Please help me bias this amp, see schematics

[paintballnsk]

Hey guys,

I'm confused how to bias this amp. This amp is based on the soldano atomic 16, but i'm confused at the additions here. How do I bias this? it's EL84. Is one of these Grid biasing and one of them Cathode Biasing?

I've never biased an amp before, but I have heavily modified this one. So I want to re-check and adjust if necessary.

Thanks again

[PRR]

Voltage on 180 ohm resistor?

[Leevi]

Measure the values like plate and cathode voltage and use the following calculator:


https://robrobinette.com/Tube_Bias_Calculator.htm


/Leevi

[paintballnsk]

13.32V with the master at 0. 29.9V with the master dimed. It looks like that 180 Ohm is actually 270. the schematic is wrong or it was already changed by someone else.

[paintballnsk]

Also plate voltage is 360 according to the diagram

[Latole]

13.32V with the master at 0. 29.9V with the master dimed. It looks like that 180 Ohm is actually 270. the schematic is wrong or it was already changed by someone else.

Bias should be made with no signal ; all volumes to 0

Wich resistor values is the good one ?

Schematic is one thing, tubes need is another thing. That is why amps may need new bias with new tubes.
A must for  A/B class amps, less for A class amp .
And user may want a hot bias or a colder bias or medium one.........

[Latole]

I don't undurstand how the amp can work with open switch an no ground !!

[JB]

I don't undurstand how the amp can work with open switch an no ground !!

The triangle symbol is ground, the switch is drawn in the "Standby" mode. 

[Latole]

I don't undurstand how the amp can work with open switch an no ground !!

The triangle symbol is ground, the switch is drawn in the "Standby" mode.

Oh ! It is a stanby, great. First time I see standby switch there.

[paintballnsk]

13.32V with the master at 0. 29.9V with the master dimed. It looks like that 180 Ohm is actually 270. the schematic is wrong or it was already changed by someone else.

Bias should be made with no signal ; all volumes to 0

Wich resistor values is the good one ?

Schematic is one thing, tubes need is another thing. That is why amps may need new bias with new tubes.
A must for  A/B class amps, less for A class amp .
And user may want a hot bias or a colder bias or medium one.........

It's 270. So at 270 that puts me at 70% dissipation. If I wanted a little more headroom, I could make that 300, which would put me at 63%. Maybe I'll go grab a 250 resistor and a trim pot. Anyone know where I can find a 100 OHM trim pot that can handle the power there? All the ones I see are only .5 watt rating.

[sluckey]

You can get a high wattage pot (rheostat) at Mouser. Not worth it IMO. Also, cathode biased power tubes typically run 90% to 110% of max. And EL84s typically use 130Ω to 150Ω cathode resistors.

[pdf64]

...If I wanted a little more headroom...

In this context, ‘headroom’ doesn’t have a precise definition that can be related to a technical characteristic; what exactly do you mean by it?
eg guys technical use ‘more headroom’ when describing changes that may reduce gain, or increase (clean) power output.

[paintballnsk]

I want the EL84's to have more dynamic range and less distortion. That's what I mean by headroom. More plucky pick attack, less mush.

[pdf64]

I want the EL84's to have more dynamic range and less distortion. That's what I mean by headroom. More plucky pick attack, less mush.

I interpret that as meaning you’d like the amp’s power to be higher?
If so, increasing the cathode bias resistor value will tend to work counter to that.
Rather it would be better to move to fixed bias.

[paintballnsk]

So help me out. Based on the schematics, how would I do that?

[shooter]

EL84's can only do so much, any tube for that matter
you have a "catch22" loop;


if you have more vdc at the cathode (bigger Rk) you might get a smidgen more clean before distortion.
the "drive" signal is about all you really can tweak to "keep it cleaner" otherwise.


that said, if you want to play;


try dropping the G2 1k to 470 ohm
try bumping Rk up in steps of ~~~~50ohms
if that's the wrong way for you, start bringing it down by 50
ALWAYS do math with each R change, so
VDC at cathode / cathode R = ~~~~tube current * plate VDC = tube dissipation (anything close to 70-90% is good :)  Remember you have 2 tubes and 1 R so current is "halved" to each tube


only make 1 change and evaluate before moving to the next, if you didn't like the change, put it back and re-test BEFORE moving on

[tubeswell]

So help me out. Based on the schematics, how would I do that?

Cathode-bias involves achieving a negative voltage at the grid - with respect to the cathode - by using a resistor in series with the cathode to make the cathode voltage positive - with respect to the grid (I.e. the ‘negative’ voltage at the grid is relative to the positive voltage at the cathode, even though the absolute voltage at the grid is the same as the ground voltage).


Fixed-bias involves achieving an absolute negative voltage at the grid by creating a negative VDC power supply, and connecting the cathode directly to ground. With this method, there is more plate-to-cathode voltage for a given B+ voltage, so more gain is possible (than with cathode-bias that relies on the same B+ voltage). So you need to disconnect the ground side of the EL84 grid leak resistors from the ground, and run these to a negative DC voltage supply point - which you need to make by creating a backwards rectified voltage that is negatively filtered* - and you need to remove the cathode resistor (and cathode bypass cap and standby switch) and connect the cathode(s) directly to ground.


* The amount of negative voltage you need is typically about 1/10th of the B+ voltage at the plates (but it needs to be a negative voltage). It should be an adjustable negative voltage so you can dial-in the optimum amount of tube current to get between about 62-85% plate dissipation, depending on your personal taste 👅 and life goals.

[pdf64]

Alternatively the self bias at the cathode can be made fixed by connecting a suitable zener in parallel with the existing bias resistor//cap. eg zener voltage somewhere between 15 and 20V, rated at least 5W.
That will stop the cathode voltage rising too much at high power output levels, and so enable a lower idle current to be used without compromising performance.
85% idle dissipation in AB fixed bias, when the plate voltage is already significantly above the limit, seems excessive?

[mresistor]

Fender bias circuit info  as some examples for you    you would probably have to play around with the bias range resistor, the one attached to the PT secondary winding.   
And if I converted from a cathode bias to fixed bias I would use 1 ohm resistors to ground on the power tube cathodes to make bias measurement easier.


https://el34world.com/charts/Biascircuits.htm