franco

[Colas LeGrippa]

Oh....btw,   thanks .

[Colas LeGrippa]

To the long bearded priest, keeper of  the  sacred knowledge :  would it be possible  to verify in your dusty book about adjustable bias,
if you have posted the right schematics for a cathode biased guitar amplifier, please ?
I tried and I tried, unsuccessfully to make it work...

[kagliostro]

That way to adjust bias requires an increased signal to drive the power section

However I don't know if this is your problem

Franco

[HotBluePlates]

There are explanations on how that sistem works ?

Can we consider it as  a form of Mixed Bias ?

Whether you consider it "mixed bias" is probably philosophical:  is "fixed bias" when a voltage is applied to the grid, or is it when there is a negative voltage supply that is applied to the grid?  I would call it "cathode bias."

Overall, bias is just "a voltage between cathode and grid" that keeps the grid more-negative than the cathode to tame plate current.

With PRR's plan, most of the "voltage between cathode and grid" is developed by the 180-220Ω cathode resistor (value due to the note about increasing the value).  This biases the tube "too cool."

There is a 2nd path for cathode current:  through the 10kΩ resistor and 10kΩ pot.  Voltage across this path must be the same as the voltage through the main cathode resistor noted above.  Current is very much smaller due to the high resistance.

There is a moving-tap across half of the extra resistive-path:  We can set that tap anywhere from 0v to half the bias voltage created by the main cathode resistor.

Now the moving tap gets connected to the grid:  Placing the grid at some positive voltage effectively subtracts from the voltage developed across the cathode resistor.

   +10v cathode, 0v grid:  grid is effectively -10v of the cathode.
   +10v cathode, +5v grid:  grid is effectively -5v of the cathode.

The grid can be placed at 0v (full bias volts, minimum plate current), at half the cathode-resistor voltage (least bias volts, maximum plate current), or anywhere in-between.  Modifying the effective volts at the grid in this manner is more-impact than trying to do the same at the cathode, doesn't have as much current flowing through the pot, and so doesn't require a high-wattage part.

I must say I don't remember to have seen that configuration previously

Sure you have:  Williamson Amplifier (except that was for balancing current among 2 push-pull tubes, so it looks slightly different).

... if you have posted the right schematics for a cathode biased guitar amplifier, please ?
I tried and I tried, unsuccessfully to make it work...

Try again, you're doing something wrong.

I have used the "Williamson version" in an amp build, and it worked well to change current in the output tubes (meaning the "balance" part of the circuit to which the grids are connected).

[Colas LeGrippa]

It woudld be very surprising that I could'nt follow a so simple schematics.
In fact, it works, but from 90mA to120mA which is definitey too high a bias for a plate to cathode voltage of 390V, for a pair of EL34..
I tried to change the 120R and 10k resistors for other values without satisfying results.
So I put back my Hi-LO switch with a 240R and 300R 25W resistors .
Colas

[PRR]

If 240 and 300 worked for you, why would you use 120? (That was for EL84.)

Use your 300r.

Leave the 10k the same. The actual value does not affect bias, it s just a voltage divider feeding near-infinite impedance.

[Colas LeGrippa]

That was the info that was missing for me,  surely obvious for others


Thanks for details


Colas