Help with Biasing a Bassman 5F6A

[carrejans]

Hi, I built a 5F6A Bassman, with help of this layout:
http://www.ceriatone.com/images/layo...one_100508.jpg

All is working fine; now I wanted to bias the power tubes. Never done this before. So, I need a little help.

I use JJ 6L6GC's. The datasheet says the maximum plate dissipation is 30W.
http://www.jj-electronic.com/pdf/6L6%20GC.pdf

I allready did some measurements.
With the bias pot at minimum I have
Anode Voltage: 445V
Cathode Current: 18.8mA
So, plate dissipation: 8.4W

With the bias pot at maximum I have
Anode Voltage: 417V
Cathode Current: 50mA
So, plate dissipation: 20.85W


I checked with my guitar, and to be honest, I didn't hear a difference between different bias settings. (I did hear an increase in noise, with higher cathode current) What kind of difference should I hear?

What is the most common bias setting? Very near the maximum dissipation of the tube? (not really possible, because the max I can get is about 21W)


Another question: For matched tubes, the cathode currents should be as close as possible, I assume? How many milliamperes may they differ, to still be considered "matched"?
(I measured them both with the minimum bias: 17.3mA and 18.8mA)


Thanks!!

[tubeswell]

Try biasing it to about 18W. 21W will still be okay, but 8W is probably too low and you will probably have issues as you crank it.

[carrejans]

Try biasing it to about 18W. 21W will still be okay, but 8W is probably too low and you will probably have issues as you crank it.

What is this 18W based on?
Thanks...

[MWJB]

15-17W dissipation is typical for these amps (so 30-35mA), the JJ will take the 21W Tubeswell mentions (your 50mA setting) but I wouldn't try this with any other current production 6L6 type tube (stay below 17W).

If you like 50mA per tube, then great, but also note that your plate voltage will be on the low side for a 5F6A...possibly not an issue if you are not going head to head on a stage with higher voltage versions.

W dissipation is simply plate current multiplied by plate voltage, as your plate voltage drops as you increase current, you need to measure both at any given time. There is no right/wrong, as long as you have 30mA min...up to what your power tubes will safely take, whatever sounds best is right.

You never usually bias fixed bias amps to max plate dissipation, & rarely even bias cathode biased 6L6 amps to max plate dissipation.

<15% is a fair standard for a matched pair of power tubes, yours are fine.

Higher currents will give a thicker, greasier tone, richer sustain with increased envelope of sag/decay. Moderate currents (30-40mA) will give better fidelity & dynamics. Lower currents (<28mA?) may sound thin & sterile with poor sustain...low currents may only be of interest to harp players, in order to curb feedback.

[tubeswell]

MWJB got it covered

[carrejans]

Thank you for all the info!! Much appreciated.

If you like 50mA per tube, then great, but also note that your plate voltage will be on the low side for a 5F6A...possibly not an issue if you are not going head to head on a stage with higher voltage versions.

What do you mean with this?



So, can I assume that tube life will be very long, due to the fact that you bias it at 17W, while it can handle 30W max?

[MWJB]

"What do you mean with this?" A 5F6A running 430-440vdc will not be as loud as a typical 5F6A running 470vdc+, sometimes over 500v for a reissue. That does not mean that the higher voltage amp will have a nicer tone, just that if you put your lower voltage amp side by side with a higher voltage amp (with similar efficiency speakers etc), the higher voltage amp will be louder. So don't be offended if some oik with an off the shelf amp isn't blown away with yours, because his blows it away volumewise (irrespective of how sweet the tone).

"So, can I assume that tube life will be very long, due to the fact that you bias it at 17W, while it can handle 30W max?" Not really, a couple of thousand hours is a fair tube life, depends on how hard you play. Be aware that, although you may have 30mA per tube at idle, when you play a note, the current will fly up way over 100mA, maybe to 200mA for short periods. This is why you bias to allow some margin in fixed bias (in cathode bias there is less current swing from idle, so tubes can be biased hotter). Tubes biased to 50mA will get significantly hotter than those biased to 30mA. It is normal to bias 6L6 to 50-70% plate dissipation in fixed bias, 6L6 Fenders.

Higher idle currents (after a point) do not make the amp louder, in fact it often has the opposite effect. How loud you need to be depends on your environment, so bias for tone in the first instance.

Bias to 10-15mA per tube and they'll last for 10 years of hard gigging, but will sound crap for guitar.

[tubeswell]

What do you mean with this?

The load on the B+ side increases as you remove the negative voltage from the grid, pulling down the B+ voltage because more current is flowing through the tube (Plate resistance will be low anyway because of the effect of the screen voltage, so its pretty much a plate current x plate voltage equation). Well that's my take on it.

So, can I assume that tube life will be very long, due to the fact that you bias it at 17W, while it can handle 30W max?

Colder biasing will increase longevity a bit. Heat is the enemy of electronics

[carrejans]

"What do you mean with this?" A 5F6A running 430-440vdc will not be as loud as a typical 5F6A running 470vdc+, sometimes over 500v for a reissue. That does not mean that the higher voltage amp will have a nicer tone, just that if you put your lower voltage amp side by side with a higher voltage amp (with similar efficiency speakers etc), the higher voltage amp will be louder. So don't be offended if some oik with an off the shelf amp isn't blown away with yours, because his blows it away volumewise (irrespective of how sweet the tone).

"So, can I assume that tube life will be very long, due to the fact that you bias it at 17W, while it can handle 30W max?" Not really, a couple of thousand hours is a fair tube life, depends on how hard you play. Be aware that, although you may have 30mA per tube at idle, when you play a note, the current will fly up way over 100mA, maybe to 200mA for short periods. This is why you bias to allow some margin in fixed bias (in cathode bias there is less current swing from idle, so tubes can be biased hotter). Tubes biased to 50mA will get significantly hotter than those biased to 30mA. It is normal to bias 6L6 to 50-70% plate dissipation in fixed bias, 6L6 Fenders.

Higher idle currents (after a point) do not make the amp louder, in fact it often has the opposite effect. How loud you need to be depends on your environment, so bias for tone in the first instance.

Bias to 10-15mA per tube and they'll last for 10 years of hard gigging, but will sound crap for guitar.

Thanks, that cleared up everything!!