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Class A 807 or 6L6 RF amp grid current




 
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Author Topic: Class A 807 or 6L6 RF amp grid current  (Read 1962 times)
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WA4WAX
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« on: July 18, 2019, 10:47:48 PM »

What would be the ball park grid current for an 807 or 6L6 run class A as a driver with about 325 Volts on the plate.  I have been told about 3 to 5 mA.

Thoughts?

Thanks!
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N1BCG
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« Reply #1 on: July 19, 2019, 09:35:26 AM »

The linear nature of Class A suggests extremely low grid current regardless of the tube for single-ended amplifiers. "Extremely low" meaning in the uA range and considered "zero bias".

Class A is as linear as they get but also low efficiency.
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« Reply #2 on: July 19, 2019, 11:01:38 AM »

class of operation is not spec'd by grid current, it is spec'd by plate conduction angle

you can have class c operation with no grid current

you can have grid current in class a operation (class a2)

back to the question ... need to check an expanded tube spec (maybe tung sol, etc) for control grid dissipation limits
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WD4DMZ
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« Reply #3 on: July 19, 2019, 11:38:38 AM »

Here is a link to 6L6 tube data. Might help.

http://tdsl.duncanamps.com/show.php?des=6L6

Rich
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« Reply #4 on: July 19, 2019, 09:22:45 PM »

Here is a link to 6L6 tube data. Might help.

http://tdsl.duncanamps.com/show.php?des=6L6

Rich


sorry Rich .... didn't see it in there
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DMOD
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« Reply #5 on: July 20, 2019, 05:43:07 PM »

What would be the ball park grid current for an 807 or 6L6 run class A as a driver with about 325 Volts on the plate.  I have been told about 3 to 5 mA.

Thoughts?

Thanks!

What is the application and are we talking AF or RF?

In pure Class A operation there is no appreciable grid current as there would be in Class AB, B, or C.
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DMOD
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« Reply #6 on: July 20, 2019, 06:49:50 PM »

Here is a Class A circuit for audio for an 807 with 6W output but as with most Class A circuits, it's inefficient and a power waster:

* 807 Class A Circuit.pdf (101.86 KB - downloaded 57 times.)
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« Reply #7 on: July 21, 2019, 01:05:03 PM »

Yes, for audio, a power waster, indeed.

But the cool thing about pure class A for audio work (and linear RF work too) is that this is the only time we can use a SIMPLE cathode resistor for grid bias -  and use an unregulated supply for the plate and screen because the three currents do not vary during operation.  Notice the cathode resistor and screen dropping resistor in DMOD's class A 807 circuit. Simple and easy. If the currents varied during operation, those voltages would vary too, causing distortion. 

Also, there is little chance for crossover distortion with the pure class A signal on all the time, whereas, with class B p-p, there are two separate signals using different physical tube components that need to be combined together - potentially creating signal errors.

Notice the big plate power resistor. Unless this is just a driver,  we still need to get it down to 8 ohms to drive a speaker somehow... unless this is a Heising modulation circuit or other higher impedance use.


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T
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« Reply #8 on: July 21, 2019, 05:57:11 PM »

Yes, for audio, a power waster, indeed.

But the cool thing about pure class A for audio work (and linear RF work too) is that this is the only time we can use a SIMPLE cathode resistor for grid bias -  and use an unregulated supply for the plate and screen because the three currents do not vary during operation.  Notice the cathode resistor and screen dropping resistor in DMOD's class A 807 circuit. Simple and easy. If the currents varied during operation, those voltages would vary too, causing distortion. 

Also, there is little chance for crossover distortion with the pure class A signal on all the time, whereas, with class B p-p, there are two separate signals using different physical tube components that need to be combined together - potentially creating signal errors.

Notice the big plate power resistor. Unless this is just a driver,  we still need to get it down to 8 ohms to drive a speaker somehow... unless this is a Heising modulation circuit or other higher impedance use.



No free lunches.

T

For sure!

He can use the circuit on Page 1 if it's truly audio but we still don't know if the OP's application is Audio or RF??? Huh


Phil - AC0OB

* 807 Class A Circuit.pdf (128.49 KB - downloaded 44 times.)
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« Reply #9 on: July 21, 2019, 08:56:09 PM »

The thread is called  'Class A 807 or 6L6 RF amp grid current' So why the question and the advice for an AF amp
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DMOD
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« Reply #10 on: July 21, 2019, 09:20:56 PM »

The thread is called  'Class A 807 or 6L6 RF amp grid current' So why the question and the advice for an AF amp

I can only speak for myself but I did not see any "RF" in the original post on day 1 and the OP never has responded to the question as to the application which would have given us some context.  Shocked



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« Reply #11 on: July 22, 2019, 08:26:38 AM »

AF or RF, class A should be about the same efficiency for either, below the frequency at which the tube starts to become less efficient, isn't it?

