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SS Followers to drive power tubes

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gingertube

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Joined 2004
#1
Guys,
I am using ZVN0545A MOSFET Source followers (direct coupled) to drive output tube grid 1 (with the output tube bias applied to the mosfet gate). I use a "Ring of Two" BJT current source for the source load.
I recently saw an article (which I can no longer find) where it was suggested that a bipolar emitter follower would do a better job than the MOSFET source follower in this duty.

Can anyone confirm, refute, offer a WAG as to whether this is indeed correct?

Thanks,
Ian
 
#2
gingertube said:
Guys,
I am using ZVN0545A MOSFET Source followers (direct coupled) to drive output tube grid 1 (with the output tube bias applied to the mosfet gate). I use a "Ring of Two" BJT current source for the source load.
I recently saw an article (which I can no longer find) where it was suggested that a bipolar emitter follower would do a better job than the MOSFET source follower in this duty.

Can anyone confirm, refute, offer a WAG as to whether this is indeed correct?

Thanks,
Ian
Click to expand...

If the driven grids stay in Class *1, it really doesn't make much difference. If the grids are driven positive, then it does make a big difference. For emitter followers, the input impedance is dependent on the load impedance. That's why you often see cascaded emitter followers used when you want a Hi-Z input. What you don't want happening is that varying load on the stage that connects to the emitter followers. This is why FETs are preferred for grid drivers: the input impedance doesn't vary with a steeply varying load, which is what Class *2 grids are.
 
#4
If I remember well I read somewhere that Allen Wright thinks that tubes sound better than SS around the position of a cathode follower. Would it be a good idea to have the Wright/Rasmussen all tube SLCF before outputtubes in a pp amp ? Or is this restricted to pre-amp territory ? Like to try that some day.
 
#5
I believe you'd need a very large amount of output tubes to go to the bother of adding SLCFs to drive them.

Our dpa300b amps get by with a cascoded 6H30 running at 15mA very nicely - and even our new 150 watt amp with paralleled push-pull KT88s also uses cascoded 6H30's, although now at 20mA.

KISS is my key principle in signal path design, and I'd never add an extra stage unless essential.

Regarding MOSFETs, my opinion is that they have NO PLACE in an audio signal path, either as source followers or gain elements - NONE!

Regards, Allen
 
#6
Allen Wright said:
.......

Regarding MOSFETs, my opinion is that they have NO PLACE in an audio signal path, either as source followers or gain elements - NONE!

Regards, Allen
Click to expand...

you are old enough and toobie enough to have all my respect and love ..... - but say that to Papa ........

I had impression that topology is always more important in your designs than parts ...... so I'm kinda surprised

:rofl:

in any case - your writings regarding Tectronix-SLCF topology urged me (many moons ago) to try any (known to me ) active part in that manner , and for my ears what you wrote is true - dang above that cascode , dang bellow that CCS - and active part is invisible

:cheers:
 
#7
Zen Mod,
>>I had impression that topology is always more important in your designs than parts ...... so I'm kinda surprised<<

So was I. I thoght theMOSFET'd SLCF was just fine, until I made up the all tube version - and then I heard the difference!

>>in any case - your writings regarding Tectronix-SLCF topology urged me (many moons ago) to try any (known to me ) active part in that manner , and for my ears what you wrote is true - dang above that cascode , dang bellow that CCS - and active part is invisible <<

Sorry, don't quite follow you here - you mean you like the MOSFET in thr SLCF or you don't?

Regards, Allen
 
#8
Curiously, I went the other way- I used to use tubes for driving AB2 output stages, but got much better results when I switched to MOSFETs.

Of course for screen drive, most CFs will curl up and die. It takes a MOSFET to deliver the current punch necessary. I've seen comparative results with tubes appropriate for that service (like 6S45), and the MOSFET still wins.
 
#10
At the risk of being off topic, what about bandwidth limiting? If you had cheap iron, an SS follower might make things worse with a wider bandpass, no? In this case, wouldn't you be forced to add rolloff at the stage before the follower?
 
#11
leadbelly said:
At the risk of being off topic, what about bandwidth limiting? If you had cheap iron, an SS follower might make things worse with a wider bandpass, no? In this case, wouldn't you be forced to add rolloff at the stage before the follower?
Click to expand...

You might have that problem regardless of whether you use a source follower or a cathode follower. Since these connections avoid Cmiller, the BW will always be higher. If you're using cheap output iron and gNFB, you will probably need some sort of frequency compensation to avoid instabilities anyway.

Get better OPTs, and don't be relying on gNFB to correct your mistakes. :spin:
 
#12
Bringing up an old thread.

Considering using MOSFETs (IRFU310 maybe) as source followers in an OTL.

Using a LTP with small pentode as current sink then 2 mosfets (1 for each split phase) as CF to drive the 6080 in a circlotron design.

