What is the normal bias current for different tubes?

[Alan0354]

I am going to compare sound of different power tubes in the same amp. I have 6L6, JJ 6V6S, 5881 and EL34. I want to know the normal bias current for 5881 and EL34. My +B is about 440V.

For 6L6, I read 33mA at -46V and 39mA at -43V. I set the JJ 6V6S to -36V for 27mA. Is that in the ball park?

I have no idea what to set for 5881 and EL34. Please advice. Also I want to confirm I can use EL34 in 6L6 amp if I adjust the bias.

Thanks

[Mick Bailey]

Is it SE or PP? Either way you need to make sure your PT can supply the increased current for EL34s. Also check your OPT current rating.

Also, is the 5881 NOS or a Sovtek 5881? The Sovtek is closer to a 6L6GC. Plate dissipation for the older 5881 is much lower.

Most 6L6 amps need the socket rewiring to take an EL34, but once done the two are interchangeable.

[Alan0354]

Is it SE or PP? Either way you need to make sure your PT can supply the increased current for EL34s. Also check your OPT current rating.

Also, is the 5881 NOS or a Sovtek 5881? The Sovtek is closer to a 6L6GC. Plate dissipation for the older 5881 is much lower.

Most 6L6 amps need the socket rewiring to take an EL34, but once done the two are interchangeable.

It is in the SF Pro Reverb 40W designed for 6L6, so it's a push pull.

What wire needs to be change to fit the EL34?

The 5881 is a Mojo Tone.

Thanks

[jazbo8]

You can get the ballpark figures for various tubes from this calculator.

[Enzo]

As far as I know, any current production 5881 is just a 6L6. Real 5881s haven't been made in years.


There is no normal bias. That is like asking what is the normal speed for a car. Normal speed for 8 cylinders? Normal speed for 6 cylinders?


I think it sucks for guitar amp, but some guys adjust bias to eliminate crossover notches - the hifi approach. other guys like the 70% dissipation rule of thumb. But of course there are 80% guys and 60% guys. On the other end, a gain monster amp like a Peavey 5150 comes stock at between 20-25% dissipation.


Your current versus bias voltage numbers will vary tube to tube. -46v could result in 33ma, or 43ma. Bias voltage is not standardized, you just set it to whatever gives you the target performance current.

At 440v B+ 33ma yields 14 watts, and 39ma yields 17 watts. Both those are fairly cool 6L6 number to a 70% guy.


If you want to pick a tube to design an amp around, let me suggest tune each tube to its best sound, then compare THAT. After all, once you make an amp, you won;t be setting it to an arbitrary, sake-of-argument idle current, you will be setting it to its best sound. So if that results in Cool 6V6 versus hot 6L6, so be it.

[Enzo]

6L6 tubes do not use pin 6. So in many amps that use them, pin 6 on th socket is used as a convenient place to solder parts, like the other end of the screen resistor in a Fender.

The problem is pin 1. The 6L6 doesn;t use that either, so pin 1 in a Fender is often where the other end of the grid stopper resistor sits. But in an EL34, pin 1 is the suppressor grid and must be tied to the cathode - pin 8 - or other alternative. Pin 8 is the most common.

So any amp to use both will have pin 1 wired to pin 8, and any other stuff that WAS there will be moved elsewhere.

[Alan0354]

6L6 tubes do not use pin 6. So in many amps that use them, pin 6 on th socket is used as a convenient place to solder parts, like the other end of the screen resistor in a Fender.

The problem is pin 1. The 6L6 doesn;t use that either, so pin 1 in a Fender is often where the other end of the grid stopper resistor sits. But in an EL34, pin 1 is the suppressor grid and must be tied to the cathode - pin 8 - or other alternative. Pin 8 is the most common.

So any amp to use both will have pin 1 wired to pin 8, and any other stuff that WAS there will be moved elsewhere.

Thanks for you detail response on the bias setting.

I think I am going to pass on the EL34, I want to Pro reverb to be as stock as possible, I don't even want to change it. I treasure my Pro Reverb, don't want to burn any plastic insulation of the wiring by accident sticking the soldering iron into that tight spot. I might do it on my Bassman platform as it's all over the place already!!!

Thanks

[g1]

As far as I know, any current production 5881 is just a 6L6. Real 5881s haven't been made in years.

The new Tung-sol 5881 re-issue is like the original, (lower plate dissipation and voltage rating) so it is not suitable for many 6L6 circuits.

What is the purpose of your test? The data will really only be applicable to this amp with this output transformer. Some of the tubes will be better matched to the OT impedance than others, so that will skew your results.

[Alan0354]

As far as I know, any current production 5881 is just a 6L6. Real 5881s haven't been made in years.


There is no normal bias. That is like asking what is the normal speed for a car. Normal speed for 8 cylinders? Normal speed for 6 cylinders?


I think it sucks for guitar amp, but some guys adjust bias to eliminate crossover notches - the hifi approach. other guys like the 70% dissipation rule of thumb. But of course there are 80% guys and 60% guys. On the other end, a gain monster amp like a Peavey 5150 comes stock at between 20-25% dissipation.


Your current versus bias voltage numbers will vary tube to tube. -46v could result in 33ma, or 43ma. Bias voltage is not standardized, you just set it to whatever gives you the target performance current.

At 440v B+ 33ma yields 14 watts, and 39ma yields 17 watts. Both those are fairly cool 6L6 number to a 70% guy.

Is the wattage for a single 6L6? What is the formula for calculate output power?

