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Powerdrive

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I don't know of any accurate method of calculating the grid current in A2. This data is in the tube curves for some old tubes that were intended to be operated in class B. A2 wasn't popular in those days. The curves for the SV572-10 are no longer on the Svetlana web site, so I don't know if they ever published grid current curves.

The current rises sharply as you drive the grid more positive. It is also highly dependent on plate voltage. The higher the plate voltage, the lower the grid current. I have measured peak grid currents as high as 100 mA in some situations.
 
Ryssen said:
SV572-10
There where gridcurves,but how do I read them?Does it mean 100mA plate current,that the gridcurrent is 25mA?Plate voltage is 850v.

At that Q-point, your Vgk is about -45Vdc, and you could keep it entirely in Class A1, so there won't be any grid current considerations other than keeping the slew rate up by providing adequate charging current for Ci + Cmiller. You probably can't go much higher than Vgk= +20V anyway, and your grid current there would be around 5.0mAp.

That SV572 looks like a good one. Might have to do something with that some day. :)
 
You probably can't go much higher than Vgk= +20V anyway, and your grid current there would be around 5.0mAp.
Now,can you explain this to me?
icon_smile_blush.gif

You mean I should choose the driver tube so that Vin never goes above 62vP,that gives Vgmin +20v.What will happend if I go above?
 
Ryssen said:
Now,can you explain this to me?
icon_smile_blush.gif

You mean I should choose the driver tube so that Vin never goes above 62vP,that gives Vgmin +20v.What will happend if I go above?

You have to look at what happens at the other end. An input of 130Vp-p is likely to hit the final's cutoff point. If you do that, you'll get lots of distortion. Unforch, the plate curves given aren't complete enough to see what happens at the low end, and if it does cutoff at Vgk= -110V.
 
The curves give a fairly constant grid current for plate voltages above about 250 volts. The grid current is determined by the positive grid voltage. 20 volts positive gives you about 6 mA, 40 volts will draw 23 mA, and 80 volts will draw about 55 mA. Your TubeCad simulation shows a peak positive grid voltage of 58.7 volts. That looks like about 38 mA of grid current (from the tube curves). That was what I thought until I built a few amps and measured the grid current. As you drive the grid positive the tube is drawing some serious plate current, and the instantaneous plate voltage is swinging down. I have seen the plate voltage go as low as 50 volts on some tubes. At the moment the plate voltage is 50 volts, and the grid voltage is 58 volts, the grid will be drawing 60 to 80 mA.

The mosfet in the PowerDrive circuit provides all of this grid current. The driver tube sees the mosfets capacitance in parallel with whatever plate load you use. The original PowerDrive circuit used an IXYS 10M45 CCS IC. These provide the driver tube with about a 1 megohm (and about 5pf) load. Unfortunately these have been hard to find lately. I have had a few hundred on back order since November. The original PowerDrive also used a Toshiba 2SK2700 mosfet. These are no longer available due to ROHS.

I have built PowerDrive circuits using DN2540 mosfets for both the CCS and the follower. They sounded good, but I have not had time for paremetric testing. I have a large collection of mosfets awaiting testing, but no time yet.

Your TubeCad simulation shows a drive signal of about 200 volts peak to peak swinging from -141 volts to + 58.7 volts. The issue is not at the negative end of the signal swing. At -141 volts, there is 1530 volts across the tube and it is still drawing current (26.8 mA). The problem is at the positive end of the drive swing. The simulation shows the plate voltage dropping to 0.33 volts at a grid voltage of +58.7 volts. I have used TubeCad a lot, and generally find the results pretty close. I have never seen a tube able to pull the plate down below 1 volt. 30 to 50 volts is possible, but many transmitting tubes don't go much below 100 volts. Try reducing the drive voltage until Vp min is near 100 volts and see what you get.
 
Vin 89v gives Vp min 103.If I use this setup the driver tube must swing 178v P-P?

Yes, this is true. 178 v P-P is not too difficult, but won't happen from a 12AX7! There are several tubes that can do this with a CCS load. I use a 45 driven by a 5842 in my 845 SE but this amp needs almost 400 volts P-P. A friend used a 6BL7 in his 845 amp. I have achieved 200 v P-P with any of the dissimilar dual triodes originally intended for the vertical scan section of TV sets. I don't know what is available in your part of the world.

I have a simulation for a totally new amp topology. There is one serious drawback. It needs as much as 700 volts peak to peak of drive! I haven't figured this one out yet.
 
