On a SF Fender Champ build I posted before. Question about math.

[catnine]

I was trying to calculate the 6V6 mA draw. It seems that a 12ax7 draws 3 mA for both plates. I have the rail screen resistor of 989 ohm it drop's 3.25 volts across it and then a 9.90K ohm drops 21.5 volt then off to the 12ax7 100K plate resistors. A 383 VDC B+ .

I pulled the 12ax7 to remove it's draw and read across the 989 ohm and get a 1 volt drop 989/1= 0.001 ma . 1ma = 0.001 watt . this is at idle . Does this seem right? Or am I doing this all wrong? Isn't the 3.25 volt drop across the 989 ohm dropper 989/3.25= 0.003286 ma for the entire draw including the 12ax7? Or do I need to consider the 21.5 volt drop across the 9.90K resister?

I can't imagine I have only 1 ma draw on the 6v6 screen.

[g1]

You are correct, you have 1mA screen current in the 6V6. Why are you concerned with the screen current?

[catnine]

You are correct, you have 1mA screen current in the 6V6. Why are you concerned with the screen current?

I was concerned because I didn't want to kill the screens . I've been reading and found that a class A SF Champ is drawing full current at idle, I assume this means the plate and the screen don't draw more as you plug in a guitar and play. I haven't proven this out by actually doing this. I've read some use screen grid stoppers to keep the screen current down just in case yet also read it changes the feel of the amp.

Since my screen is 9 volts higher than plate because the OT DC resistance is 369 ohm across the primaries and the plate draws more current @idle than the screen does though it's 1K ohm rail resistor.

I'm not really sure if since the amp draws the most current at idle if things may change with a signal and the amp cranked , I've read it does and am trying to make sense of this.

I posted in my other topic on this build that I've never seen a real SF champ with an added screen resistor on the 6v6 to drop current.

It seemed easier to me to pull the 12ax7 rather than do the math to figure out what the plate ma was. I'm not very good at the math I just copied the fender SF Champ schematic and used the same values to built this amp then read what the voltages are when done. Then adjust the 6v6 bias to keep the 6v6 under 14 watts .

It's also confusing when I use the Weber bias calculator since they aim at 90% when I thought true class A was 100% meaning a 14 watt 6v6 should be close to 14 watts not 12 . I don't know if the 90% is because of the new production 6v6 tubes as a safety factor and it assumes 5% as screen current.

[Dave H]

I've been reading and found that a class A SF Champ is drawing full current at idle, I assume this means the plate and the screen don't draw more as you plug in a guitar and play.

It's also confusing when I use the Weber bias calculator since they aim at 90% when I thought true class A was 100% meaning a 14 watt 6v6 should be close to 14 watts not 12.

Check the data sheet. The Plate current doesn't increase with signal but the screen current does.

A tube can be set up to be "true class A" at any power. It doesn't have to be 100%. For maximum power it could be set at 100% but it's kinder to the tube to set it at 90%.

[catnine]

Check the data sheet. The Plate current doesn't increase with signal but the screen current does.

A tube can be set up to be "true class A" at any power. It doesn't have to be 100%. For maximum power it could be set at 100% but it's kinder to the tube to set it at 90%.

I wouldn't know how to read a tube chart if one came up and bit me.

[Dave H]

I wouldn't know how to read a tube chart if one came up and bit me.

It won't bite. There's no need to read any charts. It's there in writing. Look at the right hand column below for 315 plate voltage for example. Zero-signal plate current 34mA, Max-signal plate current 35mA. There's hardly any difference but Zero-signal screen current is 2.2mA and max signal screen current is 6mA that's x 2.7

[catnine]

It won't bite. There's no need to read any charts. It's there in writing. Look at the right hand column below for 315 plate voltage for example. Zero-signal plate current 34mA, Max-signal plate current 35mA. There's hardly any difference but Zero-signal screen current is 2.2mA and max signal screen current is 6mA that's x 2.7

[ATTACH=CONFIG]41510[/ATTACH]

The only thing I don't understand , well quite a few .
1) Why is it that as the plate voltage goes up on this chart the screen goes down?

2) I removed the 12ax7 just so I could read the screen ma draw across the 1K screen rail dropper without the plates of the 12ax7 I read only 1 ma of screen current even though the screens voltage is 9 volts above the plate? Could it be the JJ6V6S I'm using . Also my plate voltage is more like 366 or 339 read pin 3 to pin 8. If I put the 12ax7 back in I read 3.2 ma across the 1 k . That's what the entire 2 tubes draw at idle. I know each type brand and even many of the same brand and type draw different ma.

I'm not even sure how many ma a jj6V6S safely draws because I can't make sense of the tube chart they [provide. I do know the plates can handle 500 volt and screens 450 and I'm no where near that.

[eschertron]

I know you've heard it before, but for convenience sake why not put a 100R or 1k resistor on the screen to make reading screen draw easier? A second bonus of that is - if sized right - the resistor protects against excessive screen draw becoming expensive.

My guess to why plate volts would go up (not looking at any chart here) is that as the screen current increases, the screen is capturing much more of the electron beam, 'starving' the plate. Not sure how many other tubes exhibit the same behavior.

[catnine]

I know you've heard it before, but for convenience sake why not put a 100R or 1k resistor on the screen to make reading screen draw easier? A second bonus of that is - if sized right - the resistor protects against excessive screen draw becoming expensive.



My guess to why plate volts would go up (not looking at any chart here) is that as the screen current increases, the screen is capturing much more of the electron beam, 'starving' the plate. Not sure how many other tubes exhibit the same behavior.

I can see how reading screen draw would be easier, yet already had 470 ohm on the tube socket per some fenders, it really didn't drop one volt if I look through different years where I have the plate and screen noted down . I only point this out because I would need to go much higher to read a voltage drop. Excess screen draw I can see.

I'm lost here on the plate voltage going up , I don't have that issue.

[Jazz P Bass]

Good reading: Tubes

[Dave H]

1) Why is it that as the plate voltage goes up on this chart the screen goes down?

In the 315 plate voltage example the screen voltage is only 225V (90V lower than the plate). In the two lower plate voltage examples the plate and screen are at the same voltage.

[Justin Thomas]

Good reading: Tubes

Without a lot of calculus for those of us without the benefit of a 1960s high school education! Probably one of the best sites I've ever read that distills concepts and ideas into everyday (to us) language. Fan't verify the info, but the fact that i could follow it at 3a.m. says a lot. Thanks for that!

Justin