I do use a bias probe, not just the voltage. Actually I just finished building the bias probe from a kit bought in Tube Depot. It's a nightmare building that thing. Instruction is not good, the two parts just don't fit tight. Turn out the posts from the top socket bottomed out. But it took me a long time to realize that because unless you have a X-Ray eye, you can't see the inside where it got stuck. I giggle the resistor so much I broke the lead of the resistor. I had to scramble to get a combination of resistors to get the 1ohm. You would think it's a cake walk to build such a simple thing, WRONG!!!

When you say 70%, do you calculate the plate current times the plate voltage? Like for 6V6GT that is spec for 12W. If I bias to 25mA and +B is 400V, the dissipation is 400X0.025=10W. So the bias is (10/12)X100%=83%. Am I right?

Thanks for you info.

[Mick Bailey]

If you use a bias probe you're measuring the cathode current. All of the current through the tube is being measured at this point and includes screen current as well as plate current. A bias probe will always over-read compared to (say) the transformer method. Your bias calculation is correct but doesn't factor screen current.

A bias probe also won't tell you if the plate dissipation is low, but the screen is drawing excessive current through a fault condition.

My advice for tube experimentation is to build an SE amp like the THD Univalve. I've got my own take on this idea but with fixed bias. It runs any octal tube including KT88s.

[Alan0354]

If you use a bias probe you're measuring the cathode current. All of the current through the tube is being measured at this point and includes screen current as well as plate current. A bias probe will always over-read compared to (say) the transformer method. Your bias calculation is correct but doesn't factor screen current.

A bias probe also won't tell you if the plate dissipation is low, but the screen is drawing excessive current through a fault condition.

My advice for tube experimentation is to build an SE amp like the THD Univalve. I've got my own take on this idea but with fixed bias. It runs any octal tube including KT88s.

I am doing the bias at 0 input. The screen current is less than 1/6, more like 1/10. So it will be easy to compensate this.

Actually because I broke the 1 ohm resistor when assembling the bias probe, I end up using two 1/4W 1.8ohm in parallel to give 0.9ohm. That will be 10% lower value. So if I use 1mV per mA, that will lower the reading by 10% and that covers the screen current!!!! Right?


Another question, For me to get 45mA with one 6L6, I have to adjust the bias to about -40V. That is about 66% only. Fender spec they bias to -50V give and take. Does that mean Fender bias much colder than 66% full power, OR does that mean my tube is old?

[Gingertube]

The reason many amps are biased for 70% of max output tube dissipation is that they violate the max Rg1 recommendations. If you bias at 70% the Rg1 can be twice the recommended maximum. That makes the load on the phase splitter easier. If you don't know the max Rg1 value for a tube then I would not bias at more than 70% of rated maximum - unless cathode bias is used.

Cathode Bias will always have a higher max Rg1 value and it is safer to run the tubes at higher currents.

Max Rg1 values for fixed bias / cathode bias
6V6GT 100K / 500K
6L6GC 100K / 500K
5881 100K / 500K
EL34 220K/700K
These values apply at maximum anode dissipation. At 70% dissipation the values can be multiplied by 2.
So your typical fixed bias Fender etc. with 220K Rg1 resistors on the output tube grid should never be biased above 70% dissipation limit.

Cheers,
Ian


Cheers,
Ian

[Alan0354]

The reason many amps are biased for 70% of max output tube dissipation is that they violate the max Rg1 recommendations. If you bias at 70% the Rg1 can be twice the recommended maximum. That makes the load on the phase splitter easier. If you don't know the max Rg1 value for a tube then I would not bias at more than 70% of rated maximum - unless cathode bias is used.

Cathode Bias will always have a higher max Rg1 value and it is safer to run the tubes at higher currents.

Max Rg1 values for fixed bias / cathode bias
6V6GT 100K / 500K
6L6GC 100K / 500K
5881 100K / 500K
EL34 220K/700K
These values apply at maximum anode dissipation. At 70% dissipation the values can be multiplied by 2.
So your typical fixed bias Fender etc. with 220K Rg1 resistors on the output tube grid should never be biased above 70% dissipation limit.

Cheers,
Ian


Cheers,
Ian

What is Rg1? Why there's a max value?

[Gingertube]

Rg1 is the "grid leak" resistor. That is the resistor that ties the output tube control grid (grid 1) to the -ve bias supply ( or to 0V in the caase of cathode bias). Grid current flowing out of the grid causes a voltage drop across this resistor causing the grid ro move in the positive direction. This voltage drop subtracts from the tube bias causing the tube current to increase, that causes more grid current - round and round it goes until boom! the tube runs away and melts.
The degree of grid current imposes a maximum value on Rg1 so as to not disturb the bias too much or cause run away.

At 70% dissipation there is less grid current (about half) so max Rg1 value can be relaxed a bit (about X2).

In cathode bias there is an opposing mechanism to the bias reduction due to the voltage across Rg1 (as current goes up then there is more bias voltage developed across the cathode bias resistor), that is why the Rg1 max limit is much higher for cathode bias than for fixed bias.

Cheers,
Ian

[Enzo]

remember, that 70% is idle dissipation, it is not output power.


yes, screen current and plate current are added together in cathode current, but most of us ignbore that. Then it winds up the tube is actually running a small amount cooler than our calculations, and that doesn;t hurt anything.


yes, plate voltage times plate current is plate dissipation. Note it is the voltage across the tube, not necessarily to ground. In our fixed bias amps, the cathode usually is at ground, so in those cases it is the same, but in a cathode biased amp, you would subtract the cathode voltage from the plate voltage first.