I have achieved 200 v P-P with any of the dissimilar dual triodes originally intended for the vertical scan section of TV sets.
Then you use both triods as voltage amps in series to get 200v P-P,or?
I thougt of using one triod like 6C45,417 or D3g as triod,might be hard to reach 200vP-P with one?
Or I might be happy with one triod and less power from the 572-10.:nownow:
The 45 seems hard to get(on ebay),been thinking of using 6B4G as drivertube,but it seems a little overkill with a powerdrive.
Any thougts?
:)
 
Ryssen said:
Then you use both triods as voltage amps in series to get 200v P-P,or?
I thougt of using one triod like 6C45,417 or D3g as triod,might be hard to reach 200vP-P with one?
Or I might be happy with one triod and less power from the 572-10.:nownow:
The 45 seems hard to get(on ebay),been thinking of using 6B4G as drivertube,but it seems a little overkill with a powerdrive.
Any thougts?
:)

The A Number One problem with using "power drive" is the extreme capacitiveness of power MOSFETs. That will tend to really pull down the high frequency response if the source impedance of the MOSFET driver isn't kept suitably low. That's where CCS loaded triodes like the 45 come in: that gives a Lo-Z source. That's where the problem lies: not many suitable Lo-u, low r(p), triodes out there, although you can get some nice, low r(p)'s from trioded pentodes, or some of the vertical deflection power amps, especially those like the 6BL7 (parallel both sections to cut the r(p) in half).

Any solution like that is apt to be "overkill" since there really aren't any low u, low r(p), small signal triodes out there.
 
The 45 seems hard to get(on ebay),been thinking of using 6B4G as drivertube,but it seems a little overkill with a powerdrive.

45's have become expensive and hard to find. I bought several a long time ago when you could still find them for $5 each. I tried the 2A3 (very similar to 6B4) as a driver in my 845 amp. It worked fine, but I liked the sound of the 45 better. A friend built a similar amp using 1/2 of a 12AT7 for the first stage and 1/2 of a 6BL7 for the second stage.

My amp uses a 5842 / 417 (again I got a bunch before they became trendy) for the bulk of the voltage gain followed by a 45. I did this because I was already making PC boards for a SE amp using this combination. I just added a big PowerDrive circuit and used that same board to drive the 845. The schematic is here (third schematic on page):

http://www.tubelab.com/845SE.htm

The same schematic was used for the 12AT7 - 6BL7 - 211 amp that my friend built. He had to tweak some resistor values. I have built an amplifier using the same circuit with a 6EM7. I did not try to drive a large tube with this amp. I just used it as a 2 watt SE amp. I see no reason why it wouldn't work though. A trioded 6V6 could probably be substituted for the 45.

I don't think that you will have enough gain to get there with one stage, but that depends on your source. Keep in mind that to get 200 volts P-P of clean drive voltage you need a tube that is linear over 200 volts of plate voltage swing. That usually means leaving some headroom at each end. Most tubes loose linearity as the plate voltage is pulled too low (same deal as in the output stage) and as the plate current drops off when the tube is approaching cutoff. So if you want 200 clean volts P-P, you want the plate voltage to swing from about 75 (or higher) volts up to 275 volts. This puts the idle voltage at about 175 volts. This is close to the limit with some of the tubes you mention. If you are loading the tube with a resistor or CCS you will need at least 300 volts of B+ for this stage. The follower in the PowerDrive povides all of the current gain, so the driver tube does not need to produce any power. An output type tube (45, 6B4 etc) is not needed. Any tube capable of delivering the required voltage swing will do.
 
I'm just curious- did you received my PM?

Yes, I did, but my reply was bounced as undeliverable.

Status:
5.1.1 (bad destination mailbox)
Diagnostic Code:
smtp; 550 (Denied by RBL dnsbl.sorbs.net)

I have had that happen before, but the recipients are mostly in Asia. I will try it again. If it comes back again:

Basically, Thanks for the offer, but I have been slowly migrating my web site to Microsoft Expression Web. It won't be ready any time soon, so I am still using Front Page for the existing site.
 
The A Number One problem with using "power drive" is the extreme capacitiveness of power MOSFETs.

Mosfets in general have a lot of input capacitance. The gate to source capacitance is often large, but in a source follower the source is at nearly the same AC potential as the gate, so this capacitance is "bootstrapped". The parameter of concern is the gate to drain capacitance or "reverse transfor capacitance". Some modern mosfets have this as low as 2 to 5 pf. Look for a reverse transfer capacitance that remains constant over the voltage range you are using. The DN2540 or FQP1N50 look good in this respect.

Still this capacitance (and the capacitance of the CCS) must be dealt with, so I tend to run the driver tube at at least 10 mA. That leaves out the wimpy 12AX7 class of tubes. There are plenty left to choose from however.
 
Official Court Jester
Joined 2003
Paid Member
tubelab.com said:


Yes, I did, but my reply was bounced as undeliverable.

Status:
5.1.1 (bad destination mailbox)
Diagnostic Code:
smtp; 550 (Denied by RBL dnsbl.sorbs.net)

I have had that happen before, but the recipients are mostly in Asia. I will try it again. If it comes back again:



:xeye: hysteric antispam cop of my ISP ..........


Basically, Thanks for the offer, but I have been slowly migrating my web site to Microsoft Expression Web. It won't be ready any time soon, so I am still using Front Page for the existing site.

:clown:
 
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