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Old 18th December 2008, 06:25 PM   #1
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Default Pentode Driver Calculations

I am trying to design a pentode driver stage using a D3a and I am stumped. Attached are the curves with the load line I was hoping to use. It is a B+ of 150V, bias of 1.25V with a 2500 ohm plate load. This results in a gain around 75 and a Z out around 2K5 which is about what I am looking for.

The stumbling block is that the there is only 100V on the plate and 150V on G2. The valve wizard website gives this dire warning : "
Quote:
It should be noted that if the screen grid were to be connected to a higher voltage than the anode, then anode current would drop rapidly while screen current would rise rapidly or 'runnaway' (it would be acting as the primary anode), and will quickly exceed its maximum ratings and be destroyed.
and I seem to fall afoul of it. It does look like I can simply move everything to the right by increasing B+ to ~225V, except then I am running up against dissipation limits. The datasheet recommended operating points are right at the dissipation limit, but I do like to run things a little bit conservatively which maybe isn't really an option here. The input signal will drive it over the limit, though.

The issue seems to be that, if this warning is true, there is a very small usable range for this tube and I am not sure how to get around that.

Suggestions?
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Old 18th December 2008, 06:40 PM   #2
SY is offline SY  United States
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I would suspect that this warning is more applicable to quiescent conditions. Think about it: nearly every pentode power stage has a screen voltage not too far below the plate voltage. Under a positive-going drive signal, the plate voltage immediately swings below the screen. On average, the plate may be higher in voltage, but not under all instantaneous conditions.

I wouldn't hesitate to increase the B+ and use a higher plate load (more horizontal load line).
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Old 18th December 2008, 06:47 PM   #3
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Quote:
Originally posted by SY
I wouldn't hesitate to increase the B+ and use a higher plate load (more horizontal load line).
My concern with the higher plate load is that the gain goes up really fast, as does the Z out. I am trying to drive the grid of the next stage, and too much gain is going to be a problem.

But, it does seem that higher B+ is a must.

Quote:
Originally posted by SY
I would suspect that this warning is more applicable to quiescent conditions. Think about it: nearly every pentode power stage has a screen voltage not too far below the plate voltage. Under a positive-going drive signal, the plate voltage immediately swings below the screen. On average, the plate may be higher in voltage, but not under all instantaneous conditions.
As I think about it, on my guitar amp SG is always higher than the plate without a signal. Perhaps this is just close enough that it can't runaway.

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Old 18th December 2008, 06:52 PM   #4
SY is offline SY  United States
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Quote:
I am trying to drive the grid of the next stage, and too much gain is going to be a problem.
I have two words for you: triode connection.
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Old 18th December 2008, 07:10 PM   #5
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Quote:
Originally posted by SY
I have two words for you: triode connection.
But I've already done that. I'm just experimenting at this point.
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Old 19th December 2008, 01:18 PM   #6
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally posted by dsavitsk


As I think about it, on my guitar amp SG is always higher than the plate without a signal. Perhaps this is just close enough that it can't runaway.]
You can run the screen higher than the anode if you don't intend on overdriving the valve (that's how all those old Feners were designed to be operated, remeber)
However, if you DO intend to overdrive it then you will need a big resistor in series with the screen to limit its dissipation (this will also lower the screen voltage a bit too of course, but it's either that or be prepared to buy new valves quite often. (Fenders are a bit notorious for buring out the screen sof 6V6s: http://i81.photobucket.com/albums/j2...we/Closeup.jpg)
In this case your overall voltages are fairly innofensive, so you may be ok with a modest resistor, say 1k 1W. But I will iterate that screen resistors are absolutely essential on power pentodes in guitar amps, and anywhere the screens are fed directly from the HT.
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Old 19th December 2008, 02:13 PM   #7
Yvesm is offline Yvesm  France
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Quote:
Originally posted by SY

. . .
I wouldn't hesitate to increase the B+ and use a higher plate load (more horizontal load line).
Agreed !

Quote:
Originally posted by SY


I have two words for you: triode connection.
Disagreed

But unless you need large gain AND large drive, using a pentode is not necessarly the best option

A similar tube driving a greedy triode:

As it should be:
Click the image to open in full size.

As it really is:
Click the image to open in full size.

Yves.
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Old 19th December 2008, 02:36 PM   #8
45 is offline 45  Italy
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Quote:
Originally posted by Yvesm



As it should be:
Click the image to open in full size.

Yves.
Yvesm, the static load for the EL802 is not 12K in your amp.
It is around 10K (12K in parallel with 68K).

It doens' t change a lot in this case but in other circumstances there could be a huge difference.

Cheers,
45
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Old 19th December 2008, 03:46 PM   #9
Yvesm is offline Yvesm  France
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Quote:
Originally posted by 45


Yvesm, the static load for the EL802 is not 12K in your amp.
It is around 10K (12K in parallel with 68K).

It doens' t change a lot in this case but in other circumstances there could be a huge difference.

Cheers,
45
Indeed
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Old 19th December 2008, 04:56 PM   #10
dtut is offline dtut  United States
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In at least three of the Mullard circuits, the EF86 front end has a higher quiescent voltage on the screen than on the plate. I've never quite understood how that works.
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