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 : "
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?
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 : "
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?
Attachments
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).
I wouldn't hesitate to increase the B+ and use a higher plate load (more horizontal load line).
SY said:
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.
SY said:
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.
An externally hosted image should be here but it was not working when we last tested it.
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)dsavitsk said:
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/j207/merlinblencowe/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.
This is a correct statement. Vg2 must be less than or equal to Vp at under quiescent condition.
You don't have to run Vg2 at 150V . By looking at your load line, I can tell that Vg2 can be dropped quite a bit. Probably down to the neighborhood of 70V. This would mean that you can keep the same plate load, just drop your Vg2. The cathode can be left un-bypassed, if your gain is excessive. The Zout will be aprox the same whether the cathode is bypassed or not in Pentode connection. I always jut assume Zout of a pentode stage is equal to the plate resistor value.
Don't be overly concerned about your cathode voltage with Pentodes. The key is to keep your eye on the ideal operating plate voltage. Say for instance you want quiescent Vp = 100V. If your Vg2 is 100V, this point may be at -1Vgk. Where as if you drop Vg2 to 75V, it may bias up to 100Vp at only -.5Vgk.
The easiest way to use small signal stage pentodes, is to first determine a plate load that you want. Then use a pot for the cathode resistor. Now, try different values of Rg2. Each time using the pot to dial in the desired Vp and then take measurements. Once you find satisfactory conditions, measure the pots resistance, find the nearest fixed resistor value, and replace the pot with the resistor.
Also, adding the resistance to g2 will improve clipping characteristics. A
dtut said:
Well, roughly, the sum of plate and screen currents (aka the cathode current) is constant.
At near zero plate voltage, the screen draws all the current alone .... and usually die !
Looking at the previous curves, you can see that, even with 170V on the screen, the largest part of the current still pass thru the plate as long as its voltage is above 50 Volts.
Thanks to the suppressor grid or whatever other method like grid alignement or critical spacing the manufacturer used to voids electrons bouncing back to the screen
Yves.
Thanks for the suggestions. They are very helpful.
Another quick question: does the next stage matter? Specifically, I know that I can't load a pentode with a choke, but can a pentode drive a choke phase splitter? Is there some inherent reason why this won't work (I feel like I have seen this claimed) -- assume, for the sake of argument that Z out will be reasonably low (between 2K5 and 3K7) and that the choke has more than enough inductance for this.
Another quick question: does the next stage matter? Specifically, I know that I can't load a pentode with a choke, but can a pentode drive a choke phase splitter? Is there some inherent reason why this won't work (I feel like I have seen this claimed) -- assume, for the sake of argument that Z out will be reasonably low (between 2K5 and 3K7) and that the choke has more than enough inductance for this.
dsavitsk said:
Just because some yutz calls himself the "Valve Wizard" doesn't mean that he knows what he's talking about. Running the screen at a higher voltage than the plate is done all the time (RF amps and sometimes vid amps, but not audio since this kind of operation is not good for distortion -- for that, you want that Vsgk as low as possible). Here's a loadline from my last project (see attached). Here, the screen is running at a higher voltage. In practice, the pentode section voltages and currents of the 6KE8 measures within 5.0% of design nominal values. What is important is staying out of the triode region. That will poof screens. Since this is for an active voltage regulator, it doesn't matter since the idea is to move the output voltage to minimize the Vgk of the error amp.
In the case of the included loadline, it leaves the saturation region at about the -0.75V Vgk line where Ip= 38mA. If you also have the forward transfer characteristics, then it's a simple matter to find what Vsgsg will give you an Ip= 38mA at a Vgk= 0V, so that the loadline will be where you want it. You will also have to check to see what Vgk will give the design nominal Ipq at the new, lower screen voltage.
Once you have the design prototype, you can also further tweak the Vsgk to see if you can improve distortion performance.
Attachments
When in doubt just examine the DC and AC load lines.
Basically pentodes aren't happy working into an open circuit or really high impedance. If you draw a load line for an open circuit or CCS (a horizontal line) you will see why. When there is a problem with an intended load, it will be visible on the loadlines.
Basically pentodes aren't happy working into an open circuit or really high impedance. If you draw a load line for an open circuit or CCS (a horizontal line) you will see why. When there is a problem with an intended load, it will be visible on the loadlines.
Jeb-D. said:
And this is what I've heard, but I am still not sure I understand it. With a triode, the horizontal choke or CCS load line is an illusion as it is swamped by the next stage which is the actual load. I figured that here it would work in the opposite direction with the choke being swamped by the low resistor load and just become the easiest part of the circuit to drive. I believe you that this isn't how it works, I am just not sure I can picture why.
Your thinking is correct. A choke load can be made to work if the next stages input impedance is on the low side.
A CCS on the other hand, can be made to look ok on the AC load line by using a low input impedance on the next stage. But if one examines the DC load line, it is not possible to determine the Q plate voltage per given plate current.
It is possible to use a CCS if you shunt it (DC path) with a resistor. However, this will cause higher gain than needed for almost all applications, have an extremely high output impedance and cause large variation in Q plate voltage between tubes. Perhaps there are some circumstances where these characteristics are ok. But for the most part, Pentodes are happier with just a resistor on the plate.
A CCS on the other hand, can be made to look ok on the AC load line by using a low input impedance on the next stage. But if one examines the DC load line, it is not possible to determine the Q plate voltage per given plate current.
It is possible to use a CCS if you shunt it (DC path) with a resistor. However, this will cause higher gain than needed for almost all applications, have an extremely high output impedance and cause large variation in Q plate voltage between tubes. Perhaps there are some circumstances where these characteristics are ok. But for the most part, Pentodes are happier with just a resistor on the plate.
I don't see a problem with that loading scheme.
Keep in mind that with a regulated cathode and g2, all of the tubes tolerance/variance will manifest itself in the plates quiescent condition . It may not be that big of an issue , unless you need the maximum swing possible from the stage.
Also, don't over due the current setting on the CCS too much. You wouldn't want to destroy g2 if the stage clips.
Keep in mind that with a regulated cathode and g2, all of the tubes tolerance/variance will manifest itself in the plates quiescent condition . It may not be that big of an issue , unless you need the maximum swing possible from the stage.
Also, don't over due the current setting on the CCS too much. You wouldn't want to destroy g2 if the stage clips.
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