Looking at Tubelabs powergrid I'm still not sure how to work out the optimum bias voltages I need to set up the mosfet for the 6080's.

I'll have a look later again to get my head around it but if anyone has any advice that would be appreciated.

This is a cool illustration:

http://www.falstad.com/circuit/e-mosfollower.html
 
Last edited:
#13
Would this mosfet IRF be suitable to drive the 6080's in a circlotron OTL?

Using this simulation what figures would be best suited as a CF between the LTP and 6080 drivers?

Let's say we have 100 volts on the plates of the 6080.
What max drive do we need for these? Grid voltage of around at least -30 volts at 100ma?

http://www.falstad.com/circuit/e-mosfollower.html

150/-150 v on the drain and source? How much current do I need on the source?
I'm really not sure how to do the maths for this.
 
#14
😱 not much interest from my question here so had to dig deep into google.

So if the normal negative grid bias for a 6080 is -40 volts I'd need at least double this on the source of the mosfet?
So -100 volts at least.
Am I thinking along the right lines here?


Source Follower Drive for Output Tubes

Another place MOSFETs can be used as followers is as follower drivers for output tubes. When output tubes are overdriven, their grids are often driven positive. If the coupling to the grids is by capacitor, this can lead to grid blocking/ bias shift as the grid current skyrockets when the grids go positive. This current builds up as a negative charge on the coupling capacitors and causes the tube to effectively be biased more negative (towards class B) than the static bias setting. So on overdriven notes, the output tubes are pushed into a harsh-sounding crossover distortion. In addition, the usual 12AX7 or 12AT7 phase inverter plate impedance is so high that it can't pull the output tube grid positive in the face of grid current, so the grid of the output tube is hard-clipped at Vgk=0V. This clipping can be as harsh as any solid state situation.

The solution to the class B bias shift and grid current clipping is to drive the output tube grids direct coupled from a cathode follower. The follower is set up so that its cathode sits at the normal negative bias voltage of the output tube, maybe -10V to -40V depending on the tube type. This means that the cathode resistor is tied to a more negative voltage of something more than twice the bias voltage, so it can pull the output tube grid negative. The grid of the source follower is connected where the output tube grid used to be, to the bias voltage source through large resistors and through capacitors to the phase inverter. The plate is connected to a postive voltage high enough to allow full signal operation at the cathode and low enough not to damage the tube.
Click to expand...
 
#17
Brit01 said:
😱 not much interest from my question here so had to dig deep into google.

So if the normal negative grid bias for a 6080 is -40 volts I'd need at least double this on the source of the mosfet?
So -100 volts at least.
Am I thinking along the right lines here?
Click to expand...

Brit01,

Gotta work it backwards. What is full power voltage, peak-to-peak, going to be into your load? What will the load be? What is the 6080 gain going to be in the circlotron configuration?

With the answers to those questions, you can select +/- power supply voltages for the MOSFET.

I'm not looking at, nor do I remember the configuration right now, but I think the tube has a gain of two or so. If the output stage is a follower, then gain will be less than one and the actual number is very important. If you're running global NFB, then you may need to compensate for that as well.

So, many questions need to be answered. I'd build the desired output stage and power supply for it. Then assemble a test drive stage to determine the output stage gain. Allowing 20% over minimum +/- drive needed probably lets the MOSFET work well without danger of clipping the driver stage.

Remember the gain stage will need x10 gain additional to compensate for 20dB of NFB.

I'm probably wrong on half this stuff.....
 
#18
Brit01 said:
Would this mosfet IRF be suitable to drive the 6080's in a circlotron OTL?

Using this simulation what figures would be best suited as a CF between the LTP and 6080 drivers?

Let's say we have 100 volts on the plates of the 6080.
What max drive do we need for these? Grid voltage of around at least -30 volts at 100ma?

http://www.falstad.com/circuit/e-mosfollower.html

150/-150 v on the drain and source? How much current do I need on the source?
I'm really not sure how to do the maths for this.
Click to expand...

You need enough current to drive the grids to about +10 or +15 V with respect to the cathodes. That will be a fair amount, but its not crazy- you could do it easily with a CF, as I think you already know.

Since I am in the habit of using a differential voltage amplifier with bipolar power supplies, I just use the B- for the CF. You could do the same with an IRF 820. It can handle the voltages and the current. Set it to idle at 10ma and you should be in good shape!
 
#19
Since I am in the habit of using a differential voltage amplifier with bipolar power supplies, I just use the B- for the CF. You could do the same with an IRF 820. It can handle the voltages and the current. Set it to idle at 10ma and you should be in good shape!
Click to expand...

I have a doubt about the Gate to Source max voltage on the data sheet (4V).
What does this refer to?

The voltage from the previous gain stage could be 130 volts feeding into the gate of a mosfet.

Bit unfamiliar with the workings of mosfets.
 
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