The grid current may depend on where the bias is set, to allow more drive to reach A2 operation, increasing the power until the tube current is saturated, or may be due in a smaller way to reactance of the grid at higher frequencies.

The second post (first answer) looks good, compare 6L6 grid current to 807, or see some old transmitter using the 6L6 as the RF tube. There is a maximum to grid gurrent to observe, then plate dissipation, and saturation. When one of those is reached that may be a good answer to the grid current question.
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« Reply #12 on: July 22, 2019, 12:41:12 PM »

An RF App. is Simple Enough - see page 2:

Phil - AC0OB

* 807 Class A Circuit.pdf (133.26 KB - downloaded 56 times.)
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WA4WAX
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« Reply #13 on: July 22, 2019, 01:30:34 PM »

Are you sure 100 ohms is the right value for that divider?

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DMOD
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« Reply #14 on: July 22, 2019, 02:19:35 PM »

Are you sure 100 ohms is the right value for that divider?



What divider and are you referencing page 1 or 2 and exactly what is this application? Giving us some context such as previous stage-next stage really helps us attempt to help you.

If this is for Area 51 use then send me a PM as I still have a TS clearance and crypto capabilities.


Phil

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« Reply #15 on: July 22, 2019, 02:39:32 PM »

Sorry!

That would be the divider in the RF amp circuit, which was the last one you posted.

Thanks!
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« Reply #16 on: July 22, 2019, 02:40:24 PM »

In other words, page 2.
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DMOD
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« Reply #17 on: July 22, 2019, 03:02:03 PM »

In other words, page 2.

Here is the Class A RF stage for the 6L6:

Ok on page 2 of the 807 schematic what are you referring to as I don't see any dividers.

Answer this: Exactly what is this application? Giving us some context such as previous stage-next stage really helps us attempt to help you. If this is for Area 51 use then send me a PM as I still have a TS clearance and crypto capabilities.


Phil - AC0OB

* 6L6 Class A RF Circuit.pdf (107.03 KB - downloaded 45 times.)
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« Reply #18 on: July 22, 2019, 03:15:32 PM »

It would be for an RF amp used to drive a pair of 809's.

What is dropping the 500 VDC supply voltage down to 300 VDC at the plate?

Thanks!
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« Reply #19 on: July 22, 2019, 03:34:52 PM »

It would be for an RF amp used to drive a pair of 809's.

What is dropping the 500 VDC supply voltage down to 300 VDC at the plate?

Thanks!

OK, I see what you're saying now and my apologies for not seeing that.

The 100 ohm was a bad swipe, but it is not a voltage divider and it was for de-coupling RF below the choke before the power supply.

Here are the proper values to drop the plate voltages down to 300 volts and still RF shunt the choke.

I also added a 0.01 uF to take DC off the tuning cap in case you don't want DC on it. I do that in case the tuning cap plates might short.


phil - AC0OB

* 6L6 Class A RF Circuit.pdf (104.16 KB - downloaded 41 times.)
* 807 Class A Circuit.pdf (130.98 KB - downloaded 45 times.)
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« Reply #20 on: July 22, 2019, 03:50:46 PM »

Why do you lower the anode voltage from 500VDC to 300VDC? The 807 feels very happy with 500VDC at the plate and there is a lot of heat dissipated in the 20 W resistor.
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« Reply #21 on: July 22, 2019, 04:08:08 PM »

Why do you lower the anode voltage from 500VDC to 300VDC? The 807 feels very happy with 500VDC at the plate and there is a lot of heat dissipated in the 20 W resistor.

I agree but that's the spec the OP gave. In fact, he never stated his supply voltage so I had to assume it was 500 volts for the designs.

Maybe the OP could now tell us his power supply voltage.

If he would like to run the plate at 500 volts, Page 3 has an update.

 
Phil - AC0OB

* 807 Class A Circuit.pdf (159.79 KB - downloaded 43 times.)
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« Reply #22 on: July 22, 2019, 06:14:44 PM »

Very good!

I am thinking of about 375 to 400 VDC.......I have one of those Heath-Zenith regulated supplies, so 400 is the top voltage.

What would be the driving voltage in RMS volts? Any idea?

Thanks!
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« Reply #23 on: July 23, 2019, 03:56:47 AM »

Very good!

I am thinking of about 375 to 400 VDC.......I have one of those Heath-Zenith regulated supplies, so 400 is the top voltage.

What would be the driving voltage in RMS volts? Any idea?

Thanks!

What do the spec sheets say for the intended 809 plate voltages.

https://frank.pocnet.net/sheets/049/8/809.pdf

Phil

* 807 Class A Circuit.pdf (106.12 KB - downloaded 36 times.)
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« Reply #24 on: July 23, 2019, 01:35:56 PM »

RF input to 807 grid > 20V p-p across 100k

Power Input to Dual 809 150W maximum at 750V

Maximum Output Power expected = 105 Watts


Phil - AC0OB

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* 807 809 RF Circuit.pdf (115.32 KB - downloaded 44 times.)
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