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Old 4th May 2004, 04:21 PM   #1
Fuling is offline Fuling  Sweden
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Default Mu-follower current capability?


Im a complete newbie when it comes to Mu-followers, but as far as I can understand such things would be great as drivers for power tubes that draws grid current (A2).

The power tube Im thinking of is 811A, which requires about +20-25V and some 20mA into the grid at idle.
Ive powered such tubes before from beefy cathode followers but now Im looking for more elegant solutions.

According to my way of thinking the mosfet is the one that does the job, the bottom triode only controls the mosfet. Since the grid always works on positive voltages the mosfet only has to deliver current in one direction, so to speak.

Am I completely lost here???
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Old 4th May 2004, 05:20 PM   #2
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I believe so, but they would require a high current to get the low output impedance.

I have tried a 12gn7a triode strapped with a mu follower sinking 30ma. From memory acroding to gary pimm, he mention that is mu follower's rp would be determine by the current following thru the mosfet. THe higher the current the lower the rp. with 30ma rp is around 40 ohms, as i recall. So in this case the bottom tube should source more than 20ma so that the top mosfet would be able to source 20ma for the idle state. What happens with a mu-follower is basically a common anode going to a cathode follower in this case a mosfet. This way they also actively load the tube too. Basically i figure that mu-follower work as a team, tube provides voltage gain and mosfet provides current. That way they are more optimize that way.

How about try say a 12gn7a(mu=42) with a follower sourcing the 811a. With cdp max output of 2volts, gain of 12gn7a(42 tested with mu follower sourcing 30ma) and the 811a(gain of 20 at low voltages). Suitable to used WIthout feedback but 2 stage approach. By the way 12gn7a was also tested with 45ma too, great. max plate rating is 12-14watts with triode strapped. Bewarn some 12gn7a are on the microphony side but I find them to be on the neutral side and great.

I am going to build a 811a soon. Tempted to go your way, but interstages are easy to get and wound to my specs. I am thinking of going 6em7 and 811a with IT coupling. Don't like the tubes sourcing to much current. Maybe i would give your approach a try in the future. Same as your schematic. But with the mu-follower having the same B+ as the 811a.
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Old 4th May 2004, 08:15 PM   #3
Fuling is offline Fuling  Sweden
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Thanks for the answer!

One major problem here is that the tube has to be able to work at a low plate voltage, since I want to keep the voltage drop across the 811s cathode resistor as low as possible.
If it also has to sink lots of current, well, mission impossible
The 811A draws almost 100mA at the peaks.

Maybe a more ordinary triode gain stage followed by a mosfet would be a better idea in this case?
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Old 5th May 2004, 12:36 AM   #4
aurt is offline aurt  United States
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I've been wrestling with a similar problem myself. There are some good pages at the Tube Cad Journal explaining SRPP function that might apply. On the Svetlana site there is a tech bulletin for the sv83 that is a mu follower with a pentode top. This would act more like your fet, and reduce the equation for Rak to a matter of transconductance. According to Svetlana, the circuit given will drive an SV572, but that only takes 50mA.
It occurs to me that the asymetric current possibilities might lend themselves nicely to driving class A2. I was thinking of going with split +- supplies and direct coupling, allowing the bias to be derived by a set difference in supply voltage. Thinking this way, the negative swing is through the bottom triode. This can potentially mess up this topology for pushing lots of current. Good thing driving A2 doesn't need much current when driving the grid negative. The positive swing is provided by the top element. As this is a cathode follower, current is the name of the game, and it should provide nicely. I'm thinking of an EL34 or KT88 for this tube. I'm still trying to figure out a nice triode with a Va of around 600 to 800V.
So it's actually a push-pull circuit, and as you said, the top element can deliver the current when needed.
You might search for "Alan Kimmel" and "Mu Stage" for another interesting read.
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Old 5th May 2004, 08:27 AM   #5
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the mu-follower should have twice the voltage on the plate +some add. to turn on the mosfet. Or not you are going into distortion.
For instance take your B+150Vs
The triode plate voltage of say 80 volts, the tube can only swing 50-60Vpk to pk only after that it would be distorted. Normally mu-followers used cap coupling because the B+ must be quite high around 300 and above so that the bottom tube does not run out of voltage to swing. But even with mosfets they need 20 volts extra you will still be insuffice voltage swing, pentodes would be worst like the el34 needs much more around 200volts extra
Hope this helps
By the way how does the 811a sounds ? is it driver dependant? Never heard A2 before just curious? i have some 811a waiting inline to be tried out.
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Old 5th May 2004, 10:37 AM   #6
Fuling is offline Fuling  Sweden
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I dont think 811s need much more than 40-50V peak to peak, so the voltage swing is not much of an issue. At least not when compared to other big triodes...
Current swings from about 20mA at idle to 100mA at positive peaks and almost zero at the bottom. The grids internal resistance is quite nonlinear, so a low driving impedance is necessary.

Well, I kind of liked them. Unfortunately they require a bit of feedback to tame the output impedance.

I guess they are VERY driver dependent, especially because of the nonlinear load they reresent for the driver.
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