Can someone tell me how to calculate the needed Neg bias voltage for a 6l6gc push pull amp?
More or less a Fender style amp section.
I know how to bias the tubes I just don't know how to calculate what the bias voltage should be.
More or less a Fender style amp section.
I know how to bias the tubes I just don't know how to calculate what the bias voltage should be.
The voltage will vary depending on the tube.
You probably want to bias it for a given current.
The sheet will give you a basic voltage for the current...
You probably want to bias it for a given current.
The sheet will give you a basic voltage for the current...
Some thoughts.
It depends on which Fender this amp is a clone of. Fender schematics with 6l6 show bias points 15 -20 Volts either side of 45 .
How to calculate the bias ? How do you calculate the diameter of the pipes for water in 'the project' ? It all depends.
The circuit that the tubes are in should be, like your Fender design, "more or less" designed to run the tubes in a chosen zone within their range of possible function and that design would include specification of the bias voltage needed to run them there.
There are different Fender amps. Do you have a schematic, or name/model of the actual circuit? The more you can say about the circuit you have the better help you'll get.
The posted data sheet gives some suggested operating points for the 6l6 in push pull but without knowing what high voltage supply (B+) there is and what the biasing circuit is and what its component values are you're not likely to get an answer guaranteed not to melt something in your amp.
If you look through the schematics here or here and can find one that matches your amp, you have a place to start looking, but again, it all depends on where the designer is deciding to run the tube and what the components used are specified to handle.
It depends on which Fender this amp is a clone of. Fender schematics with 6l6 show bias points 15 -20 Volts either side of 45 .
How to calculate the bias ? How do you calculate the diameter of the pipes for water in 'the project' ? It all depends.
The circuit that the tubes are in should be, like your Fender design, "more or less" designed to run the tubes in a chosen zone within their range of possible function and that design would include specification of the bias voltage needed to run them there.
There are different Fender amps. Do you have a schematic, or name/model of the actual circuit? The more you can say about the circuit you have the better help you'll get.
The posted data sheet gives some suggested operating points for the 6l6 in push pull but without knowing what high voltage supply (B+) there is and what the biasing circuit is and what its component values are you're not likely to get an answer guaranteed not to melt something in your amp.
If you look through the schematics here or here and can find one that matches your amp, you have a place to start looking, but again, it all depends on where the designer is deciding to run the tube and what the components used are specified to handle.
It also depends on how long you want you OP valves to last, run em at 110% like some guitar amps do and they won't last long. The general rule of thumb is to bias them at 75%, so for a 25w valve like the 6L6 if your HT/B+ is 450v, that would be 55mA. However that's anode current not cathode current & that quick calc doesn't take into consideration the cathode voltage which lets say is 50v, that would mean the potential across your valve is 400v, not 450v.
I usually design my PP amps to run on fixed bias with a 10r cathode resistor and variable bias supply, this makes adjustment easier. If your amp doesn't have this you'll need to do some calcs.
To make the subject even more complex guitar amps are often biased to get a certain tone, that could mean the bias isn't set for optimum life. For more info check out some of Uncle Dougs video's on the subject on Youtube.
Andy.
I usually design my PP amps to run on fixed bias with a 10r cathode resistor and variable bias supply, this makes adjustment easier. If your amp doesn't have this you'll need to do some calcs.
To make the subject even more complex guitar amps are often biased to get a certain tone, that could mean the bias isn't set for optimum life. For more info check out some of Uncle Dougs video's on the subject on Youtube.
Andy.
I was planning on running them at 70% (21w).
Attached is the schematic of the amp section
B+ is 472
Attached is the schematic of the amp section
B+ is 472
I know how to test the current, the R is just not on the schematic.
The bias suppy is the question.
The bias suppy is the question.
Well after playing with voltage deviders I found that about 48-50 is a good top voltae for the pot.
It is puting me in the right range of the 44ma.
What is irritating me is that the PT Bias secondary is rated at 60v but I am getting 88v.
It is puting me in the right range of the 44ma.
What is irritating me is that the PT Bias secondary is rated at 60v but I am getting 88v.
They are essentially running as triodes to all intents and porpoises. If your bias supply is adjustable and you can monitor cathode current just set em to 44mA.... ish, that's 21w. Whilst this isn't an absolute answer to your original question without finding a graph for DC conditions = Va 470v/Vg2 470v (forget the 2v) you'll just be winging it.
Valves aren't like three legged fuses a few 10's of volts either way in terms of Va or Vg2 is neither here now there. Same goes for bias voltage, there's no absolute, just set the bias to "ish" and the jobs a good un. As long as your within power dissipation max for the particular flavour of your 6L6, you'll be right.
More important is your Vg2 max power dissipation, lots of guitar amps sail very close to the wind where this is concerned resulting in premature valve failure and burnt out g2 stoppers.
Andy.
Edit, posts crossed. 60v AC RMS equates to 84v DC, or about 80v loaded.
Valves aren't like three legged fuses a few 10's of volts either way in terms of Va or Vg2 is neither here now there. Same goes for bias voltage, there's no absolute, just set the bias to "ish" and the jobs a good un. As long as your within power dissipation max for the particular flavour of your 6L6, you'll be right.
More important is your Vg2 max power dissipation, lots of guitar amps sail very close to the wind where this is concerned resulting in premature valve failure and burnt out g2 stoppers.
Andy.
Edit, posts crossed. 60v AC RMS equates to 84v DC, or about 80v